Anodizing from phosphate electrolyte onto various AZ series magnesium alloys and corrosion resistance

Abstract

We studied the effects of alloying element on environmental-friendly anodizing onto various AZ series magnesium alloys. This process uses mainly phosphate solution without the need for deleterious materials such as heavy metals or fluoride. We also investigated the characteristics of the coatings formed, such as their structure, composition, and corrosion resistance. Anodized coatings became dense, as the aluminum content increased. X-ray diffraction analysis revealed that the anodized coatings onto magnesium substrate containing aluminum concentration from 1 to 9 mass% had a glass-like structure. The formation of this structure is explained in terms of the film composition and freezing point. In salt spray tests, the anticorrosive performance of these coatings having an average thickness of 10 μm was changed, depending on the aluminum content, and the anticorrosive performance was improved with the increase in aluminum content. The coating onto AZ91D magnesium alloy retained the anticorrosive performance over 1000 hours.