Effects of silicate concentration on anodic films formed on AZ91D magnesium alloy in solution containing silica sol

Anodic films were prepared on the AZ91D magnesium alloy in 1.0 M and 1.5 M Na2SiO3 with varied silica sol addition under the constant current density of 20 mA/cm2 at 18 °C. The surface and cross-section morphologies of the anodic films were characterized by scanning electron microscopy (SEM) and energy dispersion spectrometry (EDS). The results showed that both the surface morphologies and the thickness of the anodic film were affected by the concentration of Na2SiO3 and the additions of silica sol. The effects of Na2SiO3 concentration and silica sol addition on the solution properties were also investigated. The results showed that the addition of silica sol into Na2SiO3 solution could decrease the surface energy and the conductivity of the solution. Moreover, the anodic film formed in 1.5 M Na2SiO3 with addition of silica sol was more uniform and compact than that formed in 1.0 M Na2SiO3 with addition of silica sol. And the electrochemical impedance spectroscopy (EIS) results also indicated that the anodic film formed in 1.5 M Na2SiO3 solution with 5 vol.% silica sol addition could provide higher corrosion resistance than that formed in 1.0 M Na2SiO3 with the same silica sol addition for the AZ91D Mg alloy substrate.