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Fundamentals of Protective Coatings for Industrial Structures



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40 24th Street, Sixth Floor Pittsburgh, PA 15222 - 4656 Phone: 412-281-2331 Fax: 412-281-9993 Online: www.sspc.org

SSPC: The Society for Protective Coatings

is an international association focused on the protection and preservation of steel, concrete, and other industrial and marine structures and surfaces through the use of protective coatings. SSPC is the leading source of information on surface preparation, coating selection, coating application, environmental regulations, and health and safety issues — as they relate to the industrial protective coatings industry. The association’s many services include standards, training courses, certification programs, publications, conferences, and a variety of online resources. SSPC currently has over 800 company members and more than 7,500 individual members worldwide. This manual copyright 2006. SSPC: The Society for Protective Coatings.

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SSPC: THE SOCIETY FOR PROTECTIVE COATINGS FUNDAMENTALS OF PROTECTIVE COATINGS FOR INDUSTRIAL STRUCTURES

Table of Contents UNIT 1: CORROSION AND CORROSION CONTROL 1.1 Purpose and Goals.....................................................................................................................................1-1 1.2 Mechanism of Metal Corrosion...................................................................................................................1-1 1.3 Common Types of Corrosion......................................................................................................................1-4 1.4 Methods of Corrosion Control....................................................................................................................1-7 1.5 Conclusion................................................................................................................................................1-13 1.6 Unit Summary...........................................................................................................................................1-13 Exercise 1A: Corrosion...................................................................................................................................1-14 Exercise 1B: Corrosion Control......................................................................................................................1-15 General References and Additional Reading.................................................................................................1-19 Appendix 1-A: Mechanical and Physical Properites of Some Plastics...........................................................1-20 UNIT 2: COATING TYPES AND THEIR MECHANISMS OF PROTECTION 2.1 Purpose and Goals.....................................................................................................................................2-1 2.2 Mechanisms of Corrosion Control by Coatings..........................................................................................2-1 2.3 Desired Film Properties..............................................................................................................................2-4 2.4 Coating Components and Their Functions...............................................................................................2-10 2.5 Mechanisms of Coating Film Formation...................................................................................................2-13 2.6 Comparisons of Generic Coating Types...................................................................................................2-16 2.7 Selection of Coating Systems..................................................................................................................2-30 2.8 Unit Summary...........................................................................................................................................2-32 Exercise 2A: Coating Components................................................................................................................2-34 Exercise 2B: Mechanisms of Coating Film Formation....................................................................................2-35 General References and Additional Reading.................................................................................................2-39 Appendix 2-A: Film-Forming Mechanism.......................................................................................................2-41 UNIT 3: SURFACE PREPARATION FOR PAINTING 3.1 Purpose and Goals.....................................................................................................................................3-1 3.2 Introduction to Surface Preparation............................................................................................................3-1 3.3 Purpose of Surface Preparation.................................................................................................................3-1 3.4 Preparing Surfaces Before Cleaning..........................................................................................................3-1 3.5 Surface Contaminants Causing Early Coating Deterioration.....................................................................3-2 C1 Fundamentals of Protective Coatings for Industrial Structures

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3.6 Surface Preparation Methods.....................................................................................................................3-4 3.7 Recommended Removal Methods for Different Contaminants..................................................................3-7 3.8 Standards for Cleaned Steel Surfaces.......................................................................................................3-9 3.9 Visual Aids to Surface Cleanliness...........................................................................................................3-12 3.10 Levels of Cleanliness Required for Different Coatings...........................................................................3-13 3.11 Air Abrasive Blasting Equipment.............................................................................................................3-13 3.12 Centrifugal Blasting Equipment..............................................................................................................3-16 3.13 Surface Profile and Blasting Abrasives..................................................................................................3-16 3.14 Air Blast Cleaning Procedures................................................................................................................3-18 3.15 Unit Summary.........................................................................................................................................3-19 Exercise 3A: Cleaning Methods.....................................................................................................................3-20 Exercise 3B: Conventional Abrasive Blasting System....................................................................................3-21 General References and Additional Reading.................................................................................................3-25 UNIT 4: APPLICATION OF COATINGS 4.1 Purpose and Goals.....................................................................................................................................4-1 4.2 Methods of Application: General Factors...................................................................................................4-1 4.3 Application of Coatings by Brush................................................................................................................4-3 4.4 Application of Coatings by Roller................................................................................................................4-4 4.5 Spray Application........................................................................................................................................4-4 4.6 Application of Coatings that Cure by Fusion..............................................................................................4-9 4.7 Handling of Paints....................................................................................................................................4-13 4.8 Application Temperatures and Humidities................................................................................................4-17 4.9 Achieving Desired Film Thickness............................................................................................................4-17 4.10 Striping...................................................................................................................................................4-18 4.11 Recommended Spraying Procedures.....................................................................................................4-18 4.12 Coating Application Defects...................................................................................................................4-20 4.13 Unit Summary.........................................................................................................................................4-20 Exercise 4A: Paint Calculations.....................................................................................................................4-21 Exercise 4B: Paint Application Methods.........................................................................................................4-22 General References and Additional Reading.................................................................................................4-26 UNIT 5: COATING INSPECTION OVERVIEW 5.1 Purpose and Goals.....................................................................................................................................5-1 5.2 Introduction to Inspection and Quality Control...........................................................................................5-1 5.3 The Specification and Its Contents.............................................................................................................5-1 5.4 Responsibilities of the Inspector.................................................................................................................5-2 5.5 Monitoring the Ambient Conditions.............................................................................................................5-3

C1 Fundamentals of Protective Coatings for Industrial Structures

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5.6 Pre-Surface Preparation Inspection...........................................................................................................5-9 5.7 Post-Surface Preparation Inspection........................................................................................................ 5-11 5.8 Pre-Painting Inspection............................................................................................................................5-13 5.9 Inspection of Paint Application.................................................................................................................5-14 5.10 Unit Summary.........................................................................................................................................5-20 Exercise 5A: Effects of Adverse Ambient Conditions.....................................................................................5-21 Exercise 5B: Equipment Used for Different Inspection Test Methods............................................................5-22 General References and Additional Reading.................................................................................................5-26 UNIT 6: COATINGS FOR INDUSTRIAL STEEL STRUCTURES 6.1 Purpose and Goals.....................................................................................................................................6-1 6.2 Introduction to Coatings for Industrial Steel Structures..............................................................................6-1 6.3 Selecting Surface Preparation of Steel......................................................................................................6-1 6.4 Coating Systems Appropriate for Steel......................................................................................................6-2 6.5 Selection of Coating Systems by Environmental Zone............................................................................6-13 6.6 Coatings for Atmospheric Zones (Mild and Severe).................................................................................6-14 6.7 Linings for Immersion Service..................................................................................................................6-16 6.8 Coatings for Marine Service.....................................................................................................................6-17 6.9 Coatings for Buried Steel.........................................................................................................................6-18 6.10 Coatings for High-Temperature Surfaces...............................................................................................6-20 6.11 Unit Summary.........................................................................................................................................6-20 Exercise 6A: Coating Repair..........................................................................................................................6-21 Exercise 6B: Selection of Coating Systems for Steel Structures...................................................................6-22 General References and Additional Reading.................................................................................................6-26 UNIT 7: COATING OF CONCRETE SURFACES 7.1 Purpose and Goals.....................................................................................................................................7-1 7.2 Components of Concrete...........................................................................................................................7-1 7.3 Similar Features of All Cementitious Surfaces...........................................................................................7-2 7.4 Placement of Concrete...............................................................................................................................7-4 7.5 Reasons for Coating Concrete...................................................................................................................7-6 7.6 Pre-Coating Materials Applied to Cementitious Surfaces...........................................................................7-7 7.7 Coatings for Cementitious Surfaces...........................................................................................................7-8 7.8 Surface Preparation for Coating............................................................................................................... 7-11 7.9 Application and Inspection of Concrete....................................................................................................7-13 7-10 Inspection of Coatings and Surfaces.....................................................................................................7-14 7-11 Unit Summary.........................................................................................................................................7-16 Exercise 7A: Concrete Terms.........................................................................................................................7-17 Exercise 7B: Products for Concrete Surfaces................................................................................................7-18 C1 Fundamentals of Protective Coatings for Industrial Structures

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General References and Additional Reading.................................................................................................7-22 Appendix 7-A: Typical Properties of Common Generic Coating Materials.....................................................7-24 Appendix 7-B: Nine Concrete Surface Profiles (CSP)....................................................................................7-25 Appendix 7-C: Five Different Coating Ranges................................................................................................7-26 UNIT 8: COATING DEGRADATION, DEFECTS, AND FAILURES 8.1 Purpose and Goals.....................................................................................................................................8-1 8.2 Definition of Commonly Used Terms..........................................................................................................8-1 8.3 Factors Accelerating Coating Deterioration................................................................................................8-2 8.4 Substrate Properties...................................................................................................................................8-4 8.5 Defects from Inappropriate Coating...........................................................................................................8-5 8.6 Coating Deterioration by Corrosion............................................................................................................8-9 8.7 Defects from Inadequate Surface Preparation...........................................................................................8-9 8.8 Defects from Improper Coating Application..............................................................................................8-10 8.9 Defects from Improper Curing..................................................................................................................8-13 8.10 Unit Summary.........................................................................................................................................8-13 Exercise 8A: Substrate Coating Concerns.....................................................................................................8-14 Exercise 8B: Coating Limitations....................................................................................................................8-15 Exercise 8C: Surface Preparation and Application Defects...........................................................................8-16 General References and Additional Reading.................................................................................................8-20 Appendix 8-A: Coating Defects to Which Different Substrates Are Susceptible.............................................8-21 Appendix 8-B: Defects to Which Various Coating Types Are Particularly Susceptible...................................8-22 Appendix 8-C: Defects Associated with Different Coating Properties/Chemistry...........................................8-24 Appendix 8-D: Defects Associated with Different Environmental Conditions.................................................8-25 Appendix 8-E: Descriptions, Causes, and Prevention/Remedies for Coating Defects...................................8-26 Appendix 8-F: Decision Tree: Cosmetic (Surface) Defects............................................................................8-39 Appendix 8-G: Decision Tree: Film Defects...................................................................................................8-40 UNIT 9: SAFETY IN PAINTING OPERATIONS 9.1 Purpose and Goals.....................................................................................................................................9-1 9.2 Introduction to Safety in Painting Operations.............................................................................................9-1 9.3 Hazard Communication..............................................................................................................................9-2 9.4 Hazards from Toxic Materials and Operations............................................................................................9-4 9.5 Surface Preparation Hazards and Safety Requirements...........................................................................9-5 9.6 Paint Application Hazards and Safety Requirements.................................................................................9-7 9.7 Hazards in High, Confined, and Remote Places........................................................................................9-8 9.8 Personal Protective Equipment (PPE)..................................................................................................... 9-11 9.9 Other Safety Issues..................................................................................................................................9-15

C1 Fundamentals of Protective Coatings for Industrial Structures

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9.10 Unit Summary.........................................................................................................................................9-16 Exercise 9: Safety..........................................................................................................................................9-18 General References and Additional Reading.................................................................................................9-22 Appendix 9-A: Standards and Information on Safety and Health...................................................................9-23 ADDITIONAL REFERENCES Monitoring and Controlling Ambient Conditions During Coating Operations Paint Failures-Causes & Remedies Techdata Sheet Glossary.......................................................................................................................................................... G-1 Answer Keys....................................................................................................................................................K-1

C1 Fundamentals of Protective Coatings for Industrial Structures

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Introduction

Introduction to Fundamentals of Protective Coatings for Industrial Structures Objective of Course This course is designed to provide individuals just entering the protective coatings field, or those with an interest in learning about protective coatings, with an overview of how coatings are used to protect steel and concrete substrates found on industrial structures. Upon completion of this course, the student will have the necessary understanding of the elements of a total protective coatings program to set up a basic program for a facility owner. Scope of Course This course presents a series of nine units that provide a basic understanding of the elements of a total protective coatings program for industrial facilities. These units include the following sections: Unit 1: Corrosion and Corrosion Control: Learn how to identify types of corrosion and how to compare and select coatings that meet the demands of your job. • Types and mechanisms of corrosion • Methods of corrosion control Unit 2: Coating Types and Their Mechanisms of Protection: What different types of protective coatings are available? What are the various mechanisms at work in the process of protection? • • • •

Mechanisms of corrosion control by coatings Components of coatings Mechanisms of coatings cure Coating types and their selection

Unit 3: Surface Preparation for Painting: Proper surface preparation is critical in achieving the level of protection available in protective coating systems. • • • •

Purpose: Better coating performance and profile Desired surface properties: cleanliness and profile Cleaning methods Cleaning standards

C1 Fundamentals of Protective Coatings for Industrial Structures F-1

Introduction

Unit 4: Application of Coatings: Learn of the advantage and limitations of the many methods of successful coating application. • Application methods • Achieving proper dry film thickness • Film defects from poor application Unit 5: Inspection and Quality Control: Learn inspection equipment and methods to ensure that all specification requirements are met. • Painting specifications • Role of the inspector • Inspection methods and instruments Unit 6: Coating of Steel Structures: Learn how the type of facility and the service environment relate to coating selection and application to steel surfaces. • Surface preparation of steel • Coating systems for steel • Coating specific steel structures Unit 7: Coating of Concrete: Learn why concrete presents unique selection and application problems and how to overcome them. • Nature of concrete • Coating systems for concrete • Coating special concrete structures Unit 8: Coating Degradation, Defects, and Failures: Familiarize the student with the factors that acceleate coating deterioration. • Technical terms associated with degradation • Common coating defects that can be avoided Unit 9: Safety in Painting Operations: Learn of safety hazards involved in coating operations, responsibilities of management and workers, and safety equipment, practices, and training. • Safety responsibilities of management and workers • Hazardous materials and operations • Safety equipment, practices, and training C1 Fundamentals of Protective Coatings for Industrial Structures F-2

Introduction

Coatings covered in this course are intended primarily for steel and concrete structures in industrial service, such as bridges, storage tanks, pipelines, offshore facilities, and industrial buildings and their components. Emphasis is placed on exterior coatings and interior linings because of the more severe conditions encountered in these services. SSPC Protective Coatings Specialist Certification Program This course and its advanced counterpart, C2 “Specifying and Managing Protective Coatings Projects,” provide the technical foundation necessary to specify and manage coating programs effectively. Knowledge of the information contained in these courses, coupled with practical experience in the protective coatings industry, should permit applicants to pass the examination for the SSPC Protective Coatings Specialist Certification Program. Other requirements (general education, experience, references, and continuing development) are described in the Candidate Handbook.

C1 Fundamentals of Protective Coatings for Industrial Structures F-3

Unit 1 - Corrosion and Corrosion Control

CORROSION AND CORROSION CONTROL 1.1 Purpose and Goals This unit discusses the types and mechanisms of metal corrosion and methods of corrosion control. Learning Outcomes Upon completion of this unit, you will be able to: •

Identify the elements of a corrosion cell

•

Describe the corrosion of metals

•

Explain how industrial coatings control corro-

Figure 1-2: The Corrosion Cycle

sion •

In this unit, we will discuss only the corrosion of

Describe alternative methods used to protect

common metals. Corro sion may be general or

carbon steel from corrosion

localized.

1.2 Mechanism of Metal Corrosion

Corrosion of metals in the United States has been

The Corrosion Process

reported to cost about 4.2% of the annual GNP1 and $276 billion annually2. About one-third of these costs

Corrosion can be defined as the deterioration of

can be avoided by proper use of currently existing

a substrate. Corrosion is a natural process that

corrosion control technology. The most widely used

displays the tendency of materials to “give up” energy

method to prevent corrosion today is the application

and return to their natural state. In figure 1-2, it takes

of protective coatings.

energy to create a finished steel product from iron oxide; however, once a finished steel product, it will

Metals corrode because they exist in chemically

release energy and convert back into its original state

unstable states and seek more stable, lower-energy

unless the process is stopped or slowed down.

states. For example, iron ore as mined from the earth is an oxide in its natural, stable state. Energy must be used in blast furnaces to reduce iron oxide to metallic iron for fabrication of products. Iron and steel products are then slowly oxidized by air, converting them back to their original, stable, lower-energy states.

Figure 1-1: Corroded Bridge 1 2

H.H. Bennet, J. Kruger, R.L. Parker, E. Passaglin, C.F. Reinmann, A.W. Ruff, and H. Yakowitz; Special Publication 511-1, National Bureau of Standards, Gaithersburg, MD, 1978. “Corrosion Costs and Preventative Strategies in the United States”, FHWA-RD-01-156, 2001 C1 Fundamentals of Protective Coatings for Industrial Structures 1-1

Unit 1 - Corrosion and Corrosion Control

It is necessary to understand a few fundamental

To analyze how the four elements of a corrosion cell

concepts of the mechanism of metal corrosion.

work together to produce the process of corrosion,

Corrosion occurs when four required components

we can use the example of a common household

are present. If any of the components is missing,

battery.

the corrosion process will not proceed. The required elements of a “corrosion cell” are:

A dry cell battery is a familiar example of a corrosion

• Anode

cell.

• Cathode • Metallic path connecting the anode and cathode •

Electrolyte

The word ACME can be of help in remembering these components (it contains the first letter of each component). The surface of carbon steel already contains three of the four elements: the anode, cathode and metallic Figure 1-3: Dry Cell Battery

pathway; only the electrolyte is missing. An electrolyte is a liquid that contains ions or “charged particles.” All salts (e.g., sodium or calcium chloride) form ions when dissolved in water. Once the electrolyte is present, the process of corrosion will proceed. Eliminating or controlling these components can control corrosion. Each method of corrosion control described later interferes with one or more of these components.

The anode and its counterpart, the cathode, represent negative and positive terminals of the “battery”. During the chemical reaction process, electrical current (or electrons) flows from the anode to the cathode via the metallic pathway or connection. The electrolyte carries ions from the cathode to the anode to complete the electrical circuit. The anode (negative terminal) decays during this process while the cathode (positive terminal) remains intact or “protected”. The only dif-

The Corrosion Cell During corrosion, electrons flow from the anode to the cathode through the metallic path and ions flow from the cathode to the anode through the electrolyte to complete the electrical circuit. In atmospheric corrosion, there is normally sufficient moisture and salts or other contaminants to serve as the electrolyte.

ference between a corrosion cell and a manufactured battery is that the reaction process is designed to produce an electrical current for a productive use in the manufactured battery. A natural corrosion cell, however, is generally destructive since the reaction process depletes or decays the anode. When the anode is depleted in a manufactured battery, the reaction will stop and the battery will “die” since it can’t produce more energy.

C1 Fundamentals of Protective Coatings for Industrial Structures 1-2

Unit 1 - Corrosion and Corrosion Control

the anode will decay while the other “cathodic” metal remains intact. The anodic metal thereby provides “cathodic protection” to the other metal.

Table 1- Galvanic Series Galvanic Series

Active

Magnesium Alloys Zinc Galvanized Steel Aluminium Alloys Cadmium Mild Steel

Figure 1-4: The Dry Cell Battery

Wrought Iron Cast Iron

Relative Corrosion Tendencies of Metal: Gal-



vanic Series



Chromium Stainless Steel Type 410 (Active) Stainless Steel Type 304 (Active)

Most metals are not found in their pure state in na-



ture, but rather as ores where they are combined with





Naval Brass Bronze

oxygen and other elements. The relative reactivity of



metals is directly proportional to the amount of energy





Copper Nickel Alloys



Nickel (Active)

required for their conversion from ore. Table 1 lists

Yellow Brass

metals in a decreasing order of energy required to

Copper

convert common metals to their pure form. Note that

Silver Solder

there are some metals that do exist in their pure form in nature such as gold, silver and copper; these metals





are at the noble (less active) end of the chart.

Chromium Stainless Steel Type 410 (Passive) Stainless Steel Type



A corrosion cell can also be formed when two dis-





Noble

similar metals are in contact with one another. When

304 (Passive) Platinum







Graphite Gold

two dissimilar metals are connected, the metal that is higher in Table 1,and thus requires more energy to convert it to a pure metal, is the one that becomes

Metal Passivation by Surface Oxidation

the anode and corrodes, while the other metal acts

The corrosion of some metals does not necessarily

as the cathode. The physical connection between the

create a problem. For example, aluminum will quickly

two metals serves as a metallic pathway and water or

oxidize (corrode) forming a layer of aluminum oxide

moisture typically serves as the electrolyte medium

on the metal surface. But the aluminum oxide layer

needed to complete the cell. The metal that acts as

essentially seals the metal surface and becomes

C1 Fundamentals of Protective Coatings for Industrial Structures 1-3

Unit 1 - Corrosion and Corrosion Control

Uniform Corrosion

protective because it stays tightly adhered and is not porous. Copper is another example of a metal

Uniform corrosion

that forms a protective oxide layer- in this case the

is a form of

characteristic green color that forms as copper weathers

corrosion in which

(known as “Patina”).

a metal is attacked at about the same rate over the entire surface. Anode and cathode areas on

Figure 1-6: Uniform Corrosion

a piece of corroding metal usually change with time, so that areas that were once anodes Figure 1-5: Oxidation

become cathodes, and vice versa. If the depth of attack at any point exceeds twice the average depth of attack, the corrosion is no longer considered to be uniform.

For most metals; however, corrosion is a problem because the oxidation of the metal surface does not

Pitting Corrosion

stop after an initial layer is formed. In the case of iron (and steel), a porous layer of iron oxide is formed which

Pitting corrosion

is loosely held to the surface. The porosity allows

(also simply called

corrosion to continue into the iron.

“pitting”) occurs on a metal when the

1.3 Common Types of Corrosion

amount of corro-

There are many forms of localized corrosion. Some

sion at one or more

forms likely to be encountered by coatings personnel

p o i n t s i s mu c h

are:

greater than the

•

Uniform (anodes and cathodes change locations)

•

Pitting corrosion

•

Dissimilar metal (galvanic)

•

Differential environment

•

Stray current (DC current from electric railways,

There are several reasons why the shifting of anodic and cathodic areas might not occur to produce uniform corrosion. These include lack of metal homogeneity (e.g., inclusions in the metal) and local breakdown of passive films. Pit

etc.) •

depth can be measured using a pit gage.

Dealloying (selective loss of metal; e.g., graphitization of cast iron)

• •

Figure 1-7: Pitting Corrosion

average corrosion.

Dissimilar Metal (Galvanic) Corrosion

Erosion-corrosion (erosion keeps corrosion cell active)

Dissimilar metal corrosion, sometimes called galvanic

Exfoliation (delamination along grain boundaries)

corrosion, occurs when two dissimilar metals are in

C1 Fundamentals of Protective Coatings for Industrial Structures 1-4

Unit 1 - Corrosion and Corrosion Control

contact with each other in an electrolyte. In such a

rubber or plastic) should be placed between them, or

corrosion cell, where a more active and a less active

they should be isolated from the electrolyte.

metal are connected electrically, the more active metal will corrode at a higher rate than if unconnected, and

As would be expected, much more corrosion occurs

the less active metal will corrode at a lower rate than

to an anode of small area in contact with a cathode

if unconnected. Thus, in a dissimilar metal corrosion

of larger area than in the reverse case. The former

cell of zinc and steel in seawater, the zinc will corrode

situation can be disastrous and must always be

preferentially, protecting the steel. As noted previ-

avoided.

ously, the greater the difference in potentials of the metals, the greater will be the corrosion rate. Some metal couples with large differences in potential can cause catastrophic corrosion, although the problem might easily be corrected. Potential differences existing on pieces of the same metal or similar metals can also cause galvanic corrosion. Such differences may originate as follows: •

New steel is anodic to old steel.

•

Steel is anodic to its surface mill scale.

•

Brightly cut surfaces (e.g., pipe threads) are anodic to uncut surfaces.

•

Cold worked areas (e.g., pipe bends) are anodic to less stressed areas.

In addition, wrenches and vises that cut into the metal should be avoided, and cold working should be mini-

Figure 1-8: Top- The bronze is the cathode

mized, since they stress the steel to create potential

and the steel is the anode. Bottom- In the

differences.

bronze, steel and zinc cell shown, both the bronze and steel are the cathodes and the zinc is the anode. Note that the steel

Galvanic metal corrosion can usually be avoided

changed from an anode to a cathode due

by selecting compatible metals that will come into

to the connection with the zinc, which has a

contact with each other. If they cannot be of the same

higher potential.

composition (i.e., be the same metal or alloy), they should be close to each other in the galvanic series for the environment in which they are to be located. If this is not possible, nonconducting insulators (e.g.,

C1 Fundamentals of Protective Coatings for Industrial Structures 1-5

Unit 1 - Corrosion and Corrosion Control

Concentration Cell Corrosion Concentration cell corrosion is often called crevice corrosion, because the differences in environment that drive this type of corrosion are often located in or adjacent to crevices. These crevices may be metalto-metal or metal-to-non-metal contacts.

frequently occurs on buried metal structures near electric railway and crane systems, improperly grounded welding generators, and adjacent cathodic protection systems. Stray current corrosion should always be suspected when accelerated corrosion is noted on buried structures in an area near such DC current systems. Suspected stray current corrosion can be verified by electrical tests that measure

The most prevalent form of concentration cell

voltage differences between different locations on

corrosion occurs with different concentrations of

the structure. Once detected, stray currents can be

oxygen. The areas inside a crevice are relatively

reduced by one of the following:

oxygen deficient and thus anodic because oxygen

• Reducing the current flow in the ground by

accelerates the cathodic reaction. Acceleration corrosion occurs here.

modifying its source. •

Modifying the current flow by electrical bonding.

•

Applying counterbalancing cathodic protection.

Three common examples of crevices in metal components are skip welding, back-to-back angles, and areas under bolt heads. Welds must be continuous and full penetration. NACE RP 017891 provides good information for protection under bolt heads and examples of weld defects and their correction. Another, usually less severe, form of concentration cell corrosion occurs with different concentrations of metal ion. A buildup of metal ions in the crevice will make the crevice cathodic to the metal outside the crevice. Thus, metal ion buildup in crevices causes corrosion to occur on the metal outside the crevice. Again, the rates of both of these types of concentration Figure 1-9: Stray current caused by electric railway

cell corrosion are greatly affected by the relative sizes of anode and cathode areas. Stray Current Corrosion Corrosion occurs on metal surfaces whenever direct current passes from them into an electrolyte. Accelerated corro sion by stray currents most

Dealloying Dealloying is the selective corrosion (loss) of a metallic constituent from an alloy, such as in a plumbing fixture. The leached metal may be aluminium, nickel, molybdenum, or zinc. One example of dealloying

C1 Fundamentals of Protective Coatings for Industrial Structures 1-6

Unit 1 - Corrosion and Corrosion Control

is the dezincification of brass. Another example is graphitization, which consists of corrosion of cast iron in which the metallic constituents are converted to corrosion products, leaving the graphite intact. Erosion-Corrosion Erosion-corrosion occurs when an abrasive material (e.g., slurry through a pipeline) impinges on an existing corrosion cell to keep the metal bright and

Figure 1-11: Rolling of Aluminum Flattened grain structure leads to exfoliation during corrosioin of aluminum plate.

the corrosion active. The scouring of the surface to remove the accumulation of corrosion products often results from wind- or water-borne sand. Some metals (e.g. gold, silver, and titanium) are

1.4 Methods of Corrosion Control

essentially unaffected by corrosion in certain

There are several methods of corrosion control, each

environments. This may be due to the stability of

of which has its own advantages and limitations.

some metals in their metallic state or to passivity

While we will discuss each method separ ately, they

imparted by the formation of protective oxide films.

are best used together, when ever appropriate, in a

However, these metals and alloys may not be stable

total corrosion control program.

in other environments, so the environment must be taken into account when choosing corrosion-resistant

Corrosion Control by Design

metal products.

A good structural design may control corrosion by Exfoliation

eliminating one or more of the components neces-

Exfoliation is an advanced form of intergranular corro-

application of other methods of corrosion control. Poor

sary for the corrosion reaction or permitting easier

sion where the metal delaminates along grain bound-

design of a metal structure can introduce elements

aries. Rolled

that accelerate corrosion. Examples of poor design

metal prod-

conditions are:

ucts such as certain types of aluminum alloy plate are particularly susceptible to exfoliation.

•

Contact of dissimilar metals

•

Incompatible environments

•

Water traps

•

Crevices

•

Rough and sharp surfaces (e.g., welds)

•

Limited access to work

Figure 1-10: Exfoliation

C1 Fundamentals of Protective Coatings for Industrial Structures 1-7

Unit 1 - Corrosion and Corrosion Control

Contact of dissimilar metals in an electrolyte can result in rapid corrosion. This phenomenon, galvanic corrosion, has already been described. This is a common occurrence that can easily be avoided by proper design. Dissimilar surface conditions (threads, scratches, etc.) can also be avoided with care.

Crevices should be avoided in structures, because these oxygen-deficient areas accelerate corrosion, as described earlier in this section. Crevices can also remain wet and thus accumulate electrolyte, much as in the water traps described above. They are difficult to pro tect by coating.

Incompatible environments can accelerate the corrosion process. Thus, aluminium should not come into direct contact with concrete, because the alkalinity of the concrete will attack the aluminium. Water traps are design features that allow rainwater or dew to collect. Since water greatly accelerates

Figure 1-13: Crevice Corrosion

corrosion, structures should be designed so that they do not collect water. Angles and other shapes that can collect water should be oriented downward. Weep holes should be placed where water collection cannot be otherwise avoided. Condensate water from air conditioners should not be allowed to run or drip on surfaces, nor should steam or other vapors be allowed to impinge on metal surfaces.

Limited access to work can prevent proper application of coatings. All areas to be coated should be readily accessible for both cleaning and painting. Difficult-to-reach areas are not only difficult to coat, but may also constitute a safety hazard because of reaching from a ladder or other platform further than can be done safely. Corrosion Control by Resistant Metals and Alloys If the environment is very severe, it may be best to control corrosion by using a metal or alloy that is more resistant to corrosion than structural steel. Depending upon the environment to which they are exposed, some metals/alloys are essentially immune to corrosion,

Figure 1-12: Features that trap and hold water and debris

while others which have higher electronegative potentials than structural steel form surface oxide films to impart corrosion resistance. When these protective layers are broken in localized areas, they must reform

C1 Fundamentals of Protective Coatings for Industrial Structures 1-8

Unit 1 - Corrosion and Corrosion Control

to provide continuous protection. Titanium, aluminum

Galvanizing

alloys, zinc, and stainless steels form such protective

Hot-dipped galvanized steel is formed by dipping

films. The protective films on stainless steels and

cleaned (usually acid­-dipped) steel into a molten

aluminum alloys are not resistant to all natural

bath of zinc. The zinc coating subse­quently formed is

environments. Even the more corrosion-resistant metals/alloys are frequently coated to provide additional protection.

resistant to corrosion under many conditions and can be topcoated with an organic coating to provide further protection or a different appearance. Its mechanisms of protection are both galvanic and barrier. This will

Titanium

be discussed in more detail later. Hot dipping is

Titanium provides a high strength-to-weight ratio and

usually preferred to other methods of galvanizing,

good resistance to many severe environments, e.g.,

because it produces a thicker layer of zinc than

seawater, hypochlorites, and nitric acid. It is readily

does electrogalvanizing (electrodeposition).

passivated with an oxide film.

it is metallurgically bonded to the steel. One of the

Also,

limitations of hot-dipping is that the component being coated must be small enough to fit into the bath. Nut,

Aluminum and Aluminum Alloys

bolts, and other fasteners are sometimes galvanized

Aluminum and its alloys are light in weight and

by tumbling with hot powdered zinc. Metallizing, and

corrosion resistant in many environments. They are,

zinc­-rich coatings, to be described in later units, are

however, attacked by strong acids and alkalis.

other ways to coat a steel surface with zinc.

Stainless Steels

Corrosion Control with Non-Metallics

Stainless steels are produced almost exclusively

There are several plastic, elastomeric, composite, and

for their corrosion resistance. These steels should

ceramic materials that are quite resistant to corrosion

contain at least 11% chromium.

that may be used effectively to replace steel. These find many specialized uses.

Weathering Steel

Plastics

Weathering (low-alloy) steels form protective oxide layers under mild atmospheric conditions that may

A plastic is a solid material that is essentially

defer or eliminate the need for coating. However,

an organic polymer of large molecular weight

these oxide layers may not be protective in some

containing hardeners, fillers, reinforcements and other

environments, particularly marine environments. In

components. At some time during its manufacture,

addition, the natural rusted appearance of weathering

it is shaped by flow. It is either thermoplastic, i.e., it

steels may not be acceptable aesthetically, and

can be repeatedly softened by heat and hardened

soluble iron compounds that wash from the surface

by cooling, or it is thermosetting, i.e., when cured

may objectionably stain contaminated surfaces.

chemically or by heating, it becomes an infusible, insoluble material.

C1 Fundamentals of Protective Coatings for Industrial Structures 1-9

Unit 1 - Corrosion and Corrosion Control

Examples of thermoplastics include fluorocarbons, polyethylenes, polypropylenes, and polyvinyl chlorides; examples of thermosetting plastics include epoxies, phenolics and polyester. Properties of these plastics can be found in Table 1 at the end of the chapter. They are used for such items as water and gas piping, gutters, and downspouts. Elastomers Natural and synthetic elastomeric (materials that

Corrosion Control with Inhibitors Inhibitors are chemicals such as phosphates that are added in small amounts, either continuously or intermittently, to acids, cooling waters, steams, or other environments to inhibit the corrosion reaction. They may reduce corrosion by forming a very thin film on the metal surface, by causing a passive layer to form on a metal surface, or by removing aggressive constituents from the environment. The best known inhibitors are those used in engine cooling systems.

substantially return to their original shape after removing force causing distortion) lining materials have been successfully used as liners for primary containment. These include natural and synthetic rubber and polyurethane coatings and sheets. Sheets must be glued or mechanically fastened to walls. Composites

Corrosion Control by Altering the Environment Changing the environment can help control corrosion and increase the effectiveness of other corrosion control systems. Dehumidification and purification of the atmosphere are two of the most common examples. For example, air conditioning facilities to keep relative humidity down can reduce tarnish and

Composites are combinations of two or more materials

corrosion of exposed metals, such as those found in

(e.g., binder with reinforcing materials and fillers)

a telephone switching facility.

differing grossly in form or composition. The different ingredients remain as separate entities and do not

Also, if dehumidification results in a 15 degree

merge together, but they do act in concert with each

Fahrenheit dew point depression and a humidity no

other. One of the most common types of composites

higher than 55%, blast cleaned surfaces can be left

is fiber-reinforced plastics (FRP). Reinforcement may

uncoated for significant periods before painting.

be from cloth, mat of strands of glass, carbon, or other materials. Fiberglass-reinforced plastics (FGRP) are by far the most commonly used FRP products. FGRP composites can be used to fabricate process vessels, piping, floor toppings, tanks, etc. Ceramics

Corrosion Control by Cathodic Protection Cathodic protection is a system for controlling corrosion of a metal surface by passing sufficient direct current onto it to convert all anode areas to cathode areas, thus eliminating the possibility of anodic loss of metal. While it is effective only on

Ceramics are products formed by the firing of natural

surfaces immersed in water or buried in soil, it has

earth materials at high temperatures. They exhibit

successfully controlled corrosion for many years on

high chemical, temperature, and electrical resistance

ships, waterfront structures, underground pipelines

and are used where these properties are desired.

and tanks, and the interiors of water storage tanks.

C1 Fundamentals of Protective Coatings for Industrial Structures 1-10

Unit 1 - Corrosion and Corrosion Control

Coatings are generally used on cathodically protected

The impressed current system of cathodic protection

structures to reduce current requirements. Thus,

utilizes direct current from an external power source.

a well-coated buried pipe may require only 0.01

The positive terminal of the power source is connected

milliamps per square foot, as compared to three

to the anodes, and the negative terminal is connected

milliamps per square foot for a bare pipe. Coatings on

to the structure to be protected.

cathodically protected structures must be resistant to the alkaline environment produced by the system.

The stable anodes used to discharge current have long service lives. High-silicon cast iron, graphite, and

There are two basic systems for supplying the

aluminium are among the most commonly used anode

necessary direct current to a structure to protect it

materials. Scrap iron, special lead alloys, platinum,

cathodically.

platinum-palladium alloy, platinized titanium, and platinized tantalum alloys are also used. Normally,

The sacrificial anode (galvanic) system of cathodic

rectifiers convert available AC power to DC power

protection requires no external power supply, but

for the systems. Batteries and solar power can also

incorporates anodes of special alloys that generate

provide energy for cathodic protection systems.

the necessary direct current by preferentially corroding by virtue of their natural voltage difference from the protected structure (Figure 1-14). Because sacrificial anodes are consumed in generating current, they have a limited service life. The active anode metals used in cathodic protection are usually magnesium, zinc, or aluminium of high purity or other special composition.

Figure 1-15: Impressed current system for protection of an underground pipeline

Figure 1-14: The electrochemical cell in cathodic protection. The structure acts as a cathode. No corrosion occurs on protected structure, as electrons that would be produced by corrosion reaction (Fe Fe++ + 2e ) cannot flow from structure to anode due to voltage gradient.

C1 Fundamentals of Protective Coatings for Industrial Structures 1-11

Unit 1 - Corrosion and Corrosion Control

2. The hydrogen gas that may be produced in improperly controlled (excess current) cathodic protection systems may disbond protective coatings. 3. More permeable coatings (e.g., oleoresinous phenolic) are more subject to electroendosmosis than less permeable coatings (e.g., epoxies). This complex form of corrosion is caused by greater ion flow on cathodically protected surfaces.

Corrosion Control by Coatings Corrosion control by coatings and linings (coatings on

Figure 1-16: Impressed current system for protection of the interior of an elevated water tank

interior surfaces) most commonly occurs by formation of a barrier separating the metal from the electrolyte. This and other mechanisms of corrosion control (inhibitors

Each of these two types of cathodic protection has its

and cathodic protection) will be described in Unit 2.

own advantages and limitations. The differences in the systems are summarized in Table 2:

Coatings have many advantages over the other previously discussed methods of corrosion control. These

Table 2. Comparing Sacrificial Anode and Impressed Current Cathodic Protection Systems Sacrificial Anode

include:

Impressed Current

No external power supply

External power supply

Limited current output

Variable voltage

Adjustable medium output

Variable current

OK for low resistivity

OK for high resistivity medium

Requires electrolytes

Requires electrolytes

Lower installation costs

Higher installation

Few interference problems

Can cause interference

Low maintenance costs

Monthly power bills

Localized protection

Protects larger structures

•

Ease of application

•

Ease of storage and handling

•

Range of acceptable ambient conditions

•

Economics

•

Easy repair

•

Selection of color, gloss, and texture

As with other methods of corrosion control, they also have limitations that must be addressed. These include:

There are three possible adverse effects on coatings of improperly designed cathodic protection systems.

•

Surface preparation requirements

•

Application requirements

•

Drying/curing requirements

•

Health/safety/environmental concerns

1. During cathodic protection (even with properly functioning systems), alkalinity (hydroxide ions) are

These will be discussed more fully in subsequent

always produced at the cathode. Thus, coatings on

units.

the cathode must be resistant to alkalinity. C1 Fundamentals of Protective Coatings for Industrial Structures 1-12

Unit 1 - Corrosion and Corrosion Control

1.5 Conclusion A successful corrosion control program utilizes as many corrosion systems as appropriate and practical. 1.6 Unit Summary Corrosion of metals is a natural process by which the metals are transformed to a more stable state. There are many forms of corrosion, as well as many methods of corrosion control. A successful total corrosion program utilizes as many of the available methods as are appropriate and practical.

C1 Fundamentals of Protective Coatings for Industrial Structures 1-13

Unit 1 - Corrosion and Corrosion Control

Unit 1 - Exercise 1A: Corrosion Match the items in Column A with the description in Column B. Column B

Column A 1.

Corrosion

A. Selective corrosion of alloy metal

2.

Corrosion cell

B. Direct current passes into electrolyte

3.

Crevice corrosion

C. Natural, costly process

4.

Dealloying

D. Abrasion keeps metal clean and active

5.

Electrolyte

E. List of tendencies for metals to corrode

6.

Erosion-corrosion

F. Dissimilar metal corrosion

7.

Exfoliation

G. Loss in sheets of certain rolled alloys

8.

Galvanic Corrosion

H. Anode, cathode, metal path, electrolyte

9.

Galvanic series

I. Localized corrosion

10.

Pitting

J. Anodes and cathodes change

11.

Stray current corrosion

12.

Uniform corrosion

K. Conductive medium L. Concentration cell corrosion

C1 Fundamentals of Protective Coatings for Industrial Structures 1-14

Unit 1 - Corrosion and Corrosion Control

Unit 1 - Exercise 1B: Corrosion Control Match the corrosion control methods in Column A with the mechanisms of corrosion control in Column B. Column A

Column B

1.

Altering the environment

A. Isolates metal from electrolyte

2.

Barrier coatings

B. Needs supply of direct current

3.

Corrosion-resistant material

C. Corroding anodes provide current

4.

Design

D. Eliminates water traps, etc.

5.

Impressed current CP

E. Dehumidification

6.

Inhibitors

F. Ceramics, composites, CRESs

7.

Sacrificial anode CP

G. Interfere with corrosion reaction

C1 Fundamentals of Protective Coatings for Industrial Structures 1-15

Unit 1 - Corrosion and Corrosion Control

Quiz 1. Corrosion is: a. a natural process. b. a very costly process. c.

controlled with present technology.

d. All of the above 2. What is required for corrosion to occur? a. Two dissimilar metals in contact with each other in a conductive medium b. An anode, a cathode, a metal path, and an electrolyte c. Immersion in water or burial in soil d. Stray current 3. What metal is passivated (rendered less corrosive) by formation of a surface oxide film? a. Zinc b. Aluminum c. Stainless steels d. All of the above 4. A type of corrosion that occurs during the graphitization of cast iron is: a. Galvanic corrosion b. Erosion corrosion c. Concentration cell corrosion d. Dealloying 5. A structural feature associated with concentration cell corrosion is: a. Welds b. Crevices c. Corners d. Water traps

C1 Fundamentals of Protective Coatings for Industrial Structures 1-16

Unit 1 - Corrosion and Corrosion Control

6. A TRUE statement about galvanic (sacrificial anode) cathodic protection is: a. External power is required. b. Remotely located anodes will protect larger areas. c. The anodes are consumed while providing protection. d. Performs well in both high and low resistivity soils. 7. Galvanizing can be described as: a. A passive oxide layer on the surface of steel b. A thin film of zinc metal on the surface of steel c. A thin layer of aluminum on the surface of steel d. A thin layer of rust on the surface of steel 8. Steel welds are grounded before coating application to: a. improve their appearance b. increase their corrosion resistance c. improve adhesion of the primer d. provide for a more continuous, uniform coating application 9. Continuous welds are preferred to skip (stitch) welds because they: a. provide a more attractive surface b. eliminate crevices c. avoid contact of dissimilar metals d. control galvanic corrosion 10. A TRUE statement about impressed current systems used for cathodic protection is: a. External power is required. b. Anodes must be close to protected structures. c. Never has interference problems with other metal structures in the area. d. It is self-regulating, so that no monitoring of potentials is ever required. 11. Which metal CANNOT be used as an anode for galvanic (cathodic) protection of steel? a. Zinc b. Aluminum c. Magnesium d. Copper

C1 Fundamentals of Protective Coatings for Industrial Structures 1-17

Unit 1 - Corrosion and Corrosion Control

12. Protective coatings: a. perform well in conjunction with cathodic protection. b. contain no hazardous materials. c. requires little surface preparation. d. have no safety concerns during application. 13. Which condition can cause deterioration of coatings on cathodically protected structures? a. Alkalinity produced at the cathode b. Generation of hydrogen gas by over protection of CP system c. Electroendosmosis d. All of above 14. A composite can be described as: a. having excellent resistance to all environments. b. A fiberglass-reinforced epoxy. c. A combination of two or more differing ingredients. d. using restricted to light-weight structures.

C1 Fundamentals of Protective Coatings for Industrial Structures 1-18

Unit 1 - Corrosion and Corrosion Control

References H.H. Bennet, J. Kruger, R.L. Parker, E. Passaglin, C.F. Reinmann, A.W. Ruff, and H. Yakowitz; Special Publication 511-1, National Bureau of Standards, Gaithersburg, MD, 1978 “Cost of Corrosion: $300 billion a year,” Materials Performance, Vol. 34, No. 5, June 1995, p. 5.

Supplemental Reading NACE RP-0178 Fabrication Details, Surface Finish Requirements and Proper Design Considerations for Tanks and Vessels to be Lined for Immersion Service (Note: A visual aid is available, which should be used with the written standard.)

C1 Fundamentals of Protective Coatings for Industrial Structures 1-19

C1 Fundamentals of Protective Coatings for Industrial Structures

1-20

90-800

10,000 6000 2000 4000 5000 7000 6000 2500

Nylon

Polyether-chlorinate

Polyethylene (low density)

Polyethylene (high density)

Polypropylene

Polystyrene

Rigid polyvinyl chloride

Vinyls (chloride)

10,000 7500 4000 3500 7000

Epoxy (cast)

Phenolics

Polyesters

Silicones

Ureas

Thermosets

130

8000

nil

nil

nil

nil

nil

100-450

2-30

1-2

10-700

15-1000

45

5

2500

Methyl methalcrylate

100-350

115

89

100

125

90

80

110

75

90

40

10

100

110

220

70

Tensile Elongation, Hardness, Strength psi % Rockwell R

Fluorocarbons

Thermoplastics

Material

0.3

0.3

0.4

0.3

0.8

good

1

0.3

1-11

1-12

16

0.4

1.5

0.5

4

Impact load, ft/lb

1500

1200

1000

1000

1000

low

400

450

200

120

25

150

400

420

60

Modulus of elasticity, psi x 102

1.48

1.75

1.1

1.4

1.1

1.8

1.4

1.05

0.91

0.95

0.92

1.4

1.14

1.19

2.13

265

350-900

350

300

350

145

150

180

150

120

---

210

325

200

270

Heat-distortion temperature, °F/264 1psi

Unit 1 - Corrosion and Corrosion Control

Table 1: Mechanical and Physical Properties of Some Plastics

Appendix 1-A

Unit 1 - Corrosion and Corrosion Control

Unit 1 Learning Outcomes

Unit 1 Corrosion and Corrosion Control

Upon completion of this unit, you will be able to: −  Identify the elements of a corrosion cell −  Describe the corrosion of metals −  Explain how industrial coatings control corrosion −  Describe alternative methods used to protect carbon steel from corrosion

Mechanism of Metal Corrosion

Corrosion

•  The corrosion process •  The corrosion cell •  Relative corrosion tendencies of metal

•  Corrosion- the deterioration of a substrate Corrosion is a natural process that displays the tendency of materials to “give up” energy and return to its natural state.

Natural Cycle of Corrosion

Cost of Corrosion •  Corrosion of metals in the United States has been reported to cost about $276 billion annually.

C1 Fundamentals of Protective Coatings for Industrial Structures 1-21

Unit 1 - Corrosion and Corrosion Control

Prevention of Corrosion

Typical Corrosion Cost Items •  •  •  •  • 

•  The most widely used method to prevent corrosion today is application of protective coatings.

Replacement of deteriorated items Maintenance of facilities Shut-down of facilities Product loss or environmental contamination Damages/injuries from accidents

Four Conditions Necessary for Corrosion

Four Basic Parts of a Corrosion Cell

Anode Cathode Metallic Path Electrolyte ACME

Dry Cell Battery (corrosion cell analogy)

The Corrosion Cell on a Metal Surface

C1 Fundamentals of Protective Coatings for Industrial Structures 1-22

Unit 1 - Corrosion and Corrosion Control

Potential Differences on a Single Piece of Metal

Galvanic Series Active



Magnesium

Zinc

Galvanized Steel

Aluminum

Mild Steel

Wrought Iron

Cast Iron

Chromium Stainless Steel Type 410

Stainless Steel Type 304 (Active)

Naval Brass

Nickel (Active)

Yellow Brass

Copper

Silver Solder

Chromium Stainless Steel Type 410

Stainless Steel Type 304 (Passive)

Graphite

Gold

•  Chemical differences (e.g., contaminants in the metal) •  Physical differences (e.g., cuts, hammering, etc.)

Noble

Common Corrosion Cell Chemistry

Common Types of Corrosion •  •  •  •  •  •  •  • 

•  Anode loses metal •  Cathode protected, becomes alkaline, may produce hydrogen •  Oxygen at cathode affects corrosion rate

Uniform Corrosion

Uniform Pitting corrosion Dissimilar metal Concentration cell corrosion Stray current Dealloying Erosion-corrosion Exfoliation

Pitting Corrosion

•  Anodes and cathodes reverse •  Not usually damaging

•  Localized accelerated corrosion •  Caused by metal ion non-uniformity •  Caused by localized breakdown of passive layer

C1 Fundamentals of Protective Coatings for Industrial Structures 1-23

Unit 1 - Corrosion and Corrosion Control

Dissimilar Corrosion Steel Pipe in Aluminum Deck

Dissimilar Metal Corrosion Anodic (electronegative) end – more active metals Magnesium Zinc Aluminum Cadmium Steel Lead Tin Nickel Brass Copper

Zinc Protects Steel

Cathodic (electropositive) end – more noble metals

Avoid Dissimilar Metal Corrosion by:

Concentration Cell Corrosion The effect of relative anode and cathode areas on corrosion

•  Choosing compatible metals •  Using rubber or plastic insulators •  Avoiding electrolyte

Common Types of Concentration Cell Corrosion

Aluminum Truss (Anode) and Stainless Steel Bolts (Cathode)

Differences in oxygen concentration

C1 Fundamentals of Protective Coatings for Industrial Structures 1-24

Differences in metal ion concentration

Unit 1 - Corrosion and Corrosion Control

Examples of Crevices in Construction

Skip Welds

•  Skip welds •  Back-to-back angles •  Areas under bolt heads

Sources of Stray Current Corrosion

Crevice Corrosion •  •  •  • 

Electric railways Electric cranes Welding generators Adjacent CP systems

Methods of Reducing Stray Current Corrosion

Example of Stray Current Corrosion

•  Reducing current flow by modifying source •  Modifying electrical flow by bonding •  Applying counterbalancing CP

Current flows from pipe through earth to negative bus -pipe corroded here

Some current leaks off rail into ground and onto pipe pipe protected here

C1 Fundamentals of Protective Coatings for Industrial Structures 1-25

Unit 1 - Corrosion and Corrosion Control

Dezincification of Brass

Dealloying •  Selective metal loss •  Dezincification of brass •  Graphitization of cast iron

Exfoliation of Aluminum

Erosion-Corrosion •  Abrasive removes corrosion products •  Metal surface actively corrodes

Corrosion Control by Avoiding Poor Design

Methods of Corrosion Control •  •  •  •  •  •  • 

•  •  •  •  •  • 

Design Resistant metals Non-metallics Inhibitors Altering the environment Cathodic protection Coatings

C1 Fundamentals of Protective Coatings for Industrial Structures 1-26

Avoid contact of dissimilar metals Incompatible environments Water traps Crevices Rough surfaces and sharp edges Limited access to work

Unit 1 - Corrosion and Corrosion Control

Dissimilar Metal Corrosion - Steel Nut on Copper Fitting

Incompatible Environments

Water Trap

Avoid Water Traps by: •  Inverting configuration •  Drilling weep holes

Weathering (Low-Alloy) Steel (Advantages)

Corrosion-Resistant Metals and Alloys •  •  •  •  • 

•  Forms protective oxide •  Coating may not be necessary •  Reduced corrosion rate

Titanium Aluminum alloys Stainless steels Weathering steel Galvanizing

C1 Fundamentals of Protective Coatings for Industrial Structures 1-27

Unit 1 - Corrosion and Corrosion Control

Hot-Dip Galvanizing of Steel

Weathering Steel (Limitations)

•  Barrier coating of zinc •  Cathodic protection •  Painted for added protection

•  Protective layer destroyed by chlorides •  Needs open, dry exposure •  May not be aesthetically pleasing

Ways of Coating Steel with Zinc •  •  •  •  • 

Corrosion-Resistant Non-Metallics •  •  •  • 

Hot-dip galvanizing Electrogalvanizing Tumbling with powdered zinc Metallizing Zinc-rich coatings

Plastics Elastomers Composites Ceramics

Thermoplastics •  •  •  • 

Thermosets •  Epoxies •  Phenolics •  Polyesters

Fluorocarbons Polyethylenes Polypropylenes Polyvinyl chlorides

C1 Fundamentals of Protective Coatings for Industrial Structures 1-28

Unit 1 - Corrosion and Corrosion Control

Elastomeric Linings

Composites

•  Natural and synthetic rubbers •  Polyurethanes

•  FRP and FGRP materials •  Used in piping, process vessels, tanks, etc.

Ceramics

Inhibitors •  Thin, protective films •  Passive layer on metal •  Remove aggressive constituents

•  Chemical resistant •  Temperature resistant •  Electrical resistant

Corrosion Control by Altering Environment

Cathodic Protection

•  Dehumidification •  Purification

•  External anode •  Entire structure becomes cathode •  Immersed or buried conductive media (electrolyte)

C1 Fundamentals of Protective Coatings for Industrial Structures 1-29

Unit 1 - Corrosion and Corrosion Control

Synergism of Coatings and Cathodic Protection

Uses of Cathodic Protection •  •  •  • 

•  CP protects at holidays •  Coatings reduce CP current requirements

Ships Waterfront structures Underground piping and tanks Water tank interiors

Two Basic Types of Cathodic Protection

Sacrificial Anode Metals

•  Sacrificial anode (galvanic) •  Impressed current (external power source)

•  Magnesium •  Zinc •  Aluminum

Impressed Current Anode Materials

A Cathodically Protected Water Tank •  •  •  •  • 

C1 Fundamentals of Protective Coatings for Industrial Structures 1-30

High-silicon cast iron Graphite Aluminum Scrap iron Pure, alloyed, or plated platinum

Unit 1 - Corrosion and Corrosion Control

Sacrificial Anode vs. Impressed Current Cathodic Protection Sacrificial Anode: −  No external power supply −  Limited current output −  Adjustable medium output −  OK for low resistivity −  Requires electrolytes −  −  −  − 

Lower installation costs Few interference problems Low maintenance costs Localized protection

Possible Adverse Effects of Cathodic Protection •  Deterioration of coatings not resistant to alkali •  Disbonding of coating by hydrogen gas from excessive current •  Electroendosmosis

Impressed Current: −  −  −  −  −  −  −  −  − 

External power supply Variable voltage Variable current OK for high resistivity medium Requires electrolytes Higher installation costs Can cause interference Monthly power bills Protects larger structures

Advantages of Coatings

Corrosion Control by Coatings •  Barrier •  Inhibitors •  Cathodic protection

•  •  •  •  •  • 

Limitations of Coatings •  •  •  • 

Ease of application Ease of storage and handling Range of acceptable ambient conditions Economics Easy repair Selection of color, gloss, and texture

Corrosion Control Programs •  A successful corrosion control program utilizes as many corrosion control systems as appropriate and practical.

Surface preparation requirements Application requirements Drying/curing requirements Health/safety/environmental concerns

C1 Fundamentals of Protective Coatings for Industrial Structures 1-31

Unit 1 - Corrosion and Corrosion Control

Unit 1 Summary •  Mechanisms of metal corrosion •  Common types of corrosion •  Methods of corrosion control

C1 Fundamentals of Protective Coatings for Industrial Structures 1-32

Unit 2 - Coating Types and Their Mechanisms of Protection

COATING TYPES AND THEIR MECHANISMS OF PROTECTION 2.1 Purpose and Goals

Industrial protective coatings have been around since the 1930s, but the industry was given a boost during

Scope

World War II. The need to keep ships out to sea longer

This unit covers the basic mechanisms of corrosion

and in dry docks less, led first to the development of

control by coatings, coating composition, different

the epoxy resin, and next to the polyamide epoxy,

coating types and curing mechanisms, and criteria

which had better adhesion, some flexibility, and

for coating selection. Learning Outcomes Upon completion of this unit, you will be able to: •

Define the three basic mechanisms of corrosion control by coatings

•

Define film properties necessary to provide good protection

•

Describe how coatings may provide galvanic (cathodic) protection

•

Define the three basic mechanisms of film formation

•

Define the different generic types available for use

•

Explain the criteria for coating selection and the conditions under which different systems may

an increased resistance to water. As the demand

be appropriate or inappropriate

for the materials of war increased, the demand for improvements to industrial coatings grew with it.

2.2 Mechanisms of Corrosion Control by

Better adhesion, faster cures, and better resistance

Coatings

to abrasion were some of the driving forces, but the need to keep materials from corroding remained the

The most widely used method to prevent corrosion

primary motivation for improvements to coatings.

today, particularly on carbon steel, is the application of high performance, protective coatings. High

As discussed in Unit 1, corrosion of metals is an

performance coatings protect thousands of structures,

electrochemical reaction that can be controlled by

including: off-shore drilling rigs, ships, storage tanks,

interfering with one or more of the four requirements

sewage systems, power plants, shipping containers,

for a corrosion cell: anode, cathode, electrolyte, and

pipelines, railway cars and commercial buildings.

metallic path.

C1 Fundamentals of Protective Coatings for Industrial Structures 2-1

Unit 2 - Coating Types and Their Mechanisms of Protection

Coatings can provide such interference by:

Inhibitive Pigment Protection

•

Providing a barrier to separate the electrolyte

Some coatings also provide protection by containing

from the metal

inhibitive pigments that disrupt or prevent typical

Providing chemical inhibitors to control the

corrosion reactions from occurring. The mechanism

anodic (corrosion) reaction

by which inhibitors work is not always clear, but the

Providing cathodic protection by converting

common theory is that the inhibitive materials react or

anode areas to cathode areas

bind with water to prevent it from further penetrating

• •

the coating film, or produce compounds that inhibit corrosion reactions. A good example of an inhibitor is

Barrier Protection

lead, which is no longer widely used in coatings since it

Barrier protection is the simplest way a coating

is a health and environmental hazard. Other inhibitive

functions as a protective layer and refers to the

pigments may include borates, chromates (which are

physical barrier that is formed on the substrate surface

restricted like lead), phosphate or molybdates.

by any coating. This physical barrier prevents air and water, which are necessary for corrosion, from

Galvanic (Cathodic) Protection

reaching the substrate. All coatings provide barrier protection, although some coating have characteristics

Coatings also may protect a substrate by providing

that enhance the barrier function of the coating. For

sacrificial or cathodic protection. This occurs when

example, coatings that contain micaceous iron oxide

the coating layer contains a metal that will act as the

(MIO) or aluminum flakes in their formulation form

anode in the corrosion process, thereby protecting

plate-like layers (called lamenar pigments) in the

the metal substrate (or cathode). Sacrificial coatings

coating film. Water or air cannot penetrate the “plates”

are always used as primers since the sacrificial metal

and must take a longer path to eventually reach the

must be in direct contact with the metal substrate. Zinc

substrate.

is the most common sacrificial metal used to protect iron-based steel materials. The best example of this is with zinc-rich primers where zinc dust is added to the coating formulation in amounts up to 90% by weight of cured film. A zinc layer can also be formed on a steel substrate by metallizing or galvanizing- both of which essentially deposit a solid zinc layer on the substrate. Metallizing is accomplished by melting the zinc (and/or aluminum) and spraying it onto the substrate surface using flame spray, electric arc or plasma arc processes. Metallizing can be performed in the shop or field at a project site. Galvanizing is completed by dipping steel parts in a molten zinc

Figure 2-1: Mechanism of micaceous iron oxide flakes providing extra barrior protection to film of coating

bath. The galvanizing process generally provides superior protection to metallizing for comparable zinc thicknesses, with the obvious limitation that it

C1 Fundamentals of Protective Coatings for Industrial Structures 2-2

Unit 2 - Coating Types and Their Mechanisms of Protection

Surface-Tolerant Coatings

can only be done in a factory or shop setting for new steel before erection.

Surface tolerant coatings are formulated to be applied to incompletely cleaned surfaces, especially where

Total Protective Coating System

abrasive blasting cannot be done. The residual

A coating system oftentimes consists of a primer

contaminants may be moisture, oil, or corrosion

and topcoat. In many instances, an intermediate

products. Surface-tolerant coatings require special

coat may be specified. However, there are single

properties such as good wetting of surfaces, good

coat systems, and four and five coat systems that

penetration of contaminants, or reaction with

are employed to protect industrial structures. When

moisture.

multiple coatings are used to create a system, they

•

must be compatible with one another. Also, each

Drying oil/alkyd coatings - good wetting, penetration

coating layer has a function that is performed at a

•

Epoxy mastics - good wetting

given thickness. Accordingly, one cannot make up

•

Penetrating epoxies/polyurethanes -

for an inadequate zinc primer thickness by adding an

penetration

extra few mils of an epoxy intermediate coat. Each

•

layer should be applied at the optimum thickness (not

Moisture-curing polyurethanes - reaction with moisture

too thick or too thin), and verified for proper thickness prior to the application of subsequent layers.

Universal Primers

Primer Function

Universal primers is a general term which may have

The function of the primer is to bond the coating

think of them as tie coats that permit use of a topcoat

system to the substrate. The primer also provides

that is not normally compatible with an existing

corrosion protection of the steel substrate using

coating. Other people, however, think of them merely

barrier, inhibitive, or galvanic protection, or a

as surface tolerant coatings.

different meanings to different people. Most people

combination of them. The primer must also be tolerant of the level of surface preparation performed

Direct-To Metal (DTM)

and must be compatible with the next layer applied, when applicable. If the primer is the only layer (as

The term DTM was first used by the manufacturer of

with a single coat system), it must be resistant to the

a product that could be applied directly to a prepared

service environment.

metal surface without priming. Now, it is used by several manufacturers for any coating for metal that

Special Primers

does not require use of a primer before application.

There are four primers that have special uses and/

Preconstruction Primers

or terminology that are not well defined throughout the coatings industry. These are surface-tolerant

Preconstruction primers, sometimes called hold coats

coatings, universal primers, and direct-to-metal

or holding primers, are thin coats applied in shops

coatings (DTMs).

to cleaned steel (usually abrasive blasted) prior to

C1 Fundamentals of Protective Coatings for Industrial Structures 2-3

Unit 2 - Coating Types and Their Mechanisms of Protection

construction. After construction, the coating is lightly

•

Resistance to water, fuel, chemicals, etc.

abrasive blasted, as required (secondary surface

•

Resistance to biological growth

preparation), and the total system is then applied. This minimizes or avoids field abrasive blasting.

Adhesion of Coatings

Preconstruction primers, such as inorganic zincs or

All coatings must have at least a minimal adhesion

alkyds, are used extensively in ship building.

to their substrates to provide long-term protection. Otherwise, they will not be able to withstand the

Intermediate Coat Function

stresses of shrinking during curing (sometimes

An intermediate coat is typically incorporated into

prolonged curing) or other dimensional changes.

a coating system for the purpose of adding barrier

Primers are often used to ensure good chemical

protection. The intermediate coat must be compatible

bonding to substrates. Adhesion of organic coatings

with both the primer and the topcoat.

is most commonly of a secondary bonding (involving slightly differently charged areas of molecules) rather than by direct chemical bonding. In either case,

Topcoat Function

a clean, profiled surface is required for adequate

The topcoat or finish coat is the first line of defense

adhesion.

in a corrosion protection system. It must also be aesthetically pleasing, and should be able to

Prior to performing an adhesion test, a test method

maintain color and gloss levels for long periods of

must be selected. There are three primary ways

time. Naturally, the topcoat must be resistant to the

to test the adhesion of a coating system. All three

service environment, and must be compatible with

methods are described in ASTM standards. The chart

the underlying layer (primer or intermediate coat, as

below summarizes the methods that are commonly

appropriate).

used to test the adhesion of industrial coatings.

2.3 Desired Film Properties

ASTM Standard

In order for protective coatings to provide long-term

D3359

“Adhesion By Tape Test”

D6677

“Knife Adhesion”

D4541

“Pull-off Strength of Coatings Using Portable Adhesion Testers”

D7234

“Pull-off Strength of Coatings on Concrete Using Portable Adhesion Testers”

protection to steel and other substrates, they should have certain specific properties. These properties vary with generic types or uses. Desired properties include: •

Good adhesion to substrate and between coats

•

Low permeability to electrolytes

•

Continuous, holiday-free film of uniform

Generally the first two methods listed above (ASTM

thickness •

Flexibility

•

Resistance to abrasion

•

Resistance to weathering

ASTM Test Method

D3359 and D6677) are considered “field” test methods, since they do not require any special equipment and can be conducted rather quickly.

C1 Fundamentals of Protective Coatings for Industrial Structures 2-4

Unit 2 - Coating Types and Their Mechanisms of Protection

pull-off test (ASTM D4541 and D7234) is used to evaluate a coating’s tensile strength, or its resistance to a perpendicular pull. Since the testing mechanisms are different (peel-back versus pull-off), the test results generated from each test should not be compared. Also, any type of adhesion test is “destructive.” That is, the coating in the area in that the test is performed will be damaged, and oftentimes it must be repaired. Therefore, adhesion testing should not be conducted unless required by the project specification. The Figure 2-2: Adhesion by Tape Test

specification should include repair procedures for the

(ASTM D3359)

affected areas. Permeability

The latter two methods (ASTM D4541 and D7234) require an instrument and the attachment of pull stubs

Organic coatings vary widely in their permeability

using an adhesive that may need to cure overnight, up

to electrolytes, and thus in their ability to provide

to 24 hours. It is common to conduct this method of

barrier protection. A low permeability is especially

adhesion testing

important in severe service such as immersion and

in a laboratory;

commonly leads to blistering and disbonding of

however, the in-

coatings. Permeability can be tested by measuring

struments used

the water vapor transmission of organic coating films

to test the pull-off

(ASTM D1653).

adhesion strength of coatings are

Film Continuity

portable and testing can and is rou-

A highly desirable coating property is the ability to

tinely done in the

form a uniformly thick, continuous film that is free

field.

of holidays. Film imperfections permit electrolyte to penetrate the barrier. Good flow and leveling of the wet coat will minimize pinholes and thin areas that

The tests listed in Figure 2-3: Pull-off Strength of Coatings Using Portable Adhesion Tester (ASTM D4547)

invariably fail first.

the table above dif evaluate two different adhesion properties, and differ they use differ-

ent testing mechanisms. The tape and knife adhesion tests (ASTM D3359 and D6677) are used to evaluate the “shear” or “peel” strength of a coating, while the

C1 Fundamentals of Protective Coatings for Industrial Structures 2-5

Unit 2 - Coating Types and Their Mechanisms of Protection

Hardness

ASTM D2583, “Barcol Hardness” is also called “impressor hardness” and is measured using a hand held

The hardness of a coating or lining film is sometimes

impressor. The impressor is designed to be used on

used as an alternative method to assess its level of

aluminum, soft metals, plastics, fiberglass, rubber and

cure. Film hardness can be assessed using a vari-

leather. It is designed for use on thicker materials,

ety of methods described by ASTM, including pencil

and therefore may not work on thin film coatings. An

hardness, Barcol hardness and durometer hardness.

impressor is easy to use, and simply requires the user

Barcol and durometer hardness testing are oftentimes

to position the device so that the indenter pointer and

restricted to thick film systems. Each of these meth-

support legs are on the same plane. The operator

ods is described below.

should press down fimly on the impressor handle, observe the dial indicator and record the peak reading.

ASTM D3363, “Test Method for Film Hardness by

If a softer coating

Pencil Test” can be used to assess the drying char-

is being tested,

acteristics of a coating, as indicated by its inherent

there may be

hardness. Pencils containing various hardnesses of

some drift from

lead (shown below), from very soft (6B) to very hard

the peak value.

(6H) are sharpened, then blunted (dressed) using a

Multiple read-

fine sandpaper.

ings should be obtained (from

ASTM D3363 Pencil Hardness Scale 6B 5B 4B 3B 2B B HB F H Soft

Medium

3 to 9 measurements, depend-

1H 3H 4H 5H 6H

ing on the real-

Hard

tive hardness of

Figure 2-5: Barcol Impressor

the material) and an average hardness value reported. The manufac-

Figure 2-4: ASTM D3363 Pencil Hardness Scale

turer of the impressor has specific requirements for verifying the accuracy of the instrument prior to use.

The pencil is held at a 45 degree angle to the coated

The operator should carefully read the instructions

surface and the edge of the blunted lead is pushed

prior to use, and should verify the accuracy of the in-

into the coated film, in an attempt to scratch or gouge

strument before acquiring hardness measurements.

the coating. If the coating is scratched or gouged, a softer lead is selected and the coating film is re-tested.

ASTM D2240, “Durometer Hardness” is measured

If the coating is not scratched or gouged, a harder

using either a Shore A or a Shore D scale durometer.

lead is selected and the coating film is re-tested. The

Similar to the Barcol Impressor, position the durometer

hardest pencil lead that cannot scratch the coatings

on the coated surface and set the red ancillary hand

is recorded as the “scratch hardness;” the hardness

on the on the durometer dial just above the black

pencil lead that cannot gouge the coating is recorded

needle (which indicates the hardness). Hold the

as the “gouge hardness.”

durometer vertically and apply the presser foot to the coated surface as rapidly as possible, maintaining the

C1 Fundamentals of Protective Coatings for Industrial Structures 2-6

Unit 2 - Coating Types and Their Mechanisms of Protection

foot of the du-

attempting to expand with a flexible undercoat. Tough

rometer parallel

films tend to be rigid, so a compromise must usually

with the coated

be made between flexibility and toughness if a durable

surface. Hold

film is desired.

the durometer in place for 1

Resistance to Abrasion

or 2 seconds,

In many service conditions, resistance to abrasion

then release

is important. Inorganic zinc coatings are among the

the downward

most abrasion-resistant industrial coatings. The

pressure. The maximum hardness value is in-

Taber abrasor (ASTM D 4060) and the falling sand

Figure 2-6: Shore Durometer

test (ASTM D 968) are two methods of measuring abrasion resistance.

dicated by the red ancillary hand (the black hand returns to zero

Resistance to Weathering

once the downward pressure is released). Obtain a minimum of five measurements and report the

All organic coatings are subject to deterioration by the

average hardness value. The manufacturer of the

sun’s ultraviolet light, which breaks chemical bonds in

durometer has specific requirements for verifying the

organic binders. Moisture (e.g., rain) will also acceler-

accuracy of the instrument prior to use. The operator

ate coating deterioration. Exterior coatings, such as

should carefully read the instructions provided with

epoxies, that have poor resistance to ultraviolet light

the instrument prior to use, and should verify the ac-

should be topcoated with a coating that will provide

curacy of the instrument before acquiring hardness

this resistance, such as an aliphatic polyurethane or

measurements.

acrylic.

Three additional tests that can be conducted to

Accelerated Weathering

measure the hardness of an organic coating are:

A variety of devices have been developed for

ASTM D2134: Determining the Hardness of Organic

laboratory use in simulating exposure of coated panels

Coatings with a Sward Type Hardness Rocker, ASTM

to different natural environments but with increased

D4366: Hardness of Organic Coatings by Pendulum

intensity to accelerate the weathering of coatings

Damping Tests and ASTM D1474: Indentation

to produce much earlier data. Unfortunately, data

Hardness of Organic Coatings.

obtained using these devices do not usually correlate well with data from slower, natural weathering. These

Flexibility

devices most commonly attempt to determine the effects of light, heat, and moisture on coatings.

Coating flexibility can be determined by bending a coated panel over a cylindrical or conical mandrel (ASTM D522). It is desirable that coating films be flexible so that they can easily expand and contract with the substrate. A rigid topcoat may crack when

C1 Fundamentals of Protective Coatings for Industrial Structures 2-7

Unit 2 - Coating Types and Their Mechanisms of Protection

Impact Resistance

Different accelerated weathering devices include: •

Outdoor exposure procedures for evaluating the exterior durability of coatings applied to substrates

A coating’s resistance to impact is a measure of both its flexibility and its adhesion. ASTM D2794

can be found in ASTM D4141. •

is commonly used to determine impact resistance.

Salt fog chambers (ASTM B117) most commonly

It involves dropping a known weight from various

spray a warm mist of atomized neutral or slightly

heights until the coating fractures or disbonds and is

acid solution containing 5% sodium chloride. This

reported in inches per pound.

method is not recommended by SSPC. •

Chemical Resistance

Cyclic tests include periodic wetting (with Harrison’s or other special solution) and drying.

Interior surfaces that store water, fuel, or chemicals

ASTM D 5894, Standard Practice for Cyclic Salt

must be lined with a coating that is resistant to the

Fog/UV Exposure of Painted Metal (Alternating

stored products.

Exposures in a Fog/Dry Cabinet and a UV/ Condensation Cabinet) covers basic principles

Resistance to Biological Growth

and operating practices for cyclic corrosion/UV

Coated surfaces at tropical and subtropical locations

exposure of paints on metal, using alternating

should have a mildewcide incorporated into them to

periods of exposure in two different cabinets:

protect them from biological defacement. Mildewcides

a cycling salt fog/dry cabinet and a fluorescent

that have been approved by the Environmental

UV/condensation cabinet. This is the test most

Protection Agency and have passed American Society

commonly used for evaluating the performance of

•

coatings in an accelerated test environment.

for Testing and Materials (ASTM D5590) mildew-

Humidity cabinets (ASTM D2247) expose coated

which were used very effectively in the past, are no

panels to warm, 100% humidity or to warm

longer permitted.

resistance tests should be used. Mercurial compounds,

condensation. •

Antifouling coatings are applied to the underwater

ASTM D822: Standard Practice for Filtered

portions of ships and other marine floating structures

Open-Flame Carbon-Arc Exposures of Paint

to control the attachment and growth of marine

and Related Coatings measures the resistance a

organisms.

coating has to light, heat and water by use of an open-flame carbon-arc testing device. •

There are basically four basic types of antifouling

ASTM D3361 preforms the same test described

coatings that have been used effectively: soluble

above; however, it incorporates a unfiltered open-

matrix, insoluble matrix (contact leaching), self-

flame carbon-arc testing device.

polishing, and fouling release. Each is distinctly different and so will be discussed separately.

C1 Fundamentals of Protective Coatings for Industrial Structures 2-8

Unit 2 - Coating Types and Their Mechanisms of Protection

In the soluble matrix antifouling coatings, the binder

Self-polishing antifouling coatings also leave a smooth

is usually limed rosin modified with other binders

finish free of fouling rather than a honey-combed

and plasticizers. The binder blend is slightly soluble

film and so are sometimes called self-smoothing.

in slightly alkaline seawater and slowly dissolves to

Because of this, it is much easier to renew the

release the copper oxide or other toxins. The matrix

antifouling coating.

merely serves as a system for delivering the toxic compounds into the sea water.

The term ablative that is often used in describing antifouling coatings means slowly eroding. Thus, both

Films of soluble matrix antifouling compounds initially

the insoluble matrix and the self-polishing coatings are

contain about 40% by weight of copper oxide. They

ablative, while the soluble matrix antifouling coatings

are relatively soft and are effective for only 1-2

are not.

years. In fouling release antifouling coatings, the low surfaceIn the insoluble matrix antifouling coatings, the binder

energy binder, usually a silicone elastomer, provides

is insoluble in sea water. These coatings usually have

a surface that is much more resistant to fouling

heavy loadings of copper oxide pigment particles in

attachment and growth. On high speed ships, very

contact with each other. These particles are slowly

little fouling remains and that which does can be

dissolved in the sea water to leave a honey-combed

removed by light scrubbing or low pressure water

film behind. The binder in the film is not chemically

washing. These coatings are ineffective on ships

changed. As the porous surface layer grows, the

in port.

rate of biocide release declines and the antifouling performance drops dramatically.

The term fouling release is really a misnomer, because these coatings neither hold nor release any fouling.

The insoluble matrix films are harder than those of the

The fouling species themselves do the attaching.

soluble matrix antifouling coatings and can be used on faster ships. They are also effective on stationary

Fouling release antifouling coatings contain no

structures and have a service life of 1-2 years.

toxic materials and so do not adversely affect the environment. Thus, there is much interest in them.

In the self-polishing antifouling coatings, the binder,

These coatings do have the limitation of being

frequently acrylic, undergoes a slow chemical

relatively soft and thus easily damaged.

degradation (hydrolysis) to become soluble in sea water as the ships pass through it at high speeds. The toxin is released into the sea water during the hydrolysis and erosion process at a controlled rate. Consequently, they have much longer service lives than earlier antifouling coatings. These coatings are ineffective on stationary structures or ships in port.

C1 Fundamentals of Protective Coatings for Industrial Structures 2-9

Unit 2 - Coating Types and Their Mechanisms of Protection

Coating solids (total solids) content. As described in ASTM D2369, a sample of coating is weighed in a shallow aluminum pan, heated to remove the volatile materials, and reweighed after cooling to give the percent coating solids. It should be noted that even the solvent-free (100% solids) coating systems never total 100% by this test method but are typically 96-98%, Pigment content. For latex coatings, the aluminum pan used in the above (coating solids) test is further heated at a much higher temperature to burn off the

Figure 2-7: The basic component of coatings

organic non-volatile solids and leave only inorganic pigment remaining. The pan is reweighed after

2.4 Coating Components and Their Functions

cooling.

Organic coatings have three basic components: the solvent, the resin (binder), and the pigment. Not

For other than latex coatings, procedures in ASTM

all coatings contain all three components. There

D2371 and D2372 are used. A weighed coating

are clear, pigment-free coatings and solvent-free

is spun in a high-speed centrifuge. The heavier

(100 percent solids) coatings, but never binder-free

solid pigment is separated from the lighter vehicle

coatings.

components and weighed after drying.

Coatings are sometimes compositionally into two

Volatile and non-volatile vehicle content. The

components by combining the solvent and the

percent by weight of volatile product (solvent) is

dissolved binder into what is called the vehicle, or

found by subtracting the percent by weight of coating

liquid component. The solvent is called the volatile

solids from 100. The percent by weight of non-volatile

vehicle, and the binder, the non-volatile vehicle.

vehicle (binder) is found by subtracting the percent

The insoluble pigment is the heavier, solid portion

by weight of pigment from the percent by weight of

that tends to settle to the bottom of containers upon

coating solids.

prolonged standing. Solvent Because only the solvent portion of coatings is lost

Organic solvents are used to dissolve binder materials

during curing, the remaining binder and pigment are

and reduce coating viscosity to permit easier

sometimes called the coatings solids. The coatings

application. They also control drying of wet film and

solids directly affect the thickness of coating films.

adhesion and durability of the dry film. Binders that are less soluble require stronger solvents or more

Simple laboratory tests are conducted to determine the

solvent to dissolve them. Solvent blends are generally

percent by weight of the three basic components:

used to control evaporation and film formation, and

C1 Fundamentals of Protective Coatings for Industrial Structures 2-10

Unit 2 - Coating Types and Their Mechanisms of Protection

are often recommended by coating manufacturers to

is added. In some cases, field thinning is prohibited.

give a combination of the above properties.

If thinning is permitted, the amount of thinner added must be carefully monitored, in order to avoid

Common organic solvents include: •

Ketones (e.g., methyl ethyl ketone)

•

Esters

•

Aromatics (e.g., toluene and xylene)

•

Aliphatic hydrocarbons (e.g., mineral spirits)

exceeding the allowable VOC content threshold. Resin (Binder) The resin or binder is a non-volatile component. It is both part of the wet film and the dry film. Oftentimes (but not always), a coating is identified generically

The amount of organic solvent (the volatile organic

by the type of resin used in the formulation. For

compounds, or VOCs) permitted in coatings is

example, a commonly specified coating system for

regulated in many countries, and in the US, more

the interior of a portable water storage tank is a two

severely in areas where air pollutants exceed healthful

or three-coat epoxy. In this case, “epoxy” is used to

levels. VOCs react in sunlight with other air pollutants

describe both the coating type and the raw material

to form ozone, a hazard to human health. Thus,

resin system used to formulate the coating. The resin

coatings with low-solvent contents are required.

system is the film-forming component of a coating. It cohesively bonds the pigmentation together and

In water-borne coatings, the binder and pigment

adhesively bonds the coating to the underlying

are usually dispersed in water. The most common

substrate or coating layer. It is essentially the “glue” of

water-borne coatings are commonly called emulsion

the coating. In many cases, the resin system dictates

or dispersion coatings. As the water and coalescing

the performance properties of a coating.

organic solvents in the wet paint evaporate, binder particles coalesce to form a film. In the US, these

Important properties imparted by the binder include:

coatings are often referred to as latex emulsions. Coatings are tested for VOC content according to ASTM D3960. Water content is determined according to ASTM D4017 (Karl Fischer Titration) and exempt solvents are detected according to ASTM D6133. Exempt solvent and water are not considered detrimental VOCs, so their concentrations are deducted from the VOC content of a coating formulation.

•

Mechanism and time of curing

•

Performance in different environments

•

Performance on different substrates

•

Compatibility with other coatings

•

Flexibility and toughness

•

Exterior weathering

•

Adhesion

•

Ease of application, topcoating, and repair

Pigment

Note- the addition of thinner in the field contributes

The pigment is also a non-volatile component of a

to the VOC content of a coating. A coating that was

coating formulation. The pigment is essentially an

formulated and selected based on the VOC content

insoluble raw material. That is, it does not dissolve

“as manufactured” may not be compliant once thinner

in the resin and solvent, but rather is suspended

C1 Fundamentals of Protective Coatings for Industrial Structures 2-11

Unit 2 - Coating Types and Their Mechanisms of Protection

in the vehicle. Many of us think that the pigment

that penetrates the coating film to take a considerably

merely gives the coating its color. While color is

longer pathway to the substrate. Finally, extenders

certainly a function of the pigment, it is only one of

such as silica, mica and clay may be incorporated

several potential functions. The pigment gives the

into the formulation to improve film build, increase

coating its ability to “hide” the underlying surface.

the solids content of the coating and/or provide added

This is called hiding power. A coating formulated

barrier protection.

with pigment that demonstrates poor “hiding” may

•

Opacity (hiding)

•

Color

•

Corrosion resistance

•

Wet paint properties

•

Weather and moisture resistance

•

Level of gloss

•

Reinforcement

require the application of multiple coats, in order to obscure the previous coating layer. For example, if a white coating is to be applied over a black coating, the coating must be formulated with good hiding pigments. Otherwise the black will show through, requiring multiple applications of the white overcoat.

Wet Paint Properties

The level of pigment, and to a lesser extent, the shape and size (e.g., fineness of grind) of the pigment also

Secondary or filler pigments (talc, silica, etc.) are

determines the gloss level of a coating. For example,

used to control viscosity, wet film leveling, and

a “flat” sheen level is created by formulating the

settling. These cheaper pigments impart relatively

coating with more pigment than say a semi-gloss or

little hiding.

gloss sheen level, which is generated by using less pigment in the formulation.

Weather and Moisture Resistance In addition to protecting the finish binder from the

The pigment in a coating may also provide corrosion

destructive effects of sunlight, pigments bulk the

protection. If used for this purpose, the pigment

barrier thickness and force penetrating moisture to

must be formulated into the primer layer (the layer

detour around it. This is especially true with pigments

adjacent to the carbon steel substrate). Inhibitors

that tend to leaf over each other like tiles on a roof

like phosphate, borates, and molybdates and other

(e.g., micaceous iron oxide). They can greatly

compounds formulated into a primer “inhibit” the

increase the path that electrolytes must take to reach

corrosion process. And zinc powder added to a

the substrate.

primer in sufficient quantities galvanically protects the underlying carbon steel. Certain pigments even provide barrier protection. That is, their inherent

Level of Gloss

shape and the way in which they orient themselves

The pigment-to-resin ratio, generally expressed

in the dry film create a barrier to moisture penetration

as pigment volume concentration (PVC), can vary

through the coating. Examples include micaceous

widely. There can be little or no pigment, or the

iron oxide and leafing aluminum pigments. These raw

PVC can approach a value called critical pigment

materials are “lamellar,” which means they are plate-

volume concentration (CPVC). Coatings at or slightly

like, lay flat in the coating film and cause any moisture

below the CPVC usually have lower impermeability

C1 Fundamentals of Protective Coatings for Industrial Structures 2-12

Unit 2 - Coating Types and Their Mechanisms of Protection

but better blistering resistance. However, if the

during film formation so that they cannot be softened

CPVC is exceeded, there is insufficient binder to

by heating. These terms are now used in the coatings

wet the pigment particles fully and bind them to the

industry to classify coating types according to the

substrate.

nature of their resins in dried or cured films, although the original meanings have changed somewhat. A

Other things being equal, the lower the PVC (the

thermoplastic coating forms a film upon evaporation

greater the binder content), the higher the gloss.

of organic solvent and remains unchanged chemically

Thus, resin-rich areas of finish coatings often have

and thus, soluble in its original solvent; a thermosetting

“hot spots” (glossier areas). Also, other things being

coating undergoes a chemical reaction to form a film

equal, the finer the dispersion of the pigment in the

that is insoluble in commonly used solvents. Common

vehicle (formerly called the “fineness of grind”), the

latex coatings are also thermoplastic, since they are

greater the gloss.

unchanged chemically during film formation.

Additives

Film Formation and Coating Solubilities

Additives may be added to coatings to impart special

Coatings that have their binders dissolved in organic

properties. Any additives formulated into the coating

solvent and form films by simple evaporation of

also become part of the dry film. Various quantities

solvent are commonly called “lacquers.” Water-borne

of additives are used by the formulator to adjust

coatings that form films by coalescence during water

the consistency, flow-out, surface wetting, color,

evaporation are commonly called “latex” or “emulsion”

ultraviolet (UV) light (or solar radiation from sunlight)

coatings. Since both of these types of binders are

resistance and flexibility, or to prevent settling in the

unchanged chemically in film formation, they can be

can (suspending agents). For example, polyurethane

redissolved in organic solvents.

coatings may be formulated with hindered amine light stabilizers (HALS) to help preserve gloss and color

Coatings that form protective films by a chemical reaction

upon exposure to sunlight, and plasticizers formulated

tend to be insoluble in common organic solvents. As

into a coating provide film flexibility. There are many

we will see, air-oxidizing coating films (oil-based

additives that a formulator can employ. These are

paints) are solvent-soluble after initial formation. With

only examples.

additional time, continued cross-linking of polymers renders them less soluble. Solubility in a strong solvent such as methyl ethyl ketone (MEK) can be

2.5 Mechanisms of Coating Film Formation

used to distinguish between some general coating

Thermoplastic/Thermosetting Coatings

types, as shown on the next page.

Two terms commonly used in describing film formation are thermoplastic and thermosetting. These terms were originally used in the plastics industry. Thermoplastic materials can be reversibly softened and hardened by heating and cooling, respectively; thermosetting materials undergo a chemical change C1 Fundamentals of Protective Coatings for Industrial Structures 2-13

Unit 2 - Coating Types and Their Mechanisms of Protection

MEK-Soluble Coatings

MEK-Insoluble Coatings

Non-convertible

• Lacquers

• Chemically-reacting

•

Evaporation of organic solvent

•

Coalescence of latex particles

•

Phase change without chemical change

• Latex products • Oil based products (initially)

products • Oil-based products (only slightly soluble after aging)

Convertible

Film Forming Mechanisms

•

Coatings are converted to solid protective films by a

Air-oxidation (polymerization) of unsaturated drying oils

process generally called film formation, or curing. Cure

•

Chemical reaction of components

is defined by SSPC Protective Coatings Glossary as

•

Reaction with moisture or carbon dioxide in air

“the process of changing the properties of a paint from its liquid state into a dry, stable, solid protective film by

Non-Convertible Coatings

chemical reaction with oxygen, moisture, or chemical

Solvent evaporation. Coatings that form protective

additives, or by the application of heat or radiation.”

films by simple evaporation of organic solvent are

By this strict definition, coatings that form protective

sometimes called lacquers. They are made by

films without undergoing a chemical reaction do not

dissolving solid resins in an appropriate solvent. After

cure, but merely dry to form the film. Do not confuse

their application, the solvent evaporates to deposit

the words “drying” and “curing.” “Drying” is the loss of

the resin in a thin film. No chemical change occurs

solvent and/or water, which may or may not produce

to the binder.

a protective film, while “curing” always produces a protective film.

Examples of coatings that cure by this mechanism

The majority of organic coatings are better classified

are:

as either non-convertible or convertible. A non-

•

Vinyls (polyvinyl chlorides)

convertible coating contains a resin that does not

•

Chlorinated rubbers

change during film formation. A convertible coating

•

Acrylics

contains a resin or resin-forming component that

•

Bituminous cutback coatings (coal tars and asphaltics dissolved in organic solvent)

undergoes chemical change during film formation. Coatings that cure by the same basic mechanism tend

Lacquers have poor solvent resistance, since they

to be compatible with each other, but not with coatings

are deposited from solvents, but are easy to topcoat

that cure by other mechanisms. Most coatings are

and repair because the topcoat solvent bites into the

converted into solid protective films by one of three

undercoat to bond tightly. Because lacquers are high

basic mechanisms: evaporation of solvent or water,

in solvent (VOC) content, their use has been greatly

reaction of drying oil in the binder with oxygen in the

curtailed.

air, or chemical reaction of components. However, there are a few other mechanisms of film formation in the lists in the next column:

C1 Fundamentals of Protective Coatings for Industrial Structures 2-14

Unit 2 - Coating Types and Their Mechanisms of Protection

Coalescence. Water-borne acrylic latex coatings

Heat and UV Cure. Some coatings require exposure

cure by solvent evaporation and form a coating film by

to heat or ultraviolet light to attain a complete cure after

a process known as coalescence. Water (the primary

application. The heat or UV light causes a chemical

solvent in these coatings) first evaporates from the

reaction and subsequent cure of the coating film.

coating film. As the water evaporates, a special coalescing solvent with a higher boiling point (e.g.,

Convertible Coatings

propylene glycol) aids in fusing the acrylic molecules

Convertible coatings cure via a wide range of

together to form a solid film. The coalescing solvent

mechanisms, including heat, oxygen, catalysts, water,

then evaporates from the coating film. Note that the coalescing process typically requires a minimum 50°F

or carbon dioxide:

air temperature. Should the air temperature fall below

•

Air-oxidation of drying oils

50°F before the coalescing process is complete,

•

Chemical reaction of components

curing may stop and may not start again once the

•

Reaction of binder with moisture or carbon dioxide

temperature recovers. This is a major concern with industrial water-borne acrylic coatings, and should be carefully considered.

Air-oxidation of drying oils. Drying oils in coatings that cure by oxidation actually react with oxygen (air)

Latex films are quite flexible, but tend to be more

to form a film. This oxidation process never stops,

permeable and less durable than most other films.

as long as the coating is exposed to oxygen. For

Once coalesced, the coatings are not readily affected

example, alkyd coatings (which typically contain

by water, but can be dissolved in relatively strong

unsaturated oils, pigments and driers) cure by

organic solvents.

oxidation. Many aged alkyd systems (those that have been in service for many years) become very brittle, as the resin continues to oxidize long after the

There are two other types of coatings with water as

coating is cured. Coatings that cure by oxidation are

the main carrier: •

Water-soluble (of limited protective value)

•

Water-reducible

also sensitive to excess film build and may wrinkle or crack. Common examples of coatings that cure by this mechanism are:

Water-soluble coatings are not durable enough for general use. Water-reducible coatings contain a solvent blend that can be thinned with water. Alkyd and epoxy formulations are available in either waterreducible or dispersion forms, where low VOC content is the driving force. Such alkyd films are cured by

•

Unmodified drying oils

•

Epoxy esters

•

Uralkyds

•

Alkyds

•

Oleoresinous phenolics

•

Silicone alkyds

air oxidation, and two-component epoxy films by chemical reaction.

C1 Fundamentals of Protective Coatings for Industrial Structures 2-15

Unit 2 - Coating Types and Their Mechanisms of Protection

Coatings that cure by air-oxidation of drying oils wet

•

Polyesters and Vinyl Esters

surfaces very well and generally perform well in mild

•

Phenolics

atmospheric environments, but have limited durability

•

Wash primers

in chemical, particularly alkaline, environments.

•

Silicones (Polysiloxanes)

Epoxy esters provide additional chemical resistance but have poor resistance to sunlight because of the

Because these coatings are thermosetting, they

epoxy component. They should not be confused with

usually have good chemical and solvent resistance.

higher-performance, two-component, thermosetting

They are also difficult to topcoat when fully cured,

epoxies. Uralkyds have good abrasion resistance

because topcoat solvent cannot bite into them. Thus,

and gloss retention, and are frequently used on wood

a topcoat is best applied while the undercoat still has

floors; however, these coatings are by no means

some residual tack. If a completely cured thermosetting

equivalent to the two-component or moisture-cured

coating is to be topcoated, it is necessary to treat the

polyurethanes to be discussed later. Oleoresinous

surface. One way is to sand it slightly; another way,

phenolic coatings are the only oil-based coatings

used by the Navy, is to first spray a thin (e.g., two-

that can be used successfully in water immersion

mil wet film thickness) tie coat on the last coat and

service.

allow it to cure to a tacky state before applying a full topcoat. The coating manufacturer provides the best recommendation for resolving this problem.

Chemical reaction of components. Coatings that cure by chemical reaction are usually the most durable, but have more stringent surface preparation

Other examples of chemically curing coatings are

and application requirements than other types. They

the different inorganic zinc-rich coatings. Some

are generally packaged in two separate containers

formulations cure by hydrolysis of the silicate binder

that are mixed together to initiate the curing reaction.

with water in the air. Some cure by reaction with

Components must be combined in the exact

carbon dioxide from the air.

proportions and manner specified by the supplier to achieve a film with optimum properties. Sometimes,

Zinc-rich organic coatings cure by the mechanism of

an “induction time” (often called “sweat-in time”) is

their organic binders. Thus, zinc-rich epoxies cure

required after mixing and before application to permit

by chemical reaction, and zinc-rich vinyls, by solvent

the reaction to get started. After mixing, there is

evaporation.

always a “pot life,” during which the coating must be applied before the reaction has advanced so far that

2.6 Comparisons of Generic Coating Types

it cannot be properly applied.

In this section, the film properties of commonly used coatings will be discussed, and then their

Common examples of coatings that cure by chemical

advantages and limitations will be summarized in

reaction of components are: •

Epoxies

•

Coal tar epoxies

•

Polyurethanes

•

Polyureas

brief tables. Obviously, these discussions and tables will only summarize overall tendencies. Coating manufacturers should be consulted on the proper use of their products. Special mention will be made

C1 Fundamentals of Protective Coatings for Industrial Structures 2-16

Unit 2 - Coating Types and Their Mechanisms of Protection

in the tables presented below of ease of formulating

•

Blast cleaned surface necessary for coating

each generic type with a low VOC (solvent) content,

•

Occasional poor adhesion

since new restrictions on VOC content may limit or Bituminous Coatings

eliminate their use.

Bituminous (asphalt and coal tar) are thermoplastic

Thermoplastic Lacquers

coatings that will be discussed separately from other

Lacquer formulations with synthetic resins (e.g.,

thermoplastic coatings because of their unique film

vinyls, chlorinated rubbers, and acrylics) form durable

properties. They sometimes have inert fillers added to

films that have good water and general chemical

reinforce the film and can be applied hot or cold.

resistance (especially to acids and alkalis) but, being thermoplastics, have poor solvent and heat resistance.

Bituminous coatings found much use in the past

They have a low film build but dry so rapidly that they

because they were cheap and easy to use. They have

can quickly be topcoated. They require a blasted

good water resistance but weather poorly (become

surface, and in some cases wash priming, for good

brittle) in sunlight. They are used much less now

adhesion. They are easy to topcoat and repair and

because of toxicity concerns and their limited exterior

can be formulated for good gloss retention. The good

durability.

weathering of acrylic lacquers is duplicated in WaterBorne Acrylic Coatings section.

Both asphalt and coal tar coatings can be applied by hot mopping, or by conventional application of a solvent solution (cutback), or a water emulsion.

The chief disadvantage of these lacquers is their high VOC content, which may eliminate them completely from industrial and architectural use. Because of the

Emulsion application seems to provide slightly

uniquely excellent performance of chlorinated rubber

improved weathering properties.

coatings on exterior concrete swimming pools, they have been granted temporary exemptions (in many

Advantages

areas of the U.S.) for this use.

•

Low cost

•

Easy to apply, topcoat, and repair

Advantages

•

Good water resistance

•

Rapid drying and recoating

•

Good film build

•

Good general chemical resistance

•

Low level of surface preparation required

•

Good in water immersion

•

Good gloss retention possible

Limitations

•

Good durability

•

Easy to topcoat and repair

•

Cutbacks are high in VOCs

•

Poor solvent resistance

•

Poor weathering

•

Available only in black

•

Toxic

Limitations •

High in VOCs

•

Poor solvent resistance

•

Low film build C1 Fundamentals of Protective Coatings for Industrial Structures 2-17

Unit 2 - Coating Types and Their Mechanisms of Protection

Water-Borne Acrylic Coatings

formation of tiny bubbles. If any water thinning is done, care must be taken to avoid overthining which

Water-borne acrylic coatings are based on latexes

can have disastrous results.

of resins derived from acrylic acid, methacrylic acid, and esters of these acids. These basic resins can

Water-borne acrylics have the benefit of not requiring

be modified in many ways to produce coatings with

flammable or toxic solvents for thinning or cleanup.

a variety of desirable products for steel, galvanized

Formulations have very low VOC content, well below

steel, aluminum, concrete, masonry and wood

any current or projected regulatory limit.

substrates.

Advantages

A wide range of gloss (from low to high) is available for primer and topcoats. Their exterior gloss and color retention is outstanding, so that they can be used as

•

Environmentally acceptable

•

Easy to apply (and clean up), topcoat, and repair

finish coats for other exterior systems. Acrylic latex systems have also been successfully used as directto-metal (DTM) systems. Optimum film quality requires careful polymer (latex) design and choice of formulation additives.

•

Dry rapidly for recoating

•

Reduced solvent odor

•

Excellent flexibility

•

Low cost

•

Safer (reduced flammability)

Coalescing solvents are required to obtain continuous, impermeable films. During the loss of these slow-

Limitations

evaporating solvents, film pores are filled and a soft,

•

Limited durability

flexible film is produced. Because of the relatively

•

Poor chemical and solvent resistance

slow curing process, acrylic and other water-borne

•

Poor wetting of surfaces

latex coatings have not performed as well as other

•

Poor immersion service

generic types in traditional accelerated laboratory

•

Best cured above 50°F (10°C)

tests. SSPC-Paint System 24 for latex painting system for industrial and marine atmospheres is a

Alkyd Coatings

performance-based specification for water-borne acrylics.

The reaction of natural drying oils and drying oil fatty

Water-borne acrylic coatings applied directly to

resins with a variety of desirable properties. They are

steel usually require a commercial blast cleaned

transformed into solid films by reacting with oxygen in

surface (SSPC-SP 6/NACE 3). Water-borne acrylic

the air (oxidative cross-linking). Resulting properties

coatings can be applied by brush (synthetic bristles

vary with types and amounts (oil lengths) of fatty acid

are recommended), roller, or spray (stainless

and synthetic polyester resin components.

acids with synthetic resins forms a wide range of alkyd

steel equipment is recommended). Application recommendations on the manufacturer’s product

The most commonly used fatty acids are linseed oil,

data sheets should be followed. Proper techniques,

safflour oil, soya oil, tall oil, tung oil, and dehydrated

including avoiding overmixing, will minimize the C1 Fundamentals of Protective Coatings for Industrial Structures 2-18

Unit 2 - Coating Types and Their Mechanisms of Protection

castor oil. Their oil lengths (percent of phthalic

Lead and chromate inhibitive pigments were once

anhydride) are:

used extensively in alkyd coatings. Zinc molybdate

•

Short oil (40 to 50%)

•

Medium oil (30 to 40%)

•

Long oil (20 to 30%)

and zinc phosphate pigments are more commonly used today. Alkyds provide a low-cost coating for wood and steel in relatively mild environments. They are hydrolyzed

Longer oil length alkyds dry more slowly, develop

in alkaline environments (saponified) and so cannot

less gloss, and are more flexible and more weather

be used on concrete or zinc metal products or

resistant than the short oil alkyds. Short oil alkyds

coatings.

dry more rapidly and develop better gloss, but tend to be brittle. All have excellent wetting and penetration

The manufacturing process for alkyds consists of

properties which makes them (particularly the long

heating together the resin’s ingredients in an inert

oil alkyds) surface tolerant. Short and long-oil alkyds

atmosphere. The progress of the reaction is followed

are not compatible with each other and should not be

by monitoring the resin’s viscosity and its acid value.

blended together.

The acid value is a measure of the concentration of unreacted acid groups in the resin. Almost any

As discussed earlier, incorporating synthetic resins

synthetic or natural resin that is able to react with an

results in modifications with specific advantages:

alcohol or acid can be incorporated. There are various

•

fatty acid blends available from different natural oils

Phenolic alkyds.

and fats as well as broad options for selecting the

−− Fast-curing with good water/corrosion

relative amounts of ingredients used in the reaction.

resistance but poor UV resistance

Alkyd resins, therefore, offer the coating formulator

−− Used as shop or universal primers for a

a vast range of film-forming properties from which

variety of services •

to select a binder. Because the production process

Epoxy esters.

is simple and most of the ingredients are readily

−− Improved chemical resistance with reduced

available, alkyd resins are relatively low cost raw

weathering

materials for coatings. They are among the few

−− Used as machinery enamels or for fuel

coating resins that are not derived from petroleum.

splash resistance •

The use of alkyd coatings is falling rapidly because it

Silicone alkyds.

is difficult to produce low-viscosity, low-VOC products

−− Excellent UV protection and gloss

with the good properties of higher-VOC products.

retention. −− Used as high quality finish coats •

Formulations with reduced VOC limits include:

Urethane alkyds (Uralkyds).

•

−− Fast-drying, good UV resistance.

Water-dispersable alkyds. Don’t perform as well as solvent-based products

−− Used as wood floor and furniture varnishes

•

High-solids formulations. Lower molecular weight resins used

C1 Fundamentals of Protective Coatings for Industrial Structures 2-19

Unit 2 - Coating Types and Their Mechanisms of Protection

Advantages •

Easy to apply, topcoat, and repair

•

Good flexibility possible

•

Good surface wetting and adhesion

•

Good gloss retention

•

Relatively cheap

•

From renewable source

resins based upon bisphenol F have lower viscosities (require less solvent), good hardness and excellent chemical and solvent resistance. However, they tend to be brittle and are more expensive than bisphenol A resins. The highly crosslinked bisphenol F novolac resins have outstanding solvent and chemical resistance. Polyamide-cured epoxies compared to amine-

Limitations

cured epoxies have better water resistance, more

•

Relatively high in VOCs

tolerant mixing ratios (usually 1:1), better flexibility,

•

Poor performance in severe environments

less tendency to amine blush, and a longer pot life.

•

Poor chemical and solvent resistance

They do, however, have greater amounts of solvent

•

Poor water immersion resistance

(VOCs), longer curing times (may require an induction

•

Poor alkali resistance

period), and less chemical resistance than aminecured epoxies.

Epoxy Coatings Aliphatic amine-cured epoxies, compared to

Epoxy coatings have become a versatile workhorse

polyamide-cured epoxies, are more chemically

for coating and lining steel structures. Most epoxy

resistant and faster curing, have lower viscosities

coatings have good bonding properties, good

(require less solvent), and form a tougher film.

resistance to water and a variety of chemicals, and

However, they have shorter pot lives, increased

form a tough durable finish. The primary limitation

tendency to amine blush, less tolerant mixing ratios,

of epoxy coatings is their relatively poor sunlight

and more toxicity (are skin irritants).

resistance. Also, they are usually quite inflexible, especially when highly cross-linked. By utilizing

Amine adduct-cured epoxies have more tolerant

available variations in epoxy resins and curing agents,

mixing ratios and have less a tendency to amine blush

coatings with a variety of properties have been

than the aliphatic amine-cured epoxies. However,

developed to meet a multitude of needs, sometime

they have a higher viscosity and longer curing time.

enhancing one property while compromising one or more other properties.

Aromatic amine-cured epoxies have greater chemical resistance but are slower curing (requiring accelera-

Epoxy mastics are high-solids, high-build (at least 5

tors) and very poor resistance to ultraviolet light.

mils DFT) formulations, often aluminum-filled, that are surface tolerant and usually compatible with most

Ketamine-cured epoxies have longer pot lives and

other coatings.

less toxicities but are slower curing and require moisture for curing.

Epoxy resins based on bisphenol A are versatile and the ones most commonly used today. Epoxy

C1 Fundamentals of Protective Coatings for Industrial Structures 2-20

Unit 2 - Coating Types and Their Mechanisms of Protection

Cycloaliphatic amine-cured epoxies have good tough-

applications

ness and thermal resistance but are moisture sensi-

•

May be edge retentive

tive and slow curing (may require an accelerator).

•

High film build per coat

Limitations

Thick film epoxy coatings used for concrete floors and lining systems are similar in their basic chemistries and properties to those of the epoxy coatings used for steel. The physical properties for the epoxies for concrete floorings and linings are enhanced by using fillers and aggregates. These are incorporated into coating formulations to provide special features such as: •

Lower cost

•

Dimensional stability by lowering shrinkage

•

Electrostatic dissipation from conductive fillers

•

Wear resistance

•

Slip resistance with aluminum oxide granules

•

Limited pot life

•

Chalk freely in sunlight

•

Limited flexibility

•

Cure best about 50°F (10°C)

•

Topcoating a problem

•

Blast cleaned surface needed

•

Subject to amine blush

Coal Tar Epoxy Coatings Coal tar epoxy coatings are basically epoxies (with all the properties of epoxies) with coal tar added to the resin. The coal tar reduces cost, improves water resistance, and provides greater film build. Because of the coal tar, these products tend to become brittle in sunlight, and there is concern about the toxic effects

Different flooring systems described in SSPC/PCSI

of the coal tar. They are used today mostly on steel

TU 10 include:

piling and tank linings.

•

Self-leveling flooring

•

Slurry flooring

Advantages

•

Broadcast flooring

•

Mortar flooring

•

Low in VOCs

•

Spray–applied flooring

•

Good water and chemical resistance

•

Good film build

•

Good abrasion resistance

•

Relative low cost

Advantages •

Low in VOCs

•

Good solvent and water resistance

Limitations

•

Many have good chemical resistance

•

Tough, durable, slick film

•

Toxic; personal protection needed

•

Good adhesion

•

Slow-curing

•

Good abrasion resistance

•

Limited pot life

•

Rapid curing time

•

Blast cleaned surface needed (steel)

•

No induction times

•

Topcoating a problem

•

No pot life problems with plural component

•

Available only in black

C1 Fundamentals of Protective Coatings for Industrial Structures 2-21

Unit 2 - Coating Types and Their Mechanisms of Protection

Two-Component Polyurethane Coatings

•

Polyesters have the best chemical resistance (and may have good color and gloss retention)

Two-component polyurethane coatings are formed

and are used where chemical resistance is

by the reaction of two components, a polyisocyanate

important (areas susceptible to splashes, spills,

and a polyol. A polyisocyanate contains two or more

and graffiti).

isocyanate groups (-N=C=O) groups, and a polyol contains two or more hydroxyl (-OH) groups. Because

The application temperature range is normally from

each component contains more than one functional

40°F (10°C) to 110°F (43°C). Polyurethanes have

group, the reaction product can be cross-linked to

been developed for application below 40°F (10°C).

form three-dimensional thermoset polymers. Greater

Moisture condensation on wet polyurethane coatings

cross-linking results in a harder, more chemically-

may result in a drop in gloss or microblistering. Thus,

resistant polymer.

the surface temperature should be at least 5°F (3°C) above the dew point during application.

The isocyanates can be aliphatic (containing only saturated bonds) or aromatic (containing benzene-like

Moisture-Curing Polyurethanes

unsaturated rings). While both of these general types may have similar properties, the more costly aliphatic

Moisture-curing polyurethane polymers are based

polyurethane coatings have much better weathering

upon the reaction of isocyanates with moisture in the

characteristics (color and gloss retention) and are

air. The isocyanate group (-N=C=O) reacts with any

used extensively as topcoats for exterior coating

available compound containing active hydrogen such

systems. Aromatic polyurethanes chalk and yellow

as moisture, and so moisture-curing polyurethanes

in sunlight or bright artificial lighting and so find much

must be sealed in closed, dry containers. The

use in lining systems because of their good chemical

isocyanates used in these resins are multifunctional

resistance.

(have more than one isocyanate group on the molecule). This permits cross-linking into a three-

The most common polyol co-reactants are polyethers,

dimensional structure with good water and chemical

acrylics (polyacrylates), and polyesters. Of these

resistance. Greater cross-linking results in harder,

three, generally:

more chemically-resistant polymers. As with the

•

two-component products, they can be aliphatic or

Polyethers have better resistance to hydrolysis

aromatic.

but poorer weathering. Thus, they are used on roofs and secondary containment structures

Curing of moisture-curing polyurethane coatings

where water accumulates. •

occurs in stages. The isocyanate groups first react

Acrylics (polyacrylates) have the best color and

with moisture to form an amine and carbon dioxide.

gloss retention and so are used on exteriors

The amine then reacts with other isocyanate groups

of water tanks, railroad locomotives, and other

to form polyureas, and this process continues until

structures where aesthetics is important. Clear

all of the isocyanate groups are consumed. The

coats can enhance the color and gloss retention

carbon dioxide formed during curing must escape

in the undercoat.

from the wet film to prevent bubbling problems in the

C1 Fundamentals of Protective Coatings for Industrial Structures 2-22

Unit 2 - Coating Types and Their Mechanisms of Protection

cured films.

•

Aliphatics have good gloss and color retention

Moisture-curing polyurethanes are slightly moisture

•

Aromatics have good chemical resistance

tolerant, because the surface moisture reacts with

•

Good durability

the isocyanate. Products are available with a wide

•

Good abrasion resistance

range of properties from soft and flexible to hard and

•

Low-temperature curing achievable

chemically resistant. All generally have good wear and abrasion resistance. Other good features of the

Limitations

polyurethanes include:

•

Highly toxic; need personal protection

•

•

Moisture-sensitive; gloss may drop

•

Skilled applicator needed

•

Limited pot life

•

Blast cleaned surface required (steel)

•

More expensive than epoxies

Primers (generally aromatic) with good adhesion available with barrier (aluminum and micaceous iron oxide) and galvanic (zinc) pigments

•

Intermediate coats with good barrier protection

•

Clear wood finishes with good gloss retention

•

Waterproof elastomeric coatings and linings

•

Concrete floor coatings and tank linings

•

Weather-resistant topcoats (with limited color

Two-Component Polyurea Coatings Two-component polyurea coatings are cured by

availability)

the reaction of an isocyanate component and an amine resin component. The isocyanate component

Moisture-curing polyurethanes can be applied at very

consists of a prepolymer formed by partially reacting

low temperatures (below freezing) but their curing

a polyisocyanate (compound with more than one

rates are greatly reduced. Moisture contact must be

isocyanate group) with a polyol (compound with more

avoided by slow speed stirring that does not produce a

than one alcohol group). This reduces the number

vortex. Boxing should not be used to mix coatings.

of free (unreacted) isocyanate groups, thereby reducing the toxicity, and providing a 1:1 volume ratio

The relatively high cost of moisture-curing

of components. Varying the isocyanate, polyol, and

polyurethanes can be offset by these advantages:

amine compositions can produce a variety of products

•

No required induction time

•

Quick re-coatibility and placement into service

•

Wider range of acceptable application

with different desirable properties. Polyurea polymers have two distinct types of chemistry, aromatic and aliphatic. As with polyurethanes, the

temperatures

aromatic and aliphatic natures are associated with the chemistry of the isocyanate component. Aromatic

Two-Component Polyurethane Coatings:

polyurea coatings usually have better chemical

Advantages

resistance than aliphatic polyurea coatings and are

•

Can be low in VOCs

cheaper. However, they chalk and yellow in sunlight.

•

Good water resistance

•

Good hardness or flexibility

Aliphatic polyurea coatings cure faster and are stable in ultraviolet light. Both types have good flexibility and

C1 Fundamentals of Protective Coatings for Industrial Structures 2-23

Unit 2 - Coating Types and Their Mechanisms of Protection

strength and such short pot lives that they must be

•

Skilled applicator needed

applied by plural-component spray. Polyurethanes

•

Blast cleaned surface required (steel)

and polyureas can be co-polymerized to yield hybrid

•

Toxicity of isocyanate component

polymers that have been used extensively in liners. Silicone Coatings

Fast-curing polyurea coatings must be applied by

Silicones are inorganic polymers containing a silicon-

a plural-component spray system that mixes the

to-oxygen backbone (…-Si-O-Si-O-…) rather than

properly proportioned, heated components at the

the carbon-to-carbon backbone (…-C-C-...) found in

gun. Whenever application is briefly stopped, the

organic polymers. Silicon-oxygen polymers have two

mixed coating remaining in the gun must be removed

side-groups attached to each silicon atom. Varying

mechanically or by air or solvent purging.

the silicon-oxygen chain lengths, the two side groups, and cross-linking can produce a variety of silicone

Polyaspartic Coatings

resins with excellent high-temperature and weather

The later development of polyaspartic coatings

resistance.

provided aliphatic polyurea systems that produced thin films with longer cure times, so that they can often

Some of the more common applications for 100

be applied by brush, roller, or conventional/airless

percent silicone (no organic co-polymers) coatings

spray. Also, they have high gloss and excellent gloss

include high-temperature stacks, mufflers and

and color retention. Polyaspartic systems are used

manifolds, boilers, ovens, and furnaces, steam

as DTM systems for rail car coating and as thin DFT

lines, heat exchangers, cooking utensils, combustion

topcoats for color stability. They are also used for

chambers, incinerators, wood-burning stoves, and

industrial floorings, parking deck coatings, and other

barbecue equipment.

non-immersion uses over concrete. Silicone formulations for use at different temperatures The VOC content of polyurea coatings is none or very

vary widely:

low, so that VOC limitations cause no problems.

•

cold blending with alkyds, epoxies, and other

Advantages •

Rapid curing time

•

Good film build

•

Good durability or flexibility

•

Good Abrasion resistance

•

Aliphatics have good weathering properties

•

Low or no VOC content

250-400°F (121-204°C) - Silicone resin 5 to 50%; organic polymers may be cost-effective

•

400-600°F (204-316°C) - Silicone resin 15 to 50%; leafing aluminum increases heat resistance of coatings; colored pigments are less stable/ require more silicone

•

600-800°F (316-427°C) - Silicone resin content 30 to 70% for aluminum and 70 to 100% for color finishes; cold blends or co-polymers of organic

Limitations •

Very short pot life

•

Must be applied by plural-gun application

resins used

C1 Fundamentals of Protective Coatings for Industrial Structures 2-24

Unit 2 - Coating Types and Their Mechanisms of Protection

•

•

800-1,000°F (427-538°C) - Silicone resin con-

Polyester and vinyl ester linings are low viscosity

tent usually 100%; aluminum pigment for upper

coatings that cure quickly at ambient temperatures to

part of range and black metal oxide pigment for

form strong, tightly adhering films with good chemical

lower part

and high temperature resistance. Thus, they are

1,000-1,4000°F (538-760°C)- Silicone resin

extensively used for tank, secondary containment, and, floor linings, as well as coatings for structural

content is 100%; ceramic frits that fuse into

steel, walls, and ceilings.

substrate-Si -O-Si bond give prolonged service at higher temperatures

Significant limitations of polyester and vinyl ester coatings are associated with the stresses created

Abrasive blasting to SSPC-SP 10 is the normally

by their high shrinkage and heat produced during

required preparation of steel surfaces. Application

curing. These stresses can cause the rigid coatings

is usually by spray, but brush and roller application

to crack and disbond unless they are strengthened by

may be adequate.

addition of fillers or reinforcement. More flexible lining variations cannot usually be used, because they have

Advantages •

High solids (low VOCs)

•

Good chemical/weather/heat resistance

•

Good general industrial use

much lower chemical resistances. Different systems for increasing strength or impermeability of polyester and vinyl ester linings include: •

and other minerals can be used to extend the

Limitations •

High level of surface preparation required

•

Relatively short pot life

•

Relatively slow cure

•

Relatively high cost per coat

Un-reinforced filled systems. Fillers of silica base resin or reduce cost, curing shrinkage, and coefficient of film thermal expansion.

•

Reinforced composite systems. Fiberglass chopped fibers or woven cloth reinforcement can increase strength and reduce cracking. During

Polyester and Vinyl Ester Coatings

lining installation, the fiberglass is saturated with catalyzed resin and rolled to wet glass strands to

Polyesters and vinyl esters (a special form of

wet them well and to remove air bubbles.

polyesters) are thermosetting coatings that cure by cross-linking after application to a steel or concrete

•

Flake-reinforced linings and coatings. Flake

surface. Separately mixed peroxide initiator and

reinforcement can reduce moisture and chemical

accelerator start the curing of the polyester pre-

permeation.

polymer. Styrene, present along with the pre-polymer, causes cross-linking to form a three-dimensional solid

Blast cleaning to an SSPC-SP 5 is almost always

film. Since styrene is a compatible liquid, a solvent is

required for polyester and vinyl ester coatings

not normally required for these coatings.

applied on steel. Angular grit abrasive should be used to give the coarse, angular profile of 3 mils (75

C1 Fundamentals of Protective Coatings for Industrial Structures 2-25

Unit 2 - Coating Types and Their Mechanisms of Protection

micrometers) necessary to provide strong adhesion

If the finish coat is eroded to exposed glass fibers,

during stresses from dimensional changes due to

general coating deterioration will proceed rapidly. If

curing and temperature fluctuations.

deterioration becomes significant, the surface should be lightly sanded and another finish coat applied.

Skilled personnel can apply polyester and vinyl ester Phenolic and Phenolic Epoxy Coatings

coatings by rolling, spraying, or troweling. Proper lining thickness is essential to provide effective

Phenolic resins were among the first synthetic resins

protection to these linings and coatings.

used for coatings. They are generally based on the reaction of a phenol with formaldehyde.

Advantages •

Can be low in VOCs

•

High film build

•

Water sensitivity during application

•

Good solvent and chemical resistance

•

Good abrasion resistance

Phenolic coatings are usually heat-cured to provide products with good adhesion and resistance to water, chemicals, and heat. Baked phenolic coatings are used to line cans, drums, piping, and tanks. Epoxy phenolic coatings are epoxy modifications of

Limitations

phenolic coatings. They are hard but flexible, and

•

Limited pot life

are resistant to abrasion, water, solvents, chemicals,

•

Skilled applicator needed

and heat. Both phenolic and epoxy phenolic coatings

•

Blast cleaned surface required

discolor during heat curing and have poor resistance

•

Peroxide component is hazardous

to exterior weathering because of the chemical aromaticity of phenols. Their uses are similar to those of phenolic coatings. An advantage of these

Fiberglass-Reinforced Plastic Coatings

coatings is that they can be formulated to dry in air, not

Because of their importance, a brief discussion of

requiring heat cure. It should be noted that in epoxy

fiberglass-reinforced plastic coating/lining materials

phenolic coatings, phenolic is the chief binder; the

seems appropriate. These materials usually consist

reverse is true for phenolic epoxy coatings.

of thermosetting resins (e.g., polyester, vinyl ester, or epoxy) reinforced with fiberglass cloth, mat, or

Advantages

fibers. Their chief advantage is their high strengthto-weight ratio. Tensile strengths can be as great as 50,000 psi (345 MPa). High-glass content provides maximum chemical resistance. Fiberglass-reinforced plastic coatings find much

•

Hard coatings

•

Good chemical resistance

•

Good heat resistance

•

Good solvent resistance

Limitations

use in protecting chemical process components, floor coverings, and tank linings. If placed in exterior service, they are subject to deterioration by weathering.

•

Discolor during heat curing

•

Poor exterior weathering

•

Use limited mostly to linings

C1 Fundamentals of Protective Coatings for Industrial Structures 2-26

Unit 2 - Coating Types and Their Mechanisms of Protection

Pretreatment Wash Primers

protection of the underlying steel. As discussed in Unit 1, the zinc metal in the film is preferentially corroded

Pretreatment wash primers for steel and aluminum

to protect iron and steel because it is higher in the

(e.g., SSPC-Paint 27) have been used extensively

galvanic series. A high concentration of zinc particles

to promote adhesion of vinyl or certain other

in the film will provide the necessary conductivity for

coatings or provide temporary corrosion protection

galvanic protection. This high zinc loading contributes

before applying a full coat of primer. They are two-

to the film’s porosity and its poor internal cohesion.

component products, consisting of polyvinyl butyral in alcohol solution with a corrosion inhibitor (basic zinc

Types and Properties of Inorganic Zinc

chromate) and a solution of phosphoric acid. Upon mixing, the components react with each other and

The major categories of inorganic zinc-rich coatings

with the metal to form a tightly adhering film similar

are water-borne and solvent-borne. The classification

to that of a vinyl coating. They are applied at 0.3–0.5

scheme is derived from SSPC-Paint 20, which includes

mils [7.5- 12.5 µm] and dry quickly (in less than half

both Type I (Inorganic) and Type II (Organic). Some

an hour) to provide good temporary protection from

water-borne inorganic zinc coatings are cured after

corrosion.

application (post-cured) (Type IA from SSPC-Paint 20) by heat or an acid curing agent. Most, however,

Wash primers are very high in VOCs, but they usually

are self-curing. They simply react with carbon dioxide

have a temporary exemption from VOC regulations.

from the air (Type IB).

Because of their high VOC and chromate content, they are used much less today.

Solvent-borne alkyl-silicate inorganic zinc-rich coatings are self- curing and depend on moisture in

Advantages

the air to complete the hydrolysis reaction (Type IC).

•

Promote adhesion of some primers

When the weather is hot and dry, it may be necessary

•

Provide temporary corrosion protection

•

Fast-drying

to spray water on these coatings to complete the curing. Inorganic zinc-rich coatings of most types can be

Limitations •

formulated to be acceptably low in VOCs, particularly

Film may undergo cohesive failure when applied

the water-borne products. Their films are brittle

too thickly •

High in VOCs

•

Contains chromate

•

Uses are limited today

and may crack if applied too thickly; thus, they are generally applied at less than 5 mils [125 µm] dry film thickness, although some products can successfully be applied at greater thicknesses.

Inorganic Zinc-Rich Coatings

They provide cathodic protection to steel, but as the

Inorganic zinc-rich coatings usually have silicate

zinc corrosion products fill the natural film porosity,

resins that may be water-borne or solvent-borne.

they begin to provide barrier protection. If this barrier

These coatings form a film that provides galvanic

is broken by impact, cathodic protection will again

C1 Fundamentals of Protective Coatings for Industrial Structures 2-27

Unit 2 - Coating Types and Their Mechanisms of Protection

Organic Zinc-Rich Coatings

take over until the break is healed by filling with zinc corrosion products.

Organic zinc-rich coatings utilize an organic resin rather than an inorganic silicate binder. They protect

Over time, an inorganic zinc-rich coating will provide

steel galvanically as well as by barrier protection.

increased barrier protection while galvanic action is

Organic zinc-rich coating films can be of the

reduced as zinc is consumed. Inorganic zinc-rich

thermoplastic (utilizing vinyl or chlorinated rubber

coatings require greater steel surface cleanliness

binders) or the thermosetting type (utilizing epoxy or

than do other coating types. They must be applied by

polyurethane binders). Organic zinc-rich coatings

a skilled applicator using a constantly agitated pot to

are not as electrically conductive as inorganic zinc-

keep the heavy zinc particles suspended.

rich coatings, and thus, they have a lower level of galvanic protection.

Inorganic zinc-rich silicate coatings frequently do not bond well to each other, and it is safest to repair them

Film properties of organic zinc-rich coatings are

using an organic zinc-rich coating. When topcoating

similar in most respects to those of zinc-free organic

inorganic zinc-rich films, small bubbles may form

coatings using the same resin. Organic zinc-rich

in the wet topcoat from the escape of air or solvent

coatings do not require as high a level of blast-cleaned

vapors trapped in the porous binder. Many painters

steel surface as do inorganic zinc coatings, and they

attempt to minimize this problem by applying a

are easier to topcoat. The zinc in both generic types

mist coat (a thin, quick coat) and allowing it to dry

is attacked by acid or alkali.

before applying a full topcoat. Because of topcoating problems and good performances without topcoating

Advantages

in a variety of services, it is often best not to topcoat inorganic zinc-rich coatings. Advantages

•

Can be low in VOCs

•

Good atmospheric durability

•

Relatively easily topcoated

•

Moderate surface preparation

•

Can be low in VOCs

•

Excellent abrasion resistance

•

Excellent heat resistance

•

Good atmospheric durability

•

Requires skilled applicator

•

Useful as shop primer

•

Constant agitation necessary

•

Fast-drying

•

Unsuitable for acid or alkali

•

Can be used untopcoated

•

High initial cost

•

Normally requires a topcoat

Limitations

Limitations •

Needs very clean, blasted surface

Powder Coatings

•

Requires skilled applicator, agitated coating

•

Difficult to topcoat

Powder coatings are finely divided solid products that

•

Attacked by acid and alkali

•

High initial cost

are applied to metals or other substrates and fused by baking to form a continuous protective film. The

C1 Fundamentals of Protective Coatings for Industrial Structures 2-28

Unit 2 - Coating Types and Their Mechanisms of Protection

powder coating industry is undergoing rapid growth,

•

Good thickness control, including edges

estimated at about 450,000 metric tons worldwide in

•

Good transfer efficiency by recycling overspray

1996. These coatings are used much more in Europe

•

Good film build in one coat

and Japan than in the United States.

•

Reduced waste

•

Limitations

•

High baking temperatures limit substrates to

Powder coatings are produced as both thermoplastic and theromsetting products. Thermosetting powders

metals

currently comprise more than 90% of the market. Thermosetting powder coatings are based on a primary resin and a hardener (curing agent or cross-

•

Color changes are expensive

•

Powder suspensions in air may be explosive

•

Inside surfaces may be difficult to coat (Faraday cage effect)

linker) that undergo a chemical addition reaction during film formation. Different resin types can be used

Thermal Spray Metallic Coatings

to obtain protective films with a variety of chemical and physical properties. Thermoplastic powders are

This process, sometimes referred to as metallizing,

more easily ground to the fine particle size needed

generally uses zinc, aluminum, or an alloy of zinc

for quality electrostatic spraying than are the higher

and aluminum as a sacrificial coating applied to mild

molecular weight thermoplastic powders.

steel to provide “active” corrosion protection. Although many metals can be sprayed for other purposes,

Epoxies are the oldest and most widely used

zinc and aluminum are the most likely to be used for

thermosetting powders. They are commonly used

corrosion protection of steel. The metals are supplied

on pipes, rebar, and electrical products. As with other

in wire or powder form and can be applied by:

epoxy coatings, they do not have good ultraviolet

•

resistance. Hybrid (epoxy-polyester) binders,

Flame spray, which uses wire fed through a propane or oxyactylene flame

polyesters, and acrylics provide much better ultraviolet

•

resistance. Some thermosetting powder coatings are

Arc spray, which uses two wires, fed with a voltage and meeting to make an electric arc

formulated to cure rapidly at low temperatures by the

•

action of ultraviolet light. The first powder coatings were thermoplastic, but the

Plasma spray, which uses metal powder fed through a high-intensity electric arc

A high level of surface preparation is mandatory, and

less easily ground powders did not have the ability

the final coating surface is likely to be rough and

to produce thin films with good flow and leveling

slightly porous. The use of a sealer coat not only

during baking. Examples of these products are vinyl copolymers, polyamides, and fluoropolymers.

extends the useful life of the coating by sealing the

Advantages

the roughness of the finish. A topcoat over sealed

•

Low VOC emissions

•

Low toxicity and flammability

metallizing can provide additional protection, as well as a decorative finish.

•

Good corrosion resistance possible

porosity, but improves the appearance and reduces

C1 Fundamentals of Protective Coatings for Industrial Structures 2-29

Unit 2 - Coating Types and Their Mechanisms of Protection

Zinc

New Coating Trends

Zinc used in metallizing has a purity of 99.9%. A

Coating manufacturers are always trying to produce

coating 3 mils (0.08 mm) thick provides one and one-

new products that will sell well and meet owner

half ounces per square foot of service (460 grams per

performance requirements. Traditionally, they have

square meter), a typical thickness of galvanizing. Zinc

sought new formulations that are easy to apply,

metallizing finds much use on structural steel under atmospheric exposure.

provide longer protection, or have other desirable features. Now, they must also meet present and expected health and environmental regulations.

Aluminum Most new products are formulated to be low in

A film thickness of aluminum metallizing of 4 mils

VOCs to meet prevailing VOC limits and free of lead,

(102 micrometers) provides 1 oz/sq. ft. (305 g/sqm)

chromate, and other hazardous constituents. Thus,

of surface. Aluminum has better resistance than does

they tend to be higher in solids or water-borne. Epoxy

zinc to slightly acidic conditions, seawater, and salt

mastics are an example of such a product. These are

atmospheres.

reported to be surface-tolerant and compatible with most other coatings, as well as relatively high-build,

Zinc-Aluminum Alloy

low in VOCs, and free of hazardous materials.

Galvalume is a commercially available alloy containing 85% zinc and 15% aluminum. It is reported to have

2.7 Selection of Coating Systems

improved atmospheric corrosion resistance.

Protective coating systems can effectively protect steel substrates from corrosion in all environments

Sealers

where corrosion rates are less than 50 mils [1,250

Sealers for thermal spray coatings are low-viscosity,

µm] per year. Where higher rates occur, corrosion-

clear or pigmented coatings formulated to penetrate

resistant materials are a better choice. No coating will

into the natural pores of the metallizing. Products

perform well in all situations. This section discusses

with too great a pigment particle size or high viscosity

sources of criteria for selection of appropriate coatings

coatings that do not penetrate the surface may do more

for different applications. Such sources are especially

harm than good by permitting moisture penetration.

helpful in proper selection of new materials such

Of course, the sealers must be compatible with the

as surface-tolerant coatings and siloxanes, where

sprayed metal and any top coat to be applied over

commercially available materials do not have long-

them. Coatings that adhere well to steel may not

term case histories.

adhere well to zinc or aluminum. In selecting a coating system, the first consideration is Sealers that have performed well on metallizing

the desired properties of the system for the particular

include epoxies, pretreatment wash primers, and

service:

silicones (for high-temperature service).

•

Exterior weathering (chalking; color and gloss retention)

C1 Fundamentals of Protective Coatings for Industrial Structures 2-30

Unit 2 - Coating Types and Their Mechanisms of Protection

•

Water, fuel, solvent, or chemical resistance

waste. This invariably means that the system with the

•

Abrasion, heat, or mildew resistance

maximum maintainable life is usually the best choice.

•

Appearance (color, gloss, and texture)

Chapter 8 of SSPCs Good Painting Practice: Steel

•

Drying time

Structures Painting Manual: Volume 1 presents a

•

Ease of application and maintenance

good discussion of painting costs as of 2002.

Next, possible requirements and limitations in the

Systems for spot repair should be of the same generic

following areas should be considered:

type or curing mechanism as the damaged coatings

•

already in place, in order to avoid incompatible

Severity of environmental exposure (12 SSPC

products. One exception to this rule, previously

zones) •

Surface preparation necessary

•

Access to work

•

Drying times (especially where painting times

discussed, is that since inorganic zinc coatings frequently do not bond well to themselves, it is safest to repair them with zinc-rich organic coatings.

are limited) •

Necessary applicator skills

•

Necessary equipment

•

Scaffolding

•

Safety and environmental requirements

If environmental restrictions no longer permit the use of the coating previously applied, a compatible coating should be used. A sure method of determining compatibility is to coat a small test area of the existing coating with the selected paint. The area should be checked after a few days for bleeding of undercoat,

The severity of the environment is always a major

wrinkling, loss of adhesion, or other coating defect.

factor in the coating selection process. Table 3 in Chapter 1 of SSPCs Systems and Specifications:

Fortunately, there are several sources of information

Steel Structures Painting Manual, Volume 2 defines

that take these factors into account in providing

twelve different environmental zones and the painting

recommendations for a variety of conditions. These

system recommendations for each.

sources include:

Other local factors such as airborne chloride levels, degree of surface contamination, time of wetness, and solar radiation should also be taken into account. Examples of the coating system selection process are provided in SSPCs course, C2 “Planning and Specifying Industrial Coatings Projects.”

•

Steel Structures Painting Manual, Vols. 1 & 2

•

Publications of other technical organizations

•

Government documents

•

Consultants

• •

Coatings manufacturers Materials testing

The Steel Structures Painting Manual

Life cycle cost has always been one of the most important considerations in the selection of coating

The Steel Structures Painting Manual: Volume

systems. Today, we must include in these costs the

1, Good Painting Practice, provides much useful

expense of removal, containment, and disposal of

information on a variety of specialized subjects.

old coatings that may be considered hazardous

Volume 2, Systems and Specifications, contains much

C1 Fundamentals of Protective Coatings for Industrial Structures 2-31

Unit 2 - Coating Types and Their Mechanisms of Protection

industry-accepted criteria for surface preparation

Many other countries have produced their own

for coating, coating materials, application, and

standard documents to provide similar information.

inspection. These standards should be referenced in

Some are based on other standards, but reworked

job specifications to relate requirements to industry

to fit the particular national system. For example, the

standards.

Swedish Standard SS 05-59-00 is not only found as an ISO standard (ISO 8501-1) but can be found in Japan, Australia, UK and many other countries as a

Publications of Other Technical Organizations

national standard.

Technical organizations such as the American Society for Testing and Materials (ASTM),

Consultants

International Organization for Standardization (ISO) and NACE International have standards and/or

There are many coatings consultants available to

recommended practices that provide useful criteria

conduct surveys, make coating recommendations,

on surface preparation and coatings application and

or prepare specifications.

inspection. Coatings Manufacturers Government Documents

Coatings manufacturers are usually glad to make

In the US, many federal and state agencies have

recommendations for selection of coatings. Choices,

guidance documents for the use of protective

however, are normally limited to those available from

coatings. The Naval Facilities Engineering Command

the particular manufacturer.

(NAVFAC) and the Army Corps of Engineers have a large number of guide specifications for coating a

Materials Testing

variety of facilities (shore structures). The Naval Sea

If a company has a very unique service or environment,

Systems Command (NAVSEA) has published the

it may be advantageous to conduct field or simulated

Naval Ships Technical Manual Chapter 631, which

laboratory testing of coatings to determine which will

provides detailed information on the painting of Navy

provide the best protection. This is normally expensive

ships.

and time-consuming, but actual service always provides the surest criteria.

The former tri-service manual Paints and Protective Coatings is now available in downloadable format as UFC 3-190-06, Protective Coatings and Paints from

2.8 Unit Summary

http://dod.wbdg.org.

Coatings protect steel from corrosion by providing (1) a barrier to electrolytes, (2) a chemical inhibitor

Other federal organizations such as the Forest

to corrosion reactions, or (3) galvanic (cathodic)

Products Laboratory have specialized painting

protection. Regardless of the protective mechanism,

documents. State highway departments frequently

the protective film must bond well and possess other

have their own publications for painting highway

necessary film properties.

pavements, bridges, signs, etc.

C1 Fundamentals of Protective Coatings for Industrial Structures 2-32

Unit 2 - Coating Types and Their Mechanisms of Protection

The chief components of a coating are binder, pigment, and solvent. Because binders are the most important components in determining film properties, coatings are named generically after them. The three main mechanisms of coating cure are oxidation of drying oils, evaporation of solvent or water, and chemical reaction. Those coatings that are unchanged chemically during curing are called nonconvertible; those that undergo a chemical reaction are called convertible. The diagram of generic types of coatings with different curing mechanisms at the end of this chapter presents a good overall view of available materials. Sources of information on the selection of coatings for a particular job include technical organizations such as SSPC, government documents, consultants, coatings manufacturers, and field testing.

C1 Fundamentals of Protective Coatings for Industrial Structures 2-33

Unit 2 - Coating Types and Their Mechanisms of Protection

Unit 2 - Exercise 2A: Coating Components To each of the items listed below, mark it as B, P or S to show if it is related to Binder, Pigment or Solvent.

1.

Adhesion to substrate

2.

Chemical resistance

3.

Color

4.

Compatibility with substrates and other coatings

5.

Flexibility

6.

Level of gloss

7.

Lower viscosity for application

8.

Mechanism of curing

9.

Opacity

10.

Rate of curing

11.

Reinforcement

12.

Sacrificial galvanic protection

C1 Fundamentals of Protective Coatings for Industrial Structures 2-34

Unit 2 - Coating Types and Their Mechanisms of Protection

Unit 2 - Exercise 2B: Mechanisms of Coating Film Formation To each of the generic coating types listed below, identify its mechanism of film formation as:

A. Chemical reaction of differently-packaged parts B. Chemical reaction with moisture in the air C. Chemical reaction with oxygen in the air D. Solvent evaporation E. Water evaporation/coalescence Generic Coating Type 1.

Acrylic lacquer

2.

Acrylic latex

3.

Alkyd

4.

Amine-cured epoxy

5.

Chlorinated rubber swimming pool paint

6.

Coal tar cut-back (solvent solution)

7.

Epoxy ester

8.

Polyester

9.

Siloxane

10.

Single-component polyurethane

11.

Two-component polyurea

12.

Solvent-borne inorganic zinc-rich

C1 Fundamentals of Protective Coatings for Industrial Structures 2-35

Unit 2 - Coating Types and Their Mechanisms of Protection

Quiz 1. How do most coatings provide corrosion protection to metals? a. by electrical (galvanic) protection b. by interfering with the corrosion reaction c. by isolating the metal from the surrounding environment d. by forming a passive oxide film on the metal 2. _______________ coatings are thermoplastic. a. vinyl (solution or emulsion) b. coal tar epoxy c. epoxy ester d. inorganic zinc-rich 3. Which ingredients comprise a coating’s total solids? a. solvent and pigment b. solvent and binder c. binder plus pigment d. binder plus soluble additives 4. What ingredient is used to impart hiding (opacity) to coatings? a. pigment b. solvent c. binder d. additives 5. A coating that has good resistance to ultraviolet light is: a. aromatic polyurethanes b. acrylics c. epoxies d. coal tar epoxies

C1 Fundamentals of Protective Coatings for Industrial Structures 2-36

Unit 2 - Coating Types and Their Mechanisms of Protection

6. A coating that has good solvent resistance is: a. chlorinated rubber b. acrylic emulsion c. epoxy d. alkyd 7. What coating is unchanged chemically during curing? a. coal tar b. coal tar epoxy c. epoxy d. polyester 8. A coating that does NOT cure by reaction of its two components is: a. epoxy polyamide b. epoxy mastic c. coal tar epoxy d. epoxy ester 9. An example of a coating that provides galvanic (cathodic) protection is: a. epoxies b. alkyds c. polyurethanes d. zinc-rich coatings 10. A TRUE statement about metallizing is: a. It is tolerant of incompletely cleaned steel surfaces. b. It forms a hard, dense coating. c. It is usually sealed. d. It is resistant to both acids and alkalis. 11. _______________ is NOT a way of applying metallizing. a. flame spray b. electric arc spray c. electrostatic spray d. plasma spray

C1 Fundamentals of Protective Coatings for Industrial Structures 2-37

Unit 2 - Coating Types and Their Mechanisms of Protection

12. How does micaceous iron oxide provide corrosion protection to steel? a. by inhibiting the corrosion reaction b. by increasing the path that electrolytes must take to reach the metal c. by providing a type of cathodic protection d. by reflecting ultraviolet light 13. What type of epoxy has good edge-retention properties? a. high-solids epoxy b. epoxy mastic c. epoxy ester d. coal tar epoxy 14. What type of coating requires moisture for curing? a. two-component polyurethanes b. 100% solids epoxies c. unmodified oil-base paints d. solvent-borne zinc-rich

C1 Fundamentals of Protective Coatings for Industrial Structures 2-38

Unit 2 - Coating Types and Their Mechanisms of Protection

References ASTM B117, Standard Practice for Operating Salt Spray (Fog) Apparatus ASTM D522, Standard Test Methods for Mandrel Bend Test of Attached Organic Coatings ASTM D968, Standard Test Methods for Abrasion Resistance of Organic Coatings by Falling Abrasive ASTM B117, Standard Practice for Operating Salt Spray (Fog) Apparatus ASTM D522, Standard Test Methods for Mandrel Bend Test of Attached Organic Coatings ASTM D822, Standard Test Method for Standard Practice for Filtered Open-Flame Carbon-Arc Exposures of Paint and Related Coatings ASTM D968, Standard Test Methods for Abrasion Resistance of Organic Coatings by Falling Abrasive ASTM D1474, Standard Test Method for Measuring Indentation Hardness of Organic Coatings ASTM D1653, Standard Test Method for Measuring Water Vapor Transmission of Organic Coatings ASTM D2134, Standard Test Method for Determining the Hardness of Organic Coatings with a Sward Type Hardness Rocker ASTM D2240, Standard Test Method for Rubber Property—Durometer Hardness ASTM D2247, Standard Practice for Testing Water Resistance of Coatings in 100% Relative Humidity ASTM D2369, Standard Test Method for Volatile Content of Coatings ASTM D2371, Standard Test Method for Pigment Content of Solvent-Reducible Paints ASTM D2372, Standard Practice for Separation of Vehicle From Solvent-Reducible Paints ASTM D2583, Standard Test Method for Indentation Hardness of Rigid Plastics by Means of a Barcol Impressor ASTM D2794, Standard Test Method for Resistance of Organic Coatings to the Effects of Rapid Deformation (Impact) ASTM D3361, Standard Test Method Standard Practice for Unflitered Open-Flame Carbon-Arc Exposures of Paint and Related Coatings ASTM D3359, Standard Test Methods for Measuring Adhesion by Tape Test ASTM D3363, Standard Test Method for Film Hardness by Pencil Test ASTM D3960, Standard Test Method for Measuring VOC Content ASTM D4017, Standard Test Method for Measuring Water Content ASTM D4060, Standard Test Method for Abrasion Resistance of Organic Coatings by the Taber Abraser ASTM D4346, Standard Test Method for Measuring the Hardness of Organic Coatings by Pendulum Damping Tests ASTM D4541, Standard Test Method for Pull-Off Strength of Coatings Using Portable Adhesion Testers ASTM D5590, Standard Test Method for Measuring Mildew Resistance ASTM D5894, Standard Practice for Cyclic Salt Fog/UV Exposure of Painted Metal, (Alternating Exposures in a Fog/Dry Cabinet and a UV/Condensation Cabinet) ASTM D6133, Standard Test Method for Measuring Exempt Solvents ASTM D6677, Standard Test Method for Evaluating Adhesion by Knife

C1 Fundamentals of Protective Coatings for Industrial Structures 2-39

Unit 2 - Coating Types and Their Mechanisms of Protection

References ASTM D7234, Standard Test Method for Pull-off Strength of Coatings on Concrete Using Portable Adhesion Testers ASTM G152 Standard Practice for Operating Open Flame Carbon Arc Light Apparatus for Exposure of Nonmetallic Materials ASTM G153 Standard Practice for Operating Enclosed Carbon Arc Light Apparatus for Exposure of Nonmetallic Materials DOD-P-15328, Primer, (Wash), Pretreatment (Formula 117 for Metals) (Metric) (SSPC-Paint 27) ISO 8501-1 (SS 05-59-00), Preparation of steel substrates before application of paints and related products – Visual assessment of surface cleanliness MIL-P-24647, Paint System, Anticorrosive and Antifouling, Ship Hull SSPC-Paint 16, Coal Tar Epoxy Coating, Black or Dark Red SSPC Paint 20, Zinc Rich Coating, Type I–Inorganic, and Type II–Organic SSPC Paint 22, Epoxy Polyamide Paint, (Primer, Intermediate, Topcoat) SSPC-Paint 27, Basic Zinc-Chromate/Vinyl Butyral Wash Primer (DoD-P-15328) SSPC-Paint 32, Coal Tar Epoxy Mastic, Cold-Applied SSPC-Paint 33, Coal Tar Emulsion Coating SSPC-Paint 36, Two-Component Weatherable Aliphatic Polyurethane Topcoat, Performance-Based SSPC-Paint 38, Single-Component Moisture-Cure Weatherable Aliphatic Polyurethane Topcoat, PerformanceBased SSPC-Paint 39, Two-Component Aliphatic Polyurea Topcoat, Fast or Moderate Drying, Performance-Based

C1 Fundamentals of Protective Coatings for Industrial Structures 2-40

Unit 2 - Coating Types and Their Mechanisms of Protection

•

ASTM B 117, Standard Practice for Operating Salt Spray (Fog) Apparatus

•

ASTM D 522, Standard Test Methods for Mandrel Bend Test of Attached Organic Coatings

•

ASTM D 968, Standard Test Methods for Abrasion Resistance of Organic Coatings by Falling Abrasive

•

ASTM D 2240, Standard Test Method for Rubber Property—Durometer Hardness

•

ASTM D 2247, Standard Practice for Testing Water Resistance of Coatings in 100% Relative Humidity

•

ASTM D 2369, Standard Test Method for Volatile Content of Coatings

•

ASTM D 2371, Standard Test Method for Pigment Content of Solvent-Reducible Paints

•

ASTM D 2372, Standard Practice for Separation of Vehicle From Solvent-Reducible Paints

•

ASTM D 2583, Standard Test Method for Indentation Hardness of Rigid Plastics by Means of a Barcol Impressor

•

ASTM D 2794, Standard Test Method for Resistance of Organic Coatings to the Effects of Rapid Deformation (Impact)

•

ASTM D 3359, Standard Test Methods for Measuring Adhesion by Tape Test

•

ASTM D 3363, Standard Test Method for Film Hardness by Pencil Test

•

ASTM D 4060, Standard Test Method for Abrasion Resistance of Organic Coatings by the Taber Abraser

•

ASTM D 4541, Standard Test Method for Pull-Off Strength of Coatings Using Portable Adhesion Testers

•

ASTM D 5894, Standard Practice for Cyclic Salt Fog/UV Exposure of Painted Metal, (Alternating Exposures in a Fog/Dry Cabinet and a UV/Condensation Cabinet)

•

ASTM D 6677, Standard Test Method for Evaluating Adhesion by Knife

•

ASTM G 152 Standard Practice for Operating Open Flame Carbon Arc Light Apparatus for Exposure of Nonmetallic Materials

•

ASTM G 153 Standard Practice for Operating Enclosed Carbon Arc Light Apparatus for Exposure of Nonmetallic Materials

•

DOD-P-15328, Primer, (Wash), Pretreatment (Formula 117 for Metals) (Metric) (SSPC-Paint 27)

•

ISO 8501-1 (SS 05-59-00), Preparation of steel substrates before application of paints and related products – Visual assessment of surface cleanliness

•

MIL-P-24647, Paint System, Anticorrosive and Antifouling, Ship Hull

•

SSPC-Paint 16, Coal Tar Epoxy Coating, Black or Dark Red

•

SSPC Paint 20, Zinc Rich Coating, Type I–Inorganic, and Type II–Organic

•

SSPC Paint 22, Epoxy Polyamide Paint, (Primer, Intermediate, Topcoat)

Appendix 2-A

C1 Fundamentals of Protective Coatings for Industrial Structures 2-41

Unit 2 - Coating Types and Their Mechanisms of Protection

Unit 2 Objectives

Unit 2 Coating Types and Their Mechanisms of Protection

•  Upon completion of this unit, you will be able to: −  Define the three basic mechanisms of corrosion control by coatings −  Define the film properties necessary to provide good protection −  Describe how coatings may provide galvanic (cathodic) protection −  Define the three basic mechanisms of film formation −  Define the different generic types available for use −  Explain the criteria for coating selection and the conditions under which different systems may be appropriate or inappropriate

Mechanisms of Corrosion Control by Coatings

Barrier Protection •  Minimize electrolyte penetration •  Increase film thickness or number of coats •  Flake or plate-like pigments

•  Barrier protection •  Chemical inhibitors •  Galvanic (cathodic) protection

Inhibitive Pigments •  •  •  • 

Galvanic (Cathodic) Protection

Slightly soluble pigments used Lead, chromate and borates (restricted) Phosphates and molybdates Only suitable for primers

•  Zinc at high loading •  Galvanizing •  Metallizing

C1 Fundamentals of Protective Coatings for Industrial Structures 2-42

Unit 2 - Coating Types and Their Mechanisms of Protection

Protection From Individual Coats of System

Inorganic Zinc Coating Cathodically Protecting Steel

•  Primer - adhesion to substrate, inhibitor protection, barrier protection •  Intermediate - barrier protection/film build/ compatibility •  Finish - barrier protection and protection from weathering (loss of color or gloss)

Examples of Surface Tolerant Coatings

Special Primers •  •  •  • 

•  •  •  • 

Surface tolerant coatings Universal primers Direct-to-metal (DTM) Preconstruction primers

Universal Primers

Drying oils/alkyds Epoxy mastics Penetrating epoxies/polyurethanes Moisture-cured polyurethanes

Direct-To-Metal Primers

•  General term for one or both of the following:

•  Primer is same as topcoat

−  Tie coat for two normally incompatible coats −  Surface tolerant coating

C1 Fundamentals of Protective Coatings for Industrial Structures 2-43

Unit 2 - Coating Types and Their Mechanisms of Protection

Preconstruction Primers •  •  •  • 

Desired Film Properties • •  •  •  •  •  •  • 

Thin hold coat applied in shop Typically inorganic zinc; may be alkyd Minimizes field abrasive blasting Used extensively in ship building

Mechanisms of Coating Adhesion

Good adhesion Low permeability Continuous film Flexibility Abrasion resistance Weathering resistance Resistance to water, fuel, chemicals Resistance to biological growth

Tests for Coating Adhesion

•  Secondary chemical •  Primary chemical

•  •  •  • 

Adhesion Tape Test

Tape test (ASTM D 3359) Knife test (ASTM D 6677) Pull-off test (ASTM D 4541) Pull-off test for coatings on concrete (ASTM D 7234)

Adhesion Pull-off Test

C1 Fundamentals of Protective Coatings for Industrial Structures 2-44

Unit 2 - Coating Types and Their Mechanisms of Protection

Adhesion Failure due to Permeability

Permeability •  A low permeability is important in severe service such as immersion and commonly leads to blistering and disbonding of coatings.

Permeability

Film Continuity

•  Permeability can be tested by measuring the water vapor transmission or organic coating films (ASTM D 1653)

•  Coating films must be continuous (free of holidays) as well as relatively impermeable to provide barrier protection.

Testing for Film Discontinuities

Hardness Tests •  Hardness tests may be used to determine the extent of curing. −  ASTM D3363 (Pencil Test for Coatings) −  ASTM D2583 (Barchol Impressor) −  ASTM D2240 (Durometer) Tests for Thick Rubber and Plastic Films

C1 Fundamentals of Protective Coatings for Industrial Structures 2-45

Unit 2 - Coating Types and Their Mechanisms of Protection

ASTM D3363 Pencil Hardness Scale

Conducting Pencil Hardness Test

6B 5B 4B 3B 2B B HB F H 2H 3H 4H 5H 6H Soft

Medium

Hard

ASTM D2583 Barcol Hardness

ASTM D2240 Durometer Hardness

Additional Hardness Tests for Organic Coatings

Flexibility •  ASTM D522 •  Coated metal specimen is bent over cylindrical or conical mandrel •  Rigid coating may fail by cracking and/or disbonding

•  ASTM D2134: Determining the Hardness of Organic Coatings with a Sward Type Hardness Rocker •  ASTM D4366: Hardness of Organic Coatings by Pendulum Damping Tests •  ASTM D1474: Indentation Hardness of Organic Coatings

C1 Fundamentals of Protective Coatings for Industrial Structures 2-46

Unit 2 - Coating Types and Their Mechanisms of Protection

Abrasion Resistance

Weathering Resistance

ASTM D968 Falling Sand Test for Architectural Coatings

•  Sun’s ultraviolet rays break chemical bonds in organic coating binders •  Moisture/salt in air penetrate and degrade coatings •  Accelerated laboratory test methods do not duplicate natural weathering well.

ASTM D4060 Taber Abraser for Industrial Coatings

Effects of Weathering

ASTM B117 Salt Fog Cabinet

ASTM D5894 Cyclic Salt Spray Chamber

ASTM D2247 Humidity Cabinet

C1 Fundamentals of Protective Coatings for Industrial Structures 2-47

Unit 2 - Coating Types and Their Mechanisms of Protection

ASTM D2794 Gardner Variable Impactor

Chemical Resistance •  Needed for tank linings •  Needed for attack by fumes and vapors •  Needed for secondary containment

Resistance to Biological Growth

Navy Anti-Fouling Coatings •  Ablative coatings with cuprous oxide (MILP-24647) usually used on hull •  Polyamide/plasticizer typically nonvolatile vehicle •  Typically applied at 10 mils WFT to give 5 mils DFT •  Formula 121A vinyl-cuprous oxide still used in low-flow areas

•  Mildew resistance (Mercurials no longer permitted) •  Anti-fouling paints (Biological fouling reduces speed and maneuverability and increases fuel consumption)

New Foul Release Coatings Require:

Drawing: Three Basic Components of Coatings

•  Low surface energy coatings such as silicones •  High modulus of elasticity (elastomeric) •  A minimum thickness to provide needed elasticity

Vehicle

C1 Fundamentals of Protective Coatings for Industrial Structures 2-48

{

}

}

Volatiles that evaporate into air

Total solids in film

Unit 2 - Coating Types and Their Mechanisms of Protection

Coating Solids (total solids) Content •  •  •  • 

Pigment After Burn-Off of Organic Binder

ASTM D2369 Sample weighed in shallow aluminum pan Sample is heated Sample reweighed after cooling

Pigment Separated by Ultracentrifuge

Calculating Volatile Content •  Percent by weight of volatile product (solvent) is found by: −  100 - Percent by weight of coating solids

Calculating Non-Volatile Content

Function of Solvent •  •  •  • 

•  Percent by weight of non-volatile vehicle (binder) is found by: −  Percent by weight of coating solids - Percent by weight of pigment

Dissolve binder Reduce viscosity Control drying of wet film Control adhesion and durability of dry film (Solvents evaporate into air to create air pollutants.)

C1 Fundamentals of Protective Coatings for Industrial Structures 2-49

Unit 2 - Coating Types and Their Mechanisms of Protection

Testing for Solvent Content

Common Solvents •  •  •  • 

•  ASTM D3960 (VOC Content) •  ASTM D4017 (Water Content- Karl Fischer Titration) •  ASTM D6133 (Exempt Solvent Content)

Ketones (e.g., methyl ethyl ketone) Esters Aromatics (e.g., toluene, xylene) Aliphatic hydrocarbons (e.g., mineral spirits)

Coating Properties Related to Binder

Binder •  •  •  •  •  •  •  • 

•  The binder is the film-forming component of coatings. (Term includes resin and polymers.)

Mechanism and time of curing Performance in different environments Performance on different substrates Compatibility with different coatings Flexibility and toughness Exterior weathering Adhesion Ease of application, topcoating and repair

Coating Properties Imparted by Pigments

Pigment •  Heavier, solid portion of coating •  May be natural earth material or synthetic

•  •  •  •  •  •  • 

C1 Fundamentals of Protective Coatings for Industrial Structures 2-50

Opacity (hiding) Color Corrosion resistance Wet paint properties Weather and moisture resistance Level of gloss Reinforcement

Unit 2 - Coating Types and Their Mechanisms of Protection

Opacity in Coatings

Restricted Inhibitive Pigments •  •  •  •  • 

•  Obscures substrates •  Protects organic binders from the sun’s ultraviolet rays •  Titanium dioxide used in most whites and light tints

Level of Coating Gloss

Alternative Inhibitive Pigments •  •  •  •  •  • 

Red lead White lead Zinc chromate Strontium chromate Basic lead silico-chromate

•  Other things being equal:

Zinc oxide Zinc phosphate Zinc molybdate Calcium borosilicate Zinc phosphosilicate Barium metaborate

−  The lower the PVC, the higher the gloss −  The finer the dispersion of the pigment in the vehicle (fineness of grind), the higher the gloss

Additives Incorporated Into Wet Paint

Coatings with Different Pigment Volume Ratios

•  •  •  •  •  •  • 

Wetting agents Biocides Driers Plasticizers Mildewcides Fine abrasives Glass beads

C1 Fundamentals of Protective Coatings for Industrial Structures 2-51

Unit 2 - Coating Types and Their Mechanisms of Protection

Mechanisms of Coating Film Formation

Film-Forming Mechanisms

Terms: −  −  −  − 

Thermoplastic and thermosetting coatings Curing process Non-convertible coatings Convertible coatings

MEK-Soluble vs. MEK-Insoluble Coatings MEK-Soluble Coatings −  Lacquers −  Latex products −  Oil-based products (initially)

Curing Curing - the process of changing the properties of a paint from its liquid state into a dry, stable, solid protective film by chemical reaction with oxygen, moisture, or chemical additives, or by the application of heat or radiation.

MEK-Insoluble Coatings −  Chemically-reacting products −  Oil-based products (after aging)

Drying vs. Curing

Drying vs. Curing “Drying” is not the same as “curing.”

“Drying”- loss of solvent and/or water, which may or may not produce a protective film “Curing”- always produces a protective film

C1 Fundamentals of Protective Coatings for Industrial Structures 2-52

Unit 2 - Coating Types and Their Mechanisms of Protection

Mechanisms of Film Formation in NonConvertible (Thermoplastic) Coatings

Mechanisms of Curing in Convertible (Thermosetting) Coatings

•  Evaporation of solvent •  Latex coalescence •  Phase change

•  Air oxidization of unsaturated drying oils •  Chemical reaction of components •  Reaction with moisture or carbon dioxide in air

Curing of Lacquers and Water Emulsions

Film Formation Process in WaterDispersed Paints

Examples of Water-Borne (Latex) Coatings

Types of Water-Borne Coatings

•  Acrylics •  PVAs (polyvinyl acetate)

•  Water-soluble •  Water-reducible

C1 Fundamentals of Protective Coatings for Industrial Structures 2-53

Unit 2 - Coating Types and Their Mechanisms of Protection

Curing by Chemical Reaction

Examples of Air-Drying Coatings •  •  •  •  •  • 

Unmodified drying oils Alkyds Epoxy esters Uralkyds Oleoresinous phenolics Silicone alkyds

CURING OF PAINTS BY CHEMICAL REACTION

Liquid Resin A

•  •  •  •  •  • 

Epoxies Coal tar epoxies Polyurethanes Polyureas Polyesters and vinyl esters Phenolics Wash primer Silicones (Polysiloxanes)

Lacquers Bituminous coatings Water-borne acrylics Alkyds Epoxies Coal tar epoxies

Generic Coating Types (cont.) •  •  •  •  •  •  •  •  • 

Solid Resin A-B

Generic Coating Types To Be Discussed

Chemically Reacting Coatings •  •  •  •  •  •  •  • 

Liquid Resin B

Lacquers

Polyurethanes/polyureas Silicone (Polysiloxane) coatings Polyesters/vinyl esters Phenolic and phenolic epoxy coatings Pretreatment wash primers Inorganic zinc-rich coatings Organic zinc-rich coatings Powder Coatings Thermal spray metallics

Advantages: −  −  −  −  −  − 

C1 Fundamentals of Protective Coatings for Industrial Structures 2-54

Rapid drying and recoating Good general chemical resistance Good in water immersion Good gloss retention possible Good durability Easy to topcoat and repair

Unit 2 - Coating Types and Their Mechanisms of Protection

Lacquers

Bituminous Coatings

Limitations: −  −  −  −  − 

Advantages: −  −  −  −  − 

High in VOCs Poor solvent resistance Low film build Blast cleaned surface necessary for coating Occasional poor adhesion

Bituminous Coatings

Water-Borne Acrylic Coatings Advantages:

Limitations: −  −  −  −  − 

Low cost Easy to apply, topcoat, and repair Good water resistance Good film build Low level of surface preparation required

−  −  −  −  −  −  − 

Cutbacks are high in VOCs Poor solvent resistance Poor weathering Available only in black Toxic

Environmentally acceptable Easy to apply (and clean up), topcoat, and repair Dry rapidly for recoating Reduced solvent odor Excellent flexibility Low cost Safer (reduced flammability)

Water-Borne Acrylic Coatings

Alkyd Coatings

Limitations: −  −  −  −  − 

Advantages: −  −  −  −  −  − 

Limited durability Poor chemical and solvent resistance Poor wetting of surfaces Poor in immersion service Best cured above 50°F (10°C)

Easy to apply, topcoat, and repair Good flexibility possible Good surface wetting and adhesion Good gloss retention Relatively cheap From renewable source

C1 Fundamentals of Protective Coatings for Industrial Structures 2-55

Unit 2 - Coating Types and Their Mechanisms of Protection

Types of Epoxies

Alkyd Coatings •  •  •  •  • 

Limitations: −  −  −  −  − 

Relatively high in VOCs Poor performance in severe environments Poor chemical and solvent resistance Poor water immersion resistance Poor alkali resistance

•  •  •  • 

Advantages of High-Solids Epoxy Coatings

Epoxy Coatings

•  Rapid cure time •  No induction times •  No pot life problems using plural component systems •  Good edge retention (70% minimum for ERC) •  High film thickness per coat

Advantages: −  −  −  −  −  − 

Epoxy-polyamides (good water resistance) Amine-cured epoxies (better chemical resistance) Amine-adduct epoxies (partially reacted pdts.) Ketimine epoxies (blocked curing agent) Cycloaliphatic amine-cured epoxies (good lowtemperature curing) Phenolic epoxies (hard, dense, chemical-resistant) Novalac epoxies (best chemical/heat resistance) Epoxy mastics (high-build) High solids epoxies

Low in VOCs Good solvent and water resistance Many have good chemical resistance Tough, durable, slick film Good adhesion Good abrasion resistance

Epoxy Coatings

Coal Tar Epoxy Coatings

Limitations: −  −  −  −  −  −  − 

Advantages:

Limited pot life Chalk freely in sunlight Limited flexibility Cure best above 50oF (10oC) Topcoating a problem Blast cleaned surface needed Subject to amine blush

−  −  −  −  − 

C1 Fundamentals of Protective Coatings for Industrial Structures 2-56

Low in VOCs Good water and chemical resistance Good film build Good abrasion resistance Relative low cost

Unit 2 - Coating Types and Their Mechanisms of Protection

Coal Tar Epoxy Coatings

Two-Component Polyurethanes •  Polyisocanate •  Polyol

Limitations: −  −  −  −  −  − 

Toxic; personal protection needed Slow-curing Limited pot life Blast cleaned surface needed Topcoating a problem Available only in black

Two-Component Polyurethanes

Two-Component Polyurethanes

Advantages: −  −  −  −  −  −  −  − 

Limitations:

Can be low in VOCs Good water resistance Good hardness or flexibility Aliphatics have good gloss and color retention Aromatics have good chemical resistance Good durability Good abrasion resistance Low-temperature curing achievable

−  −  −  −  −  − 

Two-Component Polyureas

Highly toxic; need personal protection Moisture-sensitive; gloss may drop Skilled operator needed Limited pot life Blast cleaned surface required More expensive than epoxies

Two-Component Polyureas •  Reaction products of isocyanates and amineterminated co-reactants •  Extremely fast curing •  Water-sensitivity during application •  High-build, 100% solids •  Can be soft to hard elastomers •  Can be used in hybrids with polyurethanes •  Used on concrete floors or containments

•  Aromatic •  Aliphatic

C1 Fundamentals of Protective Coatings for Industrial Structures 2-57

Unit 2 - Coating Types and Their Mechanisms of Protection

Polyaspartic Coatings

Polyaspartic Coatings

Advantages: −  −  −  −  −  − 

Limitations: −  −  −  −  − 

Rapid curing time Good film build Good durability or flexibility Good abrasion resistance Aliphatics have good weathering properties Low or no VOC content

Silicone Coatings

Silicone Coatings

Advantages: −  −  −  −  − 

Limitations: −  −  −  − 

High-solids (low VOCs) Good chemical resistance Good weather resistance Good heat resistance Good general industrial use

Polyester and Vinyl Ester Coatings

High level of surface preparation Relatively short pot life Relatively slow cure Relatively high cost per coat

Polyester and Vinyl Ester Coatings

Advantages: −  −  −  −  − 

Very short pot life Must be applied by plural-gun application Skilled applicator needed Blast cleaned surface required (steel) Toxicity of isocyanate component

Limitations: −  −  −  − 

Can be low in VOCs High film build Good water resistance Good solvent and chemical resistance Good abrasion resistance

C1 Fundamentals of Protective Coatings for Industrial Structures 2-58

Limited pot life Skilled operator needed Blast cleaned surface required Peroxide component is hazardous

Unit 2 - Coating Types and Their Mechanisms of Protection

Fiberglass-Reinforced Plastic Coatings

Fiberglass-Reinforced Plastic Coatings

•  Polyester, •  Vinyl ester, or •  Epoxy linings

•  Advantages: −  Glass cloth, mat, or fibers −  High glass/high strength −  High resin/high chemical resistance

Phenolic and Epoxy Phenolic Coatings

Fiberglass-Reinforced Plastic Coatings •  Limitations:

Advantages: −  −  −  − 

−  Deterioration by weathering −  Rapid deterioration

Phenolic and Epoxy Phenolic Coatings

Hard coatings Good chemical resistance Good heat resistance Good solvent resistance

Pretreatment Wash Primers

Limitations:

Advantages:

−  Discolor during heat curing −  Poor exterior weathering −  Use limited mostly to linings

−  Promote adhesion of some primers −  Provide temporary corrosion protection −  Fast-drying

C1 Fundamentals of Protective Coatings for Industrial Structures 2-59

Unit 2 - Coating Types and Their Mechanisms of Protection

Pretreatment Wash Primers

Types of Inorganic Zinc •  1-A Water-borne, post-cured •  1-B Water-reducible, self-cured •  1-C Solvent-reducible, self-cured

Limitations: −  Film may undergo cohesive failure when applied too thickly −  High in VOCs −  Uses are limited −  Contains chromate

Inorganic Zinc Coatings

Inorganic Zinc Coatings

Advantages: −  −  −  −  −  −  − 

Limitations:

Can be low in VOCs Excellent abrasion resistance Excellent heat resistance Good atmospheric durability Useful as shop primer Fast-drying Can be used untopcoated

−  −  −  −  − 

Organic Zinc-Rich Coatings

Organic Zinc-Rich Coatings

Advantages: −  −  −  − 

Needs very clean, blasted surface Requires skilled applicator, agitated pot Difficult to topcoat Attacked by acid and alkali High initial cost

Limitations: −  −  −  −  − 

Can be low in VOCs Good atmospheric durability Relatively easily topcoated Moderate surface preparation

C1 Fundamentals of Protective Coatings for Industrial Structures 2-60

Requires skilled operator Constant agitation necessary Unsuitable for acid or alkali High initial cost Normally requires a topcoat

Unit 2 - Coating Types and Their Mechanisms of Protection

Advantages of Powder Coatings (Mostly Thermosetting) •  •  •  •  •  • 

Limitations of Powder Coatings •  High baking temperatures limit substrates to metals •  Color changes are expensive •  Powder suspensions in air may be explosive •  Inside surfaces difficult to coat

Low VOC emissions Low toxicity and flammability Good corrosion resistance Good thickness control, including edges Good film build in one coat Reduced waste

Requirements for Sealers for Metallizing

Thermal Spray Metallizing •  Zinc or aluminum •  Wire or powder form

•  Good penetration (low viscosity and pigment particle size) •  Compatibility with sprayed metal and any topcoat to be applied

Applied by: −  Flame spray −  Arc spray −  Plasma spray

New Coating Trends

Sealers for Metallizing •  Epoxies •  Pretreatment wash primer •  Silicones for high temperatures

•  Low VOC Coatings − High-Solids − Water-Borne

•  Non-Toxic Ingredients

C1 Fundamentals of Protective Coatings for Industrial Structures 2-61

Unit 2 - Coating Types and Their Mechanisms of Protection

Important Factors in Coating Selection •  •  •  •  •  • 

Possible Coating Requirements or Limitations •  •  •  •  •  •  • 

Exterior weathering Water, fuel, solvent, or chemical resistance Abrasion, heat, or mildew resistance Appearance Drying time Ease of application and maintenance

Severity of Environment •  •  •  •  •  • 

Other Considerations •  Life cycle cost •  Spot repair

SSPC 12-zone classification Variables Soluble salts Moisture Air contaminants Solar radiation

Information Sources •  •  •  •  •  • 

Severity of environment Surface preparation Access to work Drying time Applicator skills Scaffolding Safety and environmental requirements

Unit 2 Summary •  Three (3) mechanisms of corrosion control by coatings •  Three (3) basic components of coatings •  Two (2) basic mechanisms of coatings cure •  Basic understanding of generic types

SSPC Other organizations Government Consultants Coatings manufacturers Materials testing

C1 Fundamentals of Protective Coatings for Industrial Structures 2-62

Unit 3 - Surface Preparation for Painting

SURFACE PREPARATION FOR PAINTING 3.1 Purpose and Goals

3.2 Introduction to Surface Preparation

Scope

Surface preparation is one of the most important factors in performance of industrial coatings, both on metal

This unit covers the following subjects:

and on concrete surfaces. It includes (1) pre-clean­ing

•

Preparation of surfaces before cleaning

or solvent cleaning to remove surface contaminants,

•

Contaminants adversely affecting coating

(2) cleaning of a surface to the desired level, and

performance

(3) producing a profile (texture) for good adhesion of

•

Surface preparation methods

coatings.

•

Standards for cleaned steel surfaces

•

Required cleanliness levels for different coatings

•

Conventional abrasive blasting equipment

•

Centrifugal blasting equipment

•

Blasting abrasives

•

Blasting procedures

Poor surface preparation can result in premature coating failure. In general terms, the better the surface preparation, the longer the life of the coating system. 3.3 Purpose of Surface Preparation

Learning Outcomes

The purpose of surface preparation is two-fold: to

Upon completion of this unit, you will be able to:

the requirements of the specification. Sometimes

• • •

• •

clean and to roughen the substrate according to

Describe the need and importance of surface

the methods used to prepare surfaces for coating

preparation

application achieve these criteria simultaneously (as

Define surface preparation methods appropriate

with abrasive blast cleaning), while other times these

for different substrates

steps must be preformed separately (as with chemical

Define the standards for different levels of steel

stripping). In either case, the inspector must treat

cleanliness, and how to determine whether they

these as two distinct “acceptance criteria,” as the level

have been achieved

of cleaning may be adequate, but the roughness may

Explain the importance of profile height and how

be insufficient or excessive. Alternatively, the surface

to achieve a particular height by blasting steel

roughness may be on target, but the level of cleaning

Describe the basic components of equipment

may be inadequate.

for abrasive blasting and the basics of blasting operations

3.4 Preparation of Surfaces Before Cleaning and Painting Before cleaning and painting of surfaces, all structural modifications must have been completed. Repair of damaged substrates must also have been completed.

C1 Fundamentals of Protective Coatings for Industrial Structures 3-1

Unit 3 - Surface Preparation for Painting

“Sharp” edges and corners, such as those generated by

Inside corners are also difficult to coat with a uniformly

torch cutting operations are difficult to coat, as the coat-

thick film free of holidays. Coving corners makes them

ing tends to pull away

easier to apply good coating films.

(draw thin) on the corners during ap-

Crevices and pits should be filled by weld metal or

plication because of

other filler material depending upon the substrate. This

surface tension that is

provides both a more paintable surface and controls

created on the sharp

corrosion or other substrate deterioration. Treatment of

edge. Some coat-

irregular concrete surfaces is discussed in more detail

ings have relatively

in Unit 7.

good “edge retention” properties, while others that shrink a lot

Welds often have sharp projections that stick out of the

Figure 3-1: Grinding of Edges

wet paint. Welds should be ground to a smooth crown

during the drying and

for easier painting. Weld spatter bonds loosely to the

curing processes may have relatively poor edge reten-

steel but tightly enough that it has to be ground off.

tion properties. The problem of reduced coating thick-

Otherwise, when it disbands from the surface, it will

ness at edges can be addressed in three manners: •

Rounding the edges by grinding or sanding

•

Application of a stripe coat before or after priming

take the coating with it and create holidays.

(see Unit 4) •

Using edge-retentive coatings (ERCs) (see Unit 4)

Figure 3-3: Weld Spatter

3.5 Surface Contaminants Causing Early Coating Deterioration Surface contaminants that may cause early deterioration of coatings include:

Figure 3-2: Flame Cut Edges

C1 Fundamentals of Protective Coatings for Industrial Structures 3-2

•

Rust

•

Mill scale

•

Grease and oil

•

Dirt and dust

Unit 3 - Surface Preparation for Painting

•

Water

installed coating system and with proper wetting of the

•

Soluble salts

substrate by the coating during application (causing a

•

Paint chalk

defect known as “fisheyes”), these contaminants can

•

Loose, cracked, or peeling paint

be driven into the surface during surface preparation and/or can contaminate the abrasive media used for

Rust

surface preparation (power tool cleaning and abrasive

Rust consists of the corrosion products of steel (iron

abrasive will be recycled and reused; it too can deposit

blast cleaning). This is particularly problematic when the

oxides). Whether loose or relatively tightly adherent, it

the contamination onto cleaned surfaces.

must be removed for satisfactory coating performance. It is not a good base for applying coatings be­cause it

Dirt and Dust

expands and becomes porous as the iron reacts with

Dirt and dust will also prevent tight bonding of coatings,

water and oxygen (corrosion).

and must be removed completely. So-called “over-rust primers” (also referred to as “rust Water

converters”) do not perform as well as conventional coatings applied over clean steel, and the effective­ness

Steel surfaces must be dry before painting. Moisture

of rust converters is unproven.

may either produce flash rusting before painting or accelerate underfilm corrosion after painting, especially

Mill Scale

when soluble salts are present. Water can also prevent an organic coating from properly “wetting out” the

Mill scale is a bluish, somewhat shiny oxide residue

surface on metal or concrete surfaces.

that forms on steel surfaces during hot rolling. Although initially it is rather tightly adhering, it soon cracks, pops,

Soluble Salts

and disbonds. Unless completely removed before painting, it will later cause the coating to crack and

Soluble salts are deposited from the atmosphere as

expose the underlying steel. Steel is anodic to mill

marine or industrial contaminants onto exterior surfaces.

scale and so corrodes more rapidly in this combination

They are not completely removed from steel surfaces

of “dissimilar metals.”

by abrasive blast or power tool cleaning and accelerate corrosion of the cleaned steel. Soluble salts should be

Grease, Oil, Cutting Compounds and/or Lubri-

removed to specified or manufacturer-recommended

cants

levels before coating. These salts can be removed by washing with water or a water solution of a commercial

Grease, oil, cutting compounds or lubricants used in

product designed for this purpose. Waterjetting will also

the steel fabrication process, or that have become

remove these salts. The tolerance of different generic

deposited onto existing surfaces while in service

coating types in different environments to different levels

can adversely affect the performance of the newly-

of soluble salts has not been well established.

installed coating system unless they are detected and adequately removed prior to surface preparation. In addition to potentially interfering with adhesion of the

C1 Fundamentals of Protective Coatings for Industrial Structures 3-3

Unit 3 - Surface Preparation for Painting

Paint Chalk

effects. This is never the case with heavy oil deposits or grease, or with water-borne or solvent-free paints.

The sun’s ultraviolet light causes all exterior organic coatings to chalk to some extent. Chalk is the residue

Solvent for cleaning, commonly mineral spirits, is usually

left after deterioration of the coating’s surface organic

applied with rags, which are changed frequently as they

binder. All loose chalk must be removed before coating.

become contaminated. A final rinse is always made with

It is often specified that old paint have an ASTM D4214

fresh solvent.

rating of no less than 8 before topcoating it.

Organic solvents should not come into contact with

Deteriorated Paint

the eyes or skin, be used near sparks or flames, or be

All old, loose paint must also be removed before

inhaled unnecessarily. Solvent­cleaned metals usually

maintenance painting. Before removing any old paint,

require further surface cleaning before coating.

it must be determined whether the paint contains significant amounts of lead or other toxic material. If

Steam Cleaning

so, then special precautions must be taken to protect

Steam cleaning is another effective method of removing

workers, others in the area, and the environment during

grease and oil. Commercial detergents or solvent can

its removal.

be added to the steam to improve the cleaning power. In addition, steam cleaning may be used to remove

3.6 Surface Preparation Methods

dirt and grime from coated surfaces. Steam-cleaned

Surface preparation methods employed by a painting

steel is usually further cleaned by other methods before

contractor or facility owner can range from simple

coating.

solvent cleaning to hand and power tool cleaning, dry and wet abrasive blast cleaning, chemical stripping,

Alkali Cleaning

water jetting and other more non-traditional methods

Alkaline cleaners effectively remove grease and oil

such as sponge jetting and cryogenic blast cleaning

from contaminated steel by wetting, emulsifying, and

using dry ice pellets. The degree of cleaning required

dispersing them. They may cause significant damage to

is dependent on the service environment, the coating

chemically reactive metals, such as aluminum or zinc,

system and the intended service life of the coating once

or to wood or concrete.

installed. The actual surface preparation requirements are always placed in the job specificaiton.

Detergent/Water Cleaning

Solvent Cleaning

Aqueous solutions of household detergents may be

Solvent cleaning is used chiefly to remove grease and

effective in the removal of light deposits of grease and oil. They seldom have adverse effects on substrates.

oil from contamin­ated surfaces. Occasionally, some

Citric acid is a safer and more environmentally friendly

solvent-containing coatings can dissolve thin deposits of

alternative to acid etching.

oil and incorporate them into the coating without adverse

C1 Fundamentals of Protective Coatings for Industrial Structures 3-4

Unit 3 - Surface Preparation for Painting

Hand Tool Cleaning Hand tool cleaning is typically performed with wire brushes, scrapers, and other tools that do not depend on electric or pneumatic power to operate. These hand tools are only intended to remove loosely adhering corrosion products, old paint and flaking mill scale, and are not intended to produce an anchor pattern in the steel. Hand tools are frequently used to prepare

Figure 3-4: Chemical Stripping

surfaces for spot touch-up during maintenance painting activities.

carcinogens (cancer causing agents), their use as paint strippers declined. Other paint strippers came

Power Tool Cleaning

onto the market, and were formulated to work on a variety of surfaces, including wood, steel, etc. These

Power tool cleaning is typically performed with grinders,

paint strippers included caustic-based (pH 14), which

pneumatic chisels, needle scalers, and rotopeen tools

attacked the resin component of drying-oil (e.g., alkyd)

that require an electric pneumatic power source to

coatings, destroying their backbone and causing the

operate. Most of these tools can remove both loosely

coating system to debond from the underlying substrate.

and tightly adhering corrosion products, and paint

Caustic-based paint removers are the consistency of a

and mill scale from the steel surfaces. Stratified rust,

heavy paste that is sprayed or troweled onto the surface.

pack rust and rust scale are removed using these

After a few hours of “dwell time,” the stripper is removed

types of tools. Some of these tools can also produce

from the surfaces using scrapers, air or water pressure

an anchor pattern into the steel by “peening” the

or even blasting with ice crystals. Multiple applications

surface. Additionally, these tools can be purchased

can be required, depending on the coating system

with vacuum ports and hoses for attachment to HEPA

and thickness. Neutralization of the surface after the

(High Efficiency Particulate Air) filtered vacuums so that

stripper has been removed is required for proper coating

the fine, airborne particles that are generated during

performance. Alkaline stippers are not effective on

surface preparation activities are collected at the point

epoxy or most other thermosetting coatings.

of generation.

Environmentally and user-friendly chemical strippers

Chemical Stripping

are available that have a neutral pH and little odor.

Removal of coatings using chemical strippers has been

They rely on the metal surface below the coating to

widely used outside of the industrial coatings arena.

act as bond breakers. They are slower to work with on

Methylene chloride-based paint strippers were used for

thicker films and thus may require several applications

removing coatings in the residential, commercial and

to remove multiple layers.

light industrial markets for years, and the commercial aircraft industry used these strippers to remove

Chemical strippers do not generate a surface profile,

coatings from the exterior of the fuselage. However,

and will not remove rust or mill scale. Therefore,

when chlorinated solvents became recognized as

mechanical methods of surface preparation may be C1 Fundamentals of Protective Coatings for Industrial Structures 3-5

Unit 3 - Surface Preparation for Painting

required after the coating has been removed or a coating

•

Radial water injection (water rings)

system that is tolerant of intact mill scale and rust must

•

Coaxial water injection (water introduced into throat of nozzle)

be selected, provided it will perform adequately in the •

service environment.

Slurry blasting (water introduced into air/abrasive stream substantially upstream of nozzle)

The use of chemical strippers is described in SSPC Technical Update (TU 6), “Chemical Stripping of Organic

Dehumidification by refrigeration or desiccation of

Coatings from Steel Structures.”

enclosed spaces may be used to control rust bloom. Typically, rust bloom will be prevented from forming

Abrasive Blasting

on an abrasive blast cleaned steel surface with a dew

Abrasive blasting is usually the preferred method of

temperature and a relative humidity not to exceed

point of 15 to 20°F (9 to 12°C) below the prevailing

preparing steel and some other metal surfaces for

55%. Surface contamination will lower the critical

cleaning. Abrasive blast cleaning of metal structures

relative humidity. Dehumidification is described in

usually increases the service lives of their coatings

SSPC-TR 3: Dehumidification and Temperature Control

significantly over those cleaned by other methods. The

During Surface Preparation, Application, and Curing of

impact of high-velocity abrasive particles can completely

Coatings/Linings of Steel Tanks, Vessels, and Other

remove all rust, scale, dirt, and old coating, but not grease

Enclosed Spaces.

or oil, which must be removed by solvent cleaning before blasting. Abrasive blasting also roughens the surface to

Water Cleaning and Jetting

produce a texture that promotes tight coating adhesion. Aluminum and other soft metals require softer abrasives

Water cleaning and waterjetting are often used

(e.g., plastic beads) or lower impact velocities in the

to clean metal and concrete surfaces for coating.

removal of old coatings to prevent metal damage. Care

Low pressure water cleaning (LP WC) involves

must be taken with abrasive blasting of concrete to avoid

pressures up to 5,000 psi (34 MPa). The water may

substrate damage.

be heated and detergent may be added to aid in the cleaning. LPWC, sometimes called power washing, is effective in removing dirt and visible mildew on coated

Uncontrolled exterior abrasive blasting is not usually

metals and is generally safe on adjacent wood and

permitted, especially if there are toxic materials such

concrete/masonry. A spot check should be done to

as lead in the paint being removed. The particulate dust

void damage. High-pressure water cleaning (HP WC)

must be captured by some sort of containment and

uses pressures from 5,000 to 10,000 psi (34 to 70

properly disposed.

MPa). High-pressure waterjetting (HP WJ) is defined as cleaning from 10,000 to 30,000 psi (70 to 210 MPa).

Water is sometimes injected into the abrasive blast

For pressures over this limit, the cleaning is defined as

stream to control the particulate matter emitted during

ultrahigh-pressure waterjetting (UHP WJ).

blasting. SSPC-TR 2/NACE 6G198 describes three basic types of wet abrasive blast cleaning:

C1 Fundamentals of Protective Coatings for Industrial Structures 3-6

Unit 3 - Surface Preparation for Painting

Waterjetting Steel Cleanliness Levels: •

WJ-1 Bare metal with no visible contamination

•

WJ-2 No more than 5% visible trace residues

•

WJ-3 No more than 33% visible trace residues

•

WJ-4 All loose residues (mill scale, rust, and paint)

use of a mixture of water and abrasive to acheive a specified cleanliness level prior to the application of a protective coating or lining system. 3.7 Recommended Removal Methods for Different Contaminants

removed

This chart lists the contaminants that are best removed by the various cleaning methods:

Water cleaning has the advantage over abrasive blasting of removing soluble salts that can later cause osmotic blistering of the coating. As with all wet cleaning methods, it may be necessary to add corrosion inhibitors to the water to prevent flash rusting (N = No flash rust, L = Light, M = Moderate, H = Heavy) of steel. Corrosion inhibitors are not required for some protective coatings and may in some cases induce osmotic blistering.

Cleaning Methods

Contaminants

Degreasing

Grease and oil

Power washing

Dirt and mildew

Hand and power tools

Loose rust, mill scale, and loose paint

Low-pressure water cleaning

Dirt and mildew

High-pressure waterjetting Marine fouling, loose rust and paint

Environmentally acceptable inhibitors include sodium and potassium nitrites and phosphates. Waterjetting does not produce a profile, but may expose a previous profile.

Ultrahigh pressure waterjetting

Rust, tight paint

Abrasive blasting

Rust, mill scale, tight paint

Recommended Cleaning Methods for Metals Wet Abrasive Blast Cleaning

Each type of metal has its own best method of surface

Abrasive may be injected into the stream of water or

preparation for coating. Metals of different degrees of

used separately after water cleaning to remove mill

hardness affect the surface preparation method.

scale and roughen the metal surface. Without abrasive injection, the surface of water-cleaned steel will not be

Steel. The most productive method of cleaning uncoated

roughened. Cleaning steel for coating may be achieved

steel is solvent washing or steam cleaning followed by

with water pressures as great as 40,000 psi (2,800 MPa)

abrasive blasting. In­deed, SSPC standards for hand

and water volumes of only 2 to 5 U.S. gallons (7.5 to 55

and power tool cleaning and for blast cleaning always

liters) per minute. Extreme caution must be maintained

first require solvent cleaning. The preferred method of

with these high pressures to avoid injuries to personnel

cleaning damaged areas of coated steel is also abrasive

and structures.

blasting. Waterjetting and wet abrasive blasting are excellent alternatives, especially where dry abrasive

SSPC and NACE are working on a joint standard that

blasting cannot be tolerated. Other cleaning methods,

will be released in 2009 on Wet Abrasive Blast Cleaning

such as hand or power tool cleaning, may be more

that will contain requirements for Wet Abrasive Blast

practical for spot repair of coatings.

Cleaning of uncoated or coated steel surfaces by the

C1 Fundamentals of Protective Coatings for Industrial Structures 3-7

Unit 3 - Surface Preparation for Painting

Galvanized steel. The recommended method of

Steel alloys. Low-alloy (weathering) steel normally

cleaning un­coated galvanized steel varies with the

is not coated for protection, but relies on a natural

condition of its surface. Simple solvent cleaning may be

oxide film formed in mild lo­ca­tions. If unsightly rust

adequate for new, clean galvanizing. This will remove

streaking requires removal and coating of the steel,

any oil applied to the galvanizing to protect it during

cleaning should be done with high-pressure or ultrahigh-

ex­terior storage. Other temporary protective systems,

pressure waterjetting. Adding abrasive may be required

such as chromate treatment, must be removed as

to produce a surface profile. Coating systems normally

recom­mended by their manufacturers. Epoxy and latex

used on structural steel can then be applied.

coatings will normally bond well to the smooth, clean galvanized steel without roughening, although some

Stainless steel may require coating for a pleasing

applicators may think it necessary to use a phosphoric

appearance. To provide adequate texture for primer

acid or other chemical treatment.

bonding to this or other very hard metals, blasting with a very hard, nonferrous abrasive, such as aluminum oxide, garnet, or silicon carbide, is required.

Loose zinc corrosion products or coating on weathered galvanized steel should be removed by bristle or

Recommended Cleaning Methods for

wire brushing or by water cleaning as vigorously as

Concrete/Masonry

necessary for complete removal of contaminants. If rusting is present on older galvanized steel, it should

Concrete must be structurally sound, and any surface

be carefully spot-cleaned by water cleaning/jetting or

imperfections must be repaired before cleaning and

sweep abrasive blasting to minimize removal of intact

coating. These and related concerns with surface

galvanizing. Deteriorated coating on galvanizing should

preparation of concrete are described more fully in

also be re­moved in this manner.

Unit 7 and in SSPC-SP 13 “Surface Preparation of Concrete.”

Uniform corrosion of unpainted galvanizing from prolonged exposure may eventually expose the

Low-pressure water or steam cleaning (ASTM D4258)

underlying brownish iron-zinc alloy. If this occurs, the

is used to remove loose contaminants from concrete

surface should be painted as soon as possible.

surfaces to receive light-duty ser­vice. High-pressure water cleaning (ASTM D4259) is used to re­move old

Aluminum and other soft metals. New, clean

coatings or other tightly held materials from concrete

aluminum and other soft metals may be adequately

surfaces to receive more severe service.

cleaned for coating by solvent washing. Deter­gent washing may be required for removal of dirt or loose

Abrasive blasting (ASTM D4259 and D4261) or

cor­ro­sion products. Abrasive blasting with plastic beads

acid etching (ASTM D4260) may also be used on

or other soft abrasive may be necessary to remove old

concrete/masonry to obtain a sur­face profile and clean

coatings. Blasting with hard abrasives (e.g., steel grit

surfaces for coating. Take care to avoid dam­ag­ing

or shot) will damage soft metals. These metals may

surfaces with high-pressure water or abrasive. Grease

be wash-­primed to promote adhesion of oil-based or

and oil must be removed with detergents or steam

latex coatings.

before blasting; solvent cleaning will merely transfer the grease and oil inside the con­crete.

C1 Fundamentals of Protective Coatings for Industrial Structures 3-8

Unit 3 - Surface Preparation for Painting

Concrete surfaces must be completely dry when coating with non­water-borne mater­ials. The Plastic Sheet Method (ASTM D 4263), In-Situ Probe Method (ASTM F2170) and Calcium Chloride Method (ASTM F1869) are the procedures used to detect or measure moisture in concrete. These methods are discussed more fully in Unit 7.

SSPC Standard

Extent of Removal of Contaminants

SP 7 Brush-off blast

Removes all loose mill scale, rust and paint

SP 14 Industrial Blast

Removes all contaminants except for traces of tightly adhering mill scale, rust and paint

SP 6 Commercial blast Removes all visible contaminants except for shadows, streaks, and stains up to 33%

3.8 Standards for Cleaned Steel Surfaces The most frequently used standards for cleaned steel

SP 10 Near-white blast Removes all visible contaminants except for shadows, streaks and stains up to 5%

surfaces are those of SSPC. They include standards for all common methods of cleaning. Volume 2 of SSPCs Steel Structures Painting Manual: Systems and

SP 5 White metal blast Removes all visible contaminants

Specifications contains all these standards, as well as other useful information.

Solvent Cleaning (SSPC-SP 1)

The SSPC surface preparation standards for mechanical cleaning of steel surfaces are summarized below in

Solvent cleaning removes all visible oil, grease, soil,

descending order of cleanliness, for convenience of the

drawing and cutting compounds and other soluble

reader (i.e., best at bottom of list):

contaminants. This may be accomplished with solvent, steam, emulsion or alkaline cleaners. It should precede all other SSPC surface preparation proce­dures and is

SSPC Standard

Extent of Removal of Contaminants

SP 2 Hand tool cleaning

Removes all loose mill scale, rust and paint

12, 14 and 15, so that they are free of grease and oil.

SP 3 Power tool cleaning

Removes all loose mill scale, rust and paint

Hand Tool Cleaning (SSPC-SP 2)

SP 15 Commercial grade power tool cleaning

Removes all visible contaminants except for stains up to 33%; minimum 1 mil profile

specifically required in SSPC-SP 2, 3, 5, 6, 7, 10, 11,

Hand tool cleaning removes all loose materials—mill scale, rust, paint, and other loose contaminants. Tightlyheld mill scale, rust, and paint are not removed. A

SP 11 Power tool Removes all visible cleaning to bare metal contaminants; minimum 1 mil profile

variety of tools are available for this cleaning method. This method is usually satisfactory when using a paint that has good surface wetting (e.g., oil-based paint) on a structure located in a mild environment.

The SSPC surface preparation standards for abrasive blast cleaning of steel are summarized below in decending order of cleanliness, for the convenience of the reader (i.e., best at bottom of list):

C1 Fundamentals of Protective Coatings for Industrial Structures 3-9

Unit 3 - Surface Preparation for Painting

Power Tool Cleaning (SSPC-SP 3)

Brush-Off Blast Cleaning (SSPC-SP 7/NACE No. 4)

Power tool cleaning also removes loosely-bonded mill

Brush-off blast cleaning, with its requirement for solvent

scale, rust, and paint but is much faster than hand tool

cleaning, removes all visible (without magnification)

cleaning.

oil, grease, dirt, dust, and loose mill scale, rust, and paint. Tightly-adhering mill scale, rust, and paint are not

Commercial Grade Power Tool Cleaning

removed. The requirements are simi­lar to those of hand

(SSPC-SP 15)

and power tool cleaning. This method is best used in a mild environment with paints that wet surfaces well.

Commercial grade power tool cleaning with its requirement for solvent cleaning, removes all visible

Industrial Blast Cleaning (SSPC-SP 14/NACE No. 8)

contaminants but allows 33% staining. It requires a 1 mil minimum surface profile.

Industrial blast cleaning, with its requirement for solvent cleaning, removes all visible (without magnification)

Power Tool Cleaning to Bare Metal (SSPC-SP 11)

oil, grease, dirt, dust, loose mill scale, rust, and paint. Traces of tightly adhering mill scale, rust and coating

Power tool cleaning to bare metal with its requirement

residues are permitted to remain on 10% of each 6400

for solvent cleaning removes all visible (without magni­

mm2 (9 in2) of surface area if evenly distributed. This

fication) oil, grease, dirt, dust, mill scale, paint, and other

method is best used in a mild environment with paints

foreign ma­terials and provides a roughened surface

such as alkyds that wet surfaces well.

suitable for painting. If the steel is pitted, slight rust or paint residue may remain in the bottoms of pits. This

Commercial Blast Cleaning (SSPC-SP 6/NACE No. 3)

surface cleanliness compares roughly to that received from commercial blast cleaning (SSPC-SP 6) in terms

Commercial blast cleaning, with its requirement

of the perfor­mance received from coating systems

for solvent cleaning, removes all visible (without

applied to it. It requires a 1 mil minimum profile. Power

magnification) oil, grease, dirt, dust, mill scale, rust,

tool cleaning specifications will be discussed in greater

paint, and other foreign matter. However, no more than

detail in Unit 6.

one-third of each nine-square-inch area cleaned may have random staining consisting of slight shadows,

Pickling (SSPC-SP 8)

slight streaks, or minor discolorations caused by rust, mill scale, or previously applied paint.

Pickling cleans surfaces by chemical reaction, electroly­ sis, or both to visibly (without magnification) remove all

Near-White Blast Cleaning (SSPC-SP 10/NACE No. 2)

mill scale and rust. Heavy deposits of rust, mill scale, and paint are first removed by other methods (SSPC-

Near-white blast cleaning with its requirement for solvent

SP 2, 3, 6, or 7). Pickling is conducted only in shops.

cleaning is the second-highest level of abrasive blasting

The following standards are issued jointly by SSPC

cleanliness. It removes all visible (without magnification)

and NACE.

oil, grease, dirt, dust, mill scale, rust, paint, and other foreign contaminants. No more than five percent of

C1 Fundamentals of Protective Coatings for Industrial Structures 3-10

Unit 3 - Surface Preparation for Painting

each nine-square­-inch cleaned area may have random

Substrates before Application of Paints and Related

staining of slight shadows, stains, or discolorations from

Products- Tests for the Assessment of Surface

rust, mill scale, or previous paint.

Cleanliness.” The identification system and the definitions vary considerably from those of SSPC. Therefore these standards are described separately

White Metal Blast Cleaning (SSPC-SP 5/NACE No. 1)

in this training.

White metal blast cleaning with its requirement for solvent cleaning is the highest level of abrasive blast

ISO St 2:

cleaning. It removes all visual (without magnification)

Thorough Hand and Power Tool

Cleaning

oil, grease, dirt, dust, mill scale, rust, paint, and other

“When viewed without magnification, the surface shall

foreign matter.

be free from visible oil, grease and dirt, and from poorly High- and Ultrahigh-Pressure Waterjetting (SSPC-

adhering mill scale, rust, paint coatings and foreign

SP 12/NACE No. 5)

matter.”

This standard defines four grades of surface cleanliness

ISO St 3: Very Thorough Hand and Power Tool

using high pressure waterjetting, an alternative to

Cleaning

abrasive blast cleaning, WJ-1 through WJ-4. Only WJ-1 and WJ-2 are used for high-performance coatings in

“(Same) As for St 2, but the surface shall be treated

industrial or marine environments. It also lists three

much more thoroughly to give a metallic sheen arising

conditions of salt contamination, NV-1 (none detectable),

from the metallic substrate.”

NV-2 (less than 7 µg/cm chloride, less than 10 µg/cm 2

2

ferrous, and less than 17 µg/cm2 sulfate salts), and

ISO F1: Flame Cleaning

NV-3 (less than 50 µg/cm2 chloride and sulfate salts).

“When viewed without magnification, the surface shall

Waterjetting is effective in removing water-soluble

be free from mill scale, rust, paint coatings and foreign

surface contaminants from severely pitted substrates.

matter. Any remaining residues shall show only as

It can remove rust, existing coatings or linings, and

a discoloration of the surface (shades of different

surface grease and oil. Waterjetting alone does not

colours).”

create a surface profile; thus, it is recommended primarily for recoating or relining projects where there

ISO Sa 1: Light Blast Cleaning

is an adequate preexisting profile.

“When viewed without magnification, the surface shall

Preparation of Steel Substrates before Application of Paints and Related Products (ISO Standard

be free from visible oil, grease and dirt, and from poorly adhering mill scale, rust, paint coatings and foreign

8502)

matter.”

The International Standards Organization has also published written surface cleanliness standards for preparation of steel surfaces. These standards are housed in ISO Standard 8502, “Preparation of Steel

C1 Fundamentals of Protective Coatings for Industrial Structures 3-11

Unit 3 - Surface Preparation for Painting

ISO Sa 2: Thorough Blast Cleaning

To use any of the VIS Standards, you must first establish the initial condition of steel using visual

“When viewed without magnification, the surface shall

standards. Steels of different condi­t ions appear

be free from visible oil. Grease and dirt, and from most

significantly different from one another after similar

of the mill scale, rust, paint coatings and foreign matter.

cleaning.

Any residual contamination shall be firmly adhering.”

Initial Condition of Steel Before Preparation (A–G)

ISO Sa 2.5: Very Thorough Blast Cleaning “When viewed without magnification, the surface shall be free from visible oil, grease and dirt, and from mill scale, rust, paint coatings and foreign matter. Any remaining traces of contamination shall show only as stains in the form of spots or stripes.”

Condition A:

Completely covered with adherent mill scale; little or no rust visible

Condition B:

Covered with both mill scale and rust

Condition C:

Completely covered with rust; little or no pitting

Condition D:

Completely covered with rust; pitting visible

Condition E:

Previously painted; light-colored paint applied over blast-cleaned surface, paint mostly intact

Condition F:

Previously painted; zinc-rich paint applied over blast-cleaned surface, paint mostly intact

Condition G:

Paint applied over mill scale-bearing steel; system thoroughly weathered, thoroughly blistered, or thoroughly stained

ISO Sa 3: Blast Cleaning to Visually Clean Steel “When viewed without magnification, the surface shall be free from visible oil, grease and dirt, and shall be free from mill scale, rust, paint coatings and foreign matter. It shall have a uniform metallic colour.” 3.9 Visual Aids to Surface Cleanliness To help in identifying the level of cleaned steel, SSPC

After determining the proper initial grade of steel,

has prepared SSPC-VIS 1: Guide and Reference

compare the cleaned steel with the pictorial standards for

Photographs for Steel Surfaces Prepared by Dry

that grade. Pictorial standards are available for cleaning

Abrasive Blast Cleaning; SSPC-VIS 3: Guide and

levels of conditions A through D for blast cleaning; and

Reference Photographs for Steel Surfaces Prepared

A through G for power and hand tool cleaning; and C

by Power and Hand Tool Cleaning l; SSPC-VIS 4:

and D for wet abrasive blast cleaning.

Guide and Reference Photographs for Steel Surfaces Prepared by Waterjetting; and SSPC-VIS 5: Guide and

Photographs in SSPC-VIS 1 also show the different

Reference Photographs for Steel Surfaces Prepared

appearances of SP 5 surfaces obtained with non-

by Wet Abrasive Blast Cleaning. VIS 1 contains

metallic and metallic abrasives and differences in

photographic standards for four levels of blast cleaning.

profile, angle of view, and diffusion of light.

As their titles suggest, VIS 3 contains photos of hand and power-tool cleaned steel; VIS 4 has photos of

Although less desirable than referencing the appropriate

waterjet cleaned steel; and VIS 5 has photos of wet

published visual standard or reference photographs,

abrasive blasting.

another way to set up a standard for blast-cleaned steel is to blast a section of the structure to an acceptable

C1 Fundamentals of Protective Coatings for Industrial Structures 3-12

Unit 3 - Surface Preparation for Painting

level, as determined by the engineers, and cover it

Listed below is the relative order of surface prepa-

with a clear lacquer to save it as a standard during the

ration required, from the most tolerant to the least

blasting operation. A 12-inch steel test plate can also

tolerant:

be blasted to an acceptable level and sealed in a water­ proof, grease proof bag or wrapper.

Coating Type •

Drying oil

SSPC-VIS 4 is a guide and visual reference for steel

•

Alkyd

cleaned by waterjetting, developed jointly by SSPC and

•

Coal tar

NACE. This standard features photographs that illustrate

•

Asphaltic

the appearance of painted and unpainted rusted steel

•

Water-borne acrylic

surfaces before and after waterjetting, with additional

•

Epoxy mastic

photographs that depict the occurrence of light,

•

Vinyl lacquer

moderate and heavy flash rusting after waterjetting.

•

Chlorinated rubber

•

Epoxy

•

Coal tar epoxy

•

Polyurethane/Polyurea

•

Organic zinc-rich

•

Inorganic zinc-rich (solvent-based)

•

Inorganic zinc-rich (water-based)

SSPC-VIS 5 is a guide and visual reference for steel cleaned by wet abrasive blasting, developed jointly by SSPC and NACE. This standard features photographs of steel with initial conditions C and D cleaned by wet abrasive blasting to SP 6 and SP 10. Photographs also depict light, medium, and heavy flash rusting.

Higher levels of cleaning usually result in longer performances, especially with high-performance

SSPC-VIS 1, 3, 4 and 5 serve only as aids in establishing

coatings. Also, higher levels are required in severe

the level of cleanliness. The SSPC definition is the legal

environments than in mild environments.

standard. In order to avoid confusion, the appropriate visual standards should be referenced in the contract specification.

The substrate cleanliness level should never be less

3.10 Levels of Cleanliness Required for Different

particular primer and service.

than that recommended by the coating supplier for the

Coatings 3.11 Air Abrasive Blasting Equipment

Different types of coatings require different levels of cleaning. Drying oil coatings are probably most tolerant

Air abrasive blasting equipment has five basic com-

of contamination, and therefore require a lower level of

ponents:

surface preparation.

•

Air compressor (1)

•

Air hose (4)

•

Blasting machine (2)

•

Blast hose (3)

•

Nozzle (5)

C1 Fundamentals of Protective Coatings for Industrial Structures 3-13

Unit 3 - Surface Preparation for Painting

A clean, dry, white blotter or cloth is held 18 inches in

Air Compressor

front of the nozzle with only the air flowing for one to two

The air compressor takes in, compresses, and then

minutes. Any stain or moisture picked up on the blotter

releases large vol­umes of air by piston or rotary action

or cloth indicates the presence of contaminants.

to the blasting machine. The continuous and constant supply of high pressures and volumes of air to propel

SSPC AB 1, AB 2 and AB 3 defines the cleanliness

abrasives from the blasting pot through the blasting

requirements for blast cleaning abrasives and are

hose and nozzle to the metal surface is one of the most

defined below.

critical parts of the blas­ting operation. Typical blasting pressures at the nozzle are 90–100 psi (6.5–7 MPa).

A simple test of oil contamination of abrasive is to conduct the “ASTM D7393 Standard Practice for

The rate of blast cleaning is directly related to the nozzle

Indicating Oil in Abrasives Test (also known as the Vial”

air pressure and volume. A 10 psi (0.7 mPa) drop in

test) by placing a small amount of it in a glass jar filled

pressure will reduce the cleaning rate by 15 percent.

with clean water and shake the jar. A sheen that rises to the surface of the water indicates oil contamination

Sources of pressure loss in air abrasive blasting in-

on the abrasive.

clude: •

Worn compressor parts

•

Hoses small in diameter and/or great in length

•

Couplings joining lengths of hose

An abrasive test for conductivity (soluble salt contamination) is conducted by ASTM D4940, “Standard Test Method for Conductimetric Analysis of Water Soluble Ionic Contamination of Blasting Abrasive.” According to SSPC AB 1, AB 2 or AB 3, the tolerable threshold for conductivity is 1,000 microsiemen. This is discussed more fully in Unit 5. Air Delivery Hose The hose delivering air from the compressor to the blasting machine need not be as durable as the blasting

Figure 3-5: Steel Grit

hose, as it is not eroded by abrasive or dragged along

Figure 3-6: Steel Shot

Air and Abrasive Cleanliness Oil and water traps (e.g., aftercoolers) are used to remove contaminants originating at the compressor or other components that would otherwise be trans­ferred to cleaned surfaces. They require frequent inspection and clean­ing. Detection of oil and water contaminants can be achieved with a simple blotter test (ASTM D4285).

Figure 3-7: Abrasive Blaster

C1 Fundamentals of Protective Coatings for Industrial Structures 3-14

Unit 3 - Surface Preparation for Painting

the ground. It should have as large a diameter as

recommended for use because its reduced diameter

practical (typically two inches ID when the hose is

introduces frictional losses of pressure. Shorter nozzles

under 50 feet long) and as short a length as practical

can usually be used to get into tight places.

to reduce frictional losses. As few as pos­sible couplings should be used to reduce loss of air pressure at these

Nozzle

connec­tions.

Nozzles are available in several lengths,

Blasting Machine

designs, sizes of

The blasting machine, or “sand pot,” holds the abrasive.

openings, and lining

A valve at its bottom controls the amount of abrasive

materials. Nozzle

fed into the blast hose. The chief parts of a typical

length of five to

gravity-fed blasting machine are:

eight inches (125

•

Moisture separator

mm to 200 mm)

•

Exhaust valve

•

Filling head

•

Metering valve

•

Hose/tank coupling

Figure 3-8: Nozzles

are generally used for removing tightly adhering rust and scale. Shorter nozzles (three inches [75 cm] or less) are more appropriate for use behind beams and in other inaccessible places where a whip might otherwise be used.

A continuous, uniform flow of abrasive, such as that obtained from an automatic metering valve, is required

The tapered shape of the venturi nozzle is much more

for efficient cleaning. Blasting machine capacities vary

efficient than the cylindrical shape of the straight bore

from 50 pounds (20 kg) to several tons of abrasive.

nozzle. It creates a larger, more uniform blast pattern

Smaller machines require more time to keep them filled and operating.

and can increase cleaning rates by 30–50 percent.

Blast Hose

The nozzle orifice size is chosen according to the

The blast hose carrying the air and abrasive from the

maintains constant pressure is usually best. A 1/2-inch

available volume of air. The largest practical size that

blasting machine to the nozzle must be sturdy, flexible,

(12 mm) nozzle with proper air supply may clean four

and treated to prevent electrical shock. A typical three-

times the area of a 1/4-inch (6 mm) nozzle. Nozzle

ply blast hose with a 1.25 inch (32 mm) ID is sold in

sizes are given by manu­fac­turers in units of 1/16 inch.

short, join­able sections to minimize frictional losses.

Thus, a 1/2 inch (12 mm) nozzle is a No. 8, while a

It always has a static elec­tricity-conducting exterior

1

layer.

/4 inch (6 mm) nozzle is a No. 4.

The nozzle lining, particularly at the throat, is gradually

A short length of light, flexible hose called a “whip”

worn away by the abrasive. Since enlarging optimum

is sometimes joined at the nozzle. It permits easier

openings reduces blasting efficiency, the liner is usually

handling, particularly in tight areas, but is not

replaced after the original diameter has increased­to the C1 Fundamentals of Protective Coatings for Industrial Structures 3-15

Unit 3 - Surface Preparation for Painting

next size. Tungsten carbide and Norbide liners may have

achieved in fixed painting facilities (“shops”) in a

service lives of 300 and 750 to 1,000 hours, respectively,

controlled envi­ronment. Machines with motor-driven,

as compared to six to eight hours for cast iron liners.

bladed wheels hurl abrasive at high speed by centrifugal

Abuse of the more brittle tungsten car­bide and Norbide

force.

nozzles by dropping or banging can significantly re­duce their service lives. Nevertheless, they are much more

The advantages of centrifugal blasting over air blasting

economical to use in the long run.

include:

All nozzles are equipped with “deadman switches,” which require depressing to allow a flow of air and abrasive. If the operator drops the nozzle, the flow of air and abrasive is immediately stopped.

•

Savings of time, labor, energy, and abrasive

•

Superior, more uniform cleaning

•

Reduction of blasting waste by recycling abrasive

•

Protection of the environment

Limitations of centrifugal blasting include:

A hypodermic needle gauge is used to measure pressure at the nozzle. The gauge is inserted into the blast hose immediately in front of the nozzle in the direction of abrasive flow to minimize damage to it from the flowing abrasive. As

•

Limited portability

•

High initial cost

•

Difficult to use on irregular surfaces

•

Primarily for new steel

Centrifugal blast cleaning is much faster than air blast

stated earlier in 3.10, productive blasting is usually done

cleaning. This is true with fixed shop equipment and

at 90–100 psi (6.5-7 MPa).

with portable vacuum blasting systems available for field use, which may collect, clean, and recycle abrasive.

3.12 Centrifugal Blasting Equipment

Production using containment systems is slower, but

Automatic cleaning by centrifugal blasting is an

the technique is very effective in eliminating blasting

alternative to conventional air blasting. It is normally

dust and cleaning welds, and in removing hazardous waste from toxic paints. Portable centrifugal blasting equipment is also available for regular steel surfaces like the decks of ships, the sides of storage tanks, or concrete floors. 3.13 Surface Profile and Blasting Abrasives Surface Profile The profile of abrasive-blasted steel is the contour on a plane perpendicular to the surface. It is classified by depth and texture. Profile depth is a measure of the

Figure 3-9: Centrifugal Blasting

C1 Fundamentals of Protective Coatings for Industrial Structures 3-16

Unit 3 - Surface Preparation for Painting

roughness of the blasted surface based on the average

Natural mineral abrasives. Silica is readily available,

distance between peaks and valleys.

cheap, and effective, but health concerns about its breakdown particulates are greatly restricting its use.

For good primer adhesion to blast-cleaned steel, a

Staurolite, a nonsilica sand natural abrasive, is faster

minimum profile is required; with too great a profile

cutting with less breakdown and dusting. Garnet is a

depth, pinpoint rusting may occur on poorly covered

tough, angular, natural abrasive that can be recycled

peaks. There is an optimum profile height and/or an

a few times. Classes and grades of mineral abrasives

optimum abrasive recommended by manufacturers of

are defined in SSPC-AB 1, Type I.

primers for steel surfaces. Slag abrasives. Copper, nickel, and coal slags, industry by-products, are fast-cutting but have a high breakdown

Round shot may peen the steel surface to a wavy

rate and cannot be recycled. Classes and grades of slag

profile. It is very effective in removing brittle deposits

abrasives are defined in SSPC-AB 1, Type II.

such as mill scale. Grit is angular and so produces a more jagged finish, generally prefer­red for tight

Metallic abrasives. Steel shot and grit abrasives are

coating adhesion. The variety of grit materials available

efficient, hard, and dust-free. They must be kept dry

produces a variety of surface patterns. A mixture of grit

during storage to pre­vent rusting. Indeed, all abrasives

and shot is usually used in centrifugal blasting.

should be stored in their original, sealed bags on pallets off the ground until ready for use. Standards of

Mineral and slag abra­s ives are semiangular and

cleanliness for recycled ferrous metallic abrasives are

produce surface patterns somewhere between those

given in SSPC-AB 2. Standards for newly manufactured

of shot and grit.

or remanufactured steel abrasive (grit and shot) are given in SSPC-AB 3.

Blasting Abrasive Properties

Standards for recyclable encapsulated abrasive

Four types of abrasives are commonly used in blast

media consisting of steel grit or aluminum oxide in a

cleaning. Each is described below and in SSPC

compressible open-cell matrix (i.e., “sponge”) are given

specifications.

in SSPC-AB 4.

Abrasive Specifications: •

Synthetic abrasives. Aluminum oxide and silicon

SSPC-AB 1: Natural Minerals (Type I) Slags (Type

carbide are non­- metallic­ abrasives with cleaning

II)

properties similar to those of metallics but without the

•

SSPC-AB 2: Recycled Steel Grit/Shot

problem of rusting. They are hard, fast-cutting, and low

•

SSPC-AB 3: New/Remanufactured Steel Grit/

dusting, but are expensive and must be recycled to be

Shot

economical.

•

SSPC-AB 4: Recyclable encapsulated abrasive media consisting of steel grit or aluminum oxide in

A larger-sized abrasive will cut deeper than a smaller-

a compressible open-cell matrix (i.e., “sponge”)

sized abrasive, but the greatest rate of cleaning is generally achieved with as small a size as possible to give the desired cleanliness and anchor profile. C1 Fundamentals of Protective Coatings for Industrial Structures 3-17

Unit 3 - Surface Preparation for Painting

Particles in the 40–50-mesh range are commonly used

The table below list abrasives that can be used to

today. Those larger than 16–18 mesh have a slow

achieve a given profile:

cleaning rate and may gouge the surface; those 100 mesh or finer may not be able to give the desired profile

Approximate Profile Height of Blasted Steel

or remove tight mill scale.

Using Different Abrasives 1-mil Profile (25 µm) 30/60-mesh silica sand G-80 steel grit S-110 steel shot 80-mesh garnet 100-grit aluminum oxide

The manner in which abrasive particles fracture and change shape upon impact is called their breakdown characteristic. As abrasives fracture and flake, the abrasive particles become increasingly smaller in size until they are eliminated by the separator system. New abrasive is added at regular intervals to maintain

2-mil Profile (50 µm) 16/35-mesh silica sand G-40 steel grit S-280 steel shot 36-mesh garnet 36-grit aluminum oxide

a “working mix” of new (larger) and recycled (smaller) particles. During continuous operations, such as a production line, new abrasive should be added hourly, rather than daily or at change of shift, in order to maintain a consistent

3–4-mil Profile (75-100 µm) 8/20-mesh silica sand G-25 steel grit S-330 steel shot 16-mesh garnet 16-grit aluminum oxide

profile. The sizes and shapes of the abrasive particles are largely responsible for the type of blast profile produced on steel. Suppliers of primers for steel usually

1.5-mil Profile (37 µm) 16/35-mesh silica sand G-50 steel grit S-170 steel shot 36-mesh garnet 50-grit aluminum oxide 2.5-mil Profile (62 µm) 8/35-mesh silica sand G-40 steel grit S-280 steel shot 16-mesh garnet 24-grit aluminum oxide

recommend one or more abrasives or mixes or specify a desired profile height for blast cleaning.

3.14 Air Blast Cleaning Procedures Conventional blast cleaning is best done systematically,

If the profile pro­duced by abrasive blasting is too great,

heeding the concerns described below:

barrier protection of the steel may be inadequate, and pinpoint rusting may result.

Angle of attack. The nozzle-to-surface angle may range from 45–90°, depending on the work. An 80–90° angle is suit­able for re­moving rust and mill scale and cleaning pits. A slightly down­ward angle will direct the dust away from the blaster and permit better visi­bility. An angle of 45–60° is best for peeling heavy layers of coating or rust. General cleaning is best done at 60–70°.

C1 Fundamentals of Protective Coatings for Industrial Structures 3-18

Unit 3 - Surface Preparation for Painting

Nozzle-to-surface distance. The closer the nozzle

3.15 Unit Summary

to the work, the greater will be the abrasive density,

Without proper surface preparation, the protection

but the smaller will be the blast pattern. While a close

afforded by coatings will be significantly diminished. The

distance (e.g., six inches [150 mm]) may be necessary

chief functions of surface preparation are:

for removing tight scale, 18 inches (450 mm) or more

•

may be more appropriate for removing old paint and for

To remove surface contaminants that prevent good adhesion and/or prevent premature deterioration of

general cleaning.

coating and substrate. •

Straight-line passes. Each pass with the blast nozzle

To texture the surface to provide additional area for bonding of the primer.

should occur in a straight line at the same distance from the surface. Arcing or varying the distance from the work

Typical contaminants on steel surfaces that must be

will produce nonuniform clean­ing.

removed before coating include rust, mill scale, dirt, oil, and deteriorated coating. SSPC has developed

Coating cleaned surfaces. No more steel should

standards for steel prepared to different levels of

be blasted than can be coated that day, since rusting

cleanliness. It has also developed visual aids for

can occur overnight. On hot, hu­mid days, flash rusting

verifying these levels.

can occur in a few hours. If this occurs, it must be removed by brush blasting before coating. Soluble salt contamination will also accelerate flash rusting. In production work, it only takes one spray painter to keep up with four blasters. The work should be scheduled accordingly. Dehumidification. Dehumidification of closed spaces may be used to retain a blast cleaned surface for coating at a later time. Dehumidification is described in SSPCTR 3 (NACE 6A192) “Dehumidification and Temperature Control During Surface Preparation, Application, and Curing of Coatings/Linings of Steel Tanks, Vessels, and Other Enclosed Spaces.”

C1 Fundamentals of Protective Coatings for Industrial Structures 3-19

Unit 3 - Surface Preparation for Painting

Unit 3 - Exercise 3A: Cleaning Methods Match the cleaning methods listed in Column A with the descriptions listed in Column B.

Column A

Column B

1. ____ Abrasive blasting

A. Cleans and produces profile with no dust

2. ____ Alkaline stripper

B. Removes oil based paints (e.g., alkyds)

3. ____ Hand and power tools

C. Restores but doesn’t produce profile

4. ____ SARA

D. Removes grease and oil

5. ____ SSPC-SP 1

E. Removes only loose contaminants

6. ____ Waterjetting

F. Safe chemical removal of coatings

7. ____ Wet abrasive blasting

G. Cleans and produces profile but generates dust

C1 Fundamentals of Protective Coatings for Industrial Structures 3-20

Unit 3 - Surface Preparation for Painting

Unit 3 - Exercise 3B: Conventional Abrasive Blasting System Match the component of a conventional abrasive blasting system listed below in Column A with the descrptions listed in Column B. Then rearrange the components in Column A in the order that they occur in the system, starting at the air compressor. Column A

Column B

1. ____ Aftercooler

A. Directs air/abrasive flow at target

2. ____ Air hose

B. Carries air and abrasive to nozzle

3. ____ Blast hose

C. Removes moisture from air

4. ____ Blasting machine

D. Carries air to blasting machine

5. ____ Compressor

E. Meters flow of abrasive into air stream

6. ____ Deadman switch

F. Provides high-pressure air

7. ____ Nozzle

G. Safety device for turning off air/abrasive stream

C1 Fundamentals of Protective Coatings for Industrial Structures 3-21

Unit 3 - Surface Preparation for Painting

Quiz 1. What word best describe oil, grease, and dirt on a steel surface? a. adherents b. inhibitors c. contaminants d. activators 2. The method of surface preparation described in SSPC-SP 1 is: a. industrial blast cleaning b. near-white blast cleaning c. water jetting d. solvent cleaning 3. The purpose of an aftercooler located immediately after the compressor during abrasive blasting is: a. cooling the breathing air for the blaster b. removing (condensing) any water present c. removing any carbon monoxide present d. introducing abrasive into the air stream 4. Mill scale is defined as: a. an oxide of iron produced during hot rolling of the steel b. a loosely adhering product formed during storage of new steel in a moist environment c. a protective surface produced for steel by phosphate treatment d. the roughened surface of steel produced by abrasive blasting 5. Which item is used to detect oil or moisture in the compressed air used for abrasive blasting? a. vial b. hypodermic gage c. color reagent d. blotter

C1 Fundamentals of Protective Coatings for Industrial Structures 3-22

Unit 3 - Surface Preparation for Painting

6. Which item is used to detect oil or clay on an abrasive used for abrasive blasting? a. vial b. hypodermic gage c. color reagent d. blotter 7. An example of a synthetic abrasive is: a. garnet b. aluminum oxide c. sand d. steel grit 8. An example of a natural abrasive is: a. garnet b. aluminum oxide c. coal slag d. silicon carbide 9. A whip is used during abrasive blasting... a. to reduce hose resistance to abrasive flow b. to reduce rate of nozzle deterioration c. to reach otherwise inaccessible places d. to increase the abrasive velocity 10. An advantage of water jetting compared to abrasive blasting is: a. It removes grease and oil. b. It removes soluble salts. c. It produces a higher profile. d. It is comparatively safe to use. 11. A limitation of water jetting is: a. It is very slow. b. It does not remove mill scale. c. It does not produce a surface profile. d. It uses huge volumes of water. C1 Fundamentals of Protective Coatings for Industrial Structures 3-23

Unit 3 - Surface Preparation for Painting

12. Which of the following defects is too high a steel blast profile often associated? a. osmotic blistering b. pinpoint corrosion c. galvanic corrosion d. limited coating adhesion 13. The preferred method of removing soluble salt contaminants from steel surfaces is: a. water washing b. solvent washing c. abrasive blasting d. SSPC-SP 15

C1 Fundamentals of Protective Coatings for Industrial Structures 3-24

Unit 3 - Surface Preparation for Painting

References ASTM F1869, Standard Test Method for Measuring Moisture in Concrete by the Calcium Chloride Method ASTM F2160, Standard Test Method for Measuring Moisture in Concrete by the In-Situ Probe Method ASTM D 4214, Evaluating the Degree of Chalking of Exterior Paint Films ASTM D 4258, Standard Practice for Surface Cleaning Concrete for Coating ASTM D 4259, Standard Practice for Abrading Concrete ASTM D 4260, Practice for Acid Etching Concrete ASTM D 4261, Standard Practice for Surface Cleaning Concrete Unit Masonry for Coating ASTM D 4263, Standard Test Method for Indicating Moisture in Concrete by the Plastic Sheet Method ASTM D 4285, Standard Test Method for Indicating Oil or Water in Compressed Air ASTM D 4940, Standard Test Method for Condumetric Analysis of Water Soluble Ionic Contamination of Blasting Abrasive ISO Standard 8502, Preparation of Steel Substrates Before Application of Paints and Related Products MIL-PRF-131, Barrier Materials, Waterproof, Greaseproof, Flexible, Heat-Sealable SSPC-AB 1, Mineral and Slag Abrasives SSPC-AB 2, Cleanliness of Recycled Ferrous Metallic Abrasives SSPC-AB 3, Ferrous Metallic Abrasives SSPC-SP 1, Solvent Cleaning SSPC-SP 2, Hand Tool Cleaning SSPC-SP 3, Power Tool Cleaning SSPC-SP 5, White Metal Blast Cleaning (NACE No. 1) SSPC-SP 6, Commercial Blast Cleaning (NACE No. 3) SSPC-SP 7, Brush-Off Blast Cleaning (NACE No. 4) SSPC-SP 8, Pickling SSPC-SP 10, Near-White Blast Cleaning (NACE No. 2) SSPC-SP 11, Power Tool Cleaning to Bare Metal SSPC-SP 12, Surface Preparation and Cleaning of Metals by Waterjetting Prior to Re-coating (NACE No. 5) SSPC-SP 13, Surface Preparation of Concrete (NACE No. 6) SSPC-SP 14, Industrial Blast Cleaning (NACE No. 8) SSPC-SP 15, Commercial Grade Power Tool Cleaning SSPC-TR 2, Wet Abrasive Blast Cleaning (Technical Report)(NACE 6G198) SSPC-TR 3, Dehumidification and Temperature Control During Surface, Preparation, Application and Curing of Coatings/Linings of Steel Tanks, Vessels and Other Enclosed Spaces (NACE6A192) SSPC-TU 6, Chemical Stripping of Organic Coatings from Steel Structures SSPC-VIS 1, Guide and Reference Photographs for Steel Surfaces Prepared by Dry Abrasive Blast Cleaning (Referenced in ASTM D 2200)

C1 Fundamentals of Protective Coatings for Industrial Structures 3-25

Unit 3 - Surface Preparation for Painting

References SSPC-VIS 3, Guide and Reference Photographs for Steel Surfaces Prepared by Power and Hand-Tool Cleaning (Referenced in ASTM D 2200) SSPC-VIS 4, Guide and Reference Photographs for Steel Surfaces Prepared by Waterjetting (NACE VIS 7) SSPC-VIS 5, Guide and Reference Photographs for Steel Surfaces Prepared by Wet Abrasive Blast Cleaning (NACE VIS 9)

Additional Reading Drisko, R.W., Polly, D.R., and Schwab, L.K. Salt Spray Evaluation of Coated Galvanized Steel. Journal of Protective

Coatings and Linings, February 1985, pp. 30-39. O’Donoghue, M., et. al. Chemical Strippers and Surface-Tolerant Coatings. Journal of Protective Coatings and Lin-

ings, May 2000, pp. 74-93.

C1 Fundamentals of Protective Coatings for Industrial Structures 3-26

Unit 3 - Surface Preparation for Painting

Unit 3 Topics

Unit 3 Surface Preparation for Painting

•  •  •  •  • 

Unit 3 Topics (cont.) •  •  •  • 

Preparation of surfaces Contaminants Surface preparation methods Standards for cleaned steel surfaces Cleanliness levels

Unit 3 Learning Outcomes Upon completion of this unit, you will be able to:

Air abrasive blasting equipment Centrifugal blasting equipment Blasting abrasives Blasting procedures

−  Describe the need and importance of surface preparation −  Define surface preparation methods appropriate for different substrates −  Define the standards for different levels of steel cleanliness, and how to determine whether they have been achieved −  Explain the importance of profile height and how to achieve a particular height by blasting steel −  Describe the basic components of equipment for abrasive blasting and the basics of blasting operations

Surface Irregularities to be Addressed

Surface Preparation Includes: •  Pre-cleaning to remove surface contaminants •  Cleaning surfaces to desired levels •  Producing a surface profile (texture)

•  •  •  • 

Sharp edges Inside corners Crevices, pits, and other depressions Welds and other projections

C1 Fundamentals of Protective Coatings for Industrial Structures 3-27

Unit 3 - Surface Preparation for Painting

Edge Failure of Coatings

Cause of Reduced Edge Coating Thickness

Edge Treatments for Painting

Grinding of Edges

•  Round edges by grinding •  Add stripe coat for extra thickness •  Use edge-retentive coating

Stripe Coat for Extra Thickness

New Edge Retentive Coating

C1 Fundamentals of Protective Coatings for Industrial Structures 3-28

Unit 3 - Surface Preparation for Painting

Two Welds, One Ground and One Unground

Inside Corners •  Difficult to obtain uniformly thick, voidfree films •  Coving provides more easily painted surface

Coating Failure on Improperly Prepared Weld

Weld Spatter

Surface Contaminants Causing Early Coating Deterioration •  •  •  •  •  •  •  • 

Rust on Surfaces

Rust Mill scale Grease and oil Dirt and dust Soluble salts Water Paint chalk Loose, cracked, or peeling paint

•  Becomes porous and flakes •  Poor base for coating •  Stabilizers and converters unproven

C1 Fundamentals of Protective Coatings for Industrial Structures 3-29

Unit 3 - Surface Preparation for Painting

Rusting Mill Scale on Steel

Corrosion Under Mill Scale

Grease, Oil, Cutting Compounds

Effects of Coating Over Uncleaned Steel in Inaccessible Area

•  Interfere with adhesion and wetting of the substrate (“fisheyes”) •  Contamination of abrasive

Dirt and Dust

Water

•  Dirt and dust will prevent tight bonding of coatings •  ISO 8502-3

•  Produce flash rusting •  Accelerate underfilm corrosion

C1 Fundamentals of Protective Coatings for Industrial Structures 3-30

Unit 3 - Surface Preparation for Painting

Soluble Salt Problems on Cleaned Surfaces

Remove Soluble Salts By:

•  Will accelerate corrosion of cleaned steel •  Will promote osmotic blistering of coatings

•  Dry abrasive blast cleaning is the most widely used method for preparing steel for coating and in conjunction with pressure washing or steam cleaning, can help reduce salt levels. Three other methods, which can assist in salt removal are: −  Ultra High Pressure Waterjetting −  High Pressure Waterjetting with Abrasive −  Wet Abrasive Blast Cleaning

Effect of Coating Over Soluble Salt-Contaminated Surface

Effect of Coating Over Chalk •  Chalk- the residue left after deterioration of the coating’s surface organic binder •  ASTM D4212 rating of no less than 8 before topcoating

Common Surface Preparation Methods

Deteriorated Paint •  •  •  • 

•  All old, loose paint must be removed before painting

Degreasing (SP 1) Hand and power tool cleaning (SP 2, 3, 15, 11) Chemical stripping (TU 6) Abrasive blasting--wet and dry (SP 5, 10, 6, 14, 7) •  Water cleaning and jetting (SP 12)

C1 Fundamentals of Protective Coatings for Industrial Structures 3-31

Unit 3 - Surface Preparation for Painting

Methods of Degreasing •  •  •  • 

Solvent Cleaning •  •  •  •  • 

Solvent cleaning Steam cleaning Alkali cleaning Detergent/water cleaning

Alkali Cleaning

Removes grease, oil, and dirt Use clean, soaked rags Turn and replace rags often Use fresh solvent for final rinse Wear gloves and face shield

Detergent/Water Cleaning •  May remove light deposits of grease, oil, and dirt •  Few adverse effects on substrates •  Safe to use

•  Removes grease and oil •  May damage aluminum, zinc, and wood •  Use PPE for eyes, hands, and other exposed skin

Assorted Hand Tools

Hand Tools

•  Typically performed with wire brushes and scrapers

•  Remove loosely adhering corrosion products, old paint and flaking mill scale •  Does not produce anchor profile •  Used for spot touch-up maintenance painting activities

C1 Fundamentals of Protective Coatings for Industrial Structures 3-32

Unit 3 - Surface Preparation for Painting

Rotary Power Tools and Abrasive Disks

Needle Guns With and Without HEPA Accessories

•  Typically performed with grinders, pneumatic chisels, needle scalers, and rotopeen tools

Cutters for Rotary Impact Tools

Chemical Stripping •  SSPC TU 6: Chemical Stripping of Organic Coatings from Steel Structures •  Method to remove coating •  Does not generate surface profile •  Does not remove rust or mill scale

Chemical Stripping

Types of Strippers •  Alkaline: Best for oil-based coatings •  Solvent: Best for latex paints •  SARA: Best for thermosetting coatings

C1 Fundamentals of Protective Coatings for Industrial Structures 3-33

Unit 3 - Surface Preparation for Painting

Selective Adhesion Release Agents (SARA) •  •  •  •  •  •  • 

Abrasive Blasting •  Preferred method of preparing steel and other metal surfaces for cleaning

Water emulsions of solvents Relatively low toxicities Biodegradable No objectionable odor Can remove multiple coats of many generic types Can be applied by brush, dip, or spray Often ineffective on highly cross-linked thermosets (Novolac epoxies and vinyl esters)

Wet Abrasive Blast Cleaning

Abrasive Blast Nozzle

•  Radial water injection (water rings) •  Coaxial water injection (water introduces into throat of nozzel) •  Slurry blasting (water introduced into air/ abrasive stream substantially upstream of nozzel)

Dehumidification Used to Retain Blasted Surface

Dehumidification Used to Retain Blasted Surface

C1 Fundamentals of Protective Coatings for Industrial Structures 3-34

Unit 3 - Surface Preparation for Painting

Dehumidification Used to Retain Blasted Surface

Dehumidification Can Control Rust Bloom •  When dew point is 15 to 20°F (9 to 12°C) below surface temperature •  And R/H is no more than 55%

Pressures for Cleaning with Water

Waterjetted Steel Cleaning Levels

•  Low-pressure water cleaning (LP WC): less than 5,000 psi (34 MPa) •  High-pressure water cleaning (HP WC): 5,000 to 10,000 psi (34 to 70 MPa) •  High-pressure waterjetting (HP WJ): 10,000 to 30,000 psi (70 to 210 MPa) •  Ultrahigh-pressure waterjetting (UP WJ): above 30,000 psi (210 MPa)

•  Visual cleanliness •  Flash rusting levels •  Soluble salt levels

SSPC VIS 4: Waterjetting

Waterjetted Steel Cleanliness Levels •  •  •  • 

WJ-1 Bare metal with no visible contamination WJ-2 No more than 5% visible trace residues WJ-3 No more than 33% visible trace residues WJ-4 All loose residues (mill scale, rust, and paint) removed

C1 Fundamentals of Protective Coatings for Industrial Structures 3-35

Unit 3 - Surface Preparation for Painting

•  •  •  • 

Waterjetting

Remotely Controlled Waterjetting

Robotic Waterjet Cleaning of Steel

Ultrahigh-Pressure Waterjetting of Steel

Flash Rusting Levels

Heavy Flash Rusting

No flash rust (N) Light (L) Moderate (M) Heavy (H)

C1 Fundamentals of Protective Coatings for Industrial Structures 3-36

Unit 3 - Surface Preparation for Painting

Moderate Flash Rusting

Light Flash Rusting on Wet Abrasive Blasted Steel

SSPC-VIS 5: Wet Abrasive Blast Cleaning

Water Ring for Wet Abrasive Blasting

Methods of Control of Particulate Emissions

Shop Centrifugal Blasting

•  Vacuum Blasting •  Shop Centrifugal Blasting •  Blasting Rooms •  Sponge Blasting •  Wet Abrasive Blasting

C1 Fundamentals of Protective Coatings for Industrial Structures 3-37

Unit 3 - Surface Preparation for Painting

Abrasive Blasting Room

Abrasive Blasting Cabinet

Sponge Blasting Equipment

Sponge with Embedded Abrasive

Recommended Removal Methods for Different Contaminants

Preferred Surface Preparation for Steel

Cleaning Method −  Degreasing −  Power washing −  Hand and power tools −  Low-pressure water cleaning −  High-pressure water cleaning −  High-pressure water jetting −  Ultrahigh-pressure water jetting −  Abrasive blasting

Contaminants

•  Abrasive blasting •  Water cleaning

−  Grease and oil −  Dirt and mildew −  Loose rust, mill scale, and loose paint −  Dirt and mildew −  Marine fouling, loose rust and paint −  Rust, mill scale, tight paint −  Rust, mill scale, tight paint −  Rust, mill scale, tight paint

C1 Fundamentals of Protective Coatings for Industrial Structures 3-38

Unit 3 - Surface Preparation for Painting

Recommended Surface Preparations for Galvanizing •  •  •  • 

Aluminum/Soft Metal Surface Preparation

Water cleaning/jetting Solvent cleaning Light abrasive blasting Phosphoric acid or other chemical treatment

•  Solvent or detergent washing •  Blasting with soft abrasive

Requirements for Proper Coating of Concrete

Steel Alloy Surface Preparation •  High-pressure and ultrahigh-pressure waterjetting •  Blasting with hard abrasive (Aluminum oxide, garnet, or silicon carbide)

•  Structural soundness •  Free of surface contaminants •  Textured (open pores)

SSPC-SP 1 Solvent Cleaning Methods

Concrete Cleaning Methods •  Low-pressure and high-pressure water cleaning •  High-pressure and ultra-high pressure waterjetting •  Abrasive blasting •  Detergent washing

•  Solvent wash or dip •  Steam clean •  Emulsion or alkali cleaners

C1 Fundamentals of Protective Coatings for Industrial Structures 3-39

Unit 3 - Surface Preparation for Painting

SSPC Standards for Mechanical Cleaning

SSPC Standards for Abrasive Blasted Steel in Order of Increasing Cleanliness

Extent of Removal of Contaminants

SSPC Standard − 

SP 2 Hand tool cleaning

− 

− 

SP 3 Power tool cleaning

− 

− 

SP 15 Commercial grade power tool cleaning

− 

− 

SP 11 Power tool cleaning to bare metal

− 

Removes all loose mill scale, rust, and paint Removes all loose mill scale, rust, and paint Removes all visible contaminants for stains up to 33%; minimum 1 mil profile Removes all visible contaminants; minimum 1 mil profile

SSPC Standard

Extent of Contamination Removal

−  SP 7 Brush-off blast

−  Removes all loose mill scale, rust, − and paint − −  Removes all contaminants except for traces of tightly adhering mill scale, rust and paint up to 10% − −  Removes all visible contaminants except for shadows, streaks, or stains up to 33% − −  Removes all visible contaminants except for shadows, streaks, and stains up to 5% − −  Removes all visible contaminants

−  SP 14 Industrial blast

−  SP 6 Commercial blast

−  SP 10 Near-white blast

−  SP 5 White metal blast

ISO Standard 8502 ISO St 2:

Thorough Hand and Power Tool Cleaning

ISO St 3:

Very Thorough Hand and Power Tool Cleaning

ISO F1:

Flame Cleaning

ISO Sa 1:

Light Blast Cleaning

ISO Sa 2:

Thorough Blast Cleaning

ISO Sa 2.5:

Very Thorough Blast Cleaning

ISO Sa 3:

Blast Cleaning to Visually Clean Steel

Visual Aids for Cleaned Steel •  •  •  •  • 

SSPC-VIS 1 SSPC-VIS 3 SSPC-VIS 4 SSPC-VIS 5 Preserved field standard

Initial Condition of Previously Unpainted Steel (A-D)

SSPC-VIS 1: Dry Abrasive Blast Cleaning

Condition A: Completely covered with adherent mill scale; little or no rust visible Condition B: Covered with both mill scale and rust Condition C: Completely covered with rust; little or no pitting Condition D: Completely covered with rust; pitting visible

C1 Fundamentals of Protective Coatings for Industrial Structures 3-40

Unit 3 - Surface Preparation for Painting

Using VIS 1 to Evaluate Abrasive Blast Cleanliness

Conditions A-D

SSPC VIS 3: Hand and Power Tool Cleaning

Condition G

Initial Condition of Previously Painted Steel (E-H) Condition E:

Condition F:

Condition G:

Condition H:

Steel Cleaning Levels (Low to High) for Different Generic Types of Coating

Previously painted; light colored paint applied over blast-cleaned surface, paint mostly intact Previously painted; zinc-rich paint applied over blast-cleaned surface, paint mostly intact Paint applied over mill scale-bearing steel; system thoroughly weathered, blistered, or stained Degraded paint system applied over steel; system thoroughly weathered; thoroughly blistered; or thoroughly stained

•  •  •  •  •  • 

Drying oil Alkyd Coal tar/asphaltic Water-borne acrylic Epoxy mastic Vinyl lacquer

•  •  •  •  • 

Chlorinated rubber Epoxy/coal tar epoxy Polyurethane Organic zinc Inorganic zinc

C1 Fundamentals of Protective Coatings for Industrial Structures 3-41

Unit 3 - Surface Preparation for Painting

Air Abrasive Blasting Equipment

Air Abrasive Blasting Setup

•  Five Basic Components −  −  −  −  − 

Air Compressor Air Hose Blasting Machine Blast Hose Nozzle

How an Air Compressor Works

Small Rotary Air Compressor

Large Diameter Blasting Hose and Small Diameter Coating Hose Secured to Compressor

Aftercooler Used Directly After Compressor

(Note: safety pins and cables at couplings)

C1 Fundamentals of Protective Coatings for Industrial Structures 3-42

Unit 3 - Surface Preparation for Painting

Filter and Carbon Monoxide Alarm for Blaster’s Breathing Air

Air Pressure and Volume •  A 10-psi drop in nozzle air pressure reduces the blasting rate by 15%.

Vial Test for Quality of Abrasive

Sources of Pressure Loss

•  SSPC AB1, AB2 and AB3 defines the cleanliness requirements for blast cleaning abrasives. •  ASTM D7393, Standard Practice for Indicating Oil in Abrasives

•  Worn compressor parts •  Hoses •  Couplings

Abrasive Test for Conductivity

Gravity Fed Blasting Machine

•  ASTM D4940, “Standard Test Method for Conductimetric Analysis of Water Soluble Ionic Contamination of Blasting Abrasive” •  Tolerable threshold for conductivity is 1,000 microseimens

•  Parts of a typical gravity fed blasting machine are: −  −  −  −  − 

Moisture Separator Exhaust Valve Filling Head Metering Valve Hose/Tank Coupling

C1 Fundamentals of Protective Coatings for Industrial Structures 3-43

Unit 3 - Surface Preparation for Painting

Gravity-Fed Blast Pot

Blasting Hose Layers

Whip Hose

Venturi and Straight Bore Nozzles

Deadman Switch

Hypodermic Needle Gauge for Determining Pressure at Nozzle

C1 Fundamentals of Protective Coatings for Industrial Structures 3-44

Unit 3 - Surface Preparation for Painting

Advantages of Centrifugal Blasting

Centrifugal Blast

•  Savings of time, labor, energy, and abrasive •  Superior, more uniform cleaning •  Reduction of blasting waste by recycling abrasive •  Protection of the environment

Limitations of Centrifugal Blasting •  •  •  •  • 

Surface Profile and Blasting Abrasives

Limited portability High initial costs Difficult to use on irregular surfaces Primarily for new steel Expensive to maintain

Profiles of Steel Blasted with Shot and Grit

Properties of Abrasives •  •  •  • 

Size Shape Hardness Breakdown characteristics

C1 Fundamentals of Protective Coatings for Industrial Structures 3-45

Unit 3 - Surface Preparation for Painting

Types of Commonly Used Abrasives •  •  •  • 

SSPC Abrasive Specifications

Natural mineral abrasives Slag abrasives Metallic abrasives Synthetic abrasives

AB-1

Natural Minerals (Type I) Slags (Type II)

AB-2

Recycled Steel Grit/Shot

AB-3

New/Remanufactured Steel Grit/Shot

AB-4

Recyclable encapsulated abrasive media consisting of steel grit or aluminum oxide in a compressible open-cell matrix (i.e., “sponge”)

Pinpoint Rusting Caused by Too High of a Surface Profile

Abrasive Size and Shape •  Size and shape of abrasive are largely responsible for blasted steel surface profile characteristics

Examples of Abrasives Recommended and Profiles Produced 1-mil Profile (25 µm)

1.5-mil Profile (37 µm)

2-mil Profile (50 µm)

30/60-mesh silica sand

16/35-mesh silica sand

16/35-mesh silica sand

G-80 steel grit

G-50 steel grit

G-40 steel grit

S-110 steel shot

S-170 steel shot

S-280 steel shot

80-mesh garnet

36-mesh garnet

36-mesh garnet

100-grit aluminum oxide

50-grit aluminum oxide

36-grit aluminum oxide

Examples of Abrasives Recommended and Profiles Produced

C1 Fundamentals of Protective Coatings for Industrial Structures 3-46

2.5-mil Profile (62 µm)

3-4-mil Profile (75-100 µm)

8/35-mesh silica sand

8/20-mesh silica sand

G-40 steel grit

G-25 steel grit

S-280 steel shot

S-330 or 390 steel shot

16-mesh garnet

16-mesh garnet

24-grit aluminum oxide

16-grit aluminum oxide

Unit 3 - Surface Preparation for Painting

Angle of Blasting Attack

Nozzle-to-Surface Distance

How a Straight Pass Should be Used

Cleaning and Coating Rates •  1 painter = 4 blasters

Unit 3 Summary •  •  •  •  •  •  •  •  • 

Preparation of surfaces Contaminants Surface preparation methods Standards for cleaned steel surfaces Cleanliness levels Abrasive blasting equipment Centrifugal blasting equipment Blasting abrasives Blasting procedures

C1 Fundamentals of Protective Coatings for Industrial Structures 3-47

Unit 4 - Application of Coatings

APPLICATION OF COATINGS 4.1 Purpose and Goals

best method for the particular job. The following considerations should be given in selecting the most

Scope

appropriate method:

This unit covers the different methods of applying

Appropriateness for the particular coating. Some

coatings and their advantages and limitations. It also

coatings, like inorganic zincs, can only be applied

describes different aspects of coating operations and

successfully by spray. Viscous, VOC-conform­ing

coating failures due to poor application.

coatings frequently present special application problems.

Learning Outcomes

Appropriateness for the particular structural

Upon completion of this unit, you will be able to: •

components. Brush application is commonly

Explain appropriate application systems for

preferred over other methods in tighter areas such

different jobs •

as pipe racks, stairways, and handrails.

Describe the basics of coating application operations

•

Desired appearance (gloss, color, and texture).

Define coating failures associated with poor

Special equipment may be required for the desired

application and explain how to avoid them

finish.

4.2 Methods of Application: General Factors

Speed, ease, and economics of application method.

A comprehensive recommendation of practices for

Economics usually dictate choosing a system that can

coatings application to steel surfaces in shop, field,

complete the job quickly with little or no deficiencies

and maintenance painting is presented in SSPC-PA 1.

to be corrected later.

SSPC-PA Guide 4 describes maintenance repainting with oil-based painting systems, and SSPC-PA

Simplicity of equipment/necessary applicator skills.

Guide 5 describes maintenance painting programs.

Some equipment is very specialized and requires

SSPC-PA Guide 7 describes the application of thin

special training.

film coatings to concrete. The SSPC Basic Spray Application Manual, SSPC-04-05, provides detailed

Safety/environmental requirements. Material,

recommended spraying procedures.

equipment, and operations must meet all safety and environmental regulations. High transfer efficiency

The basic choices of methods for applying coatings

equipment may be required to avoid overspray onto

are brush, roller, and spray. In both contract and

surfaces not to be painted. A safety plan is usually

in-house painting, the applicator is often given a

required for applicators.

choice of application method, and must decide the

C1 Fundamentals of Protective Coatings for Industrial Structures 4-1

Unit 4 - Application of Coatings

Weather. It is always best to apply coatings in an

(b) In coating a steel item, of the 2 gallons of coating

enclosed, climate-controlled shop environment,

consumed, only 1-1/4 gallons were actually

wherever practical. This results in better work, is more

applied to the surface.

environ­men­tally acceptable, and can be done in any

kind of weather.

Transfer efficiency = 1.25 x 100 = 62.5%





2

Relative rates of paint application by different methods Factors affecting paint transfer efficiency include:

on a flat steel surface have been estimated at: Square Feet Applied Per Hour

Square Meters Applied Per Hour

Brush

75-125

7-12

Roller

150-300

14-28

HVLP spray

185-310

17-30

Conventional air spray

200-450

18-42

500-1,250

50-120

Application Method

Air-assisted airless or Airless Spray

•

Item size

•

Item shape

•

Type of application equipment

•

Distance from spray gun to item

•

Skill of the operator

•

Spray gun pressures

The smaller and more complex the item, the lower the transfer efficiency. Also, the greater the distance



of the spray gun from the item and the greater the

Transfer Efficiency

atomizing pressure, the lower the transfer efficiency.

Paint transfer efficiency is the percent of mass or

The relative order of transfer efficiencies of different

volume of solid coating transferred from a container

types of application equipment (from high to low) is:

to a surface being coated. Transfer efficiency (%) = Mass of solid coating on item x 100

Mass of solid coating consumed

or Transfer efficiency (%) = Volume of solid coating on item x 100

Volume of solid coating consumed

•

Manual (brush or roller)

•

Electrostatic spray

•

High-volume, low-pressure (HVLP) spray

•

Air-assisted airless spray

•

Airless spray

•

Conventional air spray

Two other specialized coating application techniques

The calculations would work the same way if grams

that will be discussed are powder coating and

were kilograms or pounds, gallons were imperial

thermal spray metal­lizing. Powder coating transfer

gallons or liters:

efficiency is very high, and that of thermal spraying

(a) In coating a steel item, of the 1,000 grams of

can be relatively high, although its application rate is relatively low.

coating consumed, only 750 grams were actually applied to the steel surface.

Transfer efficiency = 750 x 100 = 75%



1,000

C1 Fundamentals of Protective Coatings for Industrial Structures 4-2

Unit 4 - Application of Coatings

Coverage Rates

ings. Brushing is not a good method for applying lacquers because of their fast evaporation,

Transfer efficiency is one of the factors affecting

or inorganic zinc coatings because the heavi-

coverage rates. Theoretically, one (US) gallon of paint

er zinc particles settle out if the product is not

will cover approximately 1,600 square feet at 1 mil

contin­uously mixed.

(25 µm) thickness. In reality, each gallon may cover a fraction of such a surface area because the paint

Because it is relatively slow, brushing is used primarily

usually has less than 100% solids. For instance, a

for striping (extra coating of sharp edges), touch-up,

paint with 75% solids will cover only 1,200 mil sq. ft.

small areas (e.g., trim), complex configurations, or where overspray may be a serious problem. It does

Also, if 2 mils of dry film thickness are achieved, the

not produce a very uniform film thickness.

spreading rate is further reduced by a factor of 2 to 600 mil sq. ft. The equation for obtaining theoretic

Natural bristles of good quality are usually preferred

spreading rate is:

for paint brushes. Chinese hog bristles are especially

Theoretical Spread Rate = 1600 x % Solids (in decimal form)

fine because they (1) are durable and resistant and

of a (US) gallon in sq. ft.

(2) have split, or “flagged,” ends that carry more paint

Dry Film Thickness in Mils

and leave finer brush marks. Other bristles may be Within the metric system, one liter of paint will cover

artificially flagged. Synthetic filaments resistant to

- in theory - 10 square meters at a thickness of 100

strong solvents may also be satisfactory. Nylon and

µm, or 1,000 sq. m per micron DFT.

polyester filaments are more water-resistant than natural fibers and so are preferred for applying latex

Theoretical Spread Rate = 1000 x % Solids (in decimal form) of a liter (coating) in m2

paints.

Dry Film Thickness in µm

Brush application may leave brush marks in paints

Since transfer efficiency is never 100%, the theoretical

that do not level well, resulting in areas of reduced

spreading rate must be reduced by an estimated factor

thickness. Thus, additional coats should be applied at

(e.g., 15%) to obtain a practical spreading rate.

right angles to the previous coat to minimize overlap of brush marks and thus thin places.

4.3 Application of Coatings by Brush Brushing is an effective, simple method of coat-

Some tips for optimum application of paint by brushing

ing application. It is particularly good for priming,

are given below.

since it works the

•

paint into surface

spinning the brush between the palms of the

irregularities. It is

hand.

also an especially good method for apply­ing oil and water­b orne coat-

Shake loose any unattached bristles by

•

Remove any stray bristles with a putty knife.

•

Dip the brush to cover one-half the bristle length. Too much paint may wet the heel of the brush and run down the handle.

Figure 4-1: Brushes

C1 Fundamentals of Protective Coatings for Industrial Structures 4-3

Unit 4 - Application of Coatings

• • •

•

Remove excess paint by tapping the brush on

inches [30 mm]) are used to coat chainlink fences.

the edge of the can.

Handle extensions 10 feet (3 m) or longer allow the

To minimize brush marks, use a light touch with

painter to reach high areas, usually at a reduction in

the bristle tips rather than pressing down hard.

application quality.

To minimize lap marks, begin at a natural boundary and work from the wet paint to a dry

A roller cannot penetrate pores, cracks, or other

surface.

surface irregularities as well as a brush. Also, rolling

Hold the brush at a 75-degree angle to the

can mix air into the paint to form holidays, which allow

surface.

moisture to penetrate the cured film.

4.4 Application of Coatings by Roller

Because rolling can produce stipple marks, multiple coats are best applied at right angles to each other.

A roller is best used on large, flat areas that do not require the smooth surface or uniform film thickness received from spraying. It also works well in interior

Some tips for optimum roller application are:

areas where overspray may present a mask­ing or

•

Choose proper fabric, size, and nap cover.

cleaning problem. It is best suited for applying oil-

•

If a tray is used, fill it one-half full of mixed

and water-borne coatings. It is not a proper method

paint; if a grid or screen is used, such as for

for applying lacquers and inorganic zinc coatings, as

heavy-bodied coatings, place it in the can at an

it is not for brushing.

angle. •

Load the roller uniformly and properly for even application at the desired thickness. Too little or too much paint may cause tracking or skidding.

•

Do not apply heavy pressure to the roller.

•

On vertical surfaces, apply the first stroke upward

•

Always finish in one direction for a uniform appearance.

Figure 4-2: Application by Roller

•

Always start and stop at natural boundaries.

4.5 Spray Application

A paint roller consists of a cylindrical sleeve or cover that slips over a rotatable cage with an attached

Spraying is a good application system for almost

handle. Rollers vary in width of pass (from 1–18

all types of coatings, and the usual choice for most

inches [25–450 mm]) and in the nap of the lambswool,

industrial coatings. It is good for high- solids and zinc-

mohair, or syn­thetic fiber cover. The nap fiber length

rich coatings, which are difficult to apply successfully

normally varies between 1/4 and 3/4 inch (6 mm–18

by brush or roller.

mm). A longer nap holds more paint but does not give as smooth a finish. A roller is best used on rough or irregular surfaces. Rollers with extra-long naps (11/4

C1 Fundamentals of Protective Coatings for Industrial Structures 4-4

Unit 4 - Application of Coatings

Many variations of spray equipment and procedures

The basic parts of conventional air spray equipment

have been developed in recent years to meet changing

are:

requirements. Each will be discussed.

•

Air compressor

•

Paint tank (pressure pot)

•

Hoses for air and fluid

•

Spray gun

Conventional Air Spray Conventional air spray was the first paint spray method used and is still the one most frequently used

An air compressor powers conventional air spray

today. Compressed air atom­izes the paint outside

equipment. It must continuously supply adequate

the gun tip and propels it to the object being painted.

air pressure for paint atomization and uniform flow

Paint is fed to the gun tip by siphon or pressure.

of paint to the spray gun. Air pressure and flow are

Conventional air spray features the finest atomization

directly related to each other. If one drops, so does

and finish and the greatest versatility, but also has the

the other. The air flow, measured in cubic feet per

greatest overspray.

minute (cfm) [or litres per minute – 300 cfm = 8,500 lpm = 25.5 m3/min], must be great enough that the pressure does not drop during triggering, resulting in short bursts rather than a continuous stream of atomized paint. Pulsation indicates an inadequate air supply. The air supply must also provide power for agitators or other accessories. Data sheets from paint suppliers provide information on recommended spraying pressures (and thus, necessary compressor capacities), air caps, fluid tips, etc. Oil or water in the air supply must be removed by separator or extractor

Figure 4-3: Conventional Spray Gun

attachments to provide clean air.

This equipment is often not well suited for application

The tank (pressure pot) holding the paint material has

of modern industrial high-build coatings, which may

two regulators to control the air and fluid pressures.

have to be significantly thinned in order to be used in

Some tanks have agi­tators for continuously mixing

conventional air spray equipment. The act of thinning

the paint to prevent settling of heavy pigments. As

may cause many problems for the film formation, and

far as possible, the tank should be close to the work

tends to reduce the high build quality that was sought

to prevent settling of the pigment in long hoses.

by the specifier. It may also result in exceeding VOC limits. Heating can sometimes be used to reduce

The air hose carries the compressed air, and the

coating viscosity.

fluid hose carries the paint to the gun. An air hose with too small an inside diameter (ID) may cause the airline pressure to drop, and thus starve the gun. As with blasting hoses, frictional losses of pressure

C1 Fundamentals of Protective Coatings for Industrial Structures 4-5

Unit 4 - Application of Coatings

can be re­duced by using large-diameter, short hoses

Coating manufacturers usually provide guides for fluid

and avoiding kinking or compres­sing them. A large

and air pressure application of their coatings. The

diameter may be necessary on a long hose to main­

procedure usually used for adjusting the fluid and

tain adequate air pressure. The ID of the hose from

air pressures, after all connections have been made

the com­pressor to the pressure tank is usually at least

and checked, is:

/8 inch (9 mm), and that from the pressure pot to the

3

gun at least 5/16 inch (8 mm).

•

Close air valve from pot to gun.

•

Open the main air valve from compressor to pot.

The ID of the fluid hose is determined by the necessary

•

Set regulators to recommended pressures.

volume and pressure of the particular paint in the gun.

•

Remove air cap.

Heavy materials may require as large an ID as /2 or

•

Trigger gun into waste container and increase

1

/4 inch (13 or 19 mm). Small guns often use a /4 inch

fluid pressure until a horizontal stream of

(6 mm) ID fluid hose. The hoses must be resistant to

coating about 3 feet (1 meter) long before

the paints and solvents that flow through them.

arcing into the container is obtained.

3

1

•

Replace air cap and start testing of spray

Conventional spray guns are complex, requiring good

pattern by slowly increasing the atomizing air

applicator skills. Supplier instructions provide much

pressure to form a proper spray pattern.

information on their operation. The gun’s ten basic parts are:

Faulty spray patterns may result from plugged

•

Air nozzle or cap

horn holes in the air cap or improper air or material

•

Fluid nozzle or tip

•

Fluid needle

•

Trigger

Advantages

•

Fluid adjustment valve

•

Finest atomization/finish

•

Air valve

•

Good operator control/versatility

•

Side port control

•

Low initial investment

•

Gun body and handle

•

•

Air inlet

Disadvantages

•

Fluid inlet

pressures.

•

Lower transfer efficiency

•

Lower application rate

•

Produces overspray

•

Viscous materials may present problem

Figure 4-4: Conventional Spray Cap

C1 Fundamentals of Protective Coatings for Industrial Structures 4-6

Unit 4 - Application of Coatings

Airless Spray

the pressure increased until the desired pattern is obtained. Coating viscosity can be reduced by careful

Airless spray atomization is produced by forcing

thinning or heating of the coating.

the paint through a small orifice by high hydraulic pressure, typically 1,500 to 3,500 psi (10 to 24 MPa). It

The fluid hose must be able to withstand the very high

features a greater production rate and, consequently,

pressures necessary to deliver the paint to the gun

a greater econ­omy than conventional air spraying.

and atomize it. While pres­sures of 1,500 to 3,500 psi

Note: See Glossary for definition of MPa.

(10 to 24 MPa) are commonly used, most air hoses can handle pressures as high as 5,000 psi (35 MPa).

The basic parts of airless spray equipment are: •

High pressure hydraulic pump

•

Coating container

•

High-pressure hose

•

Airless spray gun

A 1/8- to 1/4-inch (3–6 mm) ID hose is commonly used for medi­um viscosity paints, and a 3/8- to 1/2-inch (9–12 mm) ID hose for high-viscosity paints. The pump used to pressurize the paint atomization is rated according to the ratio of paint (fluid) pressure

The airless spray gun is

produced to the air pressure that pro­duced it. Thus, a

basically a fluid nozzle with

pump that delivers a paint pressure of 30 psi for each

a valve. The gun is either on

psi of air pressure from the compressor has a 30:1

or off, with no intermediate flow controls. A spray tip

ratio. There must be sufficient pressure and material

Figure 4-5: Airless Spray

Gun

(fluid) flow to produce a constant spray of paint.

filter screens out particles that might otherwise clog the tip. Each spray tip has

The standoff distance from the gun to the substrate

an orifice designed for a particular spray pattern;

is typically 12 inches (30 cm) and may be adjusted

changing tips is the only way to change the spray

to obtain the desired wet film thickness.

pattern. Insufficient application pressure will result in formation The orifice size controls the atomization and the

of tails (fingers), an incomplete fan with coating

amount of fluid delivered. The tip angle (10 to 80

concentrated at the top and bottom. Increasing the

degrees) controls the fan width. Tips with the same

pressure will resolve this problem.

orifice size but different angles will deliver the same amount of paint at different fan widths and

Airless spray equipment may be powered with

thicknesses. A large spray pattern is required for a

electricity, petroleum spirit (gasoline), or compressed

high application rate. Paint viscosity is the chief factor

air.

in selecting a particular tip.

Advantages

The size and shape of the orifice in the tip will

•

High application rate

determine the fan size and shape; the larger the orifice

•

Applies high-viscosity materials well

size, the greater the fluid flow. High viscosity coatings

•

Reduced overspray fog

require larger orifices. The fluid flow is initiated and

•

Better transfer efficiency on large surfaces

C1 Fundamentals of Protective Coatings for Industrial Structures 4-7

Unit 4 - Application of Coatings

Disadvantages

Advantages

•

Hazardous spray pressures

•

Good transfer efficiency

•

Reduced operator control

•

Reduced overspray and bounceback

•

Reduced-quality finish

•

Good with high-solids coatings

•

Expensive to maintain

•

Good gun control

Air-Assisted Airless Spray

Disadvantages

Air-assisted airless spray atomizes paint by a

•

High initial/maintenance costs

•

May require special training

•

Reduced application rate

combination of hydraul­ic and air pressures, typically 500 to 1,000 psi (3.5 to 7 MPa) fluid pressure and 10 to 15 psi (0.07– 0.1 MPa) air pressure, respectively. The air-assist results in finer droplets than are achieved

Electrostatic Spray

from normal airless spraying. It also permits reduction

Electrostatic spray permits the application of coatings

of the hydraulic pressure by 50 percent or more, and

to conductive surfaces.

thus gives the applicator a higher transfer efficiency and more control. It has several advantages over

Nonconductive surfaces (e.g., wood, plastics,

normal airless spraying.

and composites) may receive a surface treatment or coating to render them sufficiently conduc­tive

Advantages

to permit electrostatic spraying. Potentially, all

•

Finer atomization

coatings can be electrostatically sprayed, but some

•

Fewer runs and sags

formulations must first be modi­fied to improve their

•

Good transfer efficiency

electrical properties.

•

Better operator control

•

Able to produce finer finishes

Electrostatic spray can be used for all the previously mentioned methods of spray application. A charged

Disadvantage

probe is most commonly used for external ionization

•

of the atomized paint, which is attracted to a

Expensive to maintain

ground­ed conductive surface. It virtually eliminates

High-Volume, Low-Pressure Spray

overspray, as compared to conven­tional spraying.

High-volume, low-pressure spray atomizes paint at low

trained personnel.

The equipment presents no safety hazard to properly

air pressure and then utilizes a high-volume air supply to propel the droplets at a low velocity. It reduces

Electrostatic spray has the following characteristics:

VOCs by imparting good transfer efficiency.

Advantages •

Wraparound of edges

•

High transfer efficiency

C1 Fundamentals of Protective Coatings for Industrial Structures 4-8

Unit 4 - Application of Coatings

•

More uniform paint application

In other plural-component systems (e.g., polyurea

•

Material savings

delivery systems) the two components are delivered and mixed directly at the gun head. The relatively small amount of mixed components that remain after

Disadvantages •

High initial/maintenance costs

•

More suited to automation

•

Requires skilled operator

•

Safety precautions required

•

Limited to one coat

•

Limited to exterior surfaces

•

Requires conductive substrate

interruption of the work is removed mechanically from the gun. Recirculation of each of the heated components in insulated lines prior to combining may be necessary to achieve temperatures that reduce their viscosities to sprayable consistencies. In these cases, both mixing ratios and temperatures must be carefully monitored

Plural-Component Spray Systems

during application. Although heating of components

Plural-component spray systems proportion the

problem, because the components are sprayed as

components of two-component, chemically-curing very

they are mixed.

will significantly reduce pot life, this presents no

high-solids coatings (e.g., epoxies and polyurethanes) in the volume ratios specified by their coating

Advantages

manufacturers and deliver them to the spray gun (usually airless) for mixing and application. In some plural-component systems, the components are combined in a manifold and mixed in an in-line

•

No pot life problems

•

Good for high-viscosity materials

•

Faster cure times possible

•

Conservation of materials

•

static mixer prior to actual delivery to the gun. In

Disadvantages

these systems, a solvent hose is incorporated into the system to purge the residual mixed components from the system whenever spraying is interrupted.

•

High initial/maintenance costs

•

Proportioning/temperature controls required

•

Requires skilled operator

•

Impractical for small jobs

• 4.6 Application of Coatings that Cure by Fusion There are two relatively common types of protective coatings that require heat for their fusion to a protective film. These are organic powder coatings and thermal spray metallizing. Figure 4-6: Plural Component Equipment

C1 Fundamentals of Protective Coatings for Industrial Structures 4-9

Unit 4 - Application of Coatings

Powder Coatings

•

Coating enclosed surfaces is difficult due to Faraday cage effect

Powder coatings are finely divided solid coatings

•

that are applied to metal surfaces as dry particles,

Changing powder colors is time consuming when the overspray is to be collected, cleaned,

melted in an oven, and then cooled to form a solid

and reused

protective film.

•

Powder suspensions in air are potentially explosive

Powder coatings are dry rather than liquid products. Each particle contains the resin, pigment, modifiers,

Types of Powder Coating

and, if chemically curing, the curing agent, to form a solid film after melting and subsequent cooling.

Powder coating resins may be thermoplastic (simply

A blocking agent may be added to thermosetting

undergo melting and re-solidifying) or thermosetting

coatings to control their curing reaction during

(undergo chemical curing). They have a wide range

storage.

of chemical and physical properties.

Although powder coatings are widely used to coat

Thermoplastic Powder Coatings

consumer products (e.g., general metal finishing and

Thermoplastic powder coatings merely melt and flow

appliances), this module is limited to their use as

out when heated. They remain the same chemical

industrial coatings (e.g., oil and gas piping, automotive

composition and can be re-melted. These powders

parts, ship components, and rebar) for which they

require relatively high temperatures (194-240°F

have many uses.

[90-116°C]) for melting and flowing. Their solidified films are hard and difficult to grind to the fine particle

Powder coatings have several advantages over

sizes required for spraying thin films. Thus, they are

liquid coatings. Powder coatings also have several

commonly applied at greater film thicknesses, e.g.,

limitations.

10 or more mils (250 or more µm), than thermosetting

Advantages

coatings. The melted powders tend to be viscous and

•

have poor leveling, flow, and adhesion properties.

Good transfer efficiency resulting in reduced

Because of these limitations, they are not used as

coating consumption and waste generation •

No fire or toxicity hazards from organic solvents

•

Easy one-coat thick film application, even on

much as thermosetting powder coatings. They are usually applied by the fluidized bed method described later.

edges •

No viscosity adjustment requirements

•

Fast cure, quick turnaround

Commonly used industrial thermoplastic powders include:

Disadvantages •

•

and toughness but poor solvent resistance

Application is limited to shops with ovens and •

controlled environments •

Polyvinyl chloride - good chemical resistance Polyethylene and Polypropylene- resistant to mechanical damage but poor adhesion

Thin films are difficult to apply

C1 Fundamentals of Protective Coatings for Industrial Structures 4-10

Unit 4 - Application of Coatings

Thermosetting Powder Coatings

Application of Powder Coatings

Thermosetting powder coatings undergo chemical

Because powder coatings are sensitive to heat

reaction during heating and melting. After this

and humidity, special attention should be made to

chemical change, they cannot be re-melted by

controlling the environmental conditions in the storage

heating. Thermosetting coatings tend to have better

and application rooms. The temperature should be

chemical and physical properties than thermoplastic

below 80ºF for application. For corona application,

coatings. As with all thermosetting coatings, their

a relative humidity of 50 to 60% is desired. For

properties vary with their resins, curing agents,

tribo application, a relative humidity of 40 to 50% is

crosslink densities, etc. Their film build may range

preferred.

from 1 to 10 mils (25 to 250 µm). There are two basic methods by which powdered There are some advantages to using thermosetting

coatings are applied, electrostatic spray and fluidized

coatings over thermoplastic powder coatings.

bed. The electrostatic spray system is by far the more commonly used. Each of these application systems

Advantages •

requires skilled applicators.

Good adhesion (thermoplastic coatings often require priming)

Electrostatic Spray Guns

•

A greater range of colors and glosses available

•

Available in textured as well as smooth finishes

The two most commonly used methods of charging the

•

Available in thin (e.g., 2 to 3 mils [50 to 75 µm])

sprayed powder particles are using corona (external)

as well as thick films

charging and tribo (internal) charging guns. 1. Corona-Charging Guns

Commonly used industrial thermosetting powder

In a typical corona-charging gun, a high-voltage

coatings include: •

•

power supply and an electrode (ionizing needle)

Epoxy (also called “fusion-bonded epoxy” or

at the front of the gun is used to generate a high-

FBE). Good adhesion and chemical resistance

voltage, low-amperage electrical field (40 to 100

but chalk in sunlight

kV) that produces a cloud of charged particles

TGIC Polyesters. Good resistance to sunlight,

(a corona) in the air outside of the gun. Powder

so often used to topcoat FBEs

passing through the corona picks up a charge and is attracted to the grounded, conductive work piece to be coated.

Powder Coating Process The process of powder coating industrial work pieces

2.

consists of three basic steps:

Tribo-Charging Guns The tribo-charging electrostatic gun is designed to

•

Abrasive blast cleaning

impart an electric charge to powder particles from

•

Powder application

friction that is produced when they rub against

•

Oven curing, followed by cooling

a solid insulator (usually Teflon) inside the gun.

C1 Fundamentals of Protective Coatings for Industrial Structures 4-11

Unit 4 - Application of Coatings

Because these guns don’t produce an external

floors, hangers, and other support equipment in the

electric field (corona), the Faraday cage effect

spray booth or area should be regularly cleaned and

is reduced.

properly maintained.

Fluidized Bed Application

Thermal Spray Metallizing

The fluidized bed application system for powder

Thermal spray metallizing is a process in which

coating is much older than the electrostatic spray

metal wire or powder is melted and sprayed onto a

application system and so is used much less today.

metal surface, usually steel, where it cools to form a

It utilizes a dip tank with a porous bottom plate. Air is

protective coating. The steel surface must be very

passed through this plate to suspend the powder into

clean (SP 5/Sa 3) and have a deep surface profile.

the air in the tank. The item to be coated is heated to

The most commonly used alloys for corrosion control

the specified temperature and then hung in the tank

are zinc, pure aluminum, or an 85 percent zinc, 15

so that the suspended particles fuse to it. The gradual

percent aluminum alloy, the most common methods of

increase of coating thickness gradually builds up its

application are flame, electric arc, and plasma spray.

thermal insulation, so that later powder particles are

The coating material is available in powder or wire

less likely to stick to it. The coated item is then heated

form, with wire used most frequently. It forms a porous coating that protects steel by cathodic protection in a variety of environ­ments.

in an oven to complete the curing process. There is no overspray because of the containment,

Thermal spray metallizing is initially relatively porous.

so that there is a high recovery rate for the unused

Thus, it is often sealed with a low-viscosity sealer

powder. This variation has the advantages of being

(e.g., epoxy or silicone) and topcoated to provide longer protection, as well as an attractive finish.

able to coat only one side of a flat plate and eliminating the pre-heating of the item to be coated.

Thermal spraying of metals is best accomplished

Powder Booth

in the controlled environment of a shop, but can be

Powder booths are enclosed areas that contain the

accomplished in the field. Military Standard DOD-

electrostatic powder system and allow work pieces

STD-2138 (SH), Metal Spray Coatings for Corrosion

to enter one end and leave the other. They are used

Protection Aboard Navy Surface Ships (Metric),

for both automated and manual powder applications.

describes the wire-flame spraying of aluminum using

The booths serve both to contain and to collect

oxygen-fuel gas. SSPC-CS 23.00/AWS C2.23M/

overspray, so they must be kept clean and free of

NACE No. 12 Specification for the Application of

contamination. Containment within the booth is

Thermal Spray Coatings (Metallizing) of Aluminum,

accomplished with a fan that draws air from the booth

Zinc, and Their Alloys and Composites for the

opening into the collection area. After collection of the

Corrosion Protection of Steel describes thermal spray

powder by cartridge or cyclone collectors, the air is

metallic coating systems.

returned to the room. The air flow must not interfere with the powder spray pattern. The ceiling, walls,

C1 Fundamentals of Protective Coatings for Industrial Structures 4-12

Unit 4 - Application of Coatings

4.7 Handling of Paints

Long-term storage should be in a room or building isolated from other work areas and kept at the storage

Storage

temperature recommended per the manufacturer’s

Proper storage of coatings will (1) minimize fire

product data sheet (PDS). It should be dry, well-

hazards and (2) protect them from premature

ventilated, and protected from sunlight, sparks, and

deterioration before use. Safety requirements for paint

flames. Shelves or pallets should be used to keep

storage include:

cans away from the dampness of floors. Labels

•

should be kept intact and free of paint to permit easy

Store in Underwriters’ Laboratory (UL)-listed

identification. Material safety data sheets (MSDSs)

containers. •

should be available for each coating and solvent

No smoking or sources of ignition in storage

stored, as required by law.

area. •

Ground containers during transfer of liquids.

•

Have spill-absorbent materials (kits) available.

Coatings stock should be rotated and used so that old materials are consumed first. No coating should be used more than one year after manufacture or after

Indoor Storage •

its recommended shelf life has expired, unless it has

No more than 25 gallons (95 liters) in a room

been checked for quality.

except in an approved storage cabinet meeting 29 CFR 1926.152(b)(2)(i) requirements •

Shelf life is the maximum amount of time from

No more than 60 gallons (230 liters is close

manufacturer that a paint or coating can be stored in

enough to 227) of flammables or 120 gallons (450

a usable condition. Old, unusable coatings constitute

liters) of combusti­bles in any cabinet; no more

hazardous waste that is very expen­sive to properly

than three cabinets in a storage area

dispose. Once opened, the contents of a can should be used before opening another can of the same

Outdoor Storage •

•

coating.

No more than 1,100 gallons (4,200 liters) in one container pile or area; no more than 60 gallons

No more coating should be taken to the job site than

(230 liters) in each container

is actually required. Coatings should also be protected

Piles or groups of flammables no closer than 20

from the weather. Water-borne coatings can freeze in

feet (6 metres) from a building

cold weather, which will damage them permanently. It is a good practice to have a mixture of 1- and 5-

Fire Extinguishers •

gallon (5- and 20-liter) kits of two-component coatings

At least one fire extinguisher (rated not less

(e.g., epoxies and polyurethanes) because:

than 20-B units) must be located between 25

•

ft. and 75 ft. (8m and 24m) from a flammable

Unused mixed components cannot be stored overnight

liquid storage area, and not in the storage area

•

itself.

Estimation of proper ratios of thermosetting components from partially used containers is imprecise

C1 Fundamentals of Protective Coatings for Industrial Structures 4-13

Unit 4 - Application of Coatings

•

Opened and partially used coatings have a

Coating Mixing Step 2: Blending the

reduced shelf life

Components and Mixing the Blended Products Most coating manufacturers and/or the project

Mixing

specifications prohibit the blending of partial kits.

One of the most important steps in the successful

In fact most manufacturers require that complete

installation of a protective coating system is the proper

kits be mixed. This is for good reason. First, many

mixing or blending of the coating materials, prior

coating components may be ratioed by weight, not by

to application. The procedures for blending single

volume. So in order to mix partial kits, the contractor

component materials is usually straight forward and

would need to accurately weigh out a portion of each

needs little instruction other than ensuring the material

component prior to blending. In addition, there may

is homogeneous by mixing of the settled pigment and

be a critical minimum volume of material that must

solid materials into the liquid. Conversely, the mixing

be blended so that the chemical curing reaction

procedures for multiple component materials can

between the components proceeds to completion.

be more complex, and often requires the individual

This information is typically not published, but is no

responsible for mixing the coatings to read and

less critical. Many coatings are available in both

comprehend the product data sheets. Overmixing

larger and smaller kits. The contractor may choose

should be avoided because it entraps air in the

to have a few smaller kits on hand, although smaller

coating.

kits may be more costly. Excess mixed components cannot be saved and must be disposed of properly.

Coating Mixing Step 1: Agitating the Individual

Mixing instructions are provided on the manufacturer’s

Components

product data sheet should always be followed.

Before combining the individual components of the coating (if multi-component), the individual liquid

The Bottom Line

components must be thoroughly mixed. Power

Unless partial kit mixing is permitted by the

agitation using shear-type mixing blade such as a “Jiffy

specification and the coating manufacturer (and the

Mixer” mounted into a pneumatically-operated drill or

ratio of components is published), the contractor

stirrer is often required. Hand stirring using wooden

should always mix complete kits. The coating

paddles may be acceptable for house paints, but

manufacturer pre-measures each component to

is inadequate for

ensure the proper ratio is achieved when complete kits

mixing industrial

are used. Even if partial kit mixing is permitted and

coatings and is

the ratio is known, the contractor must have graduated

often prohibited.

containers for blending by volume or weighing devices

Zinc-rich coatings

(scales) for blending by weight before partial mixing

require special

should even be attempted.

mixing procedures described in their product sheets.

data

Once the components are combined into one container with stirring, they must be thoroughly

Figure 4-7: Blending

blended by power-driven agitation using a shear-type

C1 Fundamentals of Protective Coatings for Industrial Structures 4-14

Unit 4 - Application of Coatings

mixing blade as described earlier. Alternatively, the

reverse this procedure (empty the liquid into the zinc

materials can be blended by “boxing,” which is done

powder). After the zinc powder is thoroughly blended

by pouring the liquid material into a clean container

into the liquid components, the mix should be strained

and stirring up any settled materials. Then, combine

through a fine mesh screen to remove any un-wetted

the two portions and pout it back and forth, from

zinc particles that may clog the spray tip. Straining of

container to container until the material is completely

other coatings (non-zinc-rich) is usually not necessary

blended. This procedure is typically prohibited when

unless required by the manufacturer.

mixing inorganic zinc-rich primers, moisture cured urethanes or other coating materials that react with

Coating Mixing Step 3: Measure the Coating

moisture. Remember, once the coating material is

Temperature

blended, the “pot life clock” begins.

After the components are blended and strained (if required), the temperature of the coating should be

Some manufacturers provide “touch-up” kits or

measured. As a general rule, the warmer the coating,

cartridges, which are designed to patch or repair

the shorter the induction time and/or the pot life, since

damaged coating, without having to mix a large

heat increases the rate of the curing reaction.

kit. These cartridges are typically supplied in twocomponent tubes. A plunger pushes the components

Induction time (also called sweat-in-time) is the time

through a built-in static mixer, which blends the

to allow the reaction to reach a certain stage before

components in a 1:1 ratio. Once the blended

application of the coating. Induction times are usually

components exit the static mixer, the coating is applied

short (e.g., 30 minutes) but are longer with lower

to the damaged area using a brush.

coating temperatures.

Mixing Zinc Primers

Pot life is the time period during which the mixed

Zinc-rich primers containing a separate zinc powder

coating can be successfully applied.

component require the special mixing procedures provided by the manufacturer. After blending the liquid

Coating Mixing Step 4: Determine Whether Pot

component(s), the zinc powder is slowly sifted into

Agitation During Application is Required

the liquid while under agitation. It is important not to

Because zinc is a dense pigment, it may settle out of the blended material during application. In this case, the original mix ratio of the product becomes distorted, and much of the protection afforded by the primer is lying on the bottom of the pot or can. To prevent settling out of the zinc, most coating manufacturers require automated, constant agitation of the mixed material during application. In fact many spray systems are equipped with motor driven agitation

Figure 4-8: Straining

blades. Note however that many single component,

C1 Fundamentals of Protective Coatings for Industrial Structures 4-15

Unit 4 - Application of Coatings

moisture cured zinc-rich primers contain anti-settling

to maintain the volatile organic compound threshold

agents and do not require the pot agitation during

established by the Federal and/or local air quality

spray-out. In fact, agitation can draw airborne

regulations (which may be more restrictive than the

moisture into the coating, causing it to gel. Therefore,

Federal limit). Therefore, prior to adding thinner,

pot agitation is not recommended for these products,

the contractor must determine whether thinning is

even though they contain zinc pigment. The product

permitted. Some manufacturers prohibit thinner

data sheet will often provide guidance as to whether

addition, and claim that the coatings can be applied as

or not pot agitation during application is required.

supplied. Conversely, while the coating manufacturer may allow the coating to be thinned, the specification may prohibit the use of thinner. Heat may also be

Thinning (Reducing) Mixed Coatings

used to reduce coating viscosity, but may affect pot

Similar to mixing, thin-

life.

ning of a coating material is perceived to be

Determine the Type and the Amount of Thinner

rather straightforward

to Add

and requires little explanation. However,

Once it is determined that thinner addition is permitted

the type and amount

and necessary, the next step is to determine the

of thinner added to the coating impacts

type and amount of thinner to add. This information

Figure 4-9: Thinning

can be gleaned from the manufacturer’s product

the volatile organic

data sheet. The type of thinner to add may be

compound, (VOC)

dependent on the air temperature during application.

content, the target wet film thickness of the coating,

Manufacturers may have a slow evaporating thinner

and over thinning or under thinning a coating can

for warmer temperatures and a faster one for colder

adversely affect the application and the performance

temperatures. Only the manufacturer’s recommended

characteristics. Therefore, thinning of a coating is an

thinner should be used. In fact, use of thinners other

important area to discuss and is equally important to

than those recommended can void the warranty

verify that it is done properly.

on the coating, and can result in reduced coating system performance. Thinners added to coatings should be clean, new, and in their original containers.

Determine Whether Thinning is Permitted

Contaminated, recycled or used thinners should never

Fairly restrictive air quality regulations have caused

be used for reducing coatings.

manufacturers to reformulate coating materials with less solvent (higher solids content) and even different

The amount of thinner to add is also based on the

solvents. To this end, field addition of thinner for the

application temperatures, the method of application

purposes of reducing the viscosity of the coating

and the local air quality regulations. Most coating

material may be restricted or even prohibited. If

manufacturers will indicate the VOC content of the

thinning is allowed, there is often a maximum type

coating material as formulated, and the adjusted VOC

and amount of thinner that can be added in order

content based on the amount of thinner added. Any

C1 Fundamentals of Protective Coatings for Industrial Structures 4-16

Unit 4 - Application of Coatings

regulated thinner that is added to the coating in the

and moisture-cured polyurethanes re­quire minimum

field will increase the VOC content. Conversely, the

relative humidities for proper curing (e.g., 30 per­

use of a non-regulated solvent (e.g., water) will not

cent).

increase the VOC content of the coating. But there is still a maximum amount that can be added to maintain

Dew point is the temperature at or below which

sag resistance and performance of the coating while

moisture will condense from the air. If a surface is

in service.

too close in temperature to the dew point, water may condense on it, causing poor adhesion of the applied

Tinting

coating or other defects.

Paints should be obtained in the desired colors. Tints

Too high a humidity can also slow drying of water

should be added only after paints are thoroughly

emulsion coatings.

mixed. The manufacturer’s instructions should be followed, since not all tints are compatible with all

Measurement of temperatures and humidities is

coatings.

discussed in Unit 5.

Different tints are often used for adjacent coats of a multiple-coat system. If this is done, skips in the

4.9 Achieving Desired Film Thickness

topcoat become very apparent.

The film thickness of a coating is extremely important. Too thin a film may provide inadequate protection,

4.8 Application Temperatures and Humidities

and too thick a film may reduce flexibility significantly, cause wrinkling or incomplete curing, or otherwise

Coatings must be applied only within the temperature

produce adverse effects. The manufacturer’s

and humidity ranges recommended by the

recommenda­tions for coating thickness must always

manufacturer. Oil-based coatings are normally

be followed. Should there be a discrepancy between

applied above 40°F (5°C); water emulsion coatings

the manufacturer’s recommended thickness and

and chemically curing epoxies are applied above 50°F

that re­quired by the specification, the matter should

(10°C). The coat­ing temperature should be brought

be resolved in writing before coat­ing operations are

to that of the substrate.

started.

In order to prevent condensation of moisture from

Before beginning a painting operation, a small

the air onto wet paint, the paint should be applied

patch of the paint should be applied to establish

when the temperature of the substrate is at least

that a continuous film is being properly applied at

5°F (3°C) above the dew point and not falling. Also,

the specified film thickness. Specifications always

coating should never be ap­plied when the prevailing

require a minimum dry film thickness or an acceptable

wind is blowing 15 mph (22 kph) or greater, or when

thickness range. Because dry film thickness can be

the temperature is expected to fall below freezing

measured only after curing of coating films, a different

before the coating is fully cured. Many inorganic zinc

technique must be used at the start of painting to

C1 Fundamentals of Protective Coatings for Industrial Structures 4-17

Unit 4 - Application of Coatings

4.10 Striping

ensure that sufficient dry film thickness will result. This technique is to measure the wet film thickness of the

A stripe coat is a coat of paint applied only to edges

coating immediately after application and calculate

or to welds on steel structures before or after a full

the expected dry film thickness from the following

coating is applied to the entire surface. Because

relation­ship (for unthinned coatings):

coatings pull back from edges, which reduces the film thickness there, striping provides additional film

Wet Film Thickness (WFT) = Dry Film Thickness x 100

thickness for barrier protection of the steel. SSPC-PA

Percent solids by volume

1 recommends: Dry Film Thickness (DFT) = WFT x percent solids by volume

•

100

bolts, rivets, and welds

The percent solids by volume is readily available from the manufacturer, who may alternately provide the wet film/dry film relationship. This information is usually provided in the manufacturer’s product data sheet.

•

Extend stripe coat one inch from edge

•

Allow striping to set-to-touch before topcoating

•

Apply before or after topcoating

•

Grinding (rounding) edges improves effectiveness of stripe coat

If a thinner is added to reduce the viscosity, the

SSPC PA Guide 11: Protecting Edges, Crevices and

product will contain a lower percent of solids (i.e., the

Irregular Steel Surfaces by Stripe Coating discusses

total volume will increase, but the volume of solids

the technique called “stripe coating” or “striping” as a

will not). The relationships will change as shown on

way of providing extra corrosion protection measures

the next page.

on edges, outside corners, crevices, bolt heads, welds, and other irregular steel surfaces, including

WFT = DFT x (100 + percent added thinner)

When edge striping, include corners, crevices,

optional surface preparation techniques for sharp

Percent solids by volume

DFT =

WFT x percent solids by volume



(100 + percent added thinner)

edges to improve coating performance. Some details, including the advantages and limitations of specific methods of obtaining additional coating thickness, are described to assist the specification writer in assuring that the project specification will address adequate

The painter or inspector should always determine

corrosion protection.

the wet film thickness of a coating when starting its application, so that necessary modifications to

4.11 Recommended Spraying Procedures

achieve the required dry film thickness can be made at that time. The measurement of wet and dry film

The most commonly recommended aspects of paint

thicknesses is described in Unit 5.

spraying, for both airless and conventional air spray, are described below.

C1 Fundamentals of Protective Coatings for Industrial Structures 4-18

Unit 4 - Application of Coatings

spraying. If the gun is held too close to the surface,

Stroking and Overlapping

excessively heavy application may result in sagging or

The quantity of coating material exiting the spray

running of the paint. If the gun is held too far from the

gun tip varies, due to the atomization air that creates

surface, a dry spray with a sandy finish and holidays

a fan-shaped spray pattern. That is, the amount of

(pinhole or skip areas) may result. This condition

material in the center of the fan is typically greater

should be corrected immediately.

than the amount of material at the ends of the fan pattern. This variation can only be corrected by the

Corners

applicator. Once the applicator makes the first pass (e.g., left to right), the applicator should overlap the

Special actions may be necessary to prevent

previous pass by 50% when making the second pass

excessive buildup of coating in corners, especially

(e.g., right to left), then overlapping that pass when

with airless spray. The surface should be sprayed

making the third pass (left to right again) and so on.

within 1 to 2 inches of the corner; then, the gun should

This overlapping technique is important. It helps

be turned sideways so that both sides of the corner

to ensure an even film build and will improve the

are sprayed at one time.

consistency of coating thickness. Spray Technique Training In order to build an even more consistent film, the

A unique laser device developed and patented by

applicator can alternate between horizontal and

the Iowa Waste Reduction Center (IWRC) attaches

vertical passes, overlapping each type of pass by

to the top of a spray gun and can be used to train

50% as described above. This is called a crosshatch

new applicators or correct bad habits of experienced

spray technique.

sprayers. The applicator attaches the device to the spray gun using the mounting hardware provided with

Triggering

the device, then establishes the correct distance from

The stroke of the spray gun should begin before the

the spray gun tip to the surface (6-8” for conventional

gun is triggered and continue briefly after releasing

and HVLP spray and 12” for airless and air-assisted

the trigger. This produces a smooth, continuous film

airless spray). The applicator then adjusts the laser

without a heavy buildup of paint at the start and end

control knob so

of each stroke. It also helps keep the fluid nozzle or

that a single laser

airless tip clean.

“dot” appears on the surface. If

Distance

the applicator

The amount of material delivered and the atomization

spray gun further

positions the

pressure determine the proper gun-to-surface

away or closer

distance for a uniform, wet film. This is usually about

to the surface

6–12 inches (150–300 mm) for conventional air

(than the pre-set

spraying and 12–15 inches (300-375 mm) for airless

distance), or arcs

Figure 4-10: LaserPaint Applicator Training Device

C1 Fundamentals of Protective Coatings for Industrial Structures 4-19

Unit 4 - Application of Coatings

the spray gun (rather than remaining perpendicular

4.12 Coating Application Defects

to the surface) during actual application, the single

Improper coating application can result in a displeasing

laser dot will double, giving the applicator a visual

appearance or film defects. It is important that painters

signal to correct the spray gun distance/position (i.e.,

be able to recognize the defects at the time they are

re-establish the correct spray gun distance/position

produced so that they can be quickly corrected.

until the laser dots converge). Also, since the laser device is positioned at the same latitude as the spray

4.13 Unit Summary

gun tip, the laser can be used to indicate the location

A coating must be applied properly to provide long-

of the middle of the spray fan, making it easier to

term protection. It may be applied by brush, roller, or a

overlap spray passes by 50%. The device can be

variety of spray equipment. Each system has its own

removed once the applicator perfects the technique.

advantages and limitations. Spraying usually has the

The device was invented with the goal of reducing

best economics, if practical.

rework and paint waste.

Not all coatings can be successfully applied by all

Airless Spray Basics (C-12)

methods of appli­cation. The painter should use the

SSPC has developed a airless spray basics course

equipment, prevailing conditions, and the mixing

that allows the students to train like the astronauts

and application practices recommended by the

and pilots have for years using a computer simulator.

manufacturer.

For the first time in the protective coatings industry, SSPC has designed a program that incorporates paint simulator hands-on training. You’ll learn the proper technique for airless spray painting by using a program that simulates real life situations and equipment used in the field. Simulation training provides instant computerized assessments of applicator transfer efficiency, coating thickness, amount of coating sprayed, and application time so that you can make quick adjustments to improve your practice. An overview of the course content is provided below: •

Introduction/Overview of Airless Spray Equipment Operational Systems

•

Proper Mixing Techniques

•

Proper Spray Techniques

•

Troubleshooting

C1 Fundamentals of Protective Coatings for Industrial Structures 4-20

Unit 4 - Application of Coatings

Unit 4 - Exercise 4A: Paint Calculations 1. An epoxy paint containing 80% solids by volume after mixing is applied at an average of 4 mils dry film thickness. Calculate its theoretical spreading rate.

2. If the transfer efficienty is 80%, calculate the actual spreading rate.

3. At what wet film thickness should the paint be applied to achieve the desired dry film thickness of 4 mils? (See Section 4.9, if necessary)

C1 Fundamentals of Protective Coatings for Industrial Structures 4-21

Unit 4 - Application of Coatings

Unit 4 - Exercise 4B: Paint Application Methods Match the paint application methods listed in Column A with the descriptions in Column B. Column A

Column B

1.

Airless spray

A. Wraps around corners

2.

Brush

B. Good for short pot life coatings

3.

Conventional air spray

C. Best quality spray finish

4.

Electrostatic spray

D. Good for tight configurations

5.

HVLP spray

E. Faster than brushing; no overspray

6.

Plural component system

F. Fast, most commonly used spray

7.

Roller

G. Good transfer efficiency, second only to electrostatic spray

C1 Fundamentals of Protective Coatings for Industrial Structures 4-22

Unit 4 - Application of Coatings

Quiz 1. What coating application method has the greatest production rate? a. conventional air spray b. airless spray c. HVLP spray d. power roller 2. What coating application methods has the highest transfer efficiency? a. conventional air spray b. airless spray c. HVLP spray d. electrostatic spray 3. Theoretical spreading rate is defined as: a. the spreading rate of a professional painter b. the spreading rate of robotic spraying c. the spreading rate on a flat surface if no material losses occurred d. the spreading rate with 10% loss of material 4. The theoretical spreading rate of a solvent-free coating applied at 10 mils dry film thickness is: a. 100 square feet per gallon b. 160 square feet per gallon c. 1,000 square feet per gallon d. 1,600 square feet per gallon 5. A coating containing 50% solids by volume and thinned 10% be applied to achieve a 4 mil dry film thickness. What is the WFT? a. 4.8 mils b. 6.0 mils c. 8.0 mils d. 8.8 mils

C1 Fundamentals of Protective Coatings for Industrial Structures 4-23

Unit 4 - Application of Coatings

6. A limitation of conventional air spraying as compared to other methods of spray application is: a. unable to produce as fine a finish b. requires more applicator skills c. limited operator control d. greater overspray 7. A limitation of airless spraying as compared to other methods of spray application is: a. hazardous high pressures b. poor application rate c. unable to apply viscous materials d. more overspray 8. The chief advantage of HVLP spraying is: a. higher application rate b. higher transfer efficiency c. best method of applying viscous coatings d. adaptability to plural component systems 9. Induction time for thermosettting coatings is defined as: a. time period after mixing and before spraying for reaction of components to receive a start b. time period during which the mixed coating can be applied successfully c. time period for proper extent of curing of the coating before topcoating d. time period during which the coating can normally be stored without significant deterioration 10. Pot life of thermosetting coatings is defined as: a. time period after mixing and before spraying for reaction of components to receive a start b. time period after mixing of components during which the coating can be applied successfully c. time period for proper extent of curing of the coating for successful topcoating d. time period during which the coating can normally be stored without significant deterioration 11. Recoat window can be defined as: a. time period after mixing and before spraying for reaction of components to get a start b. time period during which the mixed coating can be successfully applied c. time period for proper extent of curing of the coating for successful topcoating d. time period during which the coating can be successfully stored without significant deterioration

C1 Fundamentals of Protective Coatings for Industrial Structures 4-24

Unit 4 - Application of Coatings

12. Why should coatings not be overmixed? a. to avoid overheating b. to avoid build-up of static electricity c. to avoid entrapment of air bubbles d. to limit exposure time to air contaminants 13. What application method is most commonly used with coatings having a very short pot life? a. airless spray b. HVLP c. Plural-component spray d. Electrostatic spray 14. Why are coatings heated before spraying? a. to increase their curing rates b. to reduce their viscosities c. to ensure good leveling of the wet film d. to make them easier to mix

C1 Fundamentals of Protective Coatings for Industrial Structures 4-25

Unit 4 - Application of Coatings

General References and Additional Reading ASTM D 3276, Standard Guide for Painting Inspectors (Metal Substrates) SSPC-PA 1, Shop, Field and Maintenance Painting of Steel SSPC-PA Guide 4 Guide to Maintenance Repainting with Oil Base or Alkyd Painting Systems SSPC-PA Guide 5 Guide to Maintenance Painting Programs SSPC-SP SP 5 White Metal Blast Cleaning (NACE No. 1) Military Standard MIL-STD-2138, Metal Spray Coatings for Corrosion Protection Aboard Naval Ships (Metric) SSPC-CS Guide 23.00 Specification or the Application of Thermal Spray Coatings (Metallizing) of Aluminum, Zinc, and Their Alloys and Composites for the Corrosion Protection of Steel (AWS C2.23M/NACE No. 12) 29 CFR 1926.152, Flammable and Combustible Liquids SSPC-PA Guide 11, Guide to Protection of Edges, Crevices, and Irregular Steel Surfaces SSPC-Guide 15, Field Methods for Retrieval and Analysis of Soluble Salts on Steel and other Nonporous Substrates

C1 Fundamentals of Protective Coatings for Industrial Structures 4-26

Unit 4 - Applicatioin of Coatings

Unit 4 Learning Outcomes

Unit 4 Application of Coatings

Upon completion of this unit, you will be able to:

Methods of Application: General Factors

Application Rates for Different Methods on Flat Steel

•  •  •  •  •  •  •  •  • 

−  Explain appropriate application systems for different jobs −  Describe the basics of coating application operations −  Define coating failures associated with poor application and explain how to avoid them

Appropriateness for the particular coating Appropriateness for the particular structural components Desired appearance Speed, ease, and economics of application method Simplicity of equipment/necessary applicator skills Safety/environmental requirements Weather Transfer Efficiency Coverage Rates

Application Method

Square Feet Applied Per Hour

Square Meters Applied Per Hour

Brush

75-125

7-12

Roller

150-300

14-28

HVLP spray

185-310

17-30

Conventional airless spray

200-450

18-42

Air-assisted airless spray Airless spray

300-600

28-56

500-1,250

50-120

Factors Affecting Transfer Efficiency

Transfer Efficiency •  •  •  •  • 

Item size Item shape Equipment Distance Pressures

C1 Fundamentals of Protective Coatings for Industrial Structures 4-27

Unit 4 - Application of Coatings

Relative Order of Transfer Efficiency (High to Low) •  •  •  •  •  • 

Theoretical Spreading Rate

Manual (brush or roller) Electrostatic spray High-volume, low-pressure (HVLP) spray Air-assisted airless spray Airless spray Conventional air spray

Parts of a Paint Brush

Brush Marks on Paint Film

Good Brushing Techniques

Parts of a Dip Roller

•  Choose proper size and shape of high quality brush •  Natural fibers for solvent-based, synthetic fiber for water-based coatings •  Remove stray bristles •  •  •  •  • 

Cover only ½ of bristle length with paint Tap brush on can edge to remove excess paint Apply in smooth even strokes from top to bottom Begin at natural boundary and keep wet edge Hold brush at 75 degree angle to the surface

C1 Fundamentals of Protective Coatings for Industrial Structures 4-28

Unit 4 - Applicatioin of Coatings

Paint Can and Grid vs. Tray

Good Roller Techniques • Choose proper fabric, size, and nap cover Use ½-filled tray or bucket with spreader screen Load roller uniformly Use steady, light pressure Apply first stroke upward on vertical surfaces Always finish off in one direction for a uniform appearance •  Start and stop at natural boundaries •  •  •  •  • 

Use of Roller on Ship Exterior

Spraying •  Spraying is the most economical and efficient method of applying protective coatings to large industrial structures

Conventional Air Spray

Spray Application •  •  •  •  •  • 

•  Compressed air atomizes the paint outside the gun tip and propels it to the object being painted.

Conventional (air) Airless Air-assisted airless High-volume, low-pressure Electrostatic Plural-component

C1 Fundamentals of Protective Coatings for Industrial Structures 4-29

Unit 4 - Application of Coatings

Setup for Conventional Air Spray

Conventional Air Spray •  The basic parts of conventional air spray equipment are: −  −  −  − 

Air compressor Paint tank (pressure pot) Hoses for air and fluid Spray gun

Air Compressor

Pressure-Feed Tank with Double Regulation

Typical Construction of Air and Fluid Hoses

Conventional Air Spray Gun

C1 Fundamentals of Protective Coatings for Industrial Structures 4-30

Unit 4 - Applicatioin of Coatings

Parts of Conventional Gun to be Lubricated Regularly

External Mix Pressure Air Nozzle Caps

Forming Conventional Air Spray Pattern

Faulty spray patterns

•  •  •  •  • 

Close air valve from pot to gun Open main air valve from compressor to pot Set regulators to recommended pressures Remove air cap Trigger/increase fluid pressure to get 3 foot stream •  Replace cap; slowly increase atomizing air pressure to obtain proper pattern

Faulty Spray Patterns

PROBLEM: Heavy pattern on top or bottom Causes: 1. Horn holes plugged 2. Air cap fluid tip obstructed or damaged

PROBLEM: Heavy left or right side pattern Causes: 1. Opposite side horn holes plugged 2. Dirt on the fluid tip

Solutions: 1. Clean off air cap 2. Clean or replace the fluid tip and needle

Solutions: 1. Clean off the air cap 2. Clean off the fluid cap

Faulty Spray Patterns PROBLEM: Heavy center pattern

Causes: 1.  Insufficient paint 2.  Too much atomizing pressure

Causes: 1.  Atomizing air too low 2.  Material too thick 3.  Too much material

Solutions: 1.  Turn fluid adjusting valve open (counter clockwise) or increase fluid pressure 2.  Reduce air pressure or correct by using air adjusting valve.

Solutions: 1.  Increase air pressure 2.  Reduce viscosity 3.  Lower fluid flow at fluid adjusting valve and reduce fluid pressure

PROBLEM: Split bell-shaped pattern

C1 Fundamentals of Protective Coatings for Industrial Structures 4-31

Unit 4 - Application of Coatings

Conventional Air Spray

Conventional Air Spray Disadvantages:

Advantages: −  −  −  −  − 

−  −  −  − 

Finest atomization/finish Good operator control/versatility Low initial investment Usually better with filled coatings Easier to change color

Lower transfer efficiency Lower application rate Produces overspray Viscous materials may present problem

Airless Spray System Components

Air Spray and Airless Spray Patterns •  •  •  • 

Airless spray setup

High-pressure hydraulic pump Coating container High pressure hose Airless spray gun

Airless Spray Gun Components

C1 Fundamentals of Protective Coatings for Industrial Structures 4-32

Unit 4 - Applicatioin of Coatings

Airless Spray Gun with Tip and Trigger Guards

Shape of Orifice on Gun Tip

Airless Spray Pump

Forming Airless Spray Pattern •  Size and shape of orifice determine fan size and shape •  Start fluid flow and increase until desired pattern is obtained •  Coating viscosity can be reduced by careful thinning or heating

Tailing

Airless Spray Advantages:

•  Insufficient pressure may result in tails (“fingers”) •  This is an incomplete fan with coating concentrated at ends •  Increasing pressure will resolve the problem

−  −  −  − 

High application rate Applies high-viscosity materials well Reduced overspray fog Better transfer efficiency on large surfaces

C1 Fundamentals of Protective Coatings for Industrial Structures 4-33

Unit 4 - Application of Coatings

Airless Spray

Air-Assisted Airless Spray

Disadvantages: −  −  −  − 

Advantages: −  −  −  −  − 

Hazardous spray pressures Reduced operator control Reduced-quality finish Expensive to maintain

Finer atomization Fewer runs and sags Good transfer efficiency Better operator control Able to produce fine finishes

Air-Assisted Airless Spray

HVLP Spray Advantages:

Disadvantage:

−  Good transfer efficiency −  Reduced overspray and bounce-back −  Good gun control

−  Expensive to maintain

HVLP Spray

Electrostatic Spray

Disadvantages:

Advantages: −  −  −  − 

−  High initial/maintenance costs −  May require special training −  Reduced application rate

C1 Fundamentals of Protective Coatings for Industrial Structures 4-34

Wraparound of edges High transfer efficiency More uniform paint application Material savings

Unit 4 - Applicatioin of Coatings

Plural Component Proporting (Metering) Pumps

Electrostatic Spray Disadvantages: −  −  −  −  −  −  − 

High initial/maintenance costs More suited to automation Requires skilled operator Safety precautions required Limited to one coat Limited to exterior surfaces Requires conductive substrate

Basic Components of a Plural-Component Spray System •  •  •  •  •  • 

Basic Components of a Plural Component System (cont’d.) •  •  •  • 

Feed system for each component High-performance proportioning pump Chemical-resistant hoses for each component Mixer/manifold assembly Whip hose for flexibility Airless spray gun

Plural-Component Spray

Plural-Component Spray

Advantages: −  −  −  −  −  − 

Solvent purge system Filters Heaters (optional) Off-Ratio alarm/shutdown (optional)

Disadvantages: −  High initial/maintenance costs −  Proportioning/temperature controls require monitoring −  Requires skilled operator −  Impractical for small jobs −  Large footprint

No pot life problems Good for high-viscosity materials Faster cure times possible Conservation of materials High film build in one coat Precise proportioning available

C1 Fundamentals of Protective Coatings for Industrial Structures 4-35

Unit 4 - Application of Coatings

Plural-Component Pumping/ Metering System

Heated Plural Component Lines Attached to Gun

Manifold of Plural Component System

Powder Coating

Powder Coatings

Powder Coatings (cont’d.)

•  Both thermoplastic and thermoset powders available •  Usually applied by electrostatic spray; fluidized bed also used •  High level of surface preparation required •  Powder fused in oven

•  Fusion-bonded epoxies (FBE) applied to steel piping and rebar •  High transfer efficiency; no VOC concerns •  Thermal spray (metal powder) available for field use

C1 Fundamentals of Protective Coatings for Industrial Structures 4-36

Unit 4 - Applicatioin of Coatings

Powder Coatings

Powder Coatings

Advantages:

Disadvantages:

−  Good transfer efficiency −  No fire or toxicity hazards from organic solvents −  Easy one-coat thick film application −  No viscosity adjustment requirements −  Fast cure, quick turnaround

−  −  −  −  − 

Electrostatic Spray Guns Used in Powder Coatings

Application is limited to shops with ovens Thin films are difficult to apply Coating enclosed surfaces is difficult Changing powder colors is time consuming Powder suspensions in air are potentially explosive

Alternatives in Metallizing •  Melting: Flame or arc •  Metals: Zn, Al, or Zn-Al alloy •  Application: Flame, electric arc, plasma spray

•  The two most commonly used methods of charging the sprayed powder particles are: −  Corona-charging gun (external) −  Tribo-charging gun (internal)

Thermal Spray Metallizing in Shop

Electric Arc Thermal Spray

C1 Fundamentals of Protective Coatings for Industrial Structures 4-37

Unit 4 - Application of Coatings

Thermal Spray Metallizing in Field

Handling of Paints •  •  •  • 

There are paint storage safety requirements for: •  •  •  • 

Recommended Coating Storage Conditions •  •  •  • 

Flammables/combustibles Indoor storage Outdoor storage Fire extinguishers

Recommended Coating Storage Conditions (cont’d.) •  •  •  •  • 

Storage Mixing Thinning Tinting and straining

UL-listed containers No smoking Ground containers Have spill-absorbent materials available

Indoor Storage •  No more than 25 gallons in a room except in an approved storage cabinet •  No more than 60 gallons of flammables or 120 gallons of combustibles in any cabinet •  No more than three (3) cabinets in a storage area

Store in well-ventilated rooms Store off floor Use oldest material first Keep no more than 1 year Have MSDS available

C1 Fundamentals of Protective Coatings for Industrial Structures 4-38

Unit 4 - Applicatioin of Coatings

Poor Exterior Storage of Coatings

Outdoor Storage •  •  •  • 

Only if indoor not available < 1,100 gal. in one pile or area < 60 gal. in each container > 20 feet from building

Shelf Life

Use 5- and 1-Gallon Kits

•  The maximum amount of time a paint or coating can be stored in a usable container.

•  Use mixtures of 5- and 1-gallon kits of epoxies and polyurethanes because: -  Unused mixed portions of 2-package kits cannot be stored overnight -  Visual estimation of volume ratios of kit components is difficult -  Open, partially used coatings have a reduced shelf life

Mixing One-Package Paints

Mixing •  Agitating the Individual Components •  Blending the Components and Mixing the Blended Products •  Measure the Coating Temperature •  Determine Whether Pot Agitation During Application is Required

C1 Fundamentals of Protective Coatings for Industrial Structures 4-39

Unit 4 - Application of Coatings

Mixing Two-Component Coatings

When Mixing Coatings Mechanically

•  Observe proper mixing ratios •  Induction time •  Pot life affected by:

•  Use appropriately sized equipment •  Form a vortex in the coating •  Avoid overmixing that will entrap air by using a slow speed mixer •  Never use mechanical shakers

−  Temperature −  Size of batch

Air-Powered Paint Stirrer

Bottom Line •  Unless partial kit mixing is permitted by the specification and the coating manufacturer, the contractor should always mix complete kits.

Mixing Zinc Rich Primers

Thinning Tips/Concerns •  Most coatings require no thinner •  Cold weather may increase viscosity •  Use type and amount of solvent recommended by manufacturer •  Do not permit thinning to exceed VOC limitations •  Viscosity reduction by thinning is much more dramatic on high-solids than other coatings

•  After blending the liquid components, the zinc powder is slowly sifted into the liquid while under agitation

C1 Fundamentals of Protective Coatings for Industrial Structures 4-40

Unit 4 - Applicatioin of Coatings

Temperature vs. Viscosity

Factors Affecting Viscosity •  Solvent-solids ratio •  Coating temperature •  Mixing thixotropic coatings

80

50

30

20 0 50°

•  Avoid tinting by procuring different colors •  Mix thoroughly after tinting •  Check tint/coating compatibility

125°

•  Within range recommended by supplier •  At least 5° above dew point and not falling

Achieving Desired Film Thickness

Typical Temperature Limitations Oil-based Water-borne Epoxy Inorganic Zinc

95°

Temperature Requirements During Coating Application

Tinting Concerns

•  •  •  • 

70°

•  Manufacturer’s recommendation •  Wet and dry related by volume solids

40°F 40°F 50°F 35°F

NOTE: Special low-temperature cures available

C1 Fundamentals of Protective Coatings for Industrial Structures 4-41

Unit 4 - Application of Coatings

For Unthinned Coatings

For Thinned Coatings

Striping Tips

Spraying Procedures •  •  •  • 

•  Apply stripe coat to edges/welds before or after topcoating •  Include corners, crevices, bolts, rivets, etc. •  Extend striping 1-inch from edge, etc. •  Allow striping to set-to-touch before topcoating •  Rounding of edges improves stripe coat

Stroking Triggering Distance Corners

Improper Stroking (Arcing)

Proper Stroking

C1 Fundamentals of Protective Coatings for Industrial Structures 4-42

Unit 4 - Applicatioin of Coatings

Overlap of Strokes

Proper Length of Strokes and Sectioning of Large Surface

Good Spraying Technique

Sagging

Dry Spray

Good Technique for Coating Corners •  Coat flat surface to within 1-2 inches of corner •  Turn gun position to spray sideways and apply coat to corner

C1 Fundamentals of Protective Coatings for Industrial Structures 4-43

Unit 4 - Application of Coatings

Spraying Outside Corners

Spray Technique Training •  LaserPaint Applicator Training Device

Unit 4 Summary

Coating Application Defects •  Improper application a major source of coating defects •  Newer lower-VOC coatings are more difficult to apply •  Coating defects should be identified and corrected at the time they are produced

•  •  •  •  •  • 

Unit 4 Summary (cont.) •  •  •  • 

Temperature and humidity Achieving desired film thickness Spraying procedures Application defects

C1 Fundamentals of Protective Coatings for Industrial Structures 4-44

Methods of application: general factors Brush application Roller application Spray methods Fusion methods Handling of paints

Unit 5 - Coating Inspection Overview

Coating inspection Overview 5.1 Purpose and Goals

control standards, methods, and equipment to ensure that all specification requirements are met.

Scope

SSPC’s course, “Planning and Specifying Industrial

This unit covers coating specifications, responsibilities

Coatings Pro­jects,” teaches how to prepare a coating

of the inspector, and different methods and equipment

specification for a project and how to manage the

for coating inspection.

contract.

Learning Outcomes

The inspector may be hired by the owner or the contractor, but he/she tries to work with both parties

Upon completion of this unit, you will be able to:

to get the work completed smoothly and success­fully.

•

Explain the basics of a coating specification

The inspector tries to anticipate problems before

•

Define the responsibilities of the inspector in

they arise and resolve them before they cause costly

establishing that all specification requirements

delays or change orders.

are met •

Explain the operation of basic inspection equip-

5.3 The Specification and Its Contents

ment •

Definition, Purpose, and Scope of the Job

Describe basic inspection procedures

Specification

5.2 Introduction to Inspection and Quality

A job specification may be defined as a written,

Control

detailed, precise description of work to be done; it is a part of the contract, de­scribing the quality of

It is important to understand the concepts of quality

materials and mode of construction, and defines the

assurance and quality control.

desired amount of work.

Quality assurance is all the planned and systematic

There are many purposes of a specification:

actions nec­essary to provide adequate confidence that a structure, system, or com­ponent will perform

•

To obtain a specific desired product

satisfactorily in service.

•

To assure quality materials and workmanship

•

To make sure the work is completed on time

•

To avoid delays and disputes

•

To obtain minimum or reasonable costs

Quality control is the process to verify that the quality of work performed is actually what was reported by

• To avoid costly change orders and claims

quality control.

• To meet all safety, environmental, and legal requirements

This unit describes what goes into a coating specification and how the inspector uses quality

C1 Fundamentals of Protective Coatings for Industrial Structures 5-1

Unit 5 - Coating Inspection Overview

Government job specifications usually have the ad-

the document invariably requires that the supplier’s

ditional goal of making the bidding not only fair, but

products be used. Also, the supplier is usually not a

also available to as many contrac­tors as possible.

skilled specification writer, so problems in the execution of work are more likely to occur.

Construction specifications are complicated in that they comprise legal documents and must contain

Several computer programs are available to assist

both legal and technical require­ments. Imperfections

in preparing job specifications. They provide a good

in paint or other specifi­cations may result in one or

format and start for the specification writer.

more of the consequences listed below: •

Bids from unqualified contractors

Specifications are usually prepared by certified protec-

•

Unrealistically high or low bids

tive coating specialist (PCSs) or architect-engineers

•

Acceptance of substitute (inferior) products

who specialize in this work. They have the background

•

Lower-quality or less work than desired

and the engineering standards, specifications, and

•

Change orders

other criteria documents at their disposal to prepare

•

Disputes

a document for a specific job professionally, which

•

Defaulted contracts

means:

•

Claims

•

Correct (complete without error)

•

Litigation

•

Clear (unambiguous)

•

Concise (no longer than absolutely necessary)

•

Systematic

There are several methods of preparing job specifications. Each is discussed briefly below.

Sometimes there are no specifications, just a pur-

Previously prepared job specifications for the same

chase order to do work. In these cases, the man-

work can serve as a starting point for a new specifica-

ufacturer’s recommendations as described in the

tion. However, the old specification must be checked

material supplier’s product data sheet and material

for modifications in the structure, new regulatory and

safety data sheets may be the only documents

other requirements, old errors that were not corrected,

available to the contractor to work from. This is not

and new technologies that may be more appropriate

ideal but it does happen.

than those previously used.

5.4 Responsibilities of the Inspector

Another approach is to “cut and paste” portions of other job specifications to meet present needs. This

The duties and responsibilities of a protective coatings

approach is not recommended because of possible

inspector appear at first glance to be relatively straight

omissions, duplications, or not meeting all present

forward: inspect the work performed by the painting

requirements. Each job requires a specifically tailored

contractor and compare it to the specification for

specification.

conformance. However, when taking a closer look at the “global” responsibilities and role, the duties become much more involved.

Paint suppliers sometimes offer to prepare job specifications, especially for small jobs. Of course,

C1 Fundamentals of Protective Coatings for Industrial Structures 5-2

Unit 5 - Coating Inspection Overview

Inspection Hold or Check Points

Further, the roles and responsibilities can be variable, depending upon the contract and whether the

The specific duties of the QC and the QA will vary

inspector is employed by the contractor, the facility

from project to project. The coating inspection

owner or by a third-party engineering firm. For

process typically dictates that after certain activities

example, if the inspector is part of the contractor’s

(e.g., surface preparation), work should be halted,

staff they may also be responsible for site safety

inspected, rework performed as necessary and

and environmental protection controls, and will likely

accepted by the QC and QA, before the contractor

have “stop work” authority (prohibiting the contractor

can move on to the next step of the painting process.

from continuing to work until a non-conforming item

These specific inspection items are typically referred

is corrected).

to as “hold points.” Hold-point inspections can involve visual observations or tests, and the results must be

If the inspector is employed by the facility owner,

documented. In broad terms, hold-point inspections

they, too, may be responsible for the safety and

are typically performed during:

environmental protection controls, and will no doubt

1. Pre-Cleaning

have “stop work” authority. They may also have

2. Surface Preparation

other project responsibilities (unrelated to painting),

3. Primer Application

including maintenance and protection of traffic

4. Intermediate Coat Application

flow on bridge painting projects, coordinating with

5. Top Coat Application

lock operators and the Coast Guard on waterway

6. Cure

structures, inspecting the work of other trades on the same project, etc. This obviously limits their ability to perform the same level of inspections that of the

5.5 Monitoring the Ambient Conditions

Quality Control Inspector, and is why the distinction

General

between quality control and quality assurance is so important. A third-party inspector hired by a

The locally prevailing air temperature, the moisture

facility owner can have the same responsibilities

content in the air (% relative humidity), and the

as a member of the facility owner’s staff, or their

temperature at which moisture will condense on

responsibilities may be limited to inspecting the

surfaces (dew point) are commonly called the

surface preparation and coating work. In many cases,

ambient conditions. If these conditions are not

a third party inspector does not have “stop work”

within the ranges required by a coating specification

authority, but merely communicates non-conforming

during surface preparation or coating application,

items to the contractor and the facility owner. This

problems in obtaining a protective film with a long-

is where an inspector’s role is defined as “observe,

term performance are likely to occur.

assess, document and report,” or “OADR.” Because of concerns for these problems, good specifications require the monitoring of ambient conditions using specialized instruments and test methods before the start of work and periodically during the work. Measurement of these conditions is C1 Fundamentals of Protective Coatings for Industrial Structures 5-3

Unit 5 - Coating Inspection Overview

especially important when weather conditions change

When brushing, incomplete leveling may result

during the course of a work shift.

in brush marks in the cured film; when spraying, incomplete leveling may result in orange peel

Common Coating Defects Related to Adverse

(irregular thicknesses in the cured film resembling

Ambient Conditions

an orange skin).

Some of the most commonly occurring coating

High Temperatures

defects related to unfavorable ambient conditions are discussed in the next few paragraphs. The tendency

Adverse effects of high temperatures on coatings are

for such defects to occur is often related to changes

usually related to their accelerated drying or curing

in local weather after the work has begun.

rates: •

Low Temperatures

rusting of cleaned steel. •

Adverse effects of low temperatures are most

merely by evaporation of the solvent in which their

of coatings within a reasonable amount of time.

resins are dissolved. Accelerated evaporation

Slow curing of coatings. Slow curing will

of the solvent during spray application at high

permit the accumulation of wind-borne dirt,

temperatures may result in an inability of the wet

mildew spores, and other undesirable surface

film to flow together (level) to form a continuous

contaminants. It will also increase the time during

film of even thickness. This results in dry spray

which the painted surfaces must be isolated from

(formation of a rough surface with pinholes or

traffic or other construction work to avoid damage

voids).

or contamination. •

•

Incomplete curing. Many latex coatings applied

film by coalescence of their resins, as the water

films. Also, thermosetting coatings that cure by

evaporates. If temperatures exceed those

chemical reactions of two parts or by reaction with

recommended by the manufacturer, rapid drying

oxygen in the air will have a much longer curing

will cause a low quality protective film to be

time and may never cure completely.

produced.

Improper curing. At low temperatures, many

•

thermosetting (e.g., two-part) coatings may cure

Rapid curing of coatings. Most coatings cure to form a protective film by a chemical reaction,

by mechanisms other than those intended by the

either between separately-packaged paint

formulator. •

Rapid drying of latex coatings. Latex coatings (dispersions of resins in water) form a protective

below 40°F will not coalesce to form durable

•

Rapid drying of lacquers. Coatings called lacquers form a protective film on substrates

commonly related to the complete and proper curing •

Re-rusting. High temperatures accelerate the

components, or with water or oxygen in the air.

Incomplete leveling. Low temperatures may

These chemical reactions are accelerated at

reduce the viscosity (fluidity) of a coating applied

high temperatures to cause more rapid curing of

to a surface, so that it cannot flow or level properly

coatings. Rapid curing often results in significant

(flow together to form a uniformly thick film).

C1 Fundamentals of Protective Coatings for Industrial Structures 5-4

Unit 5 - Coating Inspection Overview

•

shrinkage, stresses, and other harmful effects on

reaction with oxygen in the air may react rapidly

the coating.

at the coating surface to form a skin that will not

Effects of exotherm. The reaction of separately

permit further oxygen penetration to cure the underlying coating. Shrinking of the skin results

packaged chemically-curing coating generates

in wrinkling of the coating surface to form rows

a heat called exotherm. This exotherm, along

and furrows with uncured coating beneath.

with a prevailing high temperature, may further accelerate the curing rate to cause harmful

•

•

•

effects on coatings. Also, the exotherm may

coatings. When topcoating inorganic zinc-rich

reduce the viscosity significantly to interfere with

coating films, rising temperatures may cause air

normal application or curing of the coating. The

or solvent vapors entrapped in the pores of the

greater the volumes of Parts A and B that are

films to be emitted and rise to the surface of the

mixed together for application, the greater will

wet topcoats. This will cause pinholes to form

be the exotherm and its harmful effects.

in the topcoats.

Reduced induction time. The induction time

•

during the coating of bare concrete may cause air

required to produce an exotherm that will cause

entrapped in its pores to be emitted and rise to

the chemical curing of the coating to continue to

the surface of the wet film to leave pinholes in it.

completion. Induction times will be less or none

Sealing of the concrete will reduce the tendency

at high temperatures.

to form pinholes by outgassing.

Reduced pot life. The pot life of two-part, chemically-curing coatings is the period of time

Low Humidities

after mixing and induction (if any) during which

Harmful effects of low humidities are usually associated with changes in coating curing rates:

life is exceeded, the mixed product is too viscous

•

(thick) for proper application. In addition to the

Incomplete curing of inorganic zinc-rich coatings. Solvent-borne inorganic zinc-rich

ambient temperature, the exotherm will further

coatings require moisture from the air for curing.

reduce the pot life.

On dry days, it may be necessary to spray water

Reduced recoat window for topcoating. Two-

onto these coatings for complete curing.

part, chemically-curing coatings can only be

•

topcoated successfully in the limited time range

Curing of polyurethane and polyurea coatings. Single-part polyurethane and polyurea coatings

specified by their manufacturer. If applied too

cure by reaction with moisture in the air. Unless

soon or too late, harmful results will occur to the

the humidity is above 20%, the coatings will not

protective film. The recoat window of time will

cure satisfactorily.

be significantly reduced at high temperatures. •

Outgassing from concrete. Rising temperatures

for two-part, chemically-curing coating is the time

the coating can successfully be applied. If the pot

•

Pinholes in topcoats of inorganic zinc-rich

•

Wrinkling. At high temperatures, alkyd and

Curing of water-borne coatings. On hot, dry days, water-borne coatings may cure so fast that

other drying oil-containing coatings that cure by

they do not produce durable films.

C1 Fundamentals of Protective Coatings for Industrial Structures 5-5

Unit 5 - Coating Inspection Overview

High Humidities •

High Winds

Flash rusting. High humidities will greatly

High winds (above 15 miles per hour) at job sites may

accelerate re-rusting (flash rusting) of cleaned

also have harmful effects on coating operations:

steel. This is why dehumidifiers are often used

•

to reduce the humidity in closed spaces, such as

may blow dirt, dust, and other debris onto cleaned

storage tanks. •

surfaces. Unless removed before painting, this

Reduced bonding strength. Moisture condensed

contamination will reduce the bond strength of

on cleaned surfaces, with or without rusting, may

coatings applied to the contaminated surfaces.

reduce the bonding strength of the coating to the

•

surfaces. •

•

Contamination of uncured coatings. Wind may blow dirt, dust, and other debris onto the

Blushing of lacquers. High humidities may

uncured coating. This will produce an unsightly

cause solvent blushing during spray application.

appearance to finish coats. If a topcoat is to

The rapidly evaporating solvent reduces the

applied over the contaminated coating, the

temperature at the surface of the wet coating

contamination will reduce the bonding strength

film to the dew point, and moisture condensation

of the topcoat to the undercoat.

occurs. •

Contamination of cleaned surfaces. Winds

•

Overspray.

High winds make control of

Blistering. Condensed water on surfaces being

overspray more difficult. Wet paint mist may be

coated may cause blistering of the coatings.

carried outside the paint area onto automobiles

Improper curing of coatings. If the ambient

or other unintended surfaces.

humidity is over 80%, moisture in the air will react so rapidly with the single-part (moisture-

Measuring Ambient Conditions

curing) polyurethane and polyurea coatings,

Relative Humidity and Dew Point

that these reactions compete with the intended curing reactions, and durable protective films

Instruments used to measure percent relative humidity

cannot be achieved. Because of their reactions

and dew point are called psychrometers. The three

with moisture, exposure to air of moisture-curing

basic types of psychrometers are:

polyurethane and polyurea coatings should occur

• Sling psychrometer. A sling psychrometer has

only during actual application.

two thermometers, the

Temperatures Below Dew Point

bulb of one of which

It is a common rule that the surface temperature must

sock soaked in water.

be at least 5°F (3°C) above the dew point, and not

As the thermometers are

falling, to prevent moisture condensation. Harmful

whirled through the air,

effects of moisture condensation on protective films

water evaporates from

is fitted with a cotton

were discussed in the previous paragraph.

5-1: Sling Psychrometer

C1 Fundamentals of Protective Coatings for Industrial Structures 5-6

the wet sock to lower the “wet bulb thermometer”

Unit 5 - Coating Inspection Overview

•

temperature below that of the “dry bulb ther-

increasing temperatures to move the indicator

mometer.” Tables of the U.S. Weather Bureau

needles on the thermometer faces higher on

are used to relate the amount of “temperature

the scales. Magnets secured to the backs of

depression” to relative humidity and dew point.

the thermometers hold them in place on vertical

Battery-powered psychrometer. A battery-

steel surfaces.

powered psychrometer operates on the same

•

•

Digital contact thermometers.

Battery-

principal as the sling psychrometer, but a fan is

powered digital contact thermometers utilize

used to blow air cross the thermometers.

thermocouples to measure surface temperatures.

Electronic psychrometer. Electronic psy-

This type of thermometer is usually the most accurate.

chrometers with special sensors are much more expensive and easy to use, but many are not

•

Non-contact infrared thermometers. Battery-

suitable for exterior service. Others may perform

powered, non-contact thermometers utilize

continuous or intermittent monitoring.

infrared light emitted from the surface to determine temperature. This instrument is also

Wind Velocity

good at the surface of the paint can for measuring the temperature of mixed coatings. Some

Wind meters can be used to determine when

electronic psychrometers also have surface

wind speeds exceed

temperature measuring capabilities.

specification requirements during coating application.

Control of Ambient Conditions

The meter is positioned

5-2: Wind Meter

vertically in the wind

The easiest way to conform to ambient condition

stream, and its velocity

requirements during coating operations is to blast

is read directly from the

and paint only during those times of the day when

scale. Digital wind meters

the conditions meet the specification or coating

are also available.

manufacturer’s requirements. This may require working at night.

Surface Temperature Also, at many locations during the winter, ambient

Although not really an ambient condition, the surface

conditions can only be achieved in enclosed shops

temperature of substrates is an important local

with climate controls. In these places, exterior

condition that requires periodic measurement to

painting operations should be scheduled for times of

ensure good coating performance. The three basic

the year when conditions are more appropriate. SSPC

types of instruments for measuring substrate surface

Volume 2 “Systems and Specifications” contains a

temperature are listed here in order of increasing

chapter on monitoring and controlling environmental

price: •

Dial thermometers.

conditions during coating application. Dial thermometers

have bi-metallic springs that expand with

C1 Fundamentals of Protective Coatings for Industrial Structures 5-7

Unit 5 - Coating Inspection Overview

In some cases, the ambient conditions and surface

Control of wind, as well as temperature and dew

temperature on a project site are such that work

point/relative humidity, on exterior surfaces can

should not proceed, as there is a chance that

be made using containment that isolates the work

moisture may condense on a surface during final

area. Containment can also control contamination

surface preparation. In most instances, final surface

of surrounding environments with particulate debris

preparation work is postponed until conditions

from abrasive blasting. SSPC-Guide 6, “Guide

improve. While this approach is quite common, it

for Containing Debris Generated During Paint

can adversely impact the project schedule. If the

Removal Operations” describes various methods of

area in which surface preparation is taking place is

containment and describes four classes or levels of

contained, the contractor or facility owner may elect

containment.

to control the environment using dehumidification (DH) equipment, so that work can proceed. This

Utilization of Information on Ambient

type of equipment effectively removes moisture from

Conditions

the air, thereby reducing the chance of condensing

Information on ambient conditions and past histories

moisture on a surface. While dehumidification seems

of local conditions must be effectively utilized to

like a straightforward process, the equipment must be

minimize potential coating problems associated with

appropriately sized to dehumidify the area, and must

improper ambient conditions. Coating personnel

be properly set-up and maintained. Mobilization and

must be able to anticipate likely changes in ambient

operation of DH equipment can also escalate project

conditions and their potentially adverse effects.

costs. Therefore, the inspector should not require the

Measurements of ambient conditions should be

use of DH equipment unless stipulated by the project

made more frequently as any of the measurements

specification. The contractor may elect to mobilize

approaches a permissible limit.

the equipment on his own, in order to maintain the project schedule. Ultimately, the inspector verifies

In the morning, typically, both the temperature and

that the ambient conditions and surface temperatures

the dew point rise, and the relative humidity falls.

conform to the project specification. The means and

As the sun gets higher in the sky, more favorable

methods of achieving these conditions are up to the

conditions may be anticipated. In the late afternoon,

contractor. It is beyond the scope of this training to

temperatures are expected to fall and humidities

provide comprehensive instruction on the set-up and

to rise, so that potential adverse effects must be

operation of DH equipment. However, it is important

addressed.

that an inspector have a background in DH principles and equipment. For more detailed information on dehumidification, the inspector should read SSPC

There are many case histories where the inspector

Technical Report TR 3, “ Dehumidification and

received acceptable temperature measurements (e.g.,

Temperature Control During Surface Preparation,

95°F when the maximum temperature is 100°F) in the

Application, and Curing for Coatings/Linings of Steel

morning but failed to take additional measurements

Tanks, Vessels, and Other Enclosed Spaces.” The

later in the day as the temperature rose. The higher

information provided here was extracted from the

temperatures in the afternoon exceeded the upper

report. C1 Fundamentals of Protective Coatings for Industrial Structures 5-8

Unit 5 - Coating Inspection Overview

5.6 Pre-Surface Preparation Inspection

permissible level and resulted in poor quality coating films.

General

Changes in weather conditions can be anticipated

Before the start of surface preparation for coating,

from local weather reports. Also, many locations

all necessary construction or modification of items

are susceptible to rapid weather changes (e.g.,

requiring painting should have been completed.

sudden fogs near coasts, sudden rains in tropical

This includes grinding of welds and sharp edges

areas, and sudden winds in mountainous areas). In

and filling of pits.

such areas, it may be advisable to use quick-curing coatings to minimize coating damage by rain or wind

The job site must then be inspected for complete

before curing. Using smaller quantities of mixed two-

readiness (i.e., all required operational and support

component coatings with short pot lives may limit

equipment is present, and access for inspection

wasting of mixed coatings that cannot be applied

of work is available). This includes safety aspects

during adverse conditions or kept overnight.

such as ladders and scaffolding, power, and traffic control, so the inspector can safely perform his/her

Other case histories of coating failures involve

duties. If lead-containing or other toxic paints will be

painting that started after acceptable humidity

disturbed, the inspector should review any approved

conditions were measured and continued when

compliance plans and implement appropriate work

humidity conditions changed to unacceptable

practices, protective equipment and administrative

levels. This commonly occurred when offshore fogs

controls necessary to limit his/her exposure.

unexpectedly came ashore to significantly raise the humidity. The applicators continued the painting

Abrasives

because they didn’t want to waste the mixed coating

All new mineral and slag abrasives must be inspected

that couldn’t be kept overnight.

for physical and chemical properties as described in Measurements of ambient conditions serve no

SSPC - AB 1. Recycled ferrous metallic abrasives

purpose unless coating workers utilize this information

must be checked for cleanliness and fines as

to anticipate changes that may produce harmful

described in SSPC - AB 2. Requirements for ferrous

effects and take necessary actions to avoid them.

metallic abrasive materials are given in SSPC-AB

Fortunately, suitable temperature and humidity

3.

conditions occur frequently during the painting season.

The abrasives should be properly labeled for identification. Even if a sieve analysis is provided

Tight production schedules to meet outage schedules

by the supplier, it is prudent to run a check at the

or ship deployments are often imposed on contractors.

job site or retain a sample for later analysis should

These situations can force contractors to take short

cleaning rates be lower or profile heights other than

cuts to complete the work when ambient conditions

anticipated.

are not their best. The coating specifier should be aware of coating quality in these situations.

C1 Fundamentals of Protective Coatings for Industrial Structures 5-9

Unit 5 - Coating Inspection Overview

A simple test can be conducted for contaminants or

Air and blast hoses should be checked for damage

fines in the abrasive. A spoonful of abrasive is placed

and constrictions and should be as short and as large

in a vial of distilled water and shaken vigorously. It is

a diameter as practical to reduce frictional losses of

then checked for:

air pressure. The blast hose should have a static

•

Oil or grease that forms a surface sheen

grounding system or static dissipating properties.

•

Fines suspended in or at the surface of the

Couplings should be of the external fit type, secured well, and safety-wired.

water •

Color or turbidity from dirt

•

Soluble salts by conductivity or deposition upon

Blast nozzles should be

evaporation

of the venturi type, with a

Acidity or alkalinity with pH paper

flared exit to allow more

•

rapid and uniform cleanASTM D4940: Standard Test Method for Conductimet-

ing. An orifice gauge

ric Analysis of Water Soluble Ionic Contamination of

should be used to check

Blasting Abrasives can be conducted for rapid evalu-

the nozzle size (inches 5-4: Abrasive Blast Nozzles

ation of abrasives for the presence of contamination

[mm]) and air flow (cfm at 100 psig [liters/min at

by performing a conductivity test.

7 MPa]). This wedge-shaped instrument inserted into the rear of the nozzle has a measuring range of 1/4 to

Blast-Cleaning Equipment

/8 inch (6 to 16 mm) and an air flow range of 81 to 548

5

All air compressors and blasting equipment and hoses

cfm [2,290 to 15,520 liters/min]. Nozzles should be

should be checked for proper size, cleanliness, op-

discarded after an increase of one size (e.g., 1/16 inch

eration, and safety. Hand or power tools should also

is the difference between a #6 and a #7 nozzle).

be checked for operation and safety, and should be used only as specified in their standard operating

All noz­zles must have a deadman control that will au-

procedures. These checks should be made before the

tomatically shut off the flow of air and abrasive when

start of abrasive blasting and periodically thereafter,

released. It may be air or electrically powered.

especially after a change of abrasive.

The compressed air used in abrasive blasting must be checked to determine whether oil and water traps have removed contaminants. This is done by the blotter test described in ASTM D4285. A clean, dry, white blotter or cloth is held about 18 inches (450 mm) in front of the blast noz­zle with the air flowing for one to two minutes. Oil and water contaminants are detec­ted visually on the blotter or cloth surface.

5-3: Abrasive Blast Cleaning Daily Set-up

C1 Fundamentals of Protective Coatings for Industrial Structures 5-10

Unit 5 - Coating Inspection Overview

It is normally desired that blasting pressures be

SSPC-Guide 15 “Field Methods for Retrieval and

between 90 and 100 psi (6.5 to 7 MPa) for efficient

Analysis of Soluble Salts on Substrates” describes

blasting. A pocket-sized pressure gauge with a hypo-

the most commonly used methods for retrieval and

dermic needle can be used for determining cleaning

analysis of soluble salts from contaminated surfaces.

pressure at the nozzle. The gauge is inserted in the

These methods are also described in SSPC “The

blasting hose just before the nozzle in the direction

Inspection of Coatings and Linings.” Field extraction

of the flow. Instant readings can be made up to 160

methods for procuring samples of soluble salts for

psi (11 MPa).

analysis includes:

5.7 Post-Surface Preparation Inspection After cleaning for painting, all surfaces should be inspected for conformance to the cleanliness require-

•

Swabbing

•

Bresle patch

•

Latex sleeve

In each case, the surface is extracted with a specific

ments of the specification.

volume of deionized water. The concentration of salts in the extract is then determined by one of the

Inspecting Cleaned Surfaces for Grease and Oil

following methods:

Cleaned surfaces can be tested for contamination by grease and oil which may not be readily detected visually. In the water-break test for contamination, a mist of water is sprayed onto the cleaned surface. If the water gathers together in droplets (breaks), contamination is present.

•

Electrical conductivity meters

•

Chemical analysis (titration)

•

Use of test strips

•

Use of ion detection tubes

Verifying Level of Cleanliness of Cleaned Steel Steel surface cleanliness requirements for cleaned

An alternate test for hydrocarbon grease and oil

steel (e.g., SSPC-SP 2, 3, 5, 6, 7, 10, 11, 12, 14

is the “black light test.” If ultraviolet light is shined

and 15) can be readily checked using SSPC-VIS

onto grease or oil-contaminated surfaces, they will

1, 3, 4 and 5 reference photographs, described in

fluoresce.

Unit 3. Of course, the written standard is the legal requirement.

Testing for Non-Visible Soluble Salt Contaminants Cleaned surfaces are often contaminated with

Testing for Contamination by Blasting Dust

soluble salts, especially in chemical and marine

Blasted steel surfaces can be tested to see if the

environments and on bridges where de-icing salts

blasting dust has been removed using transparent,

are used. Although these salts (typically chlorides

cellophane tape. The tape is pressed onto the

and sulfates) are not visible to the naked eye, they

cleaned steel, pulled off, and examined for pick-up

can cause flash rusting of steel or osmotic blistering

of dust. ISO 8502-3: Preparation of steel substrates

of paints applied over them.

before application of paints and related products Tests for assessment of surface cleanliness - Part C1 Fundamentals of Protective Coatings for Industrial Structures 5-11

Unit 5 - Coating Inspection Overview

3: Assessment of dust on steel surface prepared for painting (pressure-sensitive tape method) is a test procedure that can be used in the field to assess the amount of dust on a surface. Surface Profile of Abrasive Blast Cleaned Steel Three methods

5-6: Kean-Tator Comparator

for deter­mining the profile (av-

A third procedure for measuring surface profile of

erage peak-to-

abrasive blasted steel uses a profilometer. This in-

valley depth)

strument has a dial gauge and a stylus that protrudes

of abrasive

from the base of the gauge and extends into the val-

blasted steel 5-5: ASTM Method D4417

leys of the profile.

surfaces are described in

Inspecting Surface Preparation of Concrete

ASTM D4417.

Surface Preparation Requirements

The Testex Press-O-Film Replica Tape method is preferred, because it is easy to conduct, accurate, and

Surface preparation requirements for concrete clean-

produces a permanent record. The tape consists of

ing and surface roughening are defined in ASTM D

a lay­er of deformable plastic foam bonded to a Mylar

4258, 4259, 4260, 4261, 4262 and SSPC-SP 13.

backing. The tape is rubbed onto the blast-cleaned

These will be discussed more fully in Unit 7.

surface with a plastic swizzle stick to produce a reverse replicate of the profile. The tape profile is

Moisture Content

then meas­ured with a spring micrometer. Note: It is important to use the proper tape (coarse, x-coarse,

The plastic sheet method of ASTM D4263 can be

x-coarse plus) to get an accurate measurement of

used to determine if there is too much moisture in

the surface profile. The micrometer can be set to

the concrete for painting. It and other methods for

auto­mat­ically subtract the two-mil [50 µm] thickness

determining moisture content of concrete are de-

of the non-deformable Mylar backing.

scribed in Unit 7.

An alternate procedure, in which a surface profile

Inspecting Preparation of Previously Coated

comparator is used, is available for determining

Surfaces

surface profile. Comparators in­clude ISO, Clemtex,

Soundness of Remaining Coating

and Keane-Tator instruments. Basically, they use a five-power illuminated magnifier to permit visual

In preparing previously coated sur­faces for mainte-

comparison of the blast-cleaned surface to standard

nance painting, all loose painting must be removed by

profile depths. Standards are available for sand, grit,

sanding or other appropriate method. Loose paint is

and shot-blasted steel.

normally defined as coating that can be removed with

C1 Fundamentals of Protective Coatings for Industrial Structures 5-12

Unit 5 - Coating Inspection Overview

a dull putty knife. The edges of the remaining paint

using thinner. A viscosity cup is a stainless steel

should then be feathered by sanding or light abrasive

cup on a long handle that holds a known volume of

blasting to permit a smooth transition between them

coating. The cup has a precise diameter hole in the

and the repair areas.

bottom. The cup is filled with coating and a stopwatch is used to time how long it takes the coating to drain

Chalking

from the cup through the orifice. The inspector must

Painted surfaces are normally washed to remove

intended to drain, (based on the temperature of the

know the target number of seconds that the cup is

chalk and other loosely held contaminants before

coating), the manufacturer and orifice size of the

painting. Chalk is form­ed by slow degradation of the

viscosity cup to use, and must assure that the test

coating’s organic binder by the sun’s ul­traviolet light.

is conducted out of wind, so that a true end point

Loose chalk will prevent tight bonding of a topcoat.

(stream break) can be noted.

The level of chalking may be determined manually, according to ASTM D4214, by rubbing a black felt

Application Instructions

cloth across the surface of the old paint. The density of the chalk picked up on the cloth is then compared

Both the painter and the inspector should be familiar

to ASTM visual standards. A white cloth is best used

with the manufacturer’s­ application instructions,

on dark col­ors. If a rating of less than 8 is obtained,

normally included in the Product Data Sheet. This

the surface needs more wash­ing.

data sheet includes the following information: application temperature range, drying and curing schedules, recommended application equipment,

5.8 Pre-Painting Inspection

and parameters.

Several pre-painting requirements were discussed in Unit 4. Those that normally require inspection include

Not all coatings of the same generic type are applied

the following:

in the same manner. A small area of paint should

•

Coating storage conditions

first be applied and checked for proper application.

•

Mixing procedures

If spraying, the spray pattern should be adjusted, as

•

Thinning materials and amounts

necessary, to make it proper. The color of the cured

•

Tinting, or color verification

coating should be compared with the specified color

•

Straining of coatings to remove large particles

chip, either that of the approved manufacturer or

•

Viscosity

Federal Standard 595, or the appropriate national standard, depending on what was specified. The

The first five items were discussed in Unit 4.

comparison must be made after the coating has thoroughly dried.

Viscosity Another pre-painting item that may be checked is coating viscosity. This can be easily accomplished using a Zahn or Ford viscosity cup. The viscosity cup is used to measure the adjustment made to a coating

C1 Fundamentals of Protective Coatings for Industrial Structures 5-13

Unit 5 - Coating Inspection Overview

5.9 Inspection of Paint Application

Since measurement of WFT de­stroys the film integrity, the coating must be repaired after the meas­ure­ments

Inspection during and after coating application con-

have been completed.

sists chiefly of check­ing for: •

Induction time and pot life

•

Wet and dry film thicknesses

•

Holidays

•

Adhesion

•

Curing

•

Cosmetic and film defects

The most widely used type of WFT gauge, described in ASTM D 4414, consists of a thin rigid metal notched gauge, usually with four working faces. Each of the notches in each face is cut progressively deep­er in graduated steps. The face with the scale that encompasses the specified thickness is selected for use.

Induction and Pot Life For coatings that cure by chemical reaction

To conduct the measurement, the face is pressed

(thermosetting), the inspector should check to see

firmly and squarely into the wet paint immediately

that the supplier’s induction and pot life requirements

after its application. The face is then carefully re­

are met. Otherwise, the film properties will be

moved and examined visually. The WFT is the highest

compromised.

scale reading of the notches with paint adhering to it. Measure­ments should be made in triplicate.

Wet Film Thickness

Faces of gauges should be kept clean by removing

As described in Unit

An alternative, circularly notched gauge is rolled

4, wet film thick-

perpendicularly through the wet film and the clearance

ness (WFT) mea-

of the deepest face wetted is noted.

the wet paint immediately after each measurement.

surements should routinely verify that

A special wet film thickness gauge is used during

the targeted wet film thickness of the coating is being achieved

the application of an FRP laminate. The support legs

5-7: Measuring WFT

are tapered to a point to permit bypassing of the glass fibers, resulting in an accurate measurement.

during application. That is why a wet film thickness

Laminate depths from 0.76 to 6.9 mm (30 to 270 mils)

gage should be regarded as much of an applicator’s

can be measured.

tool as a paint brush or spray gun. The industry standard for measuring wet film thickness is described in

Measuring Dry Film Thickness

ASTM D4414, “Practice for Measurement of Wet Film

While the applicator is concerned with the wet film

Thickness by Notch Gages.”

thickness of the applied coating, inspection personnel Note- Measurement is less accurate on highly

are much more concerned with the end result, or

pigmented (e.g., zinc-rich) and quick-dry coatings

the dry film thickness. The measurement of wet

(e.g, polyurea).

film thickness is simply a means to an end on most metal surfaces, where measurement of the dry film

C1 Fundamentals of Protective Coatings for Industrial Structures 5-14

Unit 5 - Coating Inspection Overview

is feasible. Currently there are three standards that

•

address the nondestructive measurement of coating

Roughness of steel surface (Deeper blasted surfaces result in higher measurements.)

thickness: SSPC-PA 2, “Measurement of Dry Coating

•

Thickness with Magnetic Gages,” ASTM D7091,

Steel composition (High alloy steels may have erroneous measure­ments.)

“Standard Practice for Nondestructive Measurement

•

of Dry Film Thickness of Nonmagnetic Coatings

Thickness of steel (There is a minimum thickness for gage accuracy.)

Applied to Ferrous Metals and Nonmagnetic,

•

Nonconductive Coatings Applied to Non-Ferrous

Curvature of steel surface (Measurements may be erroneous.)

Metals,” and ASTM D6132, “Standard Test Method for

•

Surface condition (Contaminated coating

Nondestructive Measurement of Dry Film Thickness

surfaces may cause high readings; “pull-off”

of Applied Organic Coatings Over Concrete Using

magnets may adhere to tacky surfaces; probes

an Ultrasonic Gage.” Each of these standards

may indent soft paints.)

prescribes methods for verifying the accuracy of the

•

measuring devices and for obtaining coating thickness

Orientation of gage (Must be held perpendicular to surface.)

measurements. Some of the standards also provide

•

Other magnetic fields (Strong magnetic fields

guidance on the frequency of measurements (number

from direct current welding or railway systems

of measurements to obtain based on the size of the

may interfere.)

coated structure) and one standard (SSPC-PA 2) sets limits on the thickness readings obtained verses the

Too great a film thickness may cause such problems

specification requirements.

as solvent retention, incomplete curing or too rapid curing. Excessively thick films may also become

Aside from the industry standards, coating thickness

very rigid and unable to expand or contract with the

measurements should be obtained after the

substrate without cracking.

application of each coat in a multiple coat system, Pull-off Gages (PA 2, Type 1)

not just after the final coat. The vast majority of nondestructive coating thickness gages described

Magnetic pull-off gages measure film thickness by

in SSPC-PA 2 and ASTM D7091 cannot distinguish

stretching a calibra­ted spring to determine the force

individual coating layers, but rather measure the total

required to pull an attached perma­nent magnet from

“gap” between the substrate and the gage probe.

a coated steel surface.

Magnetic Dry Film Thickness Gages

Banana gages (long, narrow instruments) represent

Magnetic gages are normally used for determining

a reliable form of pull-off gage. A helical spring is

coating DFT on steel surfaces. They rely on the fact

stretched by man­ually turn­ing a graduated dial, and

that the thicker the coating, the smaller the magnetic

a pin pops up when the magnet is lifted. At least one

field above the coating. Typical error may be 3–10

company sells an automatic gage with a dial that

percent. There are several factors that adversely

turns and stops automatically. Cheaper models have

affect DFT measurements with magnetic gages.

a rubber foot con­tact for the painted surface. More

These include: C1 Fundamentals of Protective Coatings for Industrial Structures 5-15

Unit 5 - Coating Inspection Overview

expensive models have a more dur­able tungsten

Gages for Non-Ferrous Substrates

carbide foot for greater durability and precision. “V”

Electrically operated gages are also available to de-

grooves are cut in

termine the DFT of organic coatings on aluminium,

the probe housing of

copper, and stainless steel. Alternating current from

these gages and the

the instrument probe coil induces eddy currents in

electrically operated

the metal, which in turn induce magnetic fields that

flux gages described

modify the electrical characteristics of the coil. ASTM

below to permit more accurate measure­

D1400 fully describes these instruments and their

5-8: Banana Gage

operating procedure.

ment of paint DFTs on cylindrical surfaces.

Verifying Dry Film Gages for Accuracy

Electronic Gages (PA 2, Type 2)

Prior to obtaining coating thickness measurements, the user must verify the accuracy of the magnetic gage.

Electronic gages measure magnetic flux within the

Otherwise, the coating thickness measurements are

probe or the instrument itself. Flux changes vary in-

of little value. In fact, SSPC-PA 2 requires that the

versely with the distance between the probe and the

gage be verified for accuracy prior to and after each

steel. Mechanically operated instruments of this type

period of use. You will notice that we have not used

have a horseshoe magnet that is placed directly on

the phrase “calibrate the gage.” Typically, the only

the coating, and readings are made from the position

party that can truly calibrate a coating thickness

of a needle on a calibrated scale.

gage is the manufacturer or an approved laboratory. Rather, the user can verify accuracy and adjust/

Electric instruments have a separate instru­ment probe

optimize the gage, if necessary.

that houses the magnet. Thickness measurements are presented in a digital readout. Some of these gages

SSPC-PA 2 and ASTM D7091 define two types of

have a probe attach­ed to the instrument to permit

nondestructive coating thickness gages as Type 1

greater accessibility, especially in labora­tory work.

(magnetic pull-off) and Type 2 (electronic). Calibration

They may also have attachments for strip recorders for

blocks are typically used to verify the accuracy of Type

re­petitive work or alarms

1 coating thickness gages, while non-metallic foils

to produce sounds if

(also known as plastic shims) are typically used to

minimum thick­n esses

verify Type 2 gage accuracy. The user must carefully

are not met. For the

read the project specification and gage manufacturers

paint inspector, these

instructions to determine which type of gage and

more sophisticated 5-9: Electronic Type 2 Gage

accuracy verification are required.

attach­m ents may be unnecessary.

C1 Fundamentals of Protective Coatings for Industrial Structures 5-16

Unit 5 - Coating Inspection Overview

Calibration blocks are manufactured by thickness

precision-cut angular grooves in the film. The gage

gage suppliers and by the National Institute of

is not recommen­ded with very soft or brittle films that

Standards and Technology (NIST). They are

distort or crumble, respectively, when cut.

typically chrome-plated steel or plastic coated steel blocks. These calibration blocks are typically more

A line is first drawn on the painted surface for later

accurate than plastic calibration shims, and also

reference under the magnifier. A groove is then firmly

more costly. In addition, because the calibration

cut perpendicular to the line with a tungsten carbide

block surface (beneath the chrome or plastic-coated

cutter tip as it forms a tripod with two sup­port legs.

film) is not representative of abrasive blast cleaned

The width of the cut is determined visually using the

steel, the user must measure and record the effect

illum­inated magnifier portion of the instrument. Tips

of surface roughness on the coating thickness gage

with three dif­ferent cutting angles are available for

measurement. This is called a Base Metal Reading

use with films of thickness up to 50 mils (1250 µm).

or BMR. The BMR is deducted from any coating

Visual observations are multiplied by one, two, or

thickness measurement. It remains constant for

ten to provide actual thicknesses, depending on the

the entire project (assuming the same size abrasive

cutting angle of the tip. Thicknes­ses of individual

was used throughout) and is deducted from each

coats of a multi-coat system can be determined if

measurement, regardless which coat is involved.

they are differently colored.

Note- SSPC-PA 2 prohibits the use of plastic shims for accuracy verification of Type 1 (pull-off) gages. Substantial measurement error is evident when attempting to verify Type 1 gage accuracy over roughened surfaces using plastic shims. If plastic shims must be used with a Type 1 gage, they should be placed over smooth metal and gage accuracy

5-11: Tooke Gage Reading

verified, then a BMR obtained from the prepared surfaces and deducted from the coating thickness

Because this test damages the coating, it requires

as described above.

that the damaged area be repaired.

Destructive Dry Film Thickness Gage

Holiday Discontinuity Detection

There are several models of

Newly coated structures on which the coating

the Tooke Gage–sometimes

integrity is important (particularly linings or coatings

called a “Paint Inspection

in immersion conditions) should be tested with a

Gage” or “PIG”– described in

holiday detector to ensure coating film continuity. A

ASTM D4138 for measuring

holiday (sometimes called discontinuity) is a pinhole

paint DFT on any substrate. Measurements are made by microscopic observations of

or other break in the film that permits the passage of moisture to the substrate. This allows substrate

5-10: Tooke Gage

deterioration to begin. C1 Fundamentals of Protective Coatings for Industrial Structures 5-17

Unit 5 - Coating Inspection Overview

Flourescent Coatings

Holidays are not easily detected visually, and must be located with electrical instruments called holiday

Fluorescent coatings can be used to assist in the

detectors. Holiday detectors are available in two

inspection of coatings. This technique quickly identifys

types, low and high voltage, as described in ASTM

holidays and areas with low film thickness by use

D5162.

of a blacklight/ultraviolet light, and can be used in both primer and finish coats. It may also enable the

Low-voltage (30 to 70 volts) holiday detectors are used

inspector to detect incomplete removal of coatings.

on coatings up to 20 mils (500 µm) in thickness. These

SSPC-TU 11, Inspection of Fluorescent Coating

portable devices have a pow­er source (a battery), an

Systems discusses the types of coatings and the

exploring electrode (a dampened cellulose sponge),

benefits of using flourescent coating systems.

an alarm, and a lead wire with connections to join the instrument to bare metal on the coated structure.

Adhesion Testing

A wetting agent that evapor­ates on drying should be used to wet the sponge for coatings greater than

As discussed in Unit 2, there are four basic types of

10 mils (250 µm) in thickness. The wetted sponge

testing procedures for determining wheth­er coatings

is slowly moved across the coated surface so that

are satisfactorily bonded to substrates—tape tests,

the response time is not exceeded. When a holiday

knife probe test, and pull-off adhesion tests.

is touched, an electric circuit is completed through the coated metal and connected wire back to the

In one version of the tape test (ASTM D3359, Method

instrument to sound the alarm. Holidays should be

A), an “X” is cut through the coating to the substrate.

marked after detection for repair and subsequent

Pressure-sensitive­ tape is applied over the cut

retesting.

and rapidly pulled off at an angle of 180°. The cut area is then examined for extent of deter­ioration by

High-voltage

comparing it to standard figures.

(above 800 volts) holiday detectors

In the other version of the tape test (ASTM D3359,

are used on coat-

Method B), a lattice of six lines in each direction is

ings greater than 20 mils (500 µm) in thickness. The

cut through the coating film. After the tape is pulled off, the lattice area of the coat­ing is com­pared against

5-12: Holiday Detector

four standard diagrams. A kit is available with a knife

exploring elec-

and a chrome-plated steel template for cutting the

trode may consist of a conductive brush or coil spring.

lattice pat­tern.

The detector may be a pulse or direct current type. It should be moved at a rate not to exceed the pulse

In the knife probe test (ASTM D6677), an “X” with 1.5

rate. If a holiday or thin spot in the coating is detected,

inch legs is cut through the coating to the substrate.

a spark will jump from the electrode through the air

The ease of paint removal from the cut is then

space to the metal.

rated.

C1 Fundamentals of Protective Coatings for Industrial Structures 5-18

Unit 5 - Coating Inspection Overview

In the pull-off test (ASTM D4541), a metal dolly is

Solvent-Rub Test

bonded to a coated surface at a perpendicular angle

The MEK-rub test (ASTM D4752) is used for

with an adhesive, usually a two-component epoxy.

determining the cure of ethyl silicate (inorganic) zinc-

After the adhesive has fully cured, a force is gradual­ly

rich coatings. A piece of cotton cheesecloth soaked

and uniformly applied to the dolly until it is detached

with methyl ethyl ketone (MEK) solvent is rubbed

from the coat­ing or until the desired pull-off level is

back and forth against the coating at least 50 times

reached.

or until the steel substrate is exposed.

The inspector should record the amount of tensile

This procedure can be used on epoxy or other

force (in psi [MPa]) re­quir­ed to detach the dolly.

thermosetting coatings by rubbing an MEK-wetted

There is no consensus on minimum adhesion needed

cloth back and forth a few times and then determining,

to confirm an adequate bond; however, adhesion

visually, if any coating has been picked up on the

values of 200 psi (14 MPa) or less are considered

cloth.

weak, and values of 600 psi (40 MPa) or greater are considered strong. Normally, three to six replicate

Similarly, a water-rub test can be performed on water-

readings are needed to give reliable, precise results.

borne, inor­ganic zinc coatings, or a styrene-rub test

Also of interest is where the fail­ure occur­red. Failure

on fiberglass-reinforced plastic (FRP) coatings, to

can be: •

determine if full curing has occurred.

Adhesive: failure between coats or primers to substrate

•

Cohesive: split of any coat

•

Glue: glue failure

Sandpaper Test The sandpaper test is a suitable test for coatings that cure to a hard finish. When such coatings are sanded,

Curing

they form a dust if fully cured. If incompletely cured, they tend to gum up the sand paper.

Paint films should be allowed to cure to the extent recommended by the manufacturer before topcoating.

Pencil Hardness Test

This includes topcoat window times for thermosetting coatings. This will permit proper adhesion and

A series of hardness pencils (drawing leads) is

curing of the system. Rate of curing depends on

available for determining if a coating film has

temperature; at lower temperatures, a longer time

completely cured to the hardness stated by the

is required. Some coatings depend on moisture and

supplier. The softest pencil lead that scratches the

require a minimum level of relative humidity to cure.

film is a meas­ure of the film’s hardness.

Others (e.g., water-borne coatings) will not cure properly under conditions of high humidity. Complete curing is necessary before coatings can be placed into immersion service.

C1 Fundamentals of Protective Coatings for Industrial Structures 5-19

Unit 5 - Coating Inspection Overview

Inspecting for Cosmetic and Film Defects During an application, the inspector must also check for cosmetic and film defects and see that they are corrected. Early detection will allow them to be corrected before time-consuming and expensive work is required. Coating defects are described in Unit 8. 5.10 Unit Summary The coating specification defines the work to be accomplished. It is the role of the inspector to ensure that all specification requirements are met. The inspector’s verification of the adequacy of the various stages of surface preparation and coating application is critical for coating performance. Good communication between the inspector, the contractor, and the owner will facilitate a rapid and successful completion of the work. Good knowledge of standard inspection test methods and equipment will result in reliable test data.

C1 Fundamentals of Protective Coatings for Industrial Structures 5-20

Unit 5 - Coating Inspection Overview

Unit 5 - Exercise 5A: Effects of Adverse Ambient Conditions Match each adverse ambient condition listed in Column A with its possible effect in Column B. Column A

Column B

1.

Low temperature

A. Blushing

2.

High temperature

B. Paint overspray onto unintended surfaces

3.

Low humidity

C. Dry spray, reduced pot life, or wrinkling

4.

High humidity

D. Slow or incomplete curing of coatings

5.

High wind

E. Slow curing of solvent-borne inorganic zinc-rich or onepart polyurethane coatings

C1 Fundamentals of Protective Coatings for Industrial Structures 5-21

Unit 5 - Coating Inspection Overview

Unit 5 - Exercise 5B: Equipment Used for Different Inspection Test Methods Match each of the pieces of inspection equipment listed in Column A with the description of the information received using it in Column B. Column A

Column B

1.

Black felt cloth

2.

Bresle patch

B. Air pressure at blast nozzle

3.

Holiday detector

C. Dew point and relative humidity

4.

Hypodermic needle gage

D. Cleanliness of abrasive blasted steel

5.

Knife for cutting coating

E. Coating wet film thickness

6.

Notched flat gage

F. Coating dry film thickness

7.

Psychrometer

G. Profile of abrasive-blasted steel

8.

Scotch tape

H. Adhesion of coating to substrate

9.

Solvent-soaked cloth

I. Whether coating needs thinning

10.

Spring micrometer

J. Extent of soluble salt contamination

11.

SSPC-PA 2 gage

K. Dust on abrasive blast cleaned steel

12.

SSPC-VIS 1

13.

Vial (baby food jar)

14.

Viscometer

A. Contamination of abrasive

L. Extent of chalking of paint M. Discontinuities in coating N. Extent of curing

C1 Fundamentals of Protective Coatings for Industrial Structures 5-22

Unit 5 - Coating Inspection Overview

Quiz 1. The purpose of inspection is: a. to ensure the highest quality work b. to ensure that all specification requirements are met c. to ensure that the owner gets the right value for the job d. to assist the contractor in accomplishing the required work 2. Who is responsible for preparing an adequate coating project specification? a. Coating contractor b. Inspector c. Owner d. Coating manufacturer 3. Which instrument listed below is of use in determining the existing dew point? a. psychrometer b. needle gage c. Delmhorst meter d. Tooke gage 4. The photographic standard best used to assist in determining the level of abrasive blast cleaning is: a. SSPC-VIS 1 b. SSPC-VIS 2 c. SSPC-VIS-3 d. SSPC-VIS 4 5. What instrument is used to establish surface profile of abrasive blasted steel? a. Tooke gage b. ICRI specimens c. cellophane (Scotch) tape d. deformable replicate tape

C1 Fundamentals of Protective Coatings for Industrial Structures 5-23

Unit 5 - Coating Inspection Overview

6. What instrument can be used to determine the extent of chalking of a weathered coating? a. pressure-sensitive (Scotch) tape b. a felt cloth c. a micrometer d. SSPC-VIS 4 7. What instrument determines wet film thickness of a coating? a. a Tooke gage b. a gage with different notch cuts c. a banana gage d. a pencil gage 8. What instrument is used for measuring the diameter of a used blast nozzle? a. wedge-shaped probe b. gage for measuring air pressure c. micrometer d. SSPC-VIS 3 9. What is the most common cause of heavy flash rusting of steel immediately after ultrahigh-pressure water jetting? a. surface contamination with oil b. surface contamination with soluble salts c. surface contamination with blasting dust d. surface contamination with water 10. Which test method can best be used to determine if a two-component thermosetting coating is fully cured? a. vial test b. blotter test c. solvent-rub test d. pressure-sensitive (Scotch) tape test

C1 Fundamentals of Protective Coatings for Industrial Structures 5-24

Unit 5 - Coating Inspection Overview

11. Which document describes the procedure for measuring coating dry film thicknesses on steel using a magnetic gage? a. SSPC-PA 1 b. SSPC-PA 2 c. ICRI 03732 d. SSPC-SP 7 12. What is the typical air pressure range used in abrasive blast cleaning of steel? a. 40-50 psi b. 60-70 psi c. 90-100 psi d. 130-140 psi 13. Which standard describes the use of magnetic dry film thickness gauges? a. ASTM D 3359 b. ASTM D 4145 c. ASTM D 4138 d. SSPC-PA 2 14. Which test can best be used to detect grease or oil contamination? a. water-break test b. solvent-rub test c. transparent, cellophane tape test d. Testex tape test

C1 Fundamentals of Protective Coatings for Industrial Structures 5-25

Unit 5 - Coating Inspection Overview

References ASTM D1400, Standard Test Method for Nondestructive Measurement of Dry Film Thickness of Nonconductive Coatings Applied to a Nonferrous Metal Base (withdrawn, replaced by ASTM D7091-05 Standard Practice for Nondestructive Measurement of Dry Film Thickness of Nonmagnetic Coatings Applied to Ferrous Metals and Nonmagnetic, Nonconductive Coatings Applied to Non-Ferrous Metals ASTM D3276, Standard Guide for Painting Inspectors (Metal Substrates) ASTM D3359, Standard Test Methods for Measuring Adhesion by Tape Test ASTM D4138, Standard Practices for Measurement of Dry Film Thickness of Protective Coating Systems by Destructive, Cross-Sectioning Means ASTM D4214, Standard Test Methods for Evaluating the Degree of Chalking of Exterior Paint Films ASTM D4258, Standard Practice for Surface Cleaning Concrete for Coating ASTM D4259, Standard Practice for Abrading Concrete ASTM D4260, Standard Practice for Liquid and Gelled Acid Etching of Concrete ASTM D4261, Standard Practice for Surface Cleaning Concrete Unit Masonry for Coating ASTM D4262, Standard Test Method for pH of Chemically Cleaned or Etched Concrete Surfaces ASTM D4263, Standard Test Method for Indicating Moisture in Concrete by the Plastic Sheet Method ASTM D4285, Standard Test Method for Indicating Oil or Water in Compressed Air ASTM D4414, Standard Practice for Measurement of Wet Film Thickness by Notch Gages ASTM D4417, Standard Test Methods for Field Measurement of Surface Profile of Blast Cleaned Steel ASTM D4541, Standard Test Method for Pull-off Strength of Coatings Using Portable Adhesion Testers ASTM D4752, Standard Test Method for Measuring MEK Resistance of Ethyl Silicate (Inorganic) Zinc-Rich Primers by Solvent Rub ASTM D4940, Standard Test Method for Conductimetric Analysis of Water Soluble Ionic Contamination of Blasting Abrasives ASTM D5162, Standard Practice for Discontinuity (Holiday) Testing of Nonconductive Protective Coatings on Metallic Substrates ASTM D6132, Standard Test Method for Nondestructive Measurement of Dry Film Thickness of Applied Or ganic Coatings Over Concrete Using an Ultrasonic Gage ASTM D6677, Standard Test Method for Evaluating Adhesion by Knife ASTM D7091, Standard Practice for Nondestructive Measurement of Dry Film Thickness of Nonmagnetic Coatings Applied to Ferrous Metals and Nonmagnetic, Nonconductive Coatings Applied to Non-Ferrous Metals ASTM E337, Standard Test Method for Measuring Humidity with a Psychrometer (The Measurement of Wetand Dry-Bulb Temperatures) Federal Standard 595 Colors Used in Government Procurement ISO 8502-3- Dust Assessment Test

C1 Fundamentals of Protective Coatings for Industrial Structures 5-26



Unit 5 - Coating Inspection Overview

References MIL-DTL-24441, General Specification for Epoxy-Polyamide Paint SSPC AB 1, Mineral and Slag Abrasives SSPC-AB 2, Cleanliness of Recycled Ferrous Metallic Abrasives SSPC-AB 3, Ferrous Metallic Abrasives SSPC-Guide 6, Guide for Containing Debris Generated During Paint Removal Operations SSPC-Guide 15, Field Methods for Retrieval and Analysis of Soluble Salts on Steel and Other Nonporous Substrates SSPC-PA 1, Shop, Field and Maintenance Painting of Steel SSPC- PA 2, Measurement of Dry Coating Thickneass With Magnetic Gauges SSPC-Paint 20, Zinc-Rich Primers (Type I- Inorganic and Type II- Organic) SSPC-SP 1, Solvent Cleaning SSPC-SP 2, Hand Tool Cleaning SSPC-SP 3, Power Tool Cleaning SSPC-SP 5/NACE No. 1, White Metal Blast Cleaning SSPC-SP 6/NACE No. 3, Commercial Blast Cleaning SSPC-SP 7/NACE No. 4, Brush-Off Blast Cleaning SSPC-SP 8, Pickling SSPC-SP 10/NACE No. 2, Near-White Blast Cleaning SSPC-SP 11, Power Tool Cleaning to Bare Metal SSPC-SP 12/NACE No. 5, Surface Preparation and Cleaning of Metals by Waterjetting Prior to Recoating SSPC-SP 13/NACE No. 6, Surface Preparation of Concrete SSPC-SP 14/NACE No. 8, Industrial Blast Cleaning SSPC-SP 15, Commercial Grade Power Tool Cleaning SSPC-TR 3/NACE 6A192, Dehumidification and Temperature Control During Surface Preparation, Application, and Curing for Coatings/Linings of Steel Tanks, Vessels, and Other Enclosed Spaces SSPC-VIS 1, Guide and Reference Photographs for Steel Surfaces Prepared by Abrasive Blast Cleaning Steel SSPC-VIS 2, Standard Method of Evaluating Degree of Rusting on Painted Steel Surfaces SSPC-VIS 3, Guide and Reference Photographs for Steel Surfaces Prepared by Power and Hand Tool Cleaning SSPC-VIS 4, Guide and Reference Photographs for Steel Surfaces Prepared by Waterjetting SSPC-VIS 5, Guide and Reference Photographs for Steel Surfaces Preparaed by Wet Abrasive Blast Cleaning

C1 Fundamentals of Protective Coatings for Industrial Structures 5-27

Unit 5 - Coating Inspection Overview

Additional Reading The Inspection of Coatings and Linings: A Handbook of Basic Practice for Inspectors, Owners, and Specifiers. Drisko, R.W. and Jones, T.A. eds. SSPC: The Society for Protective Coatings, Pittsburgh PA, 2003. “Monitoring and Controlling Ambient Environmental Conditions During Coating Operations” in Systems and

Specifications, SSPC Painting Manual Vol. 2 (latest edition). SSPC: The Society for Protective Coatings, Pittsburgh PA. Corbett, William D. Using Coating Inspection Instruments, KTA Tator, Inc., Pittsburgh, PA, 2002 SSPC’s C2 Course, Planning and Specifying Industrial Coatings Projects

C1 Fundamentals of Protective Coatings for Industrial Structures 5-28

Unit 5 - Coating Inspection Overview

Unit 5 Learning Outcomes Upon completion of this unit, you will be able to:

Unit 5 Inspection Overview

−  Explain the basics of coating specification −  Define the responsibilities of the inspector in establishing that all specification requirements are met −  Explain the operation of basic inspection equipment −  Describe basic inspection procedures

Quality Assurance (QA)

Quality Control (QC)

•  Planned and systematic actions to provide confidence that a system will perform satisfactorily

•  The portion of QA that ensures that materials, methods, workmanship, and the final product meet specified requirements.

The Specification and Its Contents

Inspection •  The purpose of inspection of painting operations is to ensure that the specified work is completed in full accordance with the spec. requirements.

•  Definition, purpose, and scope of the job specification •  Engineering and technical standards •  Format of job specification •  Specifying coating materials

C1 Fundamentals of Protective Coatings for Industrial Structures 5-29

Unit 5 - Coating Inspection Overview

Job Specification

Purposes of the Specification • •  •  •  •  •  • 

•  Part of a legal document •  Gives a detailed description of the desired work •  Defines quality of work •  Defines quantity of work

Results of Poorly Written Specification •  •  •  •  •  •  •  •  • 

Different Ways of Preparing Job Specifications

Bids from unqualified contractors Unrealistically high or low bids Acceptance of substitute (inferior) products Lower-quality or less work than desired Change orders Disputes Defaulted contracts Claims Litigation

•  •  •  •  • 

A Well-Written Specification is: •  •  •  • 

To obtain a specific desired product To assure quality materials and workmanship To make sure the work is completed on time To avoid delays and disputes To obtain minimum or reasonable costs To avoid costly change orders and claims To meet all safety, environmental, and legal requirements

Use old job specifications Cut and paste Paint supplier Computer program Architect/engineer specifier

Inspector’s Responsibilities •  Inspecting materials, surface preparation, coating application, and final product •  Preparing reports •  Verifying that spec requirements are met •  Attempt to resolve observed deficiencies

Correct Clear Concise Systematic

C1 Fundamentals of Protective Coatings for Industrial Structures 5-30

Unit 5 - Coating Inspection Overview

Common Inspection Hold Points or Check Points •  •  •  •  •  • 

Important Ambient Conditions to be Measured During Coating Operations •  •  •  • 

Pre-Cleaning Surface Preparation Primer Application Intermediate Coat Application Top Coat Application Cure

Ambient Conditions That Cause Coating Failures •  •  •  •  •  • 

Temperature Relative humidity Dew point Wind velocity

Coating Defects from Low Temperatures •  •  •  • 

Low temperatures High temperatures Low humidities High humidities High dew points High wind velocities

Defects from High Temperatures

Incomplete leveling of wet films Slow curing of coatings Incomplete curing of coatings Improper curing of coatings

Defects from High Temperatures (cont.)

•  Re-rusting of cleaned steel •  Rapid curing of lacquers (dry spray) •  Rapid curing of latex coatings (poor quality films) •  Exotherm (heat of reaction) accelerating curing defects •  Reduced induction time

•  •  •  •  • 

Reduced pot life Reduced recoat window Wrinkling Pinholes in topcoats over IOZs Outgassing in coatings over concrete

C1 Fundamentals of Protective Coatings for Industrial Structures 5-31

Unit 5 - Coating Inspection Overview

Defects from Low Humidities

Defects from High Humidities

•  Incomplete curing of solvent-based IOZs •  Improper curing of polyurethanes and polyureas •  Rapid curing of latex coatings (poor quality films)

•  •  •  •  • 

Coating Defects from Temperatures Below Dew Point

Flash rusting of cleaned steel Reduced bonding strength of coating Blushing of lacquers Blistering of coatings Improper curing of coatings

Coating Defects from High Winds

•  Similar to those associated with high humidities •  Surface temperature should be at least 5°F above dew point and not falling

•  Contaminated clean surfaces •  Painted surfaces •  Paint overspray

Types of Instruments Used for Measuring Dew Point and Relative Humidity

Sling Psychrometer

•  Sling psychrometer •  Battery-powered psychrometer •  Electronic psychrometer

C1 Fundamentals of Protective Coatings for Industrial Structures 5-32

Unit 5 - Coating Inspection Overview

Using Psychrometric Tables of the U.S. Weather Bureau

Digital Psychrometer

Electronic Psychrometers

Wind Velocity

Types of Surface Thermometers

Remote Sensing Thermometer

•  Dial thermometer •  Digital contact thermometer •  Non-contact thermometer

C1 Fundamentals of Protective Coatings for Industrial Structures 5-33

Unit 5 - Coating Inspection Overview

•  •  •  • 

Thermometer with Magnet on Back

Dehumidification

Pre-Surface Preparation Inspection

Checks to be Made Before Start of Work •  •  •  •  • 

Completion of construction or modification Readiness of job site Abrasive cleanliness Proper size, cleanliness, and operation of blasting equipment

Safety (access ladders, scaffolds, etc.) Utilities Traffic control Disposal containers Cleanup/sanitation

Items to Check in Vial Testing of Abrasive

Inspect Abrasives for: •  Type •  Physical and chemical properties •  Cleanliness

•  •  •  •  • 

C1 Fundamentals of Protective Coatings for Industrial Structures 5-34

Oil or grease Suspended fines Color or turbidity Soluble salts Acidity or alkalinity

Unit 5 - Coating Inspection Overview

Vial Test

Blast-Cleaning Equipment •  •  •  • 

Compressors Water and moisture traps Hoses Nozzles

Nozzle Orifice Gauge

Blotter Test

Hypodermic Needle Gauge Being Inserted into Blast Nozzle

Blast-Cleaned Structure

C1 Fundamentals of Protective Coatings for Industrial Structures 5-35

Unit 5 - Coating Inspection Overview

SSPC-VIS 1

Inspecting Blast-Cleaned Steel •  Degree of cleaning •  Non-visible contaminants •  Surface profile

SSPC-VIS 3

SSPC-VIS 4

Beading of Water on Contaminated Concrete Surface

Ultraviolet Light Used in Hydrocarbon Test

C1 Fundamentals of Protective Coatings for Industrial Structures 5-36

Unit 5 - Coating Inspection Overview

Measuring Volume of Deionized Water for Salt Extraction

Field Salt Retrieval Methods •  Swabbing •  Bresle Retrieval Cell •  Sleeve Retrieval Cell

Swabbing Measured Surface Area with Deionized Water

Injecting Deionized Water into Bresle Cell

Using Sleeve Kit for Extraction of Soluble Salts

Methods for Analysis of Extracts •  The concentration of the salts in the extract is determined by: −  −  −  − 

Electrical Conductivity Meters Chemical Analysis (titration) Use of Test Strips Use of Ion Detection Tubes

C1 Fundamentals of Protective Coatings for Industrial Structures 5-37

Unit 5 - Coating Inspection Overview

Electrical Conductivity Meters

Use of Test Strips

Dust Test

Dust Test •  ISO 8502-3 −  Part 3 is the test procedure for assessment of dust on steel surfaces (pressure-sensitive tape method)

Surface Profile

Measuring Surface Profile

C1 Fundamentals of Protective Coatings for Industrial Structures 5-38

Unit 5 - Coating Inspection Overview

Kean-Tator Comparator

Replica Tape

Replica Tape

Concrete Surface Preparation Requirements •  •  •  • 

Inspecting of Coatings Before Application •  •  •  •  •  • 

Cleaning Roughening surface Measuring moisture content Will be discussed in more detail in Unit 7

ASTM D 4212 System For Measuring Coating Viscosity •  •  •  • 

Select cup with appropriate orifice size Fully immerse cup into thoroughly mixed coating Leave for 1 - 5 minutes to equilibrate Lift cup vertically no more than 6 inches above coating surface •  Measure time with stop watch from lifting of cup from coating to time when coating stream from bottom of cup breaks

Storage Mixing Thinning Tinting Straining Viscosity

C1 Fundamentals of Protective Coatings for Industrial Structures 5-39

Unit 5 - Coating Inspection Overview

Inspection of Coating Applications Consists of Checking •  •  •  •  •  • 

Wet Film Thickness

Induction time, pot life, and recoat window Film thickness Holidays Adhesion Curing Cosmetic and film defects

Pressing Wet Film Gauge into Wet Paint Film

Inspecting Notches of Wet Film Gauge

Factors Affecting Magnetic Gauge Film Thickness Measurements

Dry Film Thickness Gages

•  •  •  •  •  •  • 

Roughness of steel surface Steel composition Steel thickness Surface curvature Surface condition Orientation Other magnetic fields

C1 Fundamentals of Protective Coatings for Industrial Structures 5-40

Unit 5 - Coating Inspection Overview

SSPC-PA 2 (DFT Measurement with Magnetic Gauges) •  •  •  •  • 

Type 1 DFT Gage

Types 1 and 2 (pull-off and electronic) Calibrating gauges and measuring DFTs Spot measurement average of 3 readings Frequency of spot measurements Allowable range of spot readings (80-120%)

Type 2 DFT Gage

Calibration Blocks

Tooke Gage

Calibration Shims

C1 Fundamentals of Protective Coatings for Industrial Structures 5-41

Unit 5 - Coating Inspection Overview

Tooke Gage

View of Cut in Coating Through Illuminated Magnifier of Tooke Gauge

Holiday Detection (Spark Testing)

Low Voltage Holiday Detector

•  For immersed linings •  Low-voltage (< 20 mils) •  High-voltage (> 20 mils)

Holiday Detector in Use on Coated Steel Pipe

Flourescent Coatings •  Locates holidays and areas of low film thickness by use of a black light/UV light •  SSPC-TU 11

C1 Fundamentals of Protective Coatings for Industrial Structures 5-42

Unit 5 - Coating Inspection Overview

Tape Adhesion Equipment

Tests for Coating Adhesion •  Tape tests (ASTM D3359) •  Knife test (ASTM D6677) •  Pull-off tests (ASTM D4541)

ASTM D3359, Method A

ASTM D3359, Method B

Portable Adhesion Tester

Hydraulic Adhesion Tester

C1 Fundamentals of Protective Coatings for Industrial Structures 5-43

Unit 5 - Coating Inspection Overview

Knife Adhesion Test

Mechanisms of Adhesion Failure •  Adhesion. Between coats or primer and substrate •  Cohesion. Split of any one coat •  Glue. Failure of glue.

•  Cut “X” through coating to substrate •  Rate ease of paint removal from cut

Tests for Complete Curing of Coatings

Visual Inspection of Cured Film

•  Solvent-rub test variations −  −  −  − 

•  Cosmetic •  Film defects •  Discussed in more detail in Unit 8

Ethyl silicate inorganic zincs Epoxies Water-borne inorganic zincs FRP coatings

•  Sandpaper test for hard coatings •  Hardness pencils

Unit 5 Summary •  •  •  • 

Basics of coating specification Responsibilities of inspector Operation of basic inspection equipment Basic inspection procedures

C1 Fundamentals of Protective Coatings for Industrial Structures 5-44

Unit 6 - Coatings for Industrial Steel Structures

COATINGS FOR INDUSTRIAL STEEL STRUCTURES 6.1 Purpose and Goals

6.3 Selecting Surface Preparation of Steel

Scope

Surface Preparation of New Steel

This unit covers the various surface preparation

In new and total recoating of steel, abrasive blasting to a

and coating systems that are appropriate for

commercial blast or better is recommended. The specific

use on industrial steel structures in different

recommended level depends on:

environments and services. Student Objectives

•

The generic type of the primer

•

The severity of the environment

•

The desired length of coating service

Upon completion of this unit, you will have an Shop abrasive blasting and coating application (either primer

understanding of: •

or total system) are usually preferred to field operations. The

The appropriate coating systems for

controlled conditions and access to work in shops usually

industrial steel structures in different

result in more economical, higher quality work and avoid

environments •

abrasive blasting concerns in the field. The results of a survey

The appropriate surface preparation

of shop painting operations indicate:

methods for each of these coating systems 6.2 Introduction to Coatings for Industrial Steel Structures

•

total coating systems usually applied

•

variety of abrasives used/recycled

•

manual abrasive blasting more common in small shops; larger shops have both automated (centrifugal) and

Historically, many different surface preparation

manual capabilities

methods and coating systems have been used to protect steel structures from corrosion. More recently, government regulations associated with coating application and removal have caused

•

epoxy systems used the most

•

airless spray used the most

Surface Preparation for Maintenance Painting

more work and costs that the more expensive high-performance systems are almost always

For maintenance painting operations, cleaning of exposed

more economical in the long term than cheaper—

steel by abrasive blasting (wet or dry) or waterjetting usually

but shorter-life—systems. It should also be noted

provides the best steel surface for coating. Should this be

that the cost of the coating materials themselves

impractical or prohibitively expensive, power tool cleaning to

is usually no more than 20 percent of the cost of

bare metal (SSPC-SP 11) may provide the de­sired degree

a total recoating, so that a better material that

of cleanliness and profile. Commercial power tool cleaning

results in a significantly longer service life read­ily

(SSPC-SP 15), power tool cleaning (SSPC-SP 3) and hand

pays for itself.

tool cleaning (SSPC-SP 2) are faster and less expensive than

C1 Fundamentals of Protective Coatings for Industrial Structures 6-1

Unit 6 - Coatings for Industrial Steel Structures

SSPC-SP 11, but do not provide as high a level of clean­

However, the remaining undeteriorated coating must be

liness. However, in many cases, they are adequate for

sound, so that the risk of significant further deterioration

maintenance painting. Both SSPC-SP 11 and SSPC-SP

in the immediate future is small.

15 require a minimum profile of 1 mil. Spot repair and total refinishing will provide a more Waterjetting at ultrahigh pressures (30,000 or more psi

pleasing appearance. It may also extend the life of the

[210 or more MPa]) may also pro­duce the necessary

total system beyond that expected from spot priming

level of cleanliness. In addition, it removes soluble salts,

and spot finishing.

such as chlorides, that adversely affect coating perfor­ m­ance. Waterjetting does not produce a roughened

Total removal and replacement of the existing coating

surface, but it will expose an existing profile. Historically,

involves the least risk of the facilities deteriorating

corrosion inhibi­tors, properly selected and applied, had

significantly but also involves the greatest cost. In

to be injected into the water to prevent flash rusting

addition, it is likely to require significantly greater

before protection is provided by the primer. Today,

operational down time.

specially formulated primers perform well over light flash rusting. SSPC-SP 12/NACE No. 5 defines four

6.4 Coating Systems Appropriate for Steel

degrees of visible surface cleanliness achieved by waterjetting.

A coating system is the combination of a specified level

Alternative Approaches to Correcting Paint

cleaned surface. A typical set of paints will include a

of surface preparation and the paints applied to the primer, one or more intermediates, and a finish (topcoat)

Deterioration

paint. The coating systems most frequently used for

When unacceptable deterioration of an existing coating

steel surfaces include:

system occurs, an important decision has to be made as to the best corrective action: spot priming and spot finishing, spot priming and total refinishing, or complete coating removal and replacement. This decision is usually based on an economic analysis and future plans for the facility. Of course, another alternative is to do nothing or merely clean to restore an acceptable appearance. If modifications are to made to the facility in the near future, no action at the present time may be the best choice if there is little risk of serious facilities deterioration involved. Spot priming and spot finishing is often the most

•

Alkyd and other oil-based systems

•

Epoxy systems

•

Polyurethane systems

•

Polyurea

•

Polyester and Vinyl Esters

•

Zinc-rich systems

•

Water-borne acrylic systems

•

Baking Phenolics

•

Silicones

•

Polysiloxanes

•

Antifouling Coatings

Specifiers, owners, and organizations like SSPC are

economical method of correction, if coating deterioration

continually preparing new and upgrading old coating

is localized and the resultant patchy look is acceptable.

specifi­cations to meet new field needs and health

C1 Fundamentals of Protective Coatings for Industrial Structures 6-2

Unit 6 - Coatings for Industrial Steel Structures

and environmental require­ments. Thus, alternative

•

Epoxy esters. Improved chemical resistance with

specifications are being prepared for products that are

reduced weathering. Used as machinery enamels

lead- and chromate-free and have low VOC levels.

or for fuel splash resistance. •

Silicone alkyds. Excellent UV protection and gloss retention. Used as high quality finish coats.

Alkyds and Other Oil-Based Systems The reaction of drying oils and drying oil fatty acids

•

UV resistance. Used as wood floor and furniture

with synthetic resins forms a range of coating resins

varnishes.

loosely called alkyds. This class of resins provides a major source of decorative and protective coating materials. The term “alkyd,” in fact, describes a wide range of synthetic resins. Alkyd coatings form their film by reacting with oxygen from the air. This process is known as oxidative crosslinking or curing.

Urethane alkyds (Uralkyds). Fast-drying, good

Lead and chromate inhibitive pigments were once used extensively in alkyd coatings. Zinc molybdate and zinc phosphate pigments are more commonly used today. Formulations with reduced VOC limits include:

The most commonly used fatty acids are linseed oils, safflower oil, soya oil, tall oil, tung oil, and dehydrated castor oil. Their oil lengths (percent of phthalic anhydride) are:

•

Water-dispersable alkyds. Do not perform as well as solvent-based products in severe service.

•

High-solids formulations. Lower molecular weight resins used.

•

Short Oil (40-50%)

Alkyds provide a low-cost coating for steel in relatively

•

Medium Oil (30-40%)

mild environments. They are hydrolyzed in alkaline

•

Long Oil (20-30%)

environments (saponified) and so cannot be used on concrete or zinc products.

Longer oil length alkyds dry more slowly, develop less gloss and are more flexible and more weather resistant

SSPC has several paint specifications developed for

than short oil alkyds. Short oil alkyds dry more rapidly

alkyd coating systems. Typically, they utilize one coat of

and develop better gloss, but tend to be brittle. All

primer with inhibitive pigment (e.g., SSPC-Paint 26) and

have excellent wetting and penetration properties which

two top­coats of silicone alkyd for high or medium gloss

makes them surface tolerant. Short and long oil alkyds

(e.g., SSPC-Paint 21) or alkyd (e.g., SSPC-Paint 104)

are not compatible with each other and should not be

for low gloss. NOTE: Primer paints such as SSPC-Paint

blended together.

25 use zinc oxide as the inhibitive pigment. SSPC-Paint 25 is an example of a formu­lation using a lead and

Incorporating synthetics results in modifications with

chromate replace­ment pigment. Navy specification TT-

specific advantages:

P-645B use zinc molyodate as an inhibitive pigment.

•

Phenolic alkyds. Fast curing with good water/ corrosion but poor UV resistance. Used as shop or universal primers for a variety of services.

Epoxy Systems Epoxy coatings have become a versatile workhorse for coating and lining steel structures. Most epoxy C1 Fundamentals of Protective Coatings for Industrial Structures

6-3

Unit 6 - Coatings for Industrial Steel Structures

coatings have good bonding and form a tough durable

lives, increased tendency to amine blush, less tolerant

finish. The primary limitation of epoxy coatings is their

mixing ratios and more toxicity (are skin irritants).

relatively poor sunlight resistance. By utilizing available variations in epoxy resins and curing agents, coatings

Amine adduct-cured epoxies have more tolerant mixing

with a variety of properties have been developed to meet

ratios and have less tendency to amine blush than the

multitude of needs, sometime enhancing one property

aliphatic amine-cured epoxies. Yet, they have a higher

while compromising one or more other properties.

viscosity and longer curing time.

Epoxy mastics are high-solids, high-build (at least 5

Aromatic amine-cured epoxies have greater chemical

mils DFT) formulations, often aluminum-filled, that are

resistance but are slower curing (requiring accelerators)

surface tolerant and usually compatible with most other

and have a very poor resistance to ultraviolet light.

coatings. Ketamine-cured epoxies have longer pot lives and less Epoxy resins based on Bisphenol A are general purpose

toxicities but are slower curing and require moisture

resins. Epoxy resins based on Bisphenol F have good

for curing.

physical properties and chemical resistance. They are often used in food and beverage and pharmaceutical

Cycloaliphatic amine-cured epoxies have good toughness

facilities due to their FDA and USDA approval status.

and thermal resistance but are moisture sensitive and

The highly crosslinked Bisphenol F novolac resins are

slow curing (may require an accelerator).

chemical resistant “work horse” resins.

Two-Component Polyurethane Systems

Polyamide-cured epoxies when compared to amine-

Two-component polyurethane coatings are formed

cured epoxies have better water resistance, better

by the reaction of two components, a polyisocyanate

flexibility, less tendency to amine blush and a longer pot

and a polyol. A polyisocyanate contains two or more

life. They do, however, have greater amounts of solvent

isocyanate groups (-N=C=O) groups, and a polyol

(VOCs), longer curing times (may require an induction

contains two or more hydroxyl (-OH) groups. Because

period) and less chemical resistance than amine cured

each component contains more than one functional

epoxies. Addition of coal tar or coal tar pitch to epoxy

group, the reaction product can be cross-linked to form

polyamide formulations provides greater water and

three-dimensional thermoset polymers. Greater cross-

chemical resistance and film build, while reducing

linking results in a harder, more chemically-resistant

the coating cost. However, coal tar is toxic (requires personal protection) and is slower curing.

polymer.

Aliphatic amine-cured epoxies, compared to polyamid-

The most common polyol co-reactants are polyethers, acrylics (polyacrylates), and polyesters. Of these three,

cured epoxies, are more chemically resistant and faster

generally:

curing, have lower viscosities (require less solvent) and form a tougher film. However, they have shorter pot

C1 Fundamentals of Protective Coatings for Industrial Structures 6-4

Unit 6 - Coatings for Industrial Steel Structures

•

Polyethers have better resistance to hydrolysis but

New polyurethanes have been developed with low VOC

poorer weathering. Thus, they are used on roofs

contents. Waterborne two-component products have

and secondary containment structures where water

also been developed.

accumulates. •

Acrylics have the best color and gloss retention and so are used on exteriors of water tanks, railroad locomotives, and other structures where aesthetics is important. Clear coats can enhance the color and gloss retention in the undercoat.

•

Polyesters have the best chemical resistance (and may have good color and gloss retention) and are used where chemical resistance is important (areas susceptible to splashes, spills and graffiti).

Moisture Curing Polyurethanes Moisture curing polyurethane polymers are based upon the reaction of isocyanates with moisture in the air. The isocyanate group (-N=C=O) reacts with any available compound containing active hydrogen such as moisture, and so moisture curing polyurethanes must be sealed in closed, dry containers. The isocyanates used in these resins are multifunctional (have more than one isocyanate group on the molecule). This permits

Other variables will affect the performance of these products. Thus, the amount of excess isocyanate and the types of additives in the coatings will affect the film properties.

crosslinking into a three dimensional structure with good water and chemical resistance. Greater cross-linking results in harder, more chemically-resistant polymers. Curing of moisture-curing polyurethane coatings occurs

The two-components, after thorough mixing separately and then together, have a pot life of 4 to 6 hours before they become too viscous to apply. Mixed coatings can be applied by brush, roller or spray. Plural-component spray equipment should be used for coatings with very short pot lives. Moisture-curing (one component) polyurethanes are more commonly used as primers. Clear finish coats enhance the color and gloss retention of pigmented base coats.

in stages. The isocyanate groups first react with moisture to form an amine and carbon dioxide. The amine then reacts with other isocyanate groups to form polyureas, and this process continues until all of the isocyanate groups are consumed. The carbon dioxides formed during curing must escape from the wet film to prevent bubbling problems in the cured films. The isocyanate can be aliphatic (contain only saturated bonds) or aromatic (contain benzene-like unsaturated

The application temperature range is normally from 40°F (10°C) to 110°F (43°C). Polyurethanes have been developed for application below 40°F (10°C). Moisture condensation on wet polyurethane coatings may result in a drop in gloss or micro-blistering. Thus, the surface temperature should be at least 5°F (3°C) above the dew point during application.

rings). While both of these general types have many similar properties, the more costly aliphatic polyurethane coatings have much better weathering characteristics (color and gloss retention) and can be used as topcoats for exterior coating systems. Aromatic polyurethanes chalk and yellow in sunlight or bright artificial lighting. However, the aromatic type has somewhat better chemical resistance and toughness. It is usually preferred as a primer but must be topcoated soon after

C1 Fundamentals of Protective Coatings for Industrial Structures 6-5

Unit 6 - Coatings for Industrial Steel Structures

application to avoid the surface chalking preventing

protective equipment and respirators should be carefully

good adhesion of topcoats.

followed.

Moisture curing polyurethanes are slightly moisture

Polyurea Systems

tolerant, because the surface moisture reacts with the

Two-component polyurea coatings are cured by the

isocyanate. Products are available with a wide range of

reaction of an isocyanate component and an amine resin

properties from soft and flexible to hard and chemically

component. The isocyanate component consists of a

resistant. All generally have good wear and abrasion

pre-polymer formed by partially reacting a polyisocyanate

resistance. Other good features of the polyurethanes

(compound with more than one isocyanate group) with

include: •

a polyol (compound with more than one alcohol group).

Primers (generally aromatic) with good adhesion

This reduces the number of free (unreacted) isocyanate

available with barrier (aluminum and micaceous

groups, thereby reducing the toxicity, and providing a

iron oxide) and galvanic (zinc) pigments

1:1 volume ratio of components. Very slightly reacted

•

Intermediate coats with good barrier protection

isocyanates (with very high amounts of free isocyanate

•

Weather-resistant topcoats (with limited color

groups) called quasi-prepolymers or semi-prepolymers

availability)

provide special formulating capabilities. Varying the isocyanate, polyol and amine chemistries can produce a

Moisture-curing polyurethanes can be applied at very

variety of products with different desirable properties.

low temperatures (below freezing) but their curing rates are greatly reduced.

Polyurea polymers have two distinct types of chemistry, aromatic and aliphatic. As with polyurethanes, the

Moisture contact must be avoided by slow speed stirring

aromatic and aliphatic natures are associated with

that does not produce a vortex. Boxing should not be

the chemistry of the isocyanate component. Aromatic

used to mix these coatings.

polyurea coatings usually have better chemical resistance than aliphatic polyurea coatings and are

The relatively high cost of moisture-curing polyurethanes

cheaper. However, they chalk and yellow in sunlight.

can be offset by:

Aliphatic polyurea coatings cure faster and are stable

•

No required induction time

•

Quick re-coatability and placement into service

•

Wider range of acceptable application

in ultraviolet light. Both types have good flexibility and strength and such short pot lives that they must be applied by plural component spray. Polyurethanes and polyureas can be co-polymerized to yield hybrid

temperatures

polymers that have been used extensively in liners.

VOC levels for polyurethanes vary widely and newer

The later development of polyaspartic coatings provided

products have very little VOCs. Safety concerns during

apliphatic polyurea systems that produced thin films

application include irritations to the eyes, skin, throat

with longer cure times, so that they can often be applied

and lungs. MSDS recommendations on personal

by brush, roller or conventional/airless spray. Also,

C1 Fundamentals of Protective Coatings for Industrial Structures 6-6

Unit 6 - Coatings for Industrial Steel Structures

they have high gloss and excellent gloss and color

Polyester and vinyl ester linings are low viscosity

retention.

coatings that cure quickly at ambient temperatures to form strong, tightly adhering ( to metals and concrete)

Polyaspartic systems are used on direct-to-metal (DTM)

films with good chemical and high temperature

systems for railcar coating and as a thin DFT topcoat

resistance. Thus, they are extensively used for tanks,

for color stability.

secondary containment and floor linings, as well as coatings for structural steel, walls and ceilings.

Steel surfaces must be thoroughly blast cleaned before coating. An SSPC-SP 6/NACE No. 3 is adequate for

Significant limitations of polyester and vinyl ester

non-immersion service, but at least an SSPC-SP 10/

coatings are associated with the stresses created by

NACE No. 2 is required for immersion service.

their high shrinkage and the heat produced during curing. These stresses can cause the rigid coatings

Fast-curing polyurea coatings must be applied by a plural-component spray system that mixes the properly proportioned, heated components at the gun. Whenever application is briefly stopped, the mixed coating remaining in the gun must be removed mechanically or by air or solvent purging. The VOC content of polyurea coatings in none or very

to crack and disbond unless they are strengthened by addition of fillers or reinforcement. More flexible lining variations cannot usually be used, because they have much lower chemical resistances. Different systems for increasing the strength or impermeability of polyester and vinyl ester linings include: •

Un-reinforced filled systems. Fillers of silica and other minerals are used to extend the base resin or

low, so that VOC limitations cause no problems.

reduce cost, curing shrinkage and coefficient of film thermal expansion. Alternatively, broadcast flooring

Polyesters and Vinyl Esters

systems can be used.

Polyesters are thermosetting coatings that cure

•

chopped fibers or woven cloth reinforcement can

by crosslinking after application to a steel surface.

increase strength and reduce cracking. During

Separately mixed peroxide initiators and accelerators

lining installation, the fiberglass is saturated with

start the curing of the polyester prepolymer. Styrene,

catalyzed resin and rolled to wet glass strands well

present along with the pre-polymer, causes crosslinking to form a three dimensional solid film. Since styrene is a compatible liquid, a solvent is not normally required for these coatings.

Reinforced composite systems. Fiberglass

and to remove air bubbles. •

Flake-reinforced linings and coatings. Flake reinforcement can reduce moisture and chemical penetration.

Vinyl esters constitute a type of polyester in which the pre-polymer is formed by the esterification reaction of Bisphenol A or another epoxy with acrylic acid or methacrylic acid. These coatings are very strong and have exceptional chemical resistance.

Blast cleaning to an SSPC-SP 5/NACE No. 1 is almost always required for polyester and vinyl ester coatings. Angular grit abrasive should be used to give the coarse, angular profile of 3 mils (75 micrometers) necessary

C1 Fundamentals of Protective Coatings for Industrial Structures 6-7

Unit 6 - Coatings for Industrial Steel Structures

to provide strong adhesion during stresses from

SSPC-Paint 20 classifies three categories of inorganic

dimensional changes due to curing and temperature

zinc-rich coating, the curing of which is affected

changes.

differently by ambient conditions:

Skilled personnel can apply polyester and vinyl ester

•

coatings by rolling, spraying or troweling. In all cases,

Type 1-A post-curing, water-borne (with alkali metal silicate binder)

the procedures published in the manufacturer’s product

•

data sheet must be carefully followed to obtain optimum

Type 1-B self-curing, water-borne (with alkali metal silicate binder)

performance. Proper lining thickness is essential to

•

provide effective protection quality to the linings and

Type 1-C self-curing, solvent-borne (usually with ethyl silicate binder)

coatings.

The water-borne types (1-A and 1-B) have very low

Surface curing of the coatings is generally inhibited

VOCs, but the solventborne type (1-C) has significant

by oxygen in the air. This inhibition may be reduced

amounts of VOCs.

by incorporating paraffinic additives into the topcoats. These additives reduce air inhibition by forming a barrier

SSPC-Paint 20 also defines three levels of zinc loading.

film over the surface prior to curing. A more expensive

The higher the loading, the longer lasting galvanic

but effective procedure to prevent oxygen inhibition is

protection.

the addition to the formulation of an oxygen-reactive product that preferentially consumes the surface

Properties of inorganic zinc-rich primers that result

oxygen.

in their many uses despite their relatively high cost include:

Special safety concerns of polyester and vinyl ester coatings focus on the toxic styrene in the formulation.

•

Safer alternative monomers include vinyl toluene,

Excellent long-term protection of steel in atmospheric service

vinyl acetate and methyl methacrylate. Respirators

•

Good high-temperature resistance

and personal protective equipment should be used

•

Slip resistance

to prevent irritation of the skin, eyes and throat and

•

Weld-through formulations of pre-construction

damage to the nervous system. Safety instructions for

primers

specific products are provided in their manufacturers’ material safety data sheet.

The surface cleanliness requirements are more stringent than those required by most other coatings. For a

Inorganic Zinc-Rich Systems

severe industrial or marine environment, an SSPC-SP 10 or SSPC-SP 5 is required. The usual recommended

Inorganic zinc-rich coatings contain a high loading of

profile height is 1 to 3 mils. Oil contamination is

zinc dust in an inorganic binder, usually a silicate. They

especially detrimental to water-borne types. However,

protect steel surfaces first by galvanic action and later

all three types are less sensitive to effects of residual

by barrier protection.

soluble salt contamination than are most organic coatings.

C1 Fundamentals of Protective Coatings for Industrial Structures 6-8

Unit 6 - Coatings for Industrial Steel Structures

Inorganic zinc-rich coatings can be applied by

several advantages as well as disadvantages compared

conventional air or airless spray. Their application

to inorganic zinc-rich coatings. Advantages include:

procedure is more detailed than application of organic coatings:

•

Less stringent surface preparation requirements

•

Easier application and curing

•

The binder is first thoroughly mixed

•

Easier to topcoat

•

The zinc dust is slowly added with agitation

•

Available in more generic compositions, each

•

The mixed coating is filtered through a seive to remove clumps

•

with different properties •

The filtered coating is then continuously agitated until spray application is complete

Curing not so easily affected by ambient conditions

•

Proper dry film thickness not so critical

•

Easier to repair defects

The dry film thickness of the coating cannot be determined directly from wet film thickness measurements.

Advantages of inorganic zinc-rich coatings compared

Alternative procedures must be used to determine the

to zinc-rich coatings include:

relationship between wet film and dry film thicknesses. Excessive dry film thickness (e.g., excess of 5 mils) may result in mudcracking of the film. Inorganic zinc-rich coatings are seldom topcoated with themselves. Topcoating of inorganic zinc-rich primers often results in topcoat bubbling or blistering associated with the primer porosity. Two methods of controlling this problem are: •

Applying a mist coat followed by a full topcoat

•

Applying a barrier or tie coat to seal the porous

•

Higher level of galvanic protection

•

May be used without a topcoat

•

Faster drying

•

Better high-temperature resistance

•

Better resistance to ultraviolet light

•

Better abrasion resistance

•

Better solvent resistance

Different generic types of zinc-rich coatings include:

surface before topcoating

•

Epoxy-polyamide type most commonly used for new and maintenance work

•

Organic Zinc-Rich Coatings

Polyurethane- good general use organic zinc-rich primer; can be applied at low temperatures

Organic zinc-rich coatings contain a high loading of zinc

•

dust in an organic binder. SSPC-Paint 20 describes Type II as having a binder that can be chemically cured

touch-up of galvanizing •

(thermosetting) or may dry by solvent evaporation (thermoplastic). Thus, these coatings may have a wide

Phenolic- used to repair galvanizing and line fresh water tanks

•

variety of binder formulations. The amounts of zinc loading also can vary widely.

Oil/Alkyd- specialty use materials such as repair/

Silicone- used primarily in high-temperature service

•

Epoxy ester- primarily used as cold galvanizing compounds

Organic zinc-rich coatings provide corrosion control to steel by both barrier and galvanic protection. They have

•

Chlorinated rubber- used as cold galvanizing compounds; high VOCs restrict use today

C1 Fundamentals of Protective Coatings for Industrial Structures 6-9

Unit 6 - Coatings for Industrial Steel Structures

• •

Vinyl- used on locks and dams; high VOCs

is a performance based specification that includes

restrict use today

outdoor testing.

Phenoxy- used as primers and cold galvanizing compounds; high VOCs restrict use today.

Waterborne acrylic coatings usually require a commercial blast cleaned surface (SSPC-SP 6/NACE No. 3).

The surface preparation, mixing and application of

Special care must be taken to ensure that there is no

organic zinc-rich coatings vary with different generic

residual oil or grease on the cleaned steel surface.

types. In all cases, the instructions of the manufacturer’s product data sheet should be followed carefully.

Waterborne acrylic coatings can be applied by brush (synthetic bristles are recommended), roller or

Waterborne Acrylic Systems

spray (stainless steel equipment is recommended). Application recommendations on the manufacturer’s

Water-borne acrylic coatings are based on latexes

product data sheets should be followed. Proper

of resins derived from acrylic acid, methacrylic acid

techniques, including avoiding over mixing, will minimize

and esters of these acids. These basic resins can

the formation of tiny bubbles. If any water thinning is

be modified in many ways to produce coatings with a

done, care must be taken to avoid over thinning, which

variety of desirable products for steel, galvanized steel, aluminum, concrete, masonry and wood substrates.

can have disastrous results.

A wide range of gloss (from low to high) is available

Waterborne acrylic coating systems have a history of protecting steel, including bridges in marine

for primer and topcoats. Their exterior gloss and color

environments and chemical tank exteriors in aggressive,

retention is outstanding, so that they can be used as

Gulf Coast environments.

finish coats for other exterior systems. Acrylic latex systems have also been successfully used as directto-metal (DTM) systems.

Furthermore, waterborne acrylic formulations have very

Optimum film quality requires careful polymer (latex)

regulatory limit.

low VOC content, well below any current or projected

design and choice of formulation additives. Coalescing

Baking Phenolics

solvents are required to obtain continuous, impermeable films. During the loss of these slow-evaporating

Heating a mixture of phenol and formaldehyde with

solvents, film pores are filled and a soft, flexible film

alkali catalysts produces thermoplastic resole resins.

is produced. Because of the relatively slow curing

These hard, chemical resistant resins are the basis of

process, acrylic and other waterborne latex coatings

baking phenolic coatings. Heating a mixture of phenol

have not performed as well as other generic types

and formaldehyde with acid catalysts produces novolac

in traditional accelerated laboratory tests. However,

resins. Novolac resins are the basis of a class of epoxy

their field performances are comparable to those of

resins. Novolacs can be combined with resoles to form

other coating systems. Thus, SSPC-Paint 24 for latex

a baking resin with chemical resistance to the most

painting system for industrial and marine atmospheres

aggressive environments.

C1 Fundamentals of Protective Coatings for Industrial Structures 6-10

Unit 6 - Coatings for Industrial Steel Structures

Phenolics can also be used to modify epoxy resins

Steel surface preparation requirements for bake

that can be cured at a lower temperature, some even

phenolic coatings include abrasive blasting to SSPC-SP

at room temperature.

5 and a profile of 1 to 2 mils (25 to 50 micrometers).

Outstanding chemical and physical properties of baked

These coating are most often spray-applied at a dry

phenolic coatings include:

film thickness of 1 to 2 mils, but can also be applied by

•

Chemical resistance- Resistant to hydrocarbons, chlorinated solvents, mild alkalis, petroleum products, and organic acids, alcohols, esters and ketones.

•

Gloss- Most finishes are flat or semi-gloss but a high gloss is available in unpigmented topcoats.

•

Electrical resistance- Good electrical resistance permits use as insulators in motor windings,

Thermal conductivity- Good heat transfer permits use on finned tube and shell and tube heat exchangers.

impact and stretching without cracking. High temperature resistance- They can withstand temperatures as high as 450°F (232°C), as well as boiling water and steam up to 300°F (150°C). •

Coefficient of thermal expansion- It is slightly lower than for other organic coatings and so can expand and contract with most metals.

•

300 g/L. Less chemically resistant waterborne products are also available. Safety concerns include flammable solvent and personal

Silicones Silicones are inorganic polymers containing a silicon-

Bend and impact resistance- Pigmented, plasticized baking enamels withstand abrasion,

•

reformulations have reduced the VOC content to about

are specified in their material safety data sheets.

systems.

•

Baked phenolic coatings have historically contained low

contact. Respirators and personal protective equipment

aircraft instruments and missile guidance •

baking, which usually causes color darkening.

solids contents and thus high VOC contents. However,

Hardness- A pencil hardness of 8H to 9H provides good scratch and abrasion resistance.

•

brush or roller. Curing requires several steps of oven

Abrasion resistance- Abrasion resistance is very good, especially for the unpigmented coatings.

Because of these properties, baked phenolic coatings are used on heat transfer equipment, pumps and insulators and as linings for containers and tanks.

to-oxygen backbone (...-Si-O-Si-O-...) rather than the carbon-to-carbon backbone (...-C-C-...) found in organic polymers. Silicon-oxygen polymers have two side groups attached to each silicon atom. One or both of these side groups may be organic (usually phenyl or methyl). Phenyl side groups are better for heat resistance, and methyl side groups provide better cure properties and water repellency. Varying the silicon oxygen chain lengths, the two side groups and crosslinking can produce a variety of silicone resins with excellent high temperature and weather resistance. Some of the more common applications for 100% silicone (no organic co-polymers) coatings include hightemperature stacks, mufflers and manifolds, boilers,

C1 Fundamentals of Protective Coatings for Industrial Structures 6-11

Unit 6 - Coatings for Industrial Steel Structures

ovens and furnaces, steam lines, heat exchangers,

Abrasive blasting to SSPC-SP 10 is the normally

cooking utensils, combustion chambers, incinerators

required preparation of steel surfaces. Application is

and barbecue equipment.

usually by spray, but brush and roller application may be adequate. Film thicknesses must be no greater than those recommended by the manufacturer.

Common uses of coatings based on cold polymer blends (i.e., not copolymerized) or copolymerized organic-inorganic hybrid resins include space heaters,

Silicone coatings generally require bake curing. The

camp stoves, ranges and dryers, lanterns, light bulbs,

curing recommendations for each proprietary coating

generators and processing equipment.

can be found in the product data sheets.

Silicone formulations for use at different temperatures

Low VOC formulations are available. Silicones are

vary widely:

relatively safe to use, but the recommendations in

•

their material safety data sheet should be carefully

250-400°F (121-204°C)- Silicone resin 5 to 50%;

followed.

cold blending with alkyds, epoxies and other organic polymers may be cost-effective •

•

•

Polysiloxanes

400-600°F (204-316°C)- Silicone resin 15 to 50%; leafing aluminum increases heat resistance of

Polysiloxanes coatings are one of the newest generic

coatings; colored pigments are less stable/require

coating variations. Acrylic siloxane hybrid coatings

more silicone

are high solids, low VOC and ambient temperature

600-800°F (316-427°C)- Silicone resin content

curing products and epoxy siloxane hybrids are ultra-

30-70% for aluminum and 70-100% for color

high solids, low VOC and ambient temperature curing

finishes; cold blends or co-polymers of organic

coatings. The improved durability of the epoxy siloxane

resins used

hybrid in traditional multi-coat coating systems provides

800-1,000°F (427-538°C)- Silicone resin content

lower application and life-cycle costs for protection of

usually 100%; aluminum pigment for upper part

large structures.

of range and black metal oxide pigment for lower part •

Inorganic siloxane and organic-inorganic siloxane hybrid

1,000-14,000°F (538-76°0C)- Silicone resin

coatings have surface preparation and application

content is 100%; ceramic frits that fuse into

methods similar to those of organic coatings (e.g., brush,

substrate -Si-O-Si bond gives prolonged service

roller and conventional or airless spray). Special care

at higher temperatures

should be taken to ensure that the dry film thickness recommended by the manufacturer is achieved.

By themselves or in combination with organic film formers, silicone resin based coatings have reduced the

Antifouling Coatings

maintenance and increased the longevity of processing

Marine fouling damages ships and fixed structures to

equipment and appliances.

which they become attached and grow. Ships have their speeds reduces and their fuel consumption increased.

C1 Fundamentals of Protective Coatings for Industrial Structures 6-12

Unit 6 - Coatings for Industrial Steel Structures

Many attempts have been made to develop coatings that will provide long lasting control of fouling without adversely affecting the environment.

6.5 Selection of Coating Systems by Environmental Zone General Considerations

Fouling begins on stationary structures (e.g., ships in

The exposure environment is usually the chief

port but not at sea) with the development of a slime of

concern when selecting a coating system for industrial

micro-organisms. This is followed later by attachment

steel structures. Other factors include government

and growth of spores and larval forms of macro-plant

restrictions, ease of application and maintenance, and

and animal fouling.

cost (both initial and long-term).

Soluble matrix antifouling coatings have slightly

Locations with Differing Exposure Environments

water-soluble binders that dissolve to release their toxic chemicals, usually copper oxide. They are useful on stationary structures and slow vessels. They provide protection for only 1 to 2 years.

SSPC, in its Steel Structures Painting Manual, Volume 2, Systems and Specifications, identifies eight different locations with special environments: 1. Inland, rural regions: Regions remote from

Insoluble matrix antifouling coatings have higher copper

coastal and industrial areas and their pollutants. It

oxide loadings and are harder, so that they can be used

should be noted that some remote, isolated areas

on fast ships, as well as stationary structures. They also

now receive acid rain from wind-borne sulfur

have a service life of 1 to 2 years.

dioxide pollution from industrial or power plants. 2. Heavy industrial regions: Paint life is reduced and

Self-polishing antifouling coatings have binders that undergo degradation (hydrolysis) on ships traveling at high speeds (not docked in ports). These coatings may

corrosion rates are increased. 3. Marine atmospheric region: Contains salt mist from the ocean. 4. Fresh and salt water immersion regions:

have a service life of ten or more years.

Differences between fresh and salt water Fouling-release coatings have smooth surfaces to which fouling organisms do not attach themselves very securely. Thus, most are lost from the hulls of ships traveling at high speeds. These coatings are relatively new and undergoing further development.

immersion occur due to osmotic and electrolytic effects. 5. Alternative water immersion regions: These regions occur on structures subject to loading and tidal changes, for instance, a ship hull or an offshore oil rig.

All antifouling coatings have a leach surface layer with depleted toxic chemicals. Those of insoluble matrix antifouling coatings are thicker and rougher than those of self-polishing coatings. Thus, they place more drag on ships and present more antifouling renewal problems.

6. Condensation and high humidity regions: This refers to areas that have almost continuous condensation. 7. Chemical environments: These include contact of high concentrations of corrosive gases, fumes, or chemicals on steel.

C1 Fundamentals of Protective Coatings for Industrial Structures 6-13

Unit 6 - Coatings for Industrial Steel Structures

•

Underground region: This refers to buried

also desired. Both needs can be met by proper use of

structures, such as piping in direct contact with

available cleaning methods and coating products.

soil, that may have high salinity (low resistance) or acidity.

Before 1980, steel bridges were most frequently coated with red lead-pigmented unmodified oil or alkyd primers

This unit will discuss coating systems for steel industrial

and alkyd or silicone alkyd topcoats. After 1980, the

structures in these locations.

use of lead pigments was greatly reduced because of health and environmental concerns. Thus, lead- and chromate-free oil, alkyd, and high-performance zinc-

6.6 Coatings for Atmospheric Zones (Mild and

rich and epoxy and polyurethane systems are being

Severe)

used today. Vinyl lacquers were once used extensively

Typical structures in atmospheric service include

on bridge exteriors, but their high VOC contents have

bridges, storage tanks, piping, towers, and industrial

restricted their use.

plants.

Bridges in Mild Environments

Steel structures in mild atmospheric environments (rural, inland) can be effectively protected with an

Bridges in relatively dry locations can be coated with

alkyd (e.g., one coat of primer and two silicone-alkyd

alkyd or water­borne systems applied over a commercial

topcoats). In severe environments (with salt or industrial

blast finish (SSPC-SP 6). Power tool cleaning (SSPC-

pollutants), a high-performance coating (e.g., two coats

SP 3) may be adequate for maintenance work.

of epoxy and a finish coat of aliphatic polyurethane)

These systems are easy to repair when they become

is more appropriate. Obviously, high-performance

damaged.

systems are also appropriate for mild environments and may be economical where coating maintenance

Other coating systems used for maintenance painting

and replacement costs are very high.

(“overcoating”) include (1) epoxy mastic systems (a penetrating, 100% solids epoxy primer is sometimes

Steel pipes and other items in atmospheric service that

used to improve adhesion, and a polyurethane topcoat

do not require a painted finish can be protected with

applied to improve appearance and protection) and (2)

petrolatum pastes and micro-crystalline waxes. They are

moisture-cured polyurethanes.

usually wrapped with tape impreg­nated with this material or with a plastic overwrap. The only surface preparation

Bridges in More Severe Environments

they require is a power tool cleaning (SSPC-SP 3) to

Bridges located in more severe (damp, immersed,

remove loose surface contaminants.

or chemical) environments require high-performance systems. Application of road salts and poor bridge

Bridges

design greatly add to the severity of existing climatic

Highway and railroad bridges are coated to protect their

environments. One of the more widely recommended

steel components from corrosion. However, because

high-performance systems include (1) an inorganic

they are highly visible, an attractive finish is usually

or organic zinc-rich primer (2) an epoxy polyamide

C1 Fundamentals of Protective Coatings for Industrial Structures 6-14

Unit 6 - Coatings for Industrial Steel Structures

midcoat, and (3) an aliphatic polyurethane topcoat for

Two examples of tower structures are transmission towers

improved weathering and appearance.

for electrical power distribution and communication/ radio towers. The most recognizable tall structure is

Many bridges still have lead-containing coatings on

the elevated water storage tank. Storage tank painting

them, so that special containment, storage, and disposal

is covered in a later section of this chapter.

procedures must be followed when removing these materials.

Corrosion Protection Corrosion control is the chief reason for coating towers.

Exteriors of Storage Tanks, Piping, and Other

A good tower design will eliminate many corrosion

Steel Structures

problems. Pipe legs and other support components

The exteriors of all steel storage tanks, piping, and other

are preferred to those with edges. Flanges and other

industrial structures in atmospheric service may use

features that collect rainwater should also be properly

the same coating systems, regardless of their use or

oriented or eliminated.

the materials stored in them. They all require corrosion protec­tion and an attractive appearance, since they are

Galvanizing. Today, most new towers in the US are

quite visible to people in the area.

coated by hot-dip galvanizing the compo­nents or by thermal spray metallizing with zinc. The latter system

As with steel bridges, the selection of a coating system

must be used for components that are too long for

is dependent on the severity of the environment. In mild

dipping in tanks. A two-mil (50 µm)-thick layer of

environments, a three-coat alkyd system (preferably

galvanizing can provide long-term protection in many

with a silicone alkyd finish) or a three-coat water-borne

environments, but failures in less than 10 years have

acrylic system will perform well over a commercial blast­

been repor­ted at some tropical and marine locations.

cleaned (SSPC-SP 6) surface. Preparing galvanized steel. New galvanized or In a more severe environment, a high-performance

thermal sprayed zinc surfaces can be cleaned for

system is necessary. This may be a system with two

coating by solvent washing with mineral spirits. It

coats of epoxy and a finish coat of aliphatic polyurethane,

may be necessary, however, to wash with a solution

or a system of one coat each of zinc-rich epoxy, epoxy-

of mild detergent to re­move more tightly adhering

polyamide, and aliphatic polyurethane products.

contaminants.

Towers and Other Tall Structures

Topcoating galvanized steel. A prime coat of epoxy-

There are many towers and other tall metal structures that require coating. These structures are coated for one or more of the following reasons:

polyamide and a finish coat of aliphatic poly­urethane­ will provide 5–10 or more years of additional protection, de­p ending on the environment. A moisture-cured polyurethane system is equally effective. Two coats of

•

Corrosion protection

water-borne acrylic will also provide good protection,

•

Appearance

gloss, and color.

•

Visibility C1 Fundamentals of Protective Coatings for Industrial Structures 6-15

Unit 6 - Coatings for Industrial Steel Structures

It is desirable to have an attractive finish on these readily

It is best to have a regularly scheduled maintenance

visible structures. For tall towers, either a special lighting

inspection and painting program to keep deterioration

system or alternating orange and white bands are

under control. Loose coating should be removed by

applied to provide rapid detection by pilots flying aircraft

scraping and sanding, and a patch of the original

in the area. Refinishing of the orange and white bands

coating system should be applied by brush or roller, as

may become necessary should color fading occur.

appropriate with a two-inch overlap onto existing sound coating. Should both coating and zinc metal be lost so

Thermal spray zinc. Thermally sprayed zinc is

that localized rusting occurs, these areas should be

relatively porous and must be sealed with a low-

water blasted or brush blasted with abrasive to clean

viscos­ity coating to provide longer-term protection. A

the steel before repairing the coating.

topcoat may be added to provide additional service 6.7 Linings for Immersion Service

life. Thermal sprayed, zinc-coated towers have been successfully protected with an aluminium-filled epoxy

Linings for immersion service must be resistant to the

mastic, followed by an aliphatic poly­urethane­ finish

stored products and must prevent their contamination.

when good color and gloss retention are desired. The current industry standard for application of Thermal

Linings for Water Tank Interiors

Spray Coatings is SSPC-CS 23.00.

Almost all states require that coating systems used to Conventional coatings. Components of new steel

line potable water tanks conform to ANSI/NSF 61. Most

towers should be abrasive-blasted in a shop to a

of those approved to date have been epoxies. Approval

near-white finish (SSPC-SP 10) and coated with one

means that no material leached from the coating system

coat of zinc-rich epoxy (SSPC-Paint 20, Type II), one

will be in sufficient quantity to cause adverse health

coat of epoxy-polyamide, and one coat of aliphatic

effects. The in-service performance of the system is

polyurethane. This system is also good for repairs,

not considered by this standard.

should the original coating system become damaged. About half of the water tank systems in the United Maintenance

States are cathodically protected. If these interiors are functioning properly, there should be little corrosion in

Because they present major repair and recoating

immersed areas except on ladders or other areas not

problems (high towers are usually only climbed by

cathodically protected. Blisters may occur in im­mersed

certified steeplejacks), new towers should be cleaned

areas from coating solvent retention or excessively

and coated in a shop environment with a high-

high catho­d­ic protection voltages. The top areas of

performance system. Historically, most towers were

the tank, especially the edges of support beams that

coated with a lead-contain­ing system. This presents

are not cathodically protected, are where most of the

major field repair and removal problems. Also, field

rusting occurs.

application of coatings to tall towers is frequently done using a synthetic wool mitt. This does not produce a uniformly thick, holiday-free coating and can be very messy. Thus, it is not recommended. C1 Fundamentals of Protective Coatings for Industrial Structures 6-16

Unit 6 - Coatings for Industrial Steel Structures

Linings for Interiors of Wastewater Tanks

suitable epoxy system is described in MIL-STD-3007:

Interiors of wastewater tanks must be resistant to the corrosive product contained. They are usually lined with two or more coats of epoxy or two coats of coal tar epoxy (e.g., SSPC-Paint 16). For very severe conditions, composite systems of fiberglass (chopped, mat, or roving) with polyester, vinyl ester, or epoxy resins may be nece­s­sary. In all cases, a near-white blast cleaning (SSPC-SP 10) of the steel prior to coating is recommended.

Unified Facilities Criteria and Unified Facilities Guide Specifications. Corrosion is usually the most severe on fuel tank floors, because there is always a little water there, despite its periodic removal from sumps. Floors of old, corroded tanks should be checked ultrasonically for adequate thickness before recoating. Pits can be filled with weld metal or epoxy/polyester. If a general loss of thickness occurs, it may be necessary to replace the tank bottom or lay a fiberglass-reinforced bottom over

Linings for Chemical Storage Tanks

the deteriorated steel bottom and bring it 18 inches up

Obviously, linings for chemical storage tanks must be

the tank wall. It is desirable to have a white finish on

very resistant to the product stored. Lining manufacturers

all interior tank linings, so that they can be inspected

are best able to provide useful infor­mation about their

more easily.

products, but laboratory immersion testing under service conditions may be necessary before a new coating product is used.

6.8 Coatings for Marine Service Coatings for marine atmospheric service were described

For very severe service, fiberglass-reinforced polyester, vinyl ester, or epoxy resins may be appropriate. In all cases, the manufacturer’s instructions for surface preparation and application should be followed.

at the start of this section. There are, however, some steel structures that have both wet and dry areas. These include marine piling and ships. In such instances, it is a normal practice to use cathodic protection on the immersed portions, in addition to protective coatings on all exposed areas.

Fuel Tank Linings Interiors of tanks containing crude oils may be bare or lined with two coats of coal tar epoxy (e.g., SSPCPaint 16) or two or more coats of epoxy. Many crude oil storage tanks are only part-lined, with coatings on the floor and approximately 3 feet (1 metre) up the walls. The roof may also be coated.

Marine Piling Marine pilings are best protected by abrasive blasting in a shop to a near-white condition (SSPC-SP 10) before coating with one of the two following systems described in MIL-STD-3007: Unified Facilities Criteria and Unified Facilities Guide Specifications.

Storage tanks that contain finished products (e.g.,

•

mils (225 µm) total DFT

gasoline and jet fuels) are best lined with an epoxy system (e.g., three coats of epoxy-polyamide). Coal tar epoxies are generally not used because coal tar

Three coats of epoxy-polyamide with at least nine

•

Two coats of coal tar epoxy with at least 16 mils (400 µm) DFT

may be extracted from linings to discolor the fuel. A C1 Fundamentals of Protective Coatings for Industrial Structures 6-17

Unit 6 - Coatings for Industrial Steel Structures

Coating repair and replacement in intertidal or lower

Surface preparation alternatives include contained

areas of steel piling is very difficult. Cofferdams can

abrasive blasting, vacuum blasting, and power tool

be used to coat areas normally immersed, and a few

cleaning to bare metal (SSPC-SP 11). The highest

commercial products are avail­able that were designed

degree of cleaning is desired for these severe

to be applied underwater by divers. Such ap­plied coat­

environments. Soluble salts (particularly chlorides) can

ings do not perform as well as those applied to dry

promote early coating deterioration, so fresh water-

surfaces.

washing may be required when cleaned surfaces or new undercoats are not coated the same day.

Ships Offshore structures are best initially coated ashore

Coating systems on immersed or wet areas of ships

where good surface preparation and coating application

are usually epoxy-polyamide. Antifouling coats are

can be achieved. A three-coat system of one coat each

used over them in continually immersed areas, and

of inorganic zinc, epoxy, and aliphatic polyurethane is

nonslip coatings are often used over them on decks.

commonly used above water, and a three-coat system

Aluminum oxide grit is used to provide slip resistance

of one coat of inorganic zinc and two epoxy topcoats,

to epoxy and other deck coatings. In­organic zinc or

below water.

epoxy systems with appropriate topcoats may be used topside. A chapter on painting of ships can be found in

Most offshore structures are cathodically protected below

the Steel Structures Painting Manual, Volume I, Good

water. They may have impressed current, sacrificial

Painting Practices. A very detailed description of coating

anode, or mixed cathodic protection systems.

Navy ships can be found in Naval Ships’ Technical Manual, Chapter 631. Additional information is found in NSRP Report 00971, Paint and Surface Preparation,

6.9 Coatings for Buried Steel

a Training Program for Shipyard Personnel, and The

Coating in Conjunction with Cathodic Protection

Surface Preparation and Coating Handbook for US

A combination of cathodic protection and coatings has

Shipyards, Report to NSRP Project 3-91-3, NSRP 0412 and NAVSEA Standard Item 00932.

long been recognized as the most effective way to

Offshore Structures

the metal from electrolytes in the soil, thus reducing

Offshore structure painting differs from onshore painting

cathodic protection protects the metal exposed to the

in that logistics and living, working, and coordinating

environment at coating holidays. New pipe coatings may

work with other personnel are especially important. All

initially provide 99 percent of the pipe protection, but as

personnel on offshore structures should attempt to work

they slowly deteriorate, the cathodic protection system

with others as effectively as possible, because work

provides more and more of the protection.

protect underground steel piping. The coating isolates the current requirements for cathodic protection; the

of different trades must be coordinated during good painting weather.

C1 Fundamentals of Protective Coatings for Industrial Structures 6-18

Unit 6 - Coatings for Industrial Steel Structures

Desired Properties for Coatings for Buried Steel For coatings to perform well in partnership with cathodic protection, they must have the special properties listed below: •

Good electrical resistance

•

Good moisture resistance

•

Good heat resistance

•

Good adhesion to metal

•

Resistance to cathodic disbonding

•

Resistance to damage during handling

•

Ease of application and repair

is blasted by a mixture of shot and grit to an SSPC-SP 6 or 10 and primed with an asphalt primer prior to an extru­sion of the mastic. Whitewash should be applied to protect the coating during exterior storage. Polyolefins Extruded or sintered polyolefin coating (polyethylene [PE] and polypropylene [PP]) systems once provided protection to underground pipelines. Their films however were often disbonded or cracked. Today, they are extensively used in a 3-coat system:

Application of Coatings There are a large number of products (both conventional coatings and alternative materials) that have the desired properties for buried steel and are sold for coat­ing piping to be located underground. These products are most often applied in a shop under con­trolled conditions with auto­ma­ted blasting and recycling of abrasive and coating application. Others are de­signed to be applied in the field over a ditch.

•

1 coat of fusion-bonded epoxy (FBE)

•

1 coat FBE polymer with PE or PP

•

1 coat of PE or PP

Powder Coatings Powder coatings are finely divided 100% solids (no VOCs) mixtures containing resins, pigments and curing agents. They are applied to conductive surfaces as a dry powder that is melted, flows together and upon cooling, forms a tough, continuous film.

Protective Systems Bituminous Enamels

Surface preparation for industrial or marine use is

Enamels utilizing coal tar pitches or petroleum asphalts

manufacturer.

usually an SSPC-SP 5 or 10, as recommended by the

have been used successfully for many years on underground piping. Because these products are not resistant to ultraviolet light, when stored in sunlight for lengthy periods, they should be protected by kraft paper or whitewash. The piping is blast-cleaned to an SSPCSP 6 or 10, followed by hot application and wrapping.

Fusion-bonded, thermosetting (e.g., epoxy) coatings are usually applied by electrostatic spray to give a one or two-layer film. They have good chem­ical resistance and are easily inspected for holidays. Pipes exposed to sunlight may be hybrid (epoxy/polyester) for ultraviolet resistance.

Asphalt Mastics Asphalt mastics are thick ( 1/ 2– 5/ 8 in. [12–15 mm]) coatings of sand, fine aggre­gate, and asphalt. The pipe

C1 Fundamentals of Protective Coatings for Industrial Structures 6-19

Unit 6 - Coatings for Industrial Steel Structures

Tapes

For long-term protection, abrasive blasting the item in a shop to a white metal surface (SSPC-SP 5) and applying

Many plastic tape systems are available for applying in

one of the following is recommended:

layers to underground piping. The American Waterworks

•

Association (AWWA) C 209 Standard describes the

[330°C])

application of cold-applied tape systems to special

•

sections, connections, and fittings. Normally, a three-

Primer to provide good tape-to-pipe bonding

•

Inner layer of adhesive (15 or more mils) and

•

5–7 mils thermal spray aluminium (good up to 1100°F [600°C])

•

plastic tape for controlling corrosion •

6–8 mils thermal spray zinc (good up to 650°F [330°C])

layer system is applied as follows: •

3–5 mils inorganic zinc coating (good up to 650°F

3–5 mils of inorganic silicone coating (good up to 1500°F [815°C])

Outer layer of plastic tape and adhesive for mechanical protection

Organic coatings are available that provide resistance to a range of lower temperatures.

Tapes are typically two inches wide with one inch of spiral overlap. Equip­ment for spiral tape application

Coatings applied to piping that is insulated before use

is available for both shop and field use. Joints are

behave quite differently than those on piping that is not

frequently wrapped manually.

insulated.

Heat-Shrink Sleeves

6.11 Unit Summary

Heat-shrink sleeves may be used to protect weld areas

There are many different coating systems available

from corrosion (AWWA C 216). There are two types:

for protecting steel surfaces from corrosion. Proper

•

Seamless tubular sleeve slid over pipe end

selection of a system for a particular structure must

•

Wraparound sleeve, heat sealed or zippered

be based on both the environment to be encountered and the type of service performed. The coating system

6.10 Coatings for High-Temperature Surfaces

will only provide the desired protection if it is properly applied to an adequately prepared surface.

Most organic coatings do not perform well on hot steel surfaces such as pipes, stacks, and mufflers. There are several products marketed for this purpose, which are aluminium-filled silicone alkyds applied in two coats. As the temperature of the steel is raised, the alkyd resin burns to leave an aluminium-filled silicone film. Because this film is only about one mil thick, it seldom provides long-term protection. An inorganic silicone coating will perform better.

C1 Fundamentals of Protective Coatings for Industrial Structures 6-20

Unit 6 - Coatings for Industrial Steel Structures

Unit 6- Exercise 6A: Coating Repair

About 30 ft2 of a three-coat epoxy coating on the interior steel wall of an aircraft hangar suffered abrasion damage to expose the bare steel. List what steps you are likely to take to repair the damaged coating.

C1 Fundamentals of Protective Coatings for Industrial Structures 6-21

Unit 6 - Coatings for Industrial Steel Structures

Unit 6- Exercise 6B: Selection of Coating Systems for Steel Structures Match each steel structure listed in Column A below with the best recommended coating system in Column B. Column A

Column B

1.

Bridge in mild environment

A. Fusion bonded epoxy, tape or bituminous coating system

2.

Bridge in severe environment

3.

Hot stack at 550°F

C. Three-coat alkyd system

4.

Drinking water tank lining

D. Two coats of epoxy and one coat of aliphatic polyurethane

5.

Jet fuel tank lining

6.

Galvanized steel tower

7.

Marine piling

8.

Severe chemical tank lining

9.

Underground piping

10.

Waste water tank

B. Zinc or aluminum metallizing or silicone coating

E. An ANSI/NSF 61 approved epoxy polyamide system F. Two coats of water-borne acrylic or a coat of epoxy polyamide and a topcoat of aliphatic polyurethane G. Three coats of epoxy polyamide or two coats of coal tar epoxy H. Three coats of amine or polyamide-cured epoxy I. Fiberglass-reinforced polyester, vinyl ester or epoxy system J. Coal tar epoxy, epoxy, polyester or vinyl ester systems

C1 Fundamentals of Protective Coatings for Industrial Structures 6-22

Unit 6 - Coatings for Industrial Steel Structures

Quiz 1. An advantage of shop over field surface preparation and coating (primer only or total system) of steel components is: a. containment of blasting dust emissions b. better access to work c. better control of environmental conditions d. all of above 2. Which coating for metal requires the highest level of surface preparation? a. alkyd coatings b. acrylic emulsion coatings c. epoxy mastic coatings d. zinc metallizing 3. Why is an aliphatic polyurethane coating often used as a topcoat for an exterior epoxy coating system? a. to impart better flexibility to the system b. to impart better abrasion resistance c. to impart better resistance to the sun’s ultraviolet light d. to use a cheaper barrier coating 4. Coal tar coatings are applied to: a. linings for fuel tanks b. fuel tank exteriors c. above ground fuel piping d. underground fuel piping 5. Water-emulsion acrylic coatings: a. require a very high level of surface preparation cleanliness. b. perform well in severe environments. c. have good resistance to ultraviolet light. d. have poor gloss retention.

C1 Fundamentals of Protective Coatings for Industrial Structures 6-23

Unit 6 - Coatings for Industrial Steel Structures

6. What coating provides the longest protection to steel in water immersion? a. epoxy polyamide b. epoxy ester c. alkyd d. inorganic zinc-rich 7. The greatest concern for ultrahigh-pressure water jetting of new steel is: a. the cost of additional safety monitoring b. poor removal of soluble salts c. lack of a surface profile produced d. a dry surface required for coating application 8. What structure would a 3-coat alkyd system be applied to? a. bridge in mild environment b. interior of a water storage tank c. interior of a fuel tank d. buried steel pipe line 9. Which technical organizations certifies interior linings for potable (drinking) water tanks? a. SSPC/NACE b. ICRI/ACI c. ANSI/NSF d. ASTM/SSPC 10. A concern about coating underground steel piping in a shop rather than at the burial site is: a. the inability to produce quality surface preparation and coating application in a shop b. coating damage from long term exposure to air c. limited surface preparation and coating application skills of shop personnel d. damage to the coatings during transportation to the field site

C1 Fundamentals of Protective Coatings for Industrial Structures 6-24

Unit 6 - Coatings for Industrial Steel Structures

11. A significant advantage of petrolatum or wax and tape type coatings for piping as compared to more conventional coatings is: a. better resistance to fuels b. better resistance to chemicals c. less stringent surface preparation requirements d. more attactive appearance 12. What protective coating systems will perform best at 1,000 degrees F (540 degrees C)? a. epoxy mastic b. epoxy phenolic c. inorganic zinc-rich d. aluminum metallizing 13. What type of abrasive is most commonly used to impact slip-resistance to coatings in high-traffic areas? a. silica sand b. aluminum oxide grit c. steel grit d. glass beads

C1 Fundamentals of Protective Coatings for Industrial Structures 6-25

Unit 6 - Coatings for Industrial Steel Structures

References AWWA C209 Cold-Applied Tape Coatings for the Exterior of Special Sections, Connections, and Fittings for Steel Water Pipelines AWWA C216, Heat Shrinkable Cross Linked Polyolefin Coatings for the Exterior of Special Sections, Connections, and Fittings for Steel Water Pipelines MIL-DTL-24441, General Specification for Epoxy Polyamide Paint MIL-STD-3007, Unified Facilities Criteria and Unified Facilities Guide Specifications NSF/ANSI 61, Drinking Water System Components–Health Effects SSPC-CS 23.00/AWS C2.23M/NACE No. 12, Specification for the Application of Thermal Spray Coatings (Metallizing) of Aluminum, Zinc, and Their Alloys and Composites for the Corrosion Protection of Steel SSPC Paint 16 Coal Tar Epoxy-Polyamide Black (Or Dark Red) Paint SSPC Paint 20 Zinc-Rich Primers (Type I - Inorganic and Type II - Organic) SSPC Paint 21, White or Colored Silicone Alkyd Paint SSPC-Paint 24, Latex Semigloss Exterior Topcoat SSPC Paint 25, Zinc Oxide, Alkyd, Linseed Oil Primer for Use Over Hand Cleaned Steel SSPC Paint 26, Slow-Drying Linseed Oil Black Maintenance Primer (Without Lead or Chromate Pigment) SSPC Paint 104, White or Tinted Alkyd Paint SSPC Painting Manual, Volume I, Good Painting Practices SSPC Painting Manual, Volume 2, Systems and Specifications SSPC-PS 24.00. Latex Painting System for Industrial and Marine Atmospheres, Performance-Based SSPC-SP 3 Power Tool Cleaning SSPC-SP 5 White Metal Blast Cleaning (NACE No. 1) SSPC-SP 6 Commercial Blast Cleaning (NACE No. 3) SSPC-SP 10 Near-White Blast Cleaning (NACE No. 2) SSPC-SP 11 Power Tool Cleaning to Bare Metal SSPC-SP 12 Surface Preparation and Cleaning of Metals by Waterjetting Prior to Recoating SSPC-SP 15, Commercial Grade Power Tool Cleaning TT-P-645, Primer, Zinc Molybdate, Alkyd Type

Additional Reading Naval Ships’ Technical Manual S98086-VD-STM-030 Chapter 631 NSRP Report 00971, Paint and Surface Preparation, a Training Program for Shipyard Personnel The Surface Preparation and Coating Handbook for US Shipyards, Report to NSRP Project 3-91-3, NSRP 0412 SSPCs Selecting Coatings for Industrial and Marine Structures

C1 Fundamentals of Protective Coatings for Industrial Structures 6-26

Unit 6 - Coatings for Industrial Steel Structures

Unit 6 Learning Outcomes

Unit 6 Coatings for Industrial Steel Structures

Upon Completion of this unit will have an understanding of: −  Coatings for industrial steel structures in different environments −  Surface preparation methods for these coating systems

Justification for High-Performance Coatings

Selecting Surface Preparation

•  Lower maintenance costs •  Longer service lives •  Coating materials