Mechanism of microarc oxidation on AZ91D Mg alloy induced by β-Mg₁₇Al₁₂ phase

This work proposed a strategy of indirectly inducing uniform microarc discharge by controlling the content and distribution of β-Mg₁₇Al₁₂ phase in AZ91D Mg alloy. Two kinds of nano-particles (ZrO₂ and TiO₂) were designed to be added into the substrate of Mg alloy by friction stir processing (FSP). Then, Mg alloy sample designed with different precipitated morphology of β-Mg₁₇Al₁₂ phase was treated by microarc oxidation (MAO) in Na₃PO₄/Na₂SiO₃ electrolyte. The characteristics and performance of the MAO coating was analyzed using scanning electron microscopy (SEM), energy dispersive spectrometer (EDS), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), contact angle meter, and potentiodynamic polarization. It was found that the coarse α-Mg grains in extruded AZ91D Mg alloy were refined by FSP, and the β-Mg₁₇Al₁₂ phase with reticular structure was broken and dispersed. The nano-ZrO₂ particles were pinned at the grain boundary by FSP, which refined the α-Mg grain and promoted the precipitation of β-Mg₁₇Al₁₂ phase in grains. It effectively inhibited the “cascade” phenomenon of microarcs, which induced the uniform distribution of discharge pores. The MAO coating on Zr-FSP sample had good wettability and corrosion resistance. However, TiO₂ particles were hardly detected in the coating on Ti-FSP sample.