Abstract:Microarc oxidation (MAO) process was conducted on AZ91D magnesium alloy in an electrolyte composed of Na₂SiO₃, NaAlO₂, Na₂B₄O₇, NaOH, C₃H₈O₃ and C₆H₅Na₃O₇ by AC pulse electrical source. The surface and cross-sectional morphologies, film thickness, chemical composition and structure of the coatings were characterized by scanning electron microscopy(SEM), layer thickness metry, energy disperse spectroscopy(EDS) and X-ray diffraction(XRD). The corrosion resistances of the coatings in a 3.5% NaCl neutral solution were evaluated by electrochemical impedance spectroscopy (EIS) and potentiodynamic polarization test. The results showed that an optimized electrolyte with a composition of 15 g/L Na₂SiO₃, 9 g/L NaAlO₂, 2 g/L Na₂B₄O₇, 3 g/L NaOH,    5 mL/L C₃H₈O₃ and 7 g/LC₆H₅Na₃O₇ was developed by means of orthogonal experiment. The coating obtained in the optimized electrolyte had a dense structure and revealed a lower current density, decreased by two orders of magnitude as compared with the magnesium substrate. Meanwhile, the corrosive potentials of the coated samples increased nearly by 73 mV. EIS result showed that the corrosion resistance of the coating was mainly determined by the inner dense layer. The coating primarily contained elements Mg, Al, O and Si and XRD analyses indicated that the coating was mainly composed of MgO, Mg₂SiO₄ and MgAl₂O₄.

Key words: magnesium alloy; microarc oxidation; dual electrolyte; corrosion resistance