Influence of Microstructure on Corrosion Property of Mg-Al-Zn Alloy

Influence of microstructure on the corrosion property of Mg-Al-Zn alloy was investigated using potentiodynamic polarization experiments, galvanic coupling experiments, and scanning electron microscopy in sodium chloride solutions. Pitting was the most common form of attack in chloride solution, and filiform corrosion was also occurred in AZ91D-T4 alloy. On the contrary, filiform attack in the bulk matrix was predominant corrosion form in AZ91D-T6 alloy, and the number and size of pit were decreased than those of AZ91D-T4 alloy. Galvanic coupling effect between $Mg_{17}Al_{12}$ and matrix was existed, but the propagation of galvanic corrosion was localized only near the $Mg_{17}Al_{12}$ phase in AZ91D-6T alloy. The corrosion resistance of Mg-Al matrix increased with decreasing Al content in the matrix. And, it could be regarded that Al content in the matrix is decreased by precipitation of $Mg_{17}Al_{12}$ during the aging treatment and it decreases the anodic reaction rate of the matrix and galvanic effect in AZ91D-T6 alloy. It could be considered that the composition and microstructure of surface protective layer would be varied by precipitation of $Mg_{17}Al_{12}$ and subsequent decreasing of Al content in the matrix. And it would contribute the corrosion resistance of AZ91D-T6 aging alloy.