Effect of fluorine anions on anodizing behavior of AZ91 magnesium alloy in alkaline solutions

Anodization behaviors of AZ91 magnesium alloys in strongly alkaline solutions with and without the addition of fluorine ions have been studied with the emphasis on the effect of the Mg17Al12(beta) phase. In the absence of F- ions, the potential of the cell was relatively low and sparks were absent throughout galvanostatic anodizing at a current density of 1 A/dm(2). After 10 min of anodization, a significant preferential anodization within an alpha matrix resulted in a nonuniform anodic film in regions of large hemispherical pits. F- ions in strongly alkaline solutions resulted in a marked increase in the cell potential and induced sparks on the alloy. These effects associated with F- ions are likely due to the formation of Mg and Al fluorides, which effectively retards the preferential surface attacks in a strongly alkaline solution. Finally, the anodic film formed without sparking was mainly Mg hydroxide, whereas that resulting from sparking was primarily Mg oxide. (c) 2008 The Electrochemical Society.