Change in Corrosion Resistance of Solution-Treated AZ91-X%Sn Magnesium Alloys

The effects of Sn addition and solution treatment on corrosion behavior were studied in AZ91 magnesium casting alloy. The addition of 5%Sn contributed to the introduction of Mg₂Sn phase, to the reduction in dendritic cell size and to the increase in the amount of secondary phases. After the solution treatment, trace amount of Al₈Mn₅ particles were observed in the α -(Mg) matrix for the AZ91 alloy, while Mg₂Sn phase with high thermal stability was additionally found in the AZ91-5%Sn alloy. Before the solution treatment, the AZ91-5%Sn alloy had better corrosion resistance than the Sn-free alloy, which is caused by the enhanced barrier effect of the ( β+Mg₂Sn ) phases formed more continuously along the dendritic cell boundaries. It is interesting to note that after the solution treatment, the corrosion rate of both alloys became increased, but the Sn-added alloy showed higher corrosion rate than the Sn-free alloy. The microstructural examination on the corroded surfaces revealed that the remaining Mg₂Sn particles in the solution-treated AZ91-5%Sn alloy play a role in accelerating corrosion by galvanic coupling with the α -(Mg) matrix.