Corrosion and surface treatment
Which statement about stress corrosion cracking is correct?
- AIt tends to occur where shot peening has introduced compressive residual stress at the surface
- BIt occurs only when the applied stress exceeds the yield stress and the material deforms plastically
- CIt occurs when a susceptible material is under tensile stress in a specific corrosive environment
Show answer and explanation
Answer: C. It occurs when a susceptible material is under tensile stress in a specific corrosive environment
Stress corrosion cracking is the growth of cracks, even at stresses far below the yield stress, when a susceptible material under tensile stress (from external loads or from residual stresses due to welding or working) is exposed to a specific corrosive environment. Classic examples are austenitic stainless steels in hot chloride environments and brass in ammonia-containing environments (season cracking). It is dangerous because the part fractures suddenly with almost no visible corrosion. Removing any one of the three conditions prevents it, so residual stresses are reduced by stress-relief annealing or compressive residual stress is introduced at the surface by shot peening. Compressive stress does not open cracks, so the choice saying it occurs readily where there is compressive residual stress is backward. The choice saying it cannot occur below the yield stress is also wrong.