Roles of β phase on AZ91D Magnesium Alloy Pit Corrosion: In Situ Study by Electrochemical Noise Analysis and Scanning Electrochemical Microscopy

Electrochemical noise (EN) analysis was used to investigate the influence of the beta phase on AZ91D alloy pit corrosion. The results revealed that the beta phase was involved in AZ91D alloy pit corrosion in two ways: (1) during the pit initiation process, the beta phase acted as a galvanic cathode. This beta phase lowered the rate of nucleation but increased the transition ratio of nucleation to metastable pits. (2) After the formation of stable pits, the beta phase was an anodic barrier that lowered the probability of pit growth, demonstrating that the pits formed on the AZ91D alloy at this point were less likely to become stable compared with T4 heat-treated AZ91D alloy. Therefore, pit cavities developed were larger than those on T4 heat-treated AZ91D alloy.