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.