In situ preparation of a novel Ta2O5/MAO composite coating on magnesium for anti-corrosion protection

Micro-arc oxidation (MAO) is an effective approach to control the corrosion rate of magnesium and its alloys. The multi-porous structure of the MAO coating, however, can result in the infiltration of the corrosive medium into the inner regions of the coating, and the corrosion resistance deterioration. In this study, a tantalum oxide (Ta2O5) was prepared on MAO-coated pure Mg to seal the micropores. The composite coatings were characterized by SEM, EDS, XRD, XPS, and TEM. And then the degradation behavior of the samples was investigated by electrochemical experiments and in vitro degradation tests. Results demonstrate that Ta2O5 effectively closes the micro defects in the MAO coating and plays a role of physical barrier. Compared with the uncoated sample, the current density of the MAO/Ta2O5 composite coating was reduced by three orders of magnitude. In addition, the values of pH, and weight loss were also significantly reduced during in vitro immersion tests. Further, the corrosion resistance mechanisms of MAO/Ta2O5 coating were also suggested. During the immersion process, the solution penetrates the sample surface through some microdefects, resulting in the chemical reaction between the destructive solution and the sample, and in turn, improving the local degradation of the sample. As a result, inhomogeneous corrosion occurs on the surface of the composite coated sample and is accompanied by the deposition of Ca and P. These results indicate the excellent protective properties of the MAO/Ta2O5 composite coating.