Formation of relatively pure alumina films by anodic polarization

Abstract

Barrier-type film formation on aluminium in aqueous sodium and potassium hydroxide electrolytes of pH 11 was monitored using transmission electron microscopy of ultramicrotomed sections of the film attached to the substrate. Film growth to relatively low voltages in sodium hydroxide at a constant current density of 50 A m −2 and associated high current efficiency develops typical barrier-type films of relatively pure alumina, as assessed by Rutherford backscattering spectroscopy, unlike the case with other forming electrolytes in which characteristic anions are generally incorporated in the outer regions of the films. For film growth in potassium hydroxide at a constant current density of 300 A m −2 to high voltages, which is associated with an overall relatively low current efficiency of formation, an irregular film morphology develops, reminiscent of the appearance of barrier-type films that have suffered dielectric breakdown. In the present situation, where films were formed to voltages less than the breakdown voltage, it is thought that local current concentration, assisted by local heating, contributes to the morphology revealed; “conducting channels” within the film section provide the means for such non-uniform film growth, but precise reasons for their development are not yet clear.