Role of β Phase during Microarc Oxidation of Mg Alloy AZ91D and Corrosion Resistance of the Oxidation Coating

Selective growth of oxidation coating was observed on Mg alloy AZ91D when this alloy was treated by microarc oxidation (MAO) technique, and then the role of intermetallic phase Mg17Al12 (β phase) during MAO was investigated. Corrosion resistance and anti-corrosion mechanism of the MAO coating were also studied. Optical microscopy and scanning electron microscopy (SEM) were used to characterize β phase and coating microstructure. Corrosion properties of the coated alloy were studied by potentiodynamic polarization test and electrochemical impedance spectroscopy (EIS) in 3.5% NaCl solution. Results showed that sparking discharge preferentially occurred on α-Mg phase rather than on β phase at the early stage of MAO; however selective growth of the coating disappeared gradually with the increasing oxidation time and β phase would not further inhibit coating growth at the prolonged stage of MAO. Mg17Al12 phase ultimately was unable to destroy the integrity and continuity of MAO coating. The MAO coating could restrain charge transfer process and then greatly enhance corrosion resistance of AZ91D alloy. Sealing treatment of MAO coating by stearic acid could further improve the corrosion resistance of AZ91D alloy.