Comparison of corrosion performance of various conversion coatings on magnesium alloy using electrochemical techniques

Comparative studies on some of the important chemical conversion coatings, namely, phosphate–permanganate, galvanic black anodizing, dichromate treatment, and micro arc oxidized and modified acid fluoride anodizing on magnesium alloy AZ31B have been conducted. The surface morphology and composition of the coatings were examined through SEM and EDX techniques. Corrosion resistances of these coatings were compared using electrochemical impedance spectroscopy (EIS) and linear polarization studies. The corrosion resistances of the coatings investigated were found to be according to the order as follows: modified acid fluoride > micro arc oxidized phosphate > micro arc oxidized silicate > dichromate > galvanic black anodizing > phosphate-permanganate > bare magnesium.