Corrosion evaluation of AZ91D Mg alloy coated with HA, thermal reduced GO and MgF2 in simulated body fluid

HA-GO/MgF2 composite coating was prepared via a pretreatment of acid solution followed by electrophoretic deposition on AZ91D Mg alloy substrate, subsequently thermally reduced to obtain HA-RGO/MgF2 double layered composite coating. The effect of the content of RGO on the surface morphology, crystalline structures and corrosion resistance of the composite coating were evaluated. The results showed that RGO contents in the suspensions was 0.33 mg/mL, the resulting HA-RGO-0.33/MgF2 composite coating exhibited uniform and dense surface morphology, with excellent bonding to the AZ91D Mg alloy substrate. Electrochemical tests revealed that HA-RGO/MgF2 composite coating improved the corrosion resistance of AZ91D Mg alloy substrate, which mainly due to RGO can act as a fence for the corrosive ions in the SBF solution to penetrate into the composite coating through filling up pores and micro-cracks present in the composite coating. Immersion tests of the coated samples in SBF also showed that HA-RGO-0.33/MgF2 composite coating can improve the corrosion resistance of AZ91D Mg alloy substrate during the immersion of 5 days. The obtained results suggested that the HA-RGO/MgF2 double layered composite coatings possess good formation ability of apatite in the SBF solution, which were related to RGO provides favorable sites for apatite nucleation. Based on the experimental results, the corrosion mechanism of HA-RGO/MgF2 composite coating and AZ91D Mg alloy substrate was proposed.