The Colouring of Anodized Aluminium by Means of Optical Interference Effects

Summary

This paper describes a novel method of electrolytic colouring of anodized aluminium, whereby a wide range of coloured finishes can be produced. The method makes use of optical interference effects taking place between light scattered from the surfaces of the electrolytically deposited metal and the aluminium oxide/aluminium interface. In order to achieve significant colours, production of larger pores, at least in the region close to the barrier layer, is necessary. These larger pores can be produced both chemically or electrochemically, but the most convenient method is a short anodizing treatment in an electrolyte such as phosphoric acid. The electrolytic colouring conditions following pore modification are not critical and many metals and colouring conditions can be used. Ranges of rather muted interference colours, including attractive blue-grey, green-grey and yellow-bronze finishes, are produced by conventional deposition of metal into a film with suitably enlarged pores, but much brighter effects can be produced if deposition takes place under conditions in which growth of new anodic film beneath the metal deposits is also possible. In some cases this new film can be grown in a final anodizing stage following the electrolytic colouring process.