THE COMPOSITIONS OF BARRIER-TYPE ANODIC FILMS FORMED ON ALUMINUM IN MOLYBDATE AND TUNGSTATE ELECTROLYTES

During the formation of barrier-type anodic films on aluminium, low co ncentrations of electrolyte species, derived from electrolyte anions, are incorporated into the film. The incorporation process and the subs equent behaviour of the electrolyte species are of great relevance to understanding the mechanism of film formation and related phenomena, s uch as dielectric breakdown. This paper considers the incorporation of electrolyte species from molybdate and tungstate electrolytes. A capa citor model of the film is proposed to explain the resultant compositi ons of the films, which are dependent upon the concentration of adsorb ed anions at the film surface. Using Rutherford backscattering spectro scopy, it is shown that molybdenum and tungsten species are incorporat ed into films at similar, constant rates, but their distributions in t he films differ owing to their different mobilities. Both species migr ate as cations by a cooperative transport mechanism involving also Al3 + and O2-/OH- ions. The compositions of the films depend upon the curr ent density and the concentration, pH and temperature of the electroly te although, for all conditions, only small amounts of electrolyte spe cies are incorporated. The key role of the composition of the electrol yte at the film surface, which is significantly modified compared with the composition of the bulk electrolyte, is highlighted by preferenti al incorporation during film formation in the presence of both molybda te and tungstate ions.