The Use of Electron Microscopy in Studies of Electrolytically Coloured Anodized Aluminium

Summary

Deficiencies in the current understanding of electrolytic colouring are identified and considered in terms of film micro structure. Electron optical methods suitable for microscopic study are described and it is indicated how each method has particular applications in elucidating mechanisms operative during colouring and explaining the properties of coloured finishes. Backscattered electron images obtained with the scanning electron microscope clearly reveal the deposit distribution in cross-section within the film. Major differences in distribution have been observed for different black finishes depending on the solution used for electrolytic colouring. Where deposits extend to or near the film surface, they act as preferential pathways for attack leading to premature failure during the acetic acid salt spray test. Transmission electron microscopy reveals the fine structural features of coloured films. Carbon replication and ultramicrotomy allow cross-sections to be studied in detail, while ion beam thinning produces plan sections suitable for examination. Different structures, depending on the relationship between the anodizing and electrolytic colouring voltage, have been observed after colouring in relatively acidic solutions compared with more neutral pH solutions. For the former solutions, where the peak electrolytic colouring voltage is lower than that of anodizing, localised barrier layer thinning precedes deposition. Conversely, where the anodizing voltage is lower, deposition precedes new porous film formation beneath the deposits when deposition effectively ceases. The presence of deposits within a film acts as a barrier to the migration of solution species, restricting not only hydrothermal reactions during sealing but also any diffusion under the influence of an electric field.