Influence of rare earth metals on the characteristics of anodic oxide films onaluminium and their dissolution behaviour in NaOH solution

The characteristics of oxide films on Al and AlA1R alloys (R = rare earth metal = Ce, Y) galvanostatically formed (at a current density of 100 lA cmÀ2) in borate buffer solution (0.5 M H3BO3 + 0.05 M Na2B4O7Á10H2O; pH = 7.8) were investigated by means of electrochemical impedance spectroscopy. EIS spectra were interpreted in terms of an ‘‘equivalent circuit” that completely illustrate the Al(AlA1R alloy)/oxide film/electrolyte systems examined. The resistance of the oxide films was found to increase on passing from Al to AlA1R alloys while the capacitance showed an opposite trend. The stability of the anodic oxide films grown in the borate buffer solution on Al and AlA1R alloys was investigated by simultaneously measuring the electrode capacitance and resistance at a working frequency of 1 kHz as a function of exposure over a period of time to naturally aerated 0.01 M NaOH solution. Analyses of the electrode capacitance and resistance values indicated a decrease in chemical dissolution rate of the oxide films on passing from Al to AlA1R alloys.