Different stages of aluminium anodization studied with depth profiling techniques

Samples of anodic oxides of aluminium representing different stages of anodization in oxalic acid solution are analyzed to establish their concentration depth profiles. The methods used were noble gas sputter sectioning combined with Auger electron spectroscopy (AES) and Rutherford backscattering spectroscopy (RBS). Three regions in the oxide growth process have been distinguished from the recording of anodic potential vs time for constant current. These regions are accessed in the present experiments. In particular, attention is focused on the transitional stage of growth, represented by the inflection point of anodic potential vs time. This transitional stage is here found to involve laterally inhomogeneous oxidation and dissolution of the metal, resulting in a broader interface region between the oxide film and the metal than for the other two stages. Carbon is found to migrate into the film from the electrolyte leaving only the thin initially present barrier oxide unaffected.