1. 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

2. 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

3. 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.

4. 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.

5. 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.

6. Stress Corrosion Cracking

7. SCC in Stainless Steel Failure is along grain boundaries.

8. 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

9. Corrosion Fatigue

11. Corrosion Fatigue in 316L Stainless Steel

12. Corrosion Fatigue

14. Corrosion Fatigue of Copper

15. Corrosion Fatigue

16. Corrosion Fatigue Multiple Cracks

17. 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.

18. 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

19. 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

20. Hydrogen Damage

21. 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

22. Failure Statistics in Germany (a) & USA (b)