ENVIRONMENT ASSISTED CRACKING

When a metal is subjected to a tensile stress and a corrosive medium, it may experience Environment Assisted Cracking. Four types:

Stress Corrosion Cracking(SCC)Hydrogen EmbrittlementLiquid Metal EmbrittlementCorrosion Fatigue

STRESS CORROSION CRACKINGStatic tensile stress and

specific environments produce cracking

Examples:1) Stainless steels in hot

chloride2) Ti alloys in nitrogen

tetroxide3) Brass in ammonia

Stress Corrosion Cracking

Ingredients:

(1) tensile stress in the metal

(2) corrosive (electrolyte) environment.

Accelerators: presence of Chloride ion and high temp.

Victims: Stainless steel is unsafe in water above 50C and over a few amount of chloride, if any tension exists. Others: mild steel in alkaline environment, copper alloys in ammonia env.

The anode is the stresses region.

Stress Corrosion Cracking (SCC)

So a structure that has SCC sensitivity, if subjected to stresses and then exposed to a corrosive environment, may initiate cracks and crack growth well below the yield strength of the metal.

Consequently, no corrosion products are visible, making it difficult to detect or prevent; fine cracks can penetrate deeply into the part.

Design for Stress Corrosion Cracking:

Material selection for a given environment Reduce applied or residual stress - Stress relieve to

eliminate residual stress (i.e. stress relieve after heat treat).

Introduce residual compressive stress in the service.Use corrosion alloy inhibitors.Apply protective coatings.

Stress Corrosion Cracking

SCC in Stainless Steel

Failure is along grain boundaries.

Corrosion Fatigue

Synergistic action of corrosion & cyclic stress. Both crack nucleation and propagation are accelerated by corrodent

Effect on S-N diagram

Increased crack propagation

AirAir

CorrosionCorrosion

log (cycles to failure, N f)

Str

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Am

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Log (Stress Intensity Factor Range, K

log (C

rack G

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Corrosion Fatigue

Corrosion Fatigue

Corrosion Fatigue in 316L Stainless Steel

Corrosion Fatigue

Corrosion Fatigue

Corrosion Fatigue of Copper

Corrosion Fatigue

Corrosion Fatigue Multiple Cracks

Hydrogen Embrittlement

This is not exactly galvanic corrosion, but it definitely is a form of environmental attack.

Hydrogen atoms diffuse into the metal from outside. Deep in the metal, they combine to form H2 gas or combine with C, if present , to form CH4.

The pressure in this internal pockets of gas is enough to initiate cracking.

The metal is already seeing a lot of tensile stress.

Normally ductile high strength metals, particularly steels, are not so ductile anymore because of these internal cracks.

Hydrogen Ebrittlment

High strength materials stressed in presence of hydrogen crack at reduced stress levels.

Hydrogen may be dissolved in the metal or present as a gas outside.

Only ppm levels of H needed

Where does the Hydrogen come from?

Arc welding can a source. Hydrogen might be released from the electrode.

Galvanic corrosion can produce hydrogen in a reduction reaction.

Hydrogen storage

Hydrogen Damage

Liquid Metal Embrittlment

Certain metals like Al and stainless steels undergo brittle failure when stressed in contact with liquid metals like Hg, Zn, Sn, Pb, Cd etc.

Molten metal atoms penetrate the grain boundaries and fracture the metal

Fig. Shows brittle fracture in Al alloy by Pb

Failure Statistics in Germany (a) &

USA (b)