Anodization of AZ91D magnesium alloy in silicate-containing electrolytes

The effects of Al(NO3)3 and/or Na2SiO3 additions in the anodizing electrolyte on the properties of the anodic films formed on AZ91D magnesium alloy were investigated. The base electrolyte used in this investigation was 3 M KOH+0.21 M Na3PO4+0.6 M KF solution. The crystal structure, chemical state, surface morphology and cross-section microstructure of the anodic film was characterized by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and scanning electron microscopy (SEM). The corrosion behavior of anodized Mg alloy is examined using electrochemical impedance spectroscopy (EIS) in 3.5 wt.% NaCl solution. The results show that the anodic film was mainly composed of MgO. The additions of Na2SiO3 and/or 0.15 M Al(NO3)3 into to the base electrolyte enhanced the formations of Al2O3, MgAl2O4 in the silicate-containing anodized film, accompanied with silicate formation. Increased corrosion resistance of the anodic film was found when AZ91D Mg alloy was anodized in 3 M KOH+0.21 M Na3PO4+0.6 M KF+0.15 M Al(NO3)3 with 0.1 M Na2SiO3 addition.