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

Four additives (Na 2 WO 4 , nano-hydroxyapatite, K 2 TiF 6 and NaF) were added into the Na 5 P 3 O 10 + NaOH + C 3 H 8 O 3 base electrolyte according to the orthogonal design of four factors three levels (L 9 (3 4 )). 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 K 2 TiF 6 showed self-sealing effects and contributed to the corrosion resistance of the samples significantly. The addition of 0.5 g L -1 Na 2 WO 4 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 Na 2 WO 4 , 0 g L -1 NaF, 5 g L -1 n-HAp, 5 g L -1 K 2 TiF 6 . The growth mechanism of MAO coatings combining with the visual phenomenon was discussed as well.