Preparation and in vitro degradation of the composite coating with high adhesion strength on biodegradable Mg–Zn–Ca alloy

Biodegradable magnesium alloys are potential for bone implants. In order to improve the corrosion resistance and the surface bioactivity of Mg–Zn–Ca alloy, a composite coating which had high adhesion strength with the substrate was prepared on Mg–Zn–Ca alloy through combined micro-arc oxidation and electrochemical deposition. The composite coating had needle-like structure and consisted of MgO, Mg3(PO4)2, hydroxyapatite and octacalcium phosphate. The thickness of the composite coating was about 12–15 μm and the adhesion strength of the optimum composite coating with the substrate was higher than 40 MPa. The potentiodynamic polarization and hydrogen evolution tests in simulated body fluid indicated that the composite coating significantly improved the corrosion resistance of the substrate. Deposition of Ca/P salt on the composite coating showed good surface bioactivity of the composite coating in hydrogen evolution tests. The compression tests of the composite coating modified alloy immersed in simulated body fluid for different time revealed that it could meet requirement of bone implant materials in the initial bone reunion period. Thus, the surface modification of the composite coating greatly enhanced the potential of magnesium and its alloy as degradable implants.