Preparation and corrosion resistance studies of nanometric sol-gel-based CeO2 film with a chromium-free pretreatment on AZ91D magnesium alloy

Magnesium alloy, although valuable, is reactive and requires protection before it can be applied in many fields. In this study, a novel protective environmental-friendly gradient coating was performed on AZ91D magnesium alloy by non-chromate surface treatments, which consisted of phytic acid chemical conversion coating and the sol-gel-based CeO₂ thin film. The surface morphologies, microstructure and composition of the coatings were investigated by scanning electron microscopy (SEM), energy disperse spectroscopy (EDS) and X-ray diffraction (XRD), respectively. The corrosion resistance of the coatings was evaluated by potentiodynamic polarization and electrochemical impedance spectroscopy (EIS) in 3.5 wt.% NaCl solution. The effects of the concentration, layers, temperature of heat treatment of CeO₂ sol on the anti-corrosion properties of the gradient coating for magnesium were also investigated. The results showed that the gradient coating was mainly composed of crystalline CeO₂. According to the results of electrochemical tests, the corrosion resistance of AZ91D magnesium alloy was found to be greatly improved by means of this new environmental-friendly surface treatment.