Visualization and characterization of electroactive defects in the native oxide film on aluminium

Abstract

Spatially localized electrochemical activity at Al/Al₂O₃ electrodes has been investigated using scanning electrochemical microscopy (SECM) in order to establish the relationship between localized corrosion of Al (and Al alloys) with the defect structure of the native Al₂O₃ film. Local electron transfer at microscopic defects (2 to 50 μm radius) was visualized in acetonitrile solutions using the nitrobenzene/nitrobenzene radical anion (E° ∼ −1.6 V vs. Ag/Ag⁺) and tetracyanoquinodimethane/tetracyanoquinodimethane radical anion couples (E° ∼ −0.3 V) as redox mediators for imaging. SECM investigations revealed no significant differences in electrochemical activity at Al/Al₂O₃ electrodes in the two mediator solutions, indicating that electrical conduction at the defect sites is weakly dependent on interfacial potential and the electric field across the Al₂O₃ film. The density of electroactive defects observed by SECM varied by 2 to 3 orders of magnitude between electrodes prepared from the same source of Al (either 99.45% and 99.9995%) suggesting that electrical conduction in the native oxide is very sensitive to surface preparation. Defect densities as low as ∼3 sites cm⁻² were readily measured by SECM.