Effect of solution pH on electrochemical and stress corrosion cracking behaviour of a 7150 Al-Zn-Mg-Cu alloy

Potentidynamic polarization test and slow strain rate test (SSRT) have been carried out on a 7150 alloy of various tempers in 3.5 wt% NaCl solution at pH 7, pH 1 and pH 12. The measured corrosion current density (icorr) values determined from the polarization curves are higher in solution pH 1 and pH 12 compared to that in solution pH 7. Slow strain rate test (SSRT) results exhibited that highly over-aged temper has higher resistance to stress corrosion cracking (SCC) compared to that of under aged (T4) and peak aged (T6) tempers. Further, for all the alloy tempers, the SCC susceptibility was the most severe in solution pH 1, intermediate in solution pH 12 and least in solution pH 7. The observed SCC behaviour of the alloy tempers has been explained with the TEM micrographs, SEM fractographs and optical micrographs of crack morphology. Local anodic dissolution (LAD) of grain boundary precipitates exacerbated by stress and accompanied by hydrogen induced cracking (HIC) are the probable mechanisms to induce SCC in the 7150 alloy of T4 and T6 tempers.