Some Evidence for Duplex Film Structuring Within the Anodic Alumina Barrier Layer

Abstract

Direct observations of anodic barrier layer oxide, by transmission electron microscopy, provide further evidence for the duplex film structuring suggested earlier, based on infrared data. Anodic films were prepared with a boric acid electrolyte maintained at 60°C and two film layers were observed: the uppermost layer being extremely thin. Only the layer lying next to the metal (primary phase barrier layer) showed a thickness relationship with respect to forming voltage: 14.8 Aå/v. The thickness of the uppermost layer (secondary phase barrier layer) was on the order of 200Aå and was essentially independent of forming voltage. With the boric acid electrolyte employed at 90° C, a pronounced increase was found in the thickness of the uppermost layer. The thickness of the layer next to the metal surface remained consistent with the ∼14 Aå/v relationship. Infrared, coating weight, effluent gas detection, and impedance data were also obtained for these coatings and correlated with the data obtained by transmission electron microscopy.