Behavior of Maleic Acid during the Anodization of Aluminum

Abstract

Anodization of aluminium in aqueous maleic acid at constant current densities showed that the formation of the anodic film took place competitively with the oxidation of maleic acid. In dilute electrolyte, except for the initial short period where the colourless thin film was formed, the over-all current served to oxidize the double bond of maleic acid into glyoxal or carbon monoxide. In concentrated electrolyte, the brownish anodic film was readily formed and became black as it thickened. The coloured film was shown to favour the passage of ionic current and was made up to aluminium oxide complexed with maleate. The coloration of the film was considered to be induced from carboxylate anion radicals, which were coordinated to the aluminium ion as a ligand.