The effect of anodization on the corrosion behavior of die-casted AZ91D Mg alloy in aqueous solution was studied. Anodization was performed in 3 M KOH + 0.21 M Na3PO4 + 0.6 M KF base electrolyte with and without Al(NO3)3 addition. The anodized 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% NaC1 solution using electrochemical impedance spectroscopy (EIS). The results showed that the anodized film was mainly composed of MgO, and the film properties were dependent on the condition of anodization process. Electrochemical testing results showed that AZ91D Mg alloy exhibited satisfactory corrosion resistance when anodized in the base electrolyte with 0.15 M Al(NO3)3 addition.
Anodic coatings on Mg and its alloys are used to improve their corrosion resistance or to enhance the adhesion for organic finish coatings 1,2. The possible mechanisms for Mg anodization have been reported in the literature 3-5. During the anodizing process, intensive sparking occurs and results in the formation of a thick, porous film on the Mg surface. The anodized film formed is similar to sintered oxide.
The anodization behaviors of Mg and Mg-alloys depend on the process parameters employed, the chemical compositions of the materials anodized, and the electrolytes used. The current density and potential applied during anodization have great influence on the reactions taking place and the properties of the resulting anodized films 4, 6. The composition of the anodized film may vary due to the different Mg/A1 ratios in the substrate 7. Preferential oxidation of Mg and A1 may also result in enrichment of less active alloying elements in Mg alloy s. Furthermore, the effect of electrolyte composition on the anodizing process has been an interesting subject of investigation 6,7,9. For instance, Khaselev et al 7 have studied the effect of aluminate addition on anodizing Mg-A1 alloy in KOH electrolyte. They found that aluminate affected the sparking behavior, and the film breakdown potential increased with increasing AI(OH)3 concentration. The use of a suitable electrolyte perhaps is one of the most efficient ways to improve the anodizing efficiency and to obtain an anodic film with satisfactory properties for a specific Mg alloy. In this investigation, thus, the effect of Al(NO3)3 and its concentration on anodizing the die-casted AZ91D Mg alloy in 3 M KOH + 0.21 M Na3PO4 + 0.6 M KF electrolyte was explored. The corrosion resistances of the anodized films were also evaluated.
EXPERIMENTAL
The chemical composition of die-casted AZ91D Mg alloy used is listed in Table 1. The as-casted AZ91D plate was 1 mm thick and was cut into square samples each with a dimension of 1.5 x 1.5 cm. All the samples were mounted in epoxy resin with one surface exposed. The exposed surface was then ground with a SiC paper to a grit of 1000 before anodizing in 3 M KOH + 0.21 M Na3PO4 + 0.6 M KF base electrolyte with or without Al(NO3)3 addition. The concentration of Al(NO3)3 added varied in the range of 0 ~ 0.25 M. A stainless steel plate was used as the cathode. In anodizing, a two-step process was employed. In the first stage, a constant current density of 5 mA/cm 2 was applied until the potential was reached to a preset value of 80 V. Then, in the second stage, a constant voltage of 80 V was applied for 20 minutes.
After anodizing, the surface morphology was examined using an optical microscope (OM) and a scanning electron microscope (SEM). The crystal structure of the anodized film was determined by X-ray diffraction (XRD) with a g
