C-1 Fundamentals of Protective Coatings for Industrial Structures Version 14a 40 24th Street, Sixth Floor Pitt
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C-1
Fundamentals of Protective Coatings for Industrial Structures
Version 14a
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