Structural chemistry of anodic alumina

Magic angle spinning-nuclear magnetic resonance has been used as a structural probe for porous anodic films formed on aluminium in oxalic acid. The morphologically and compositionally characterized films reveal four-, five-and sixfold coordination of aluminium with oxygen. However, with controlled chemical dissolution of the distinct film morphology, the residual film unifies to sixfold coordination. Surprisingly, the sixfold coordination is not limited to the pure alumina film regions, but develops relatively rapidily, suggesting a ready transformation of the bulk film material on immersion in the anodizing electrolyte in the absence of the electric field.