AN X-RAY ABSORPTION STUDY OF BARRIER-TYPE ANODIC FILMS FORMED ON ALUMINUM IN CHROMATE ELECTROLYTES

An X-ray absorption study of anodic films formed on aluminium in chromate electrolytes has been undertaken to investigate the nature of the incorporated chromium species. From the shift in chromium edge position, due to change in valency state, and from the observation of a pre-edge peak, or shape resonance, associated with Cr(VI) in tetrahedral coordination, it is evident that the anodic alumina contains both Cr(III) and Cr(VI) states. More specifically, the Cr(VI) state is probably associated with chromate and dichromate anions initially adsorbed at the film/electrolyte interface and incorporated under the field into the thickening alumina film. There is an approximately uniform uptake of chromium species, with thicker films being associated largely with the Cr(III) state. In order to explain the outward mobility of chromium species during film growth, it is evident that a field-induced transformation proceeds whereby chromate and dichromate anions are transformed to outwardly mobile Cr3+ catonic species. The transformation may be associated with structural rearrangement of the alumina film and/or electron injection at the film/electrolyte interface during anodizing.