Characterization of different conversion coatings on aluminium with spectroscopic ellipsometry

Different types of conversion coating on aluminium surfaces are studied with spectroscopic ellipsometry. The first film type, the porous anodic aluminium oxide film formed in sulphuric acid, can be accurately characterized by a two-layer structure, that takes the porous part of the film, the barrier part and the substrate roughness into account. This enables the growth efficiency and the porosity of the films to be determined under the applied electrochemical conditions. Ellipsometry measurements on the second film type, the phosphate-chromate conversion film, are interpreted with the Sellmeier relation for n and k. It is revealed that this film type has almost identical optical properties with Al2O3 barrier films. The thickness and growth rate of these films are determined accurately. Indications for the presence of a double-layer structure, predicted by other investigations, are found.