influence of scandium addition on stress corrosion cracking susceptibility of al-zn-mg alloy in different corrosive environments

Stress corrosion cracking (SCC) susceptibilities of Al-Zn-Mg alloys without and with Scandium addition were evaluated in 3.5% NaCl solution at different pH and different strain rate, using slow strain rate test technique. The results indicate that Sc addition reduces grain size and width of precipitation free zones, and transforms grain boundary precipitates from continuous distribution into interrupted distribution by inhibiting recrystallization. In solution at pH 1, pH 3 and pH 7, Sc addition reduces the degree of localized corrosion of alloy surface and SCC susceptibility of Al-Zn-Mg alloy. However, in solution at pH 10 and pH 12, grain refinement significantly promotes the diffusion of hydrogen atoms into matrix, thus Sc addition increases SCC susceptibility of Al-Zn-Mg alloy. Under different strain rate conditions, Sc addition can all reduce SCC susceptibility of Al-Zn-Mg alloy in solution at pH 1, pH 3 and pH 7, and can all increase SCC susceptibility of Al-Zn-Mg alloy in solution at pH 10 and pH 12. As a result, Sc modified Al-Zn-Mg alloy in practical applications should be avoided in alkaline environments.