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 Embrittlement  Liquid Metal Embrittlement  Corrosion Fatigue

STRESS CORROSION CRACKING  Static tensile stress and specific environments produce cracking  Examples:  1) Stainless steels in hot chloride  2) Ti alloys in nitrogen tetroxide  3) 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

Corrosion Fatigue

Corrosion Fatigue in 316L Stainless Steel

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 H 2 gas or combine with C, if present, to form CH 4.  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)