Effects of nonhalide anions on the stress corrosion cracking behaviour of alloy 7475 in the tempers T651 and T7351

The stress corrosion cracking behaviour of 7475 plate material in the tempe rs T651 and T7351 was investigated performing constant load tests. Short tr ansverse specimens were permanently immersed in aerated aqueous 0.6 M sodiu m chloride solutions with additions of 0.03 M sodium sulphate, 0.03 M sodiu m nitrate and 0.1 M sodium bicarbonate. Chloride solutions containing sulph ate, nitrate or/and bicarbonate promoted environment-induced cracking. A sh ort transverse threshold stress below 50 MPa was found for 7475-T651 plate material. Neutral 0.6 M NaCl solution was less conducive to stress corrosio n cracking. Specimens of alloy 7475 in the overaged temper T7351 failed at applied stresses above 300 MPa. This failure resulted from overload fractur e caused by a reduction of cross-sectional area due to pitting corrosion, a s confirmed by fractography showing severe pitting attack. Fractographic ex aminations of 7475-T7351 specimens failed during immersion in chloride-nitr ate-bicarbonate containing solutions with or without addition of sulphate a fter long exposure time periods revealed slight transgranular stress corros ion cracking, too. Pronounced transgranular environment-induced cracking wa s observed on the fracture surfaces of specimens which were exposed to an a queous solution containing chloride, sulphate, nitrate, and bicarbonate for 30 days at applied stress and were subsequently tensile tested in an inert environment.