Corrosion of Metals Intergranular Corrosion

Copyright 2012 Industrial Metallurgists, LLC and ASM International, 2012

Corrosion of Metals

Michael Pfeifer, PhD., P.E.Industrial Metallurgists, LLC

Northbrook, IL 60062847.528.3467

www.imetllc.comwww.materialscoursesonline.com


Module 7: Intergranular Corrosion


Module learning objectives

1. Explain the mechanisms for intergranular corrosion

2. List three sets of alloys that are susceptible to intergranular corrosion

3. Explain the microstructure features and processing that leads to intergranular corrosion.

4. Describe different approaches for controlling intergranular corrosion in different alloys.


Intergranular corrosionLocalized attack at grain boundaries with relatively little corrosion of grains

Two forms of intergranular corrosion

Grains fall outMetal disintegrates

Metal loses strength

Corrosion products push out grainsMetal appears to be flaking

Called exfoliation

Reprinted with permission of ASM International®. All rights reserved.


Two metallurgical causes behind intergranular corrosion

1. Segregation of impurities to grain boundaries

• Difference in composition can lead to galvanic corrosion

• If grain boundary is anode, then it will be attacked

• If region next to the grain boundary is anode, then it will be attacked

2. Formation of precipitates on the grain boundaries

• Precipitates are different metallurgical phase than grains

• Decrease corrosion resistance of areas adjacent to grain boundaries or develop microscopic galvanic cells.


Segregation of elements to grain boundariesOccurs when metal heated during heat treatment or cooling after being cast

Increase concentration on grain boundaries

Decrease concentration within grains

May promote galvanic corrosion

Interstitials often impurities such as sulfur or phosphorous

Best controlled by minimizing amount of interstitial impurities in an alloy

Interstitial atom


Precipitates at grain boundaries

In many alloys second phase particles form along grain boundaries

Precipitates a different metallurgical phase than grains

Two possible consequences

1. Decrease corrosion resistance of adjacent area due to depletion

2. Develop microscopic galvanic cells in area of grain boundary

Discrete particles Continuous film


Sensitization of Austenitic (3xx series) stainless steelDepletion of one or more elements from areas adjacent to grain boundaries

• Due to formation of precipitates on grain boundaries

• Increases susceptibility to corrosion

In austenitic stainless steels sensitization involves depletion of chromium


Corroded grainboundaries

304 stainless steel


Chromium carbide precipitates form on grain boundaries

Required conditions

• High chromium content

• > 0.02% carbon

• Exposure between 425 and 870 C

Sensitization best known in steels with 18% chromium and 8% nickel

• 304 stainless steel contains up to 0.08% C

• Plenty of carbon available to form chromium carbide precipitates


Chromium carbide precipitates in 304 stainless steel

0.0025 inch0.064 mm

0.0007 inch0.018 mm

Courtesy of Aston Metallurgical Services

Courtesy of Aston Metallurgical Services


Sensitization process

Occurs when steel heated between 425 and 870 C (797 and 1598 F)

1. Carbon and chromium diffuse to grain boundaries

2. React to form chromium carbide (Cr4C or Cr23C6) precipiates

Chromium diffusion rate between 425 and 870 C is low

Chromium atomCarbon atom

Depleted region


GrainGrain

Chromium gives stainless steel its corrosion-resistant properties

Need more than 12% chromium to make stainless steel corrosion resistant

Depleted regions have less than 12% chromium

• Areas near grain boundaries susceptible to attack

Carbide

18

12

%Chromium


Problem 1

What is expected to occur as the time to cool a 304 stainless steel from 800 C to 400 C increases?

Depleted region decreases in width

Depleted region increases in width

No change in width of depleted region


Problem 1





OK

INCORRECT

There will be more diffusion for longer time at elevated temperatures


Problem 1





OK

CORRECT

Exposure to longer time allows for more carbon and chromium atoms to diffuse to grain boundaries.

Results in larger chromium carbides and wider depleted region


Problem 1





OK

INCORRECT

There will be more diffusion for longer time at elevated temperatures.


Problem 2

What is expected to occur as the carbon content decreases in a 304 stainless steel that is cooled from 1050 C to room temperature?

Degree of sensitization decreases

Degree of sensitization increases

Degree of sensitization the same as at higher carbon content


Problem 2





OK

CORRECT

As carbon content decreases there is less carbon to form precipitates

Precipitates will be smaller and fewer in number compared to a steel with higher carbon content


Problem 2





OK

INCORRECT

With less carbon available, will it be as easy for chromium carbide precipitates to form compared to an alloy with higher carbon content?


Problem 2





OK

INCORRECT

With less carbon available, will it be as easy for chromium carbide precipitates to form compared to an alloy with higher carbon content?


Welding is a common cause of sensitization


Top view

Side view

Heated to carbide formation temperatures

Cooling rate sufficiently high to avoid carbide precipitation


Problem 3

What is expected to occur as the welding time to join two pieces of 304 stainless steel component increases?

Decrease width of steel that is sensitized

Increase width of steel that is sensitized

No change in width of steel that is sensitized


Problem 3





OK

INCORRECT

As welding time increases, heat put into areas away from weld increases.


Problem 3





OK

CORRECT


Increased portion heated into temperature range for carbide formation.


Problem 3





OK

INCORRECT



Sensitization during annealing

Sometimes necessary to anneal stainless steel to soften for cold working

For example, deep drawing may require more than one forming step• Steel work hardens to the point that it cannot be shaped in one step

Steel annealed above 1000 C to restore ductility

Carbides will form if steel not cooled quickly enough between 870 and 425 C

Step 1 Step 2


Problem 4

What are possible methods for reducing susceptibility to sensitization of 304 stainless steel that must be annealed?

Increase carbon content or fast cool from annealing temperature

Increase carbon content or slow cool from annealing temperature

Decrease carbon content or fast cool from annealing temperature

Decrease carbon content or slow cool from annealing temperature


Problem 4






OK

INCORRECT

Increasing carbon content results in more carbon available to form chromium carbide precipitates


Problem 4






OK

INCORRECT

Increasing carbon content results in more carbon available to form chromium carbide precipitates.

Slow cool from annealing temperature gives more time for chromium carbide precipitates to form.


Problem 4






OK

CORRECT

Decreasing carbon content in less carbon available to form chromium carbide precipitates.

Fast cool from annealing temperature reduces time for chromium carbide precipitates to form.


Problem 4






OK

INCORRECT

Slow cool from annealing temperature gives more time for chromium carbide precipitates to form.


Preventing sensitization of Austenitic stainless steel

1. High-temperature solution anneal heat treatment

2. Addition of elements that are strong carbide formers (called stabilizers)

3. Reduce carbon content

Approaches especially important if control of thermal treatment is difficult

• Some components or structures require stress relief heat treatment at temperatures ideal for sensitization


Solution anneal heat treatment to dissolve precipitates

1. Heat the alloy above 1035 C (1900 F)

• Dissolve the chromium carbides

• Puts chromium back into solid solution

• Restores the chromium-depleted zone

2. Cool rapidly (quench) below 425 C (797 F)

• Prevent formation of chromium carbide precipitates

Chromium atomCarbon atom


Addition of strong carbide formers

Stabilizers

• Niobium (type 347 stainless steel)

• Titanium (type 321 stainless steel)

These elements combine with carbon

• Form niobium carbide or titanium carbide particles

• Leaves no carbon left for chromium to react

• Chromium carbides cannot form

• Chromium depletion does not occur

Permits fabrication of large vessels, repair welding, and other operations without subsequent heat treatment


Stabilized stainless steel as received from a steel mill

• Contains titanium or niobium carbides

• Essentially no chromium carbides

Must follow certain procedures if heat treatment is required

If steel is solution annealed and cooled too fast

• Titanium or niobium remain in solid solution and titanium or niobium carbide particles do not form

• When heated into sensitization temperature range, steel behaves like a steel without titanium or niobium, and sensitization results


Decrease carbon content

As carbon content decreases• Number and size of chromium carbide particles decreases

• Elevated temperature exposure time to form particles increases

Graph general guidelines. Should be verified before applied.

(Reprinted with permission of ASM International®. All rights reserved.)

Te

mp

era

ture

( C

)

Time to Sensitization

Te

mp

era

ture

( F

)


Problem 5

How quickly should a 304 stainless steel alloy with 0.030% carbon be cooled below 600 C to avoid sensitization?

Within 100 hours


Te

mp

era

ture

( C

)


Te

mp

era

ture

( F

)

Within 30 hours Less than 1 hour Within 8 hours


Problem 5


Within 100 hours


Te

mp

era

ture

( C

)


Te

mp

era

ture

( F

)


OK

INCORRECT


Problem 5


Within 100 hours


Te

mp

era

ture

( C

)


Te

mp

era

ture

( F

)


OK

INCORRECT


Problem 5


Within 100 hours


Te

mp

era

ture

( C

)


Te

mp

era

ture

( F

)


OK

INCORRECT


Problem 5


Within 100 hours


Te

mp

era

ture

( C

)


Te

mp

era

ture

( F

)


OK

CORRECT


Testing for sensitization

ASTM A262 Standard Practices for Detecting Susceptibility to Intergranular Attack in Austenitic Stainless Steels

• 6 different tests

• Tests involve exposing samples to acid solutions followed by different methods of evaluation

Tests bears little relationship to intended service environment

• Detect metallurgical conditions that can lead to intergranular corrosion

Use for process development, supplier evaluation, and problem solving


Final remarks for sensitization

A sensitized stainless steel will not necessarily exhibit intergranular corrosion in all environments

• In environments where IG corrosion does not occur, the sensitized boundary and grains exhibit passive behavior

Ferritic stainless steels can be sensitized

• Information available in references in Module 1


Intergranular corrosion of aluminum alloys

2 mechanisms

Exfoliation

Galvanic corrosionDue to formation of precipitates

on grain boundaries



Galvanic corrosion

Galvanic cells form due to differences in composition at grain boundaries

Different forms of galvanic cells

Likelihood and severity of corrosion depends on • Alloy composition

• Degree of grain boundary precipitation • Corrosiveness of environment

Discrete particles Continuous film


Al2Cu precipitates in Al-5Cu alloy

0.010 mm(0.0004 inches)



Aluminum alloys of concernPrecipitation hardenable alloys

• 2xxx alloys (Cu is major alloying element)• 7xxx alloys (Zn and Mg are alloying elements; many with ≤ 3% Cu)

• Certain 2xx, 3xx, and 7xx cast alloys with Zn, Mg, and/or Cu

5xxx alloys• Contain magnesium as the major alloying element

• Strengthened by work hardening, such as by cold-rolling• Alloys with >3% magnesium can form Mg2Al3 precipitates along grain

boundaries when exposed to moderately elevated temperatures or after long time periods (many years) at room temperature.

Although, 6xxx alloys are precipitation hardenable, they are less of a concern


Galvanic cell 1: Copper depletion at grain boundariesOccurs in alloys that contain copper and form Al2Cu precipitates

• 2xxx alloys

• Some 7xxx alloys

• Some cast alloys

Depletion occurs when alloy cooled too slow after solution anneal treatment


1) Solution heat treat

2) Fast cool

Microstructure

25 C

Aluminum

Copper

Precipitation hardening


Aluminum alloy used in solution annealed condition or aged

Microstructure after aging

Form precipitates at aging temperature


1) Solution heat treat

2) Slow cool

25 CPrecipitates present


Depleted of copper atoms

Copper atomAluminum atom

If cooling rate not fast enough

• Form Al2Cu or Al2Cu(Zn) precipitates on grain boundaries

Easier for precipitates to form on grain boundaries than within grains

• Unless cooling rate is extremely slow, precipitates do not form in grains

Grain boundary


Copper depleted regions (anodes)

GrainGrain

Copper in solid solution (Cathodes)

Galvanic cell forms when exposed to a corrosive environment


% Copper in grain phase

Grain boundary

Copyright 2012 Industrial Metallurgists, LLC and ASM International, 2012Grain boundary


For underaged alloys some copper will still remain in solution within the

grain phase.

Possibility for galvanic cells between depleted layers and rest of grain

Important that intentionally underaged components not be exposed to

corrosive environments


Problem 6

What are two approaches for preventing intergranular corrosion in aluminum-copper alloys that can be precipitation strengthened?

Fast cool alloy after solution anneal or underage

Slow cool after solution anneal or age to maximum strength

Fast cool alloy after solution anneal or age to maximum strength

Slow cool alloy after solution anneal or underage


Problem 6






OK

INCORRECT

Fast cooling after solution anneal will prevent intergranular corrosion

Underaging will result in a difference of copper in solid solution between depleted areas and areas away from grain boundaries


Problem 6






OK

INCORRECT

Slow cooling after solution anneal will result in a difference of copper in solid solution between depleted areas and areas away from grain boundaries

However, aging to maximum strength, or even overaging, will result in an equalization of the copper concentration in solid solution


Problem 6






OK

CORRECT

Fast cooling after solution anneal will prevent formation of grain boundary precipitates

Aging to maximum strength, or even overaging, will result in an equalization of the copper concentration in solid solution


Problem 6






OK

INCORRECT

Slow cooling after solution anneal and underaging will result in a difference of copper in solid solution between depleted areas and areas away from

grain boundaries



Fast cooling after solution anneal

Aging to maximum strength or overaging

Grain boundary

Preventing intergranular corrosion in alloys that form Al-Cu precipitates


Duration of precipitation heat treatment (hours)

Yie

ld s

tren

gth

(MP

a)

Yie

ld s

tren

gth

(ksi

)



Galvanic cell 2: Precipitates more active than grain interior

Cold rolled 5xxx alloys with > 3% magnesium

Precipitation hardenable copper-free 7xxx alloys

GrainGrain

Anode

Cathode

Precipitates preferentially attacked


In general, 5xxx alloys have excellent resistance to corrosion

In 5xxx alloys with >3% magnesium• Mg2Al3 precipitates can form along grain boundaries

• Form when alloy exposed to moderately elevated temperatures or after long periods of time (many years) at room temperature

• Mg2Al3 highly anodic with respect to aluminum grain phase• Precipitates corrode, weakening the grain boundaries

Time required for precipitates to form depends on magnesium content, alloy temper, exposure temperature, and initial processing


7xxx alloys without copper

Anodic MgZn2 precipitates form on grain boundaries

• Improper heat treatment


Exfoliation

Occurs predominantly in aluminum alloy components

• Highly elongated grains that are parallel to metal surface

• Present in extruded and heavily cold-worked components

Corrosion initiates on surface and proceeds along grain boundaries

• Corrosion products take up greater volume than parent metal



Susceptibility to exfoliation depends on

• Alloy composition

• Heat treatment

• Severity of corrosive environment

Not accelerated by stress and does not lead to stress corrosion cracking


Susceptible alloys

Certain extruded products in both marine and industrial environments

• 2xxx copper-magnesium alloys

• 7xxx zinc-copper-magnesium alloys

• Certain cold worked 5xxx alloys

Attack generally associated with

• Alloy fabrication method and extent of aging

• Impurities in alloy matrix

• Metallic compounds at surface and in grain boundaries

Alloys resistant to exfoliation

• 1100 (UNS A91100)

• 3003 (UNS A93003)

• 5052 (UNS A95052)


7xxx alloys

• Exfoliation susceptibility typically high in alloys aged to maximum strength

• For alloys with copper, overaging improves exfoliation resistance, but with significant decrease in strength from maximum strength

5xxx aluminum alloys

• Special processing for some alloys to control where Mg2Al3 precipitates form within grains


Intergranular corrosion of other metals and alloys

Other metals susceptible to intergranular corrosion

• Zinc alloys that contain aluminum

• Some nickel alloys

• Possible, but uncommon in some copper alloys


Module review1. Mechanisms by which an alloy made susceptible to intergranular corrosion

a. Galvanic cells due to segregation of impurities to grain boundaries

b. Depletion of an element that provides corrosion resistance

• Sensitization

• Austenitic stainless steels

c. Galvanic cells due to depletion of element near grain boundaries

• Precipitation hardenable Al alloys that form Al-Cu precipitates

d. Galvanic cells between grain boundary precipitates and regions adjacent to grain boundaries

• Precipitation hardenable Al alloys that form Mg-Zn precipitates

• 5xxx alloys with more than 3% magnesium

2. Various microstructure features enable intergranular corrosion

• Specific processing conditions lead to development of these features

3. There are different approaches for controlling intergranular corrosion

• Specific approaches that can be used depend on specific alloy and mechanical requirements for component in which alloy will be used


End of Module 7

Documents

Corrosion of Metals Intergranular Corrosion