Chemically anchoring of TiO2 coating on OH-terminated Mg3(PO3)2 surface and its influence on the in vitro degradation resistance of Mg-Zn-Ca alloy

Surface treatment is of particular interest to slow down the in vitro degradation of Mg-based implants. In this work, micro arc oxidation (MAO) coating was fabricated on Mg-Zn-Ca alloy which has been developed by our group especially for the application of bio-implants. Evident improvement in corrosion resistance was obtained although the MAO coating was porous. In order to further diminish the contact with SBF, a composite coating was prepared by introducing a TiO2 layer by liquid phase deposition (LPD) method on the MAO pre-treated alloy. The coatings were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM) and Fourier transform infrared spectroscopy (FTIR). The results of the potentiodynamic polarization experiments showed that, compared with the bare alloy, the corrosion potential of TiO2/MAO/alloy increased from −1.771 to −1.607 V. At the same time, the corrosion current density decreased from 1.43 × 10−4 A cm−2 to 5.69 × 10−7 A cm−2. The improvement of degradation resistance in SBF was ascribed to both barrier effect and enhanced interface condition. The OH-terminated MAO layer surface can make the TiO2 layer anchor tightly on the MOA surface. The chemical bonding of composite layer induced by Mg2+ cations bridges was also analyzed.