Preparation of hydrophobic anodic film on AZ91D magnesium alloy in silicate solution containing silica sol

Anodic films were prepared on the AZ91D magnesium alloy in the electrolyte of 1.0 M Na2SiO3 with and without the addition of silica sol under the constant current density of 20 mA/cm2 at 60 °C. The anodic films were characterized by scanning electron microscopy (SEM) and X-ray diffraction (XRD). The results showed that the two main constituents of the anodic films were silicon and oxygen. However, no crystal compound including either silicon or oxygen could be detected by the XRD patterns. The addition of 10 vol.% silica sol increased the thickness of the anodic film and improved the roughness of the film surface. Furthermore, such anodic film revealed some hydrophobic property, which was not observed on the anodic film formed in the base electrolyte without addition of silica sol. And the electrochemical impedance spectroscopy (EIS) results showed that the addition of 10 vol.% silica sol improved the corrosion resistance of the anodic film for the AZ91D Mg alloy obviously.