Porous anodic alumina has been obtained through anodic oxidation in a mixed solution of sulfuric acid and citric acid. Potentiodynamic polarization and electrochemical impedance spectroscopy of anodization film have been studied in a 3.5wt.% NaCl solution, which were conducted to assess the corrosion resistance of the anodized film. The voltage-time curve shows that the oxidation voltage changes from 0 V to 8.6 V dramatically, and then decreases to a steady value(7.7V) when the oxidation time reaches to 300s. The anodic polarization curves shows that corrosion potential of the anodized film is -0.7 V vs Ag/AgCl, which is more positive than that of the 2024 Al alloy. Corrosion potential of the anodized film sealed by nickel and cobalt salt is -0.4V vs Ag/AgCl. Corrosion results indicated that the capacitance of corrosion reaction was changed with the increase of oxidation time. Inerratic nanopores appeared on the surface of the 2024 Al alloy from the SEM results. We also found that the pores size increased gradually with the oxidization time. γ-Al2O3, α-Al2O3 and a small amount of amorphous phase for anodization film appeared from the X-ray diffraction (XRD) pattern.
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Xiangfeng et al. (2013) studied this question.
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