Stress corrosion cracking behavior of the wrought magnesium alloy AZ31 under controlled cathodic potentials

Stress corrosion cracking (SCC) tests were performed using compact tension (CT) specimens in a NaCl solution under hydrogen-charged conditions to investigate the SCC behavior of the wrought magnesium alloy AZ31. The effect of NaCl concentration was estimated by conducting the SCC tests under a constant cathodic potential of −1.4 V, with NaCl concentrations of 0.5, 3.0 and 8.0%. The crack propagation rate (da/dt) became accelerated with increasing concentration due to the enhanced anodic dissolution by chloride ions. Subsequently, SCC tests were performed using a 3%NaCl solution with cathodic potentials at −1.4, −2.5 and −3.0 V to investigate the effect of cathodic potentials on SCC behavior. According to the Pourbaix diagram, the cathodic potential of −3.0 V corresponds to the immunity region, whereas −1.4 V is within the corrosion region and −2.5 V is the boundary between the two regions. In the immunity region, the SCC due to hydrogen embrittlement occurred when the KIscc value was higher than that of the SCC dominated by anodic dissolution. The KIscc value decreased and the crack propagation rate increased with decreasing cathodic potential in the immunity region at which SCC occurred due to hydrogen embrittlement. At the cathodic potential of −3.0 V, the da/dt was insensitive to the stress intensity factor because highly charged hydrogen gave rise to brittle fracture.