In vitro degradation and cytocompatibility of a low temperature in-situ grown self-healing Mg-Al LDH coating on MAO-coated magnesium alloy AZ31

Basically, Mg-Al layered double hydroxide (LDH) coatings are prepared on the surface of micro-arc oxidation (MAO) coated magnesium (Mg) alloys at a high temperature or a low pH value. This scenario leads to the growth rate of LDH coating inferior to the dissolution rate of the MAO coating. This in turn results in limited corrosion resistance of the composite coating. In this study, a Mg-Al LDH coating on MAO-coated Mg alloy AZ31 is prepared through a water bath with a higher pH (13.76) at a lower temperature (60 °C). FE-SEM, EDS, XRD, XPS, and FT-IR are applied to analyze the surface morphology, chemical compositions, and growth process. Electrochemical polarization, electrochemical impedance spectroscopy (EIS) and hydrogen evolution tests are employed to evaluate the corrosion resistance of the samples. The results disclose that the MAO coating is completely covered by the nanosheet-structured LDH coating with a thickness of approximately 3.8 μm. The corrosion current density of the MAO-LDH composite coating is decreased four orders of magnitude in comparison to its substrate; the presence of a wide passivation region in anodic polarization branch demonstrates its strong self-healing ability, indicating the hybrid coating possesses excellent corrosion resistance. The formation mechanism of the LDH coating on the MAO-coated Mg alloy is proposed. Furthermore, the cytocompatibility is assessed via an indirect extraction test for MC3T3-E1 pre-osteoblasts, which indicates an acceptable cytocompatibility of osteoblasts for the composite coating.