Impedance studies of the passive film on aluminium

Impedance spectroscopy has been used to study the characteristics of aluminium in 0.10 M NaCl solution at potentials below the pitting potential (−1.50 V SCE to −0.70 V SCE). Multiple capacitive (or pseudocapacitive) impedance features have been identified and analyzed over this range of potentials. Higher frequency resistances have been compared with surface analytical data, establishing a correlation between higher frequency impedances and the concentration of chloride in the passive film. The relationship shows that both charging resistance and chloride concentration reach a maximum immediately prior to passive film breakdown. At the onset of metastable pitting, both the impedance and chloride concentration decrease from the measured maxima. In contrast to X-ray photoelectron spectroscopy studies, the impedance results do not reveal oxide thinning prior to the onset of metastable pitting. Modelling from impedance data indicates that the thickness of the oxide layer sampled by impedance is much thinner than the oxide measured with surface analytical techniques, suggesting that the impedance method senses only the space charge layer in the oxide and not the total film thickness.