Improvement in corrosion resistance of micro arc oxidation coating formed on AZ91D magnesium alloy via applying a nano-crystalline sol-gel layer

Although magnesium is used in many industries, it is reactive and requires protection against aggressive environments. In this study, oxide coating was formed on AZ91D magnesium alloy using micro-arc oxidation (MAO) process in an alkaline electrolyte. Then, in order to seal the pores in the oxide film, a sol-gel layer was applied to the surface of the MAO coating by dipping. For investigation of heat treatment temperature of the sol-gel layer on the properties of the coatings, two different temperatures (150 and 350 A degrees C) were chosen. Surface morphologies and compositions of the coatings were analyzed by Scanning Electron Microscope and X-ray Diffraction (XRD). Surface roughness of the coatings was also measured. The corrosion behavior of the coatings was evaluated with Electrochemical Impedance Spectroscopy and potentiodynamic polarization tests in 3.5 wt%NaCl solution. The porosity percent of the coatings was measured by potentiodynamic polarization tests results. It is found that the sol-gel layers successfully cover the pores of the MAO coatings. The results of the corrosion tests show that the sol-gel layers significantly increase the corrosion resistance of the substrate by reducing the percent of the porosity. The grain size measurements by XRD analysis shows that the grain size of the sol-gel layer heated in 350 A degrees C is about 50 nm.