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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. [Copyright &y& Elsevier]

The present study deals with the effect of negatively charged ions on the ceramic coating formation on 2024 aluminium alloy during microarc oxidation (MAO) process. On the basis of the experimental results, two steps (the formation of an incipient film without arc presence and the growth of a ceramic coating with arc discharge) of MAO process have been observed. For comparison, four different negatively charged ions studied. It is proved that negatively charged ions strongly participated in the formation of an incipient film with high impedance value at the first step. The growth of ceramic coating depends on the combination between Al of the substrate and O from the electrolyte, and the negatively charged ions are little consumed. As an anodic oxide coating is prepared on the sample surface instead of the incipient film, the first step occurs easily and the growth of ceramic coating is accelerated. Furthermore, the mechanism of negatively charged ions in the formation of the MAO coating has been proposed. [ABSTRACT FROM AUTHOR]