Effect of aluminum content on stress corrosion cracking of AM60B and AZ91D magnesium alloy ingots

Stress Corrosion Cracking (SCC) of magnesium alloys is commonly observed in corrosve environments such as NaCl solutions. In this paper, the effect of aluminum content on SCC of magnesium alloy ingots, AM60B and AZ91D, is evaluated using Slow Strain Rate Testing (SSRT) in two environments: (1) air and (2) 5%NaCl solution controlled from pH10 to pH11 with Mg (OH）2. The result shows that AM60B is less susceptible to SCC than AZ91D. In AZ91D specimens, accelerated corrosion is observed around Mg17Al12 compounds in microstructures adjacent to the fracture surface. To investigate the relationship between Mg17Al12 compounds and this susceptibility to SCC, heat treatment was applied to AZ91D and the solution-treated AZ91D specimen showed the same susceptibility to SCC as the as-cast specimen of AM60B alloy ingot. However, at peak-age condition, AZ91D specimen showed a 22% higher susceptibility to SCC than the as-cast AZ91D alloy ingot. Consequently, susceptibility to SCC of the Mg-Al alloys tested in the 5%NaCl solution, is found to depend on the amount of their Mg17Al12 compounds.