The influence of Zn²⁺ on stress corrosion cracking severity in a highly sensitised Al–Mg alloy

Abstract

Recent work has indicated that a galvanic as well as chemical protection mechanism may control the intergranular stress corrosioncracking (IG-SCC) mitigation achieved by Zn-rich primers on Al-Mg alloys [1]. The effect of Zn salt species content added to the bulk NaCl environment on IG-SCC severity at -0.9 VSCE was evaluated using a highly susceptible condition of AA5456-H116. Linear elastic fracture mechanics-based testing demonstrated that, for a constant [Cl -], the threshold stress intensity for IG-SCC increased under these conditions and the IG-SCC growth rate was significantly suppressed with increasing stress intensity in the presence of Zn ions. Titration of ZnCl 2 into Al-Mg simulated crack tip solution indicates that Zn salt species may reduce crack tip acidification and thereby raise the pH of the crack tip. These results support the combined galvanic and chemical protection that may be achieved through use of Zn-rich primers for IG-SCC mitigation on AA5456-H116.