THE EFFECTS OF ANODIZING ELECTROLYTE AND TEMPERATURE ON THE STRUCTURE AND COMPOSITION OF ANODIC OXIDE FILMS ON ALUMINUM

The chemical composition and structure of anodic oxide films formed on aluminum in aqueous electrolytes containing ammonium salts of citric or adipic acid have been studied. It has been shown that aluminum oxide formation is the primary anodic process during anodization in these electrolytes. The non-oxide forming anodic process was shown to be aluminum dissolution.;The structure of the films produced anodically on aluminum was found to be dependent upon both anodizing electrolyte and temperature. At temperatures up to 85(DEGREES)C, anodization in aqueous phosphate solution produced a film which was shown by transmission electron diffraction to be primarily amorphous Al(,2)O(,3). Anodization in citrate and adipate electrolytes resulted in the formation of both amorphous Al(,2)O(,3) and crystalline (gamma)'-Al(,2)O(,3). The amounts and locations of the structural components of the complex anodic films were established by transmission electron microscopy, and were shown to be dependent upon anodizing temperature and aluminum subgrain orientation.;The conductivities and activation energies for conduction in the various anodic films were determined. The reactivity of the anodic films toward boiling water was also examined. These properties were related to the composition and structure of the films.;The presence of a thin film of thermal oxide on the surface of aluminum promotes the growth of crystalline (gamma)'-Al(,2)O(,3) during subsequent anodization in an aqueous phosphate solution. It was shown through the use of phosphorus-32 tracer and chemical thinning procedures that the anodic crystalline oxide is formed beneath a layer of amorphous oxide, and that the original thermal oxide is incorporated in the anodic crystalline film. The concentration of phosphorus in the amorphous layer is greater than that which occurs in the absence of crystalline oxide. Evidence is given that the occluded phosphorus species has some ionic mobility in both oxides.;It was shown by carbon-14 radiotracer measurements that aluminum anodization in ammonium citrate or ammonium adipate electrolytes resulted in the incorporation of small amounts of impurities derived from the solute anion into the anodic film. The distributions of these carbon-containing moieties were established by electrical and physical measurements made during chemical thinning of the films.