Study on the environmentally friendly anodizing of AZ91D magnesium alloy

A new anodizing process, based on an environmentally friendly electrolyte solution that contains none of chromate, phosphate or fluoride but can enhance the corrosion protection of magnesium alloy significantly, is investigated. Anodizing behaviors of magnesium and its alloys are influenced by many factors, including the constituents and concentrations of electrolyte solution, the nature of basis metal as well as applied current density or voltage, treatment time and solution temperature, etc. Therein, the electrolyte solution plays a determinant role. Applied constant current can ensure that anodic films grow at an almost uniform rate. Higher current density and lower solution temperature benefit the film growth. Higher voltage indicates thicker film when constant current is provided. However, the voltage does not always increase linearly with increasing film thickness even under the condition of constant current density. The new anodic film, having a number of pores that are relatively uniform in size as well as distribution on a microstructure scale and do not tranverse the entire film, forms a relatively intact and compact barrier layer. By comparison with the films produced by two available classic processes, the new film can provide more effective corrosion protection to substrate.