The technology of preparing green coating by conducting micro-arc oxidation on AZ91D magnesium alloy

Micro-arc oxidation was applied to AZ91D magnesium alloy by taking K₂Cr₂O₇ as the colouring salt in the silicate system. It was shown that the green coating obtained through performing micro-arc oxidation on magnesium alloy consisted of Mg, Mg₂SiO₄, MgO, and MgCr₂O₄ based on analysis of X-ray diffraction (XRD), and scanning electron microscopy (SEM). Among which, MgCr₂O₄ was the colouring salt; there were something in the lamellar, pit, and convex forms found on the surface of the coating. The coating consisted of a porous, and a compact, layer from the outside to the inside. As demonstrated, the colour of the coating depended on the K₂Cr₂O₇ concentration: it became gradually deeper with the addition of K₂Cr₂O₇ and the increasing micro-arc oxidation time. The corrosion resistance and hardness of the green coating were greater than that of the matrix.