Corrosion resistance of aged die cast magnesium alloy AZ91D

The corrosion behaviour of die cast magnesium alloy AZ91D aged at 160 °C was investigated. The corrosion rate of the alloy decreases with ageing time in the initial stages and then increases again at ageing times greater than 45 h. The dependence of the corrosion rate on ageing time can be related to the changes in microstructure and local composition during ageing. Precipitation of the β phase (Mg17Al12) occurs exclusively along the grain boundaries during ageing. The β phase acts as a barrier, resulting in a decreasing corrosion rate in the initial stages of ageing. In the later stages, the decreasing aluminium content of α grains makes the α matrix more active, causing an increase in the corrosion rate. Electrochemical testing results also confirm the combined effects of the changes in α and β phases on the corrosion resistance of the aged die cast AZ91D alloy.