• Author / Uploaded
• Araasu Egambaram

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Eight forms of Corrosion: Eight forms of corrosion: 1. Uniform or general corrosion Identifiable by visual inspection

2. Galvanic corrosion 3. Pitting corrosion 4. Crevice corrosion

5. Intergranular corrosion Identifiable with special inspection tools

6. Selective leaching 7. Erosion corrosion 8. Stress corrosion

Wk2a

Other important forms of Corrosion: Some other forms of corrosion: 1. Impingement Corrosion 2. Corrosion Fatigue 3. Fretting corrosion 4. Hydrogen Embrittlement 5. Biological Corrosion

An example of findings in Chemical processing company

Summarization of the findings obtained from 363 corrosion failure cases investigated in a major chemical processing company .

1. Uniform Corrosion: • Uniform corrosion is one sort of electrochemical corrosion • It occurs with equivalent intensity over the entire exposed surface and often leaves behind a scale or deposit. Example: i) rusting of steel and iron ii) tarnishing of silverware. • This is a general form of galvanic corrosion – i.e. anode and cathode are random and occurs in the same material!

1. Uniform Corrosion

Weathering Steel Cleat

Carbon Steel Manhole

METAL: Carbon Steel

House Drain and Drain Cap 1 year old cap

METAL: Cast Iron; ENVIRONMENT: Residential basement – water exposure

30 yrs old cap

Uniform (or general) corrosion mechanism (Iron in water): Uniform Corrosion mechanism • In anodic reaction, electrons and metallic radicals (Fe2+) are released. • On the other hand, released electrons are consumed in cathodic reaction and OH- is produced. • Corrosion compounds FeO(OH) and/or Fe2O3 are generated by the interaction of OH- and Fe2+ radicals. Iron is corroded by oxidation reaction and rusts are generated on reduction reaction.

Uniform (or general) corrosion mechanism (Iron in water):

Q1: Define rust. Explain the influence of water and oxygen on the formation of rust. • Rust is a generic term used to describe different iron hydroxides and oxides, Fe(OH)2, Fe(OH)3, FeO(OH), Fe2O3.H2O that form when iron corrodes. • The common form of rust is a red product, Fe2O3 called hematite. • The causes of rust formation through the reaction between iron and water or iron exposed to the atmosphere could be either water condensing from air or rain. • The oxygen in the air dissolves in the water and causes rust to form.

The associated reactions in the formation of rust are given below: Anodic Dissolution of Metal (Iron) that goes into solution (water) Fe -----> Fe2+ + 2eCathodic Reduction of Oxygen dissolved in water O2 + 2H2O + 4e- ----> 4OHThe final reaction is: Fe2+ + 2OH- -----> Fe(OH)2

The formed iron hydroxide will then further react with oxygen to give the final red product (Rust): Fe2O3.H2O

1. Uniform Corrosion

Major characteristics of uniform corrosion: i. Thinning might take place until failure. ii. Uniform in nature – leaves scale or deposit over the entire exposed area – this is called rust (e.g. iron-oxide – Fe(OH)3 or Fe2O3.) iii. Corrosion rate will decrease once the oxide layer has been established. iv. Corrosion rates should not be used to estimate the extent of localized form of corrosion. v. Weight loss can indicate the thickness reduction. vi. Fairly predictable and therefore the effects can be minimized.

1. Uniform Corrosion

 After cleaning the rust: • weight loss • corrosion rate

 Corrosion penetration rate (mils/yr): Constant depending on desired units

KW CPR  At

Weight loss after exposure time t

Exposure time density Exposed area

1. Uniform Corrosion

(a) Corrosion rate in mpy: Kw 10 3 g Corrosion rate   g 1 Dt A ( ) y inch 2 3 cm 8760 

3

10  8760 cm y inch 2

3

10 3  8760  (0.3937 inch) 3  y inch 2 534 10 3 inch  y 534 milli  inch  y K  534 So, Corrosion Rate 

 milli  inch  534 w .........   Dt A y  

where w : weight loss (mg )  g  D : Density  3   cm  t : Exposure time (h) A : Area (inch 2 )  milli  inch  Corrosion rate :   y  

Not in SI Unit!!

1. Uniform Corrosion

(b) Corrosion rate in µm/y: Kw Corrosion rate   Dt A

10 3 g g 1 ( ) y cm 2 3 cm 8760

8760  10 3 cm  y 8760  10 5 m  y 87600 m  y

where w : weight loss (mg )  g  D : Density  3   cm  t : Exposure time (h) A : Area (cm 2 )  m  Corrosion rate :   y  

K  87600

So, Corrosion Rate 

 m  87600 w .........   Dt A  y 

Not in SI Unit!!

1. Uniform Corrosion

(c) Corrosion rate in µm/y: Corrosion rate 

Kw kg  kg 1 Dt A 2 ( ) y m m 3 8760

where w : weight loss (kg)  kg  D : Density  3  m  t : Exposure time (h)

8760 m  y  8.76 10 m y 9

K  8.76 109  m  8.76 109 w So, Corrosion Rate  .........   Dt A y  

A : Area (m 2 )  m  Corrosion rate :    y 

SI Unit!!

1. Uniform Corrosion

Corrosion rate 

m  m  534 w  milli  inch  534 w 534 w 3   10  ( 0 . 0254 )   25 . 4  y  y  D t A  y D t A D t A     

 m   mm  So, 1 mpy  25.4    0.0254    y   y 

1 mpy = 0.0254 mm/y = 25.4 µm/y

1. Uniform Corrosion

Uniform Corrosion: • Corrosion rate in terms of current:

i r nF

r = rate in terms of mol/m2-s i = current per unit surface area of material corroding n = # of electrons associated with ionization of metal ion

F = constant = 96,500 C/mol

1. Uniform Corrosion

Uniform corrosion prevention: i. ii.

iii. iv. v. vi.

By removing electrolyte (i.e. lower relative humidity below 30%). By choosing material that doesn’t rust in that particular environment. Potential-pH diagram can be used for selection of materials. Add design “allowance” for rust Cathodic protection. Use of coating/paints. Ensure adequate metal thickness for a specified design life

1. Uniform Corrosion

Test and Design Considerations (Uniform Corrosion) Design Considerations • Mass loss • Reduction in load bearing capacity

Measurement • Thickness loss • Weight loss/Corrosion rate

Misapplication of Data • Corrosion rate is average value • Uniform corrosion rate can’t be used to understand localized corroision

Potential-pH diagram Under certain conditions of potential and pH, some metals form protective films … i.e., they passivate:

Pourbaix diagram for the iron /water system showing the effect of potential in moving the system from a corrosive (active) region (point 1) to a passive region (point 2). 20

2. Galvanic Corrosion • Galvanic corrosion is an electrochemical oxidation-reduction (redox) process, which occurs when two dissimilar metals or alloys are brought into electrical contact and immersed into an electrolyte solution. • Electrolytes are aqueous solutions of salts, acids and bases. • It is also known as bimetallic corrosion or dissimilar metal corrosion.

• It occurs due to the difference in oxidation potentials of metallic ions between two or more metals. • The less noble metal will corrode (i.e. will act as the anode) and the more noble metal will not corrode (acts as cathode). • The greater the difference in oxidation potential, the greater is the galvanic corrosion

• Example of galvanic corrosion: i) ii) iii) iv) v)

Steel screws in brass marine hardware, steel pipe connected to copper plumbing steel propeller shaft in bronze bearing zinc coating on mild steel lead–tin solder around copper wires.

vi) Copper and steel tubing are joined in a domestic water heater, the steel will corrode in the vicinity of the junction. vii) Low-cost household batteries typically contain carbon-zinc cells. As a part of closed circuit, the zinc within the cell will corrode preferentially.

Galvanic corrosion mechanism: Iron (less noble) and copper (more noble) in a marine environment. •

In anodic reaction, electrons and metallic radicals (Fe2+) will be released into the solution.

• On the other hand, released electrons are consumed in cathodic reaction and OH- is produced. • Corrosion compound Fe(OH)2 is generated by the interaction of OH- and Fe2+ radicals

Major characteristics of galvanic corrosion: • Galvanic corrosion always will be at the joint of two different metals having different electrode potential for each. The greater the difference, the higher the driving electric force is for corrosion. • Higher contact resistance at the boundary decreases the corrosion rate. • It also depends on electric resistance of electrolyte solution. Electrolyte solution properties (PH, oxygen content, temperature, flow rate) can influence the rate of corrosion. • Corrosion rate also depends on anode-to-cathode areas ratio. Large anode connected to a small cathode result in low corrosion rate.

Prevention of galvanic corrosion: • Galvanic corrosion can be avoided by coupling metals close to the electrochemical series. • By cathodic protection (Electrically connect a third metal which is more anodic to the other two). • Fixing insulating material between two metals. • By using larger anodic metal and smaller cathodic metal.  Bad situation: Steel siding with aluminum fasteners  Better: Aluminum siding with steel fasteners

GALVANIC SERIES Galvanic Series in Seawater (supplements Faraq Table 3.1 , page 65), EIT Review Manual, page 38-2 Tendency to be protected from corrosion, cathodic, more noble end Mercury Platinum Gold Zirconium Graphite Titanium Hastelloy C Monel Stainless Steel (316-passive) Stainless Steel (304-passive) Stainless Steel (400-passive) Nickel (passive oxide) Silver Hastelloy 62Ni, 17Cr Silver solder Inconel 61Ni, 17Cr Aluminum (passive AI203) 70/30 copper-nickel 90/10 copper-nickel Bronze (copper/tin) Copper Brass (copper/zinc) Alum Bronze Admiralty Brass Nickel Naval Brass Tin Lead-tin Lead Hastelloy A Stainless Steel (active) 316 404 430 410 Lead Tin Solder Cast iron Low-carbon steel (mild steel) Manganese Uranium Aluminum Alloys Cadmium Aluminum Zinc Beryllium Magnesium

Note, positions of ss and al**

Galvanic corrosion: area effects The rate of galvanic attack depends on the relative anode-to-cathode surface areas that are exposed to the electrolyte.

A smaller anode will corrode more rapidly than a larger one. The reason for this is corrosion rate depends on current density.

High current density results for the anode when its area is small

relative to that of the cathode. The ratio of cathodic to anodic area will determine the rate of corrosion: AreaCathode /AreaAnode >> 1 Bad!

Iron (Anode)

Aluminum (Cathode)

Big Cathode, Small Anode = Big Trouble

Dry Cell - Zinc-carbon battery Zn(s) → Zn2+(aq) + 2 e- - oxidation reaction that happens at zinc = anode 2MnO2(s) + 2 H+(aq) + 2 e- → Mn2O3(s) + H2O(l)

- reduction reaction at carbon rod = cathode

Dry cell is a galvanic electrochemical cell with a pasty low-moisture electrolyte.

Steel bolt (less noble) is isolated from copper plates.

2. Galvanic Corrosion

Galvanic corrosion around the inlet of a single-cycle bilge pump.

2. Galvanic Corrosion

copper

steel plug

A galvanized steel plug was fastened to a copper fitting.

Galvanic corrosion due to differing anodic index between the bolts and the plate

2. Galvanic Corrosion

Test and Design Considerations (Galvanic Corrosion) Design Considerations • Loss of strength • Perforation in applications that are required to be sealed (e.g. valves) • Electronic components • Fasteners must be cathodic

Measurement • Galvanic series (difference in corrosion potentials between metals)

Misapplication of Data • The order of metals on the galvanic series chart changes depending on electrolyte (seawater versus salt water – NaCl)

Identifiable by visual inspection

Uniform Corrosion

Pitting Corrosion

Crevice Corrosion

Galvanic Corrosion