The Characterization of Anodic Aluminas

Abstract

High‐purity aluminum sheets were anodized in both sulfuric and deutero‐sulfuric acid electrolytes. The anodic coatings from each electrolyte were analyzed by infrared to determine the composition and nature of the anodic barrier layer. We found that deuteration of the alumina lowered the absorption frequency of the 900–1000 wavenumber barrier layer band and also reduced the amount of both the porous layer and barrier layer. We conclude, from these data, that the barrier layer is a trihydrate. The primary phase of the barrier layer, as obtained initially or by boric acid anodizing, may be a cyclic aluminic acid trihydrate. A secondary barrier phase may be a decyclized form of the primary, having more terminal groupings and a lower polymer weight.