Some effects of alloy composition on stress corrosion cracking in Al-Zn-Mg-Cu alloys

Stress corrosion cracking (SCC) of two lower-copper Al-Zn-Mg-Cu alloys, AA7079 and AA7022 (0.6-0.9 wt% Cu), and a higher-copper AA7075 (1.5 wt% Cu) alloy are reported. In aqueous chloride, copper content of grain boundary precipitates is believed to be controlling, whereas in moist air it appears that the hydrogen diffusivity could be evident from the rate of crack growth between crack arrest markings. In moist air, the rate of hydrogen entry may control crack growth rates. X-ray photoelectron spectroscopy showed that the oxide formed in ambient conditions (e.g. ∼50% RH) was more hydrated on the AA7075-T651 than AA7079-T651.