Characterization of anodic films formed on AZ91D magnesium alloy

Anodization of die-casted AZ91D magnesium alloy was performed in 3 M KOH+0.21 M Na3PO4+0.6 M KF base electrolyte with and without Al(NO3)3 addition. The anodic film was characterized by using X-ray diffraction (XRD), scanning electron microscopy (SEM), and X-ray photoelectron spectroscopy (XPS). The corrosion resistance of the various anodized alloys was then evaluated in 3.5 wt % NaCl solution using electrochemical impedance spectroscopy (EIS) and immersion testing. The results showed that the anodic film was mainly composed of MgO. The addition of Al(NO3)3 into the base electrolyte results in the formation of Al2O3 and Al(OH)3 in the anodic film. The maximum amount of Al2O3 was found in the anodic film when the alloy was anodized in the electrolyte containing 0.15 M Al(NO3)3. The results of EIS analysis and morphological examination showed that the MgO anodic film modified with Al2O3 exhibited the superior corrosiom resistance for AZ91D Mg alloy.