Structure and properties of compound coatings on Mg alloys by micro-arc oxidation/hydrothermal treatment

The compound coatings on Mg alloys have been successfully prepared by a two-step process of micro-arc oxidation (MAO) and hydrothermal treatment. The structure and composition of MAO coatings and compound coatings were studied by FESEM, XRD, EDS and XPS, respectively. The porous MAO layer on magnesium alloy comprised MgO and amorphous phosphate compounds. A hydrothermal layer with the thickness of around 4 μm was deposited on the surface of MAO coating. The effects of pH value on the hydrothermal deposition mechanism were also proposed. At pH 5.6, the hydrothermal layer was flake-shaped, with the main crystalline of brushite (DCPD, CaHPO3⋅2H2O) and hydroxyapatite (HA, Ca5(PO4)3(OH)). At pH 9.4, the hydrothermal layer was rod-shaped with the main crystalline of HA. The corrosion resistance of the samples in simulation body fluid (SBF) solution was evaluated by Tafel curve and electrochemical impedance spectroscopy (EIS) measurement. The corrosion resistance of the compound coatings was improved by one order of magnitude than MAO coating and two orders of magnitude than Mg substrate, due to the excellent sealing effect and the increase of the thickness. The compound coating at pH 5.6 has a little greater thickness and more coverage than that at pH 9.4, therefore, the former had better corrosion resistance than the latter. Also, the hydrothermal treatment improves the apatite-induced ability of the samples in SBF solution under 37 °C. Compared with MAO coating, the compound coatings presented great apatite-induced ability due to the hydrothermal treatment. The hydrothermal layer of the rod-like HA at pH 9.4 had better apatite-induced ability than that of the flake-like DCPD at pH 5.6.