Film-Rupture Mechanism of Stress Corrosion

The atmospherically formed protective films were removed by abrasion in an argon atmosphere from surfaces of an aluminum alloy, two brasses, a magnesium alloy, and low-carbon and stainless steels. The resulting surfaces were 0.12 to 0.76 volt more negative with respect to a calomel electrode than surfaces prepared and measured under normal atmospheric conditions. Appreciable changes in electrochemical solution potentials of notched specimens, stressed in tension, occurred at or just above stresses at which the true stress-true strain curves deviated from the modulus lines. These changes in potential were caused by rupturing of the protective films at the roots of the notches and were of the order of 0.16 to 0.70 volt at failure, depending on the material. Stress corrosion is postulated to occur in corrosive media, at stresses sufficient to rupture the protective film, by electrolytic action between the filmed (cathodic) and film-free (anodic) areas.> 1