Development of anodic film on Mg alloy AZ91D

At constant applied current, the evolution of morphology, structure and composition of anodic film on Mg alloy AZ91D with anodizing time was investigated using SEM, EDX and XRD. The development of anodic film on the Mg alloy was similar to that on high-purity Mg except that, attributed to alloying effect, hunch-like resultants replaced volcano-like ones to become predominant initial products at transitional stage of anodization. The black cicatrices at the anode surface were related with the inhomogeneous activation and dissolution under strong polarization conditions. The evolution of micropores in shape, size and number was associated with anodizing mechanism. The main elements in anodizing products were Mg, O, Al and Si, indicating that both alloy substrate and electrolyte solution were involved in anodization. Anodic film developed early was mainly comprised of periclase MgO and forsterite Mg2SiO4. However, amorphous compounds became dominant with treatment time increasing. In anodizing products, the element Al existed primarily in the form of amorphous compound.