Preparation and characterization of HA sol–gel coating on MAO coated AZ31 alloy

Magnesium alloys are highly reactive and require protection for their successful applications. In this work, a layer of hydroxyapatite (HA) was prepared by a sol–gel technique on the bare and the micro-arc oxidized AZ31 alloy, forming a single HA and MgO/HA composite coating on AZ31. The microstructure and composition of the composite coating were analyzed by scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS) and X-ray diffraction (XRD). The bonding between the coating and the Mg substrate was evaluated using an adhesive strength test. The electrochemical corrosion behaviors and the hydrogen evolution rate of the samples in simulated body fluid (SBF) solution were also investigated. The results demonstrated that the bonding strength of MgO/HA interface is about 40MPa, which is stronger than that of the AZ31/HA interfaces. The improvement of bonding strength could be attributed to the mechanical interlocking between the HA and the porous MgO, and the small residual stress in the composite coating resulted from the small mismatch of thermal expansion coefficient (TEC) between MgO and HA. HA coating could reduce the hydrogen evolution rates and corrosion current densities of AZ31 alloy in SBF. The polarization resistance is greatly increased by the MgO/HA composite coating that could provide more effective barrier against corrosive ions ingress than the single HA coating for AZ31 alloy.