Anodic alumina films prepared in phosphoric, oxalic, sulphuric and chromic acids have been examined using magic-angle spinning nuclear magnetic resonance. Whilst only hexacoordinated aluminium is observed in films formed in chromic acid, tetra-, penta- and hexacoordination is observed in films formed in oxalic and sulphuric acid and tetra- and pentacoordination in the film formed in phosphoric acid. The data generally suggest an inhomogeneous film material with aluminium present both within microcrystalline and within more disordered film regions. The variation of aluminium coordination appears to be related to the presence of OH within the alumina material, with an increase of anodizing temperature favouring the development of hexacoordination.
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Farnan et al. (1989) studied this question.
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