Influence of micro-arc oxidation treatment on the corrosion behavior of AZ91D magnesium alloy

To investigate the influence of micro - arc oxidation (MAO) treatment, a layer of ceramic coating was prepared on AZ91D magnesium alloy by using MAO technique in alkali - silicate solution. The corrosion resistance of MAO treated and untreated AZ91D magnesium alloy was studied in 3.5mass % NaCl solution by means of the electrochemical impedance spectroscopy (EIS) method. During the immersion process, both specimens exhibited two kinds of impedance diagrams, two capacitive loops at the initial stage and one capacitive with one inductive loop at the later stage. Compared with the untreated magnesium alloy, the MAO treated specimen exhibited positive corrosion potential and increased resistance. Both the corrosion potential and the resistance of the MAO treated specimen fluctuated at the initial stage of immersion, afterwards decreased. A two - stage deterioration mechanism, the MAO coating deterioration and the Mg alloy localized corrosion, was proposed to describe the corrosion process of the MAO treated Mg alloy.