Corrosion of Metals : Pitting

8/13/2019 Corrosion of Metals : Pitting

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Copyright 2012 Industrial Metallurgists, LLC and ASM International, 2012

Corrosion of Metals

Michael Pfeifer, PhD., P.E.

Industrial Metallurgists, LLC

Northbrook, IL 60062

847.528.3467

www.imetllc.com

www.materialscoursesonline.com




Module 6: Pitting Corrosion




Module learning objectives

1. Explain the mechanism for pitting corrosion

2. List three alloy systems that are susceptible to pitting corrosion

3. Explain the effects of chloride concentration and exposure time on pit

density and corrosion rate.

4. List four approaches for controlling pitting corrosion




Pitting

Localized form of corrosion

Small pits form on exposed surface and penetrate into metal

Most common on passivated metals

Can be destructive if it causes perforation of equipment and structures

• Minor pitting often tolerated as long as there is no perforation and change in

appearance is acceptable

304 stainless steel exposed to aerated salt water




Under certain environmental conditions some active metals lose their activity

• Become passive

• Form extremely thin tightly adhering oxide film on metal surface

Passive metal can revert to active state at localized areas of the surface

• Large increase in corrosion rate at these localized areas




Reversion to active state can occur for various reasons

Changes in environment

• Change of concentration of corrosive species

• Electrolyte chemistry may vary because of a surface deposit

Metallurgical features

•

Inclusions or second phase particles, or the regions surrounding these features, may bepreferentially attacked.

• Constituents within a grain boundary, either second-phase particles or segregation of

detrimental species, can lead to pit initiation.

Surface defects

• Discontinuity such as a scratch or a gouge




Chloride ions (Cl-) in electrolyte a common cause of pit initiation

Pitting incubation time

• Decreases as concentration of corrosive species increases

Exposed metal in pit reverts to active

Exposed active metal has lower potential than passivated metalElectrolyte within pit more corrosive than bulk electrolyte

Passive oxide

Cl-

Cl-

Cl-

Cl- Cl-

Anode

Cathode

Cl- Cl-

Cathode

Cl-

Cl-

Cl-




Anode: M Mn+ + ne-

Cathode: O2 + 2H2O + 4e- 4OH-

Mn+

Mn+ Mn+

Mn+

Cl- Cl-

Cl-

e-e-

OH-

OH-

OH-

OH- Cl- Cl-

Cl-




Problem 1Which graph represents the number of pits formed per unit area as the chloride ion

concentration (Cl-) in water increases? Cl-(3) > Cl-(2) > Cl-(1)

Time

#

p i t s / a r e a

Cl-(2)Cl-(3)

Cl-(1)

Time

#

p i t s / a r e a

Cl-(2)

Cl-(3)

Cl-(1)

Time

#

p i t s / a r e a

Cl-(2)

Cl-(1)

Cl-(3)

Time

#

p i t s / a r e a

Cl-(2)Cl-(1)

Cl-(3)






Time

#

p i t s / a r e a

Cl-(2)Cl-(3)

Cl-(1)

Time

#

p i t s / a r e a

Cl-(2)

Cl-(3)

Cl-(1)

Time

#

p i t s / a r e a

Cl-(2)

Cl-(1)

Cl-(3)

Time

#

p i t s / a r e a

Cl-(2)Cl-(1)

Cl-(3)

OK

INCORRECT

Increased number of

chloride ions means

more active species to

cause corrosion






Time

#

p i t s / a r e a

Cl-(2)Cl-(3)

Cl-(1)

Time

#

p i t s / a r e a

Cl-(2)

Cl-(3)

Cl-(1)

Time

#

p i t s / a r e a

Cl-(2)

Cl-(1)

Cl-(3)

Time

#

p i t s / a r e

a

Cl-(2)Cl-(1)

Cl-(3)

OK

CORRECT

Expect that the number

of pits increases with

time.

Increased number of

chloride ions means

more active species to

cause corrosion.






Time

#

p i t s / a r e a

Cl-(2)Cl-(3)

Cl-(1)

Time

#

p i t s / a r e a

Cl-(2)

Cl-(3)

Cl-(1)

Time

#

p i t s / a r e a

Cl-(2)

Cl-(1)

Cl-(3)

Time

#

p i t s / a r e a

Cl-(2)Cl-(1)

Cl-(3)

OK

INCORRECT

This graph shows that a

large number of pits

quickly forms and then

the number of pits

decreases with time.






Time

#

p i t s / a r e

a

Cl-(2)Cl-(3)

Cl-(1)

Time

#

p i t s / a r e a

Cl-(2)

Cl-(3)

Cl-(1)

Time

#

p i t s / a r e a

Cl-(2)

Cl-(1)

Cl-(3)

Time

#

p i t s / a r e

a

Cl-(2)Cl-(1)

Cl-(3)

OK

INCORRECT

This graph shows that a

large number of pits

quickly forms and then

the number of pits

decreases with time.




Effect of chloride concentration on pitting rate

Increase chloride concentration

• Decreased incubation time

• Increased # pits formed at any given time

Time

#

p i t s / a r e

aCl-(2)

Cl-(3)

Cl-(1)

Cl-(3) > Cl-(2) > Cl-(1)




Problem 2Increasing the temperature (T) of a solution generally increases the tendency

towards pitting. Which graph best represents this? T(3) > T(2) > T(1)

Chloride concentration

T e n d e n c y t o

p i t

T(1)

T(2)

T(3)


T e n d e n c y t o

p i t

T(3)

T(2)

T(1)


T e n d

e n c y

t o

p i t

T(1) T(2)T(3)


T e n d e n c y

t o

p i t

T(3)T(2)

T(1)







T e n d e n c y

t o

p i t

T(1)T(2)

T(3)


T e n d e n c y

t o

p i t

T(3)T(2)

T(1)


T e n d e

n c y

t o

p i t

T(1) T(2)T(3)


T e n d e n c y

t o

p i t

T(3) T(2)T(1)

OK

INCORRECT

Shows that tendency to

pit increases as the

temperature decreases




Problem 2Increasing the temperature (T) of a solution generally increases the tendency towards pitting.

Which graph best represents this? T(3) > T(2) > T(1)


T e n d e n c y t o

p i t

T(1)

T(2)

T(3)


T e n d e n c y t o

p i t T(3)

T(2)

T(1)


T e n

d e n c y

t o

p i t

T(1)T(2)

T(3)


T e n d e n

c y

t o

p i t

T(3)T(2)

T(1)

OK

CORRECT







T e n d e n c y

t o p

i t

T(1)T(2)

T(3)


T e n d e n c y

t o

p i t

T(3)T(2)

T(1)


T e n d

e n c y

t o

p i t

T(1)T(2)

T(3)


T e n d e n

c y

t o

p i t

T(3)T(2)

T(1)

OK

INCORRECT

This graph shows the

tendency to pit

decreases as the

chloride concentration

increases.







T e n d e n c y

t o

p i t

T(1)T(2)

T(3)


T e n d e n c y

t o

p i t

T(3)T(2)

T(1)

Chloride concentration T e n d e n

c y

t o

p i t

T(1)T(2)

T(3)

Chloride concentration T e n d e

n c y

t o

p i t

T(3) T(2)T(1)

OK

INCORRECT

This graph shows the

tendency to pit

decreases as thechloride concentration

increases.




Influence of metal composition

Metals that form adherent, passive oxides most susceptible to pitting

Chromium and its alloys Aluminum and its alloys

Titanium and its alloys

Metals that contain these elements in sufficient quantities

• Stainless steels containing > 12% chromium

Within each set of alloys some have better pitting resistance than others

• Specific elements present in the alloys

• Microstructural features




Aluminum alloys

Pitting resistance depends on purity

Purest metal most resistant

1xxx alloys (best pitting resistance)

5xxx alloys

3xxx alloys

6xxx alloys

7xxx alloys

2xxx alloys (lowest pitting resistance)

Decreasing pitting resistance




Controlling pitting corrosion

Often unexpected

• Affects alloys selected based on their corrosion resistance

•

Susceptibility of an alloy to localized corrosion is often overlooked

1. Select a more corrosion-resistant metal for the application

• Selecting different alloy from a family of alloys

316 stainless steel instead of 304 stainless steel

• Select different family of alloys based on same major element

3xxx series of aluminum alloys instead of 5xxx series

• Select alloy based on a different major element

Titianium alloy instead of stainless steel





2. Surface treatment to modify oxide layer

• Passivate stainless steels

• Expose steel to nitric acid solution, followed by alkaline rinse

• Removes surface iron particles

• Dissolves detrimental inclusions at the surface

• Forms a clean, slightly thicker passive oxide

• Anodize aluminum and titanium alloys

• Grow thick oxide by anodizing

• Normal oxide layer much less than 1 micrometer thick

• Thicker than 25 micrometers after anodization

3. Apply coating

• Usefulness of metal may be eliminated





4. Adjust environment

• Reduce concentration of aggressive species

• Enhance passivity and avoid passive film breakdown• Difficult to implement in practice




Module review

1. Metals that form a passive oxide are most susceptible to pitting.

2. Pitting mechanism involves a local breakdown of passive layer andformation of a corrosion cell in the area of the breakdown.

3. Chloride a major cause of pitting

• As chloride concentration increases, number of pits formed in a

particular period of time increases.

4. Four approaches to controlling pitting corrosion.

• Select a different alloy

• Surface treatment to modify the oxide layer

• Apply surface coating

• Modify environment




End of Module 6

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Corrosion of Metals : Pitting