The compositions of barrier-type anodic films formed on aluminium in molybdate and tungstate electrolytes

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Abstract
During the formation of barrier-type anodic films on aluminium, low concen­ trations of electrolyte species, derived from electrolyte anions, are incorporated into the film. The incorporation process and the subsequent behaviour of the electrolyte species are of great relevance to understanding the mechanism of film formation and related phenomena, such as dielectric breakdown. This paper considers the incorporation of electrolyte species from molybdate and tungstate electrolytes. A capacitor model of the film is proposed to explain the resultant compositions of the films, which are dependent upon the concentration of ad­ sorbed anions at the film surface. Using Rutherford backscattering spectroscopy, it is shown that molybdenum and tungsten species are incorporated into films at similar, constant rates, but their distributions in the films differ owing to their different mobilities. Both species migrate as cations by a cooperative transport mechanism involving also Al 3+ and O 2- /0H _ ions. The compositions of the films depend upon the current density and the concentration, pH and temperature of the electrolyte although, for all conditions, only small amounts of electrolyte species are incorporated. The key role of the composition of the electrolyte at the film surface, which is significantly modified compared with the composition of the bulk electrolyte, is highlighted by preferential incorporation during film formation in the presence of both molybdate and tungstate ions.