Anodizing of AZ91D Magnesium Alloy in Borate-Terephthalic Acid Electrolyte

Anodizing a AZ91D magnesium alloy in environmentally friendly borate-terephthalic acid (TPA) electrolyte was studied. The effect of TPA on the anodizing process and the properties of the resultant anodized film were investigated by scanning electron microscopy (SEM), X-ray diffraction (XRD), energy dispersive spectrometry (EDS), potentiodynamic polarization, and electrochemical impedance spectroscopy (EIS). The results showed that the anodizing process, the surface morphology, thickness, phase structure, and corrosion resistance of the anodized film were strongly dependent on the concentration of TPA. In the presence of adequate TPA, a moderate anodizing process was obtained. The current density of the anodizing process was reduced and excessive sparking in the anodizing process was obviously inhibited. In the presence of TPA, the quality of the anodized film improved. The film became more compact and smooth in structure. The thickness of the film decreased slightly. The interface between the anodized film and the magnesium substrate became indistinct indicating a better adhesion between them. The corrosion resistance of the anodized film was obviously enhanced. From these highly positive results, TPA can be used as an effective additive for the anodizing treatment of magnesium alloy. The proposed anodizing process is of importance to make the existing anodizing process 'greener' and to improve the quality of the anodized film.