environment assisted cracking when a metal is subjected to a tensile stress and a corrosive medium,...
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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
ess
Am
plitu
de
Log (Stress Intensity Factor Range, K
log (C
rack G
row
th R
ate, da/
dN)
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)