Characterization of Anodic Films on AZ31 Magnesium Alloys in Alkaline Solutions Containing Fluoride and Phosphate Anions

AZ31 magnesium alloys were galvanostatically anodized in an alkaline solution without and with the addition of phosphate and fluoride ions. The microstructure and composition of the anodic film were investigated using cross-sectional transmission electron microscopy. Results show that during anodizing at in the KOH solution, the potential of the cell was relatively low and sparks were absent throughout the anodization. Adding fluoride ions in the KOH solution resulted in a sharp increase in cell voltage during the early stages of anodizing and subsequent sparking commenced at . In contrast, phosphate ions in the solution hardly affected the potential response of the cell. The anodic film formed in the presence of fluoride ions consisted of a compact inner layer containing significant fluorine species, while that formed in the solution without fluoride ions comprised a highly porous inner layer. Sparking, which resulted from the dielectric breakdown of the compact inner layer, led to the formation of magnesium oxide layer. In contrast, the anodic film formed in the absence of sparks was primarily the magnesium hydroxide.