Improving the corrosion resistance of micro-arc oxidation coated Mg–Zn–Ca alloy

Four additives (Na2WO4, nano-hydroxyapatite, K2TiF6 and NaF) were added into the Na5P3O10 + NaOH + C3H8O3 base electrolyte according to the orthogonal design of four factors three levels (L9 (34)). Nine different micro-arc oxidation (MAO) coatings were fabricated on Mg–2Zn–0.5Ca alloys through orthogonal experiments. The effects of four additives on the microstructure, mechanical properties, corrosion resistance and biocompatibility of MAO coatings were investigated through X-ray diffraction (XRD), scanning electron microscopy (SEM) with energy dispersive X-ray spectroscopy (EDS), electrochemical corrosion test and in vitro degradation test. The addition of nano-hydroxyapatite and K2TiF6 showed self-sealing effects and contributed to the corrosion resistance of the samples significantly. The addition of 0.5 g L−1 Na2WO4 markedly elevated the bonding strength of the coatings with the substrate. The optimal combination of factors and levels considering both mechanical properties and corrosion resistance was: 0.5 g L−1 Na2WO4, 0 g L−1 NaF, 5 g L−1 n-HAp, 5 g L−1 K2TiF6. The growth mechanism of MAO coatings combining with the visual phenomenon was discussed as well.