The role of oxide films in stress corrosion cracking initiation

In the low-potential transition range, passivatable alloys undergo electrolytic crevice corrosion i.e. localized anodic attack at any area prevented from passivating by shielding effects. A possible role of stress in this range may be seen, accordingly, in producing at every film rupture uncontrollable localized attack in critical underfilm crevices at the feet of emerging slip steps. The fundamental condition for s.c.c. would be that the incubation time, i.e. the time of attack required to form a critical notch, must be shorter than the film lifetime. The resulting model for s.c.c. initiation at the active-passive transition fits in with the mostly accepted slip-dissolution mechanism. A simple mathematical treatment is given for such model and applied to three typical systems, that are the combination of steel with K₂CO₃KHCO₃, NaOH and NH₄NO₃ solutions, respectively. Calculated incubation times are consistent with experimental times to rupture and cracking rates from s.c.c. tests.