THE DEVELOPMENT OF CHEMICAL CONVERSION COATINGS ON ALUMINUM

The growth of the chemical conversion coatings on aluminium is strongly dependent upon the purity of the metal. For specimens up to 99.99% purity, the aluminium surface is not homogeneous. Flaws are always present in the thin oxide layer covering the aluminium surface; such flaws are sited above the grain boundaries and cellular boundaries associated with impurity segregation in the aluminium substrate. These flaws are of the residual type and provide easy paths for electronic conduction in an otherwise insulating oxide layer. Consequently, coating growth (i.e. the reduction of the dichromate species to hydrated chromium oxide) occurs preferentially along the grain boundaries or cellular boundaries of the aluminium substrate. However, for aluminium specimens of 99.9996% purity or higher, where the cellular structure is absent, the population density of the residual-type flaws in the oxide layer is considerably reduced. Consequently, coating growth proceeds by the tunnelling of electrons through the thin, insulating, passive oxide layer to produce a coating of more uniform appearance than associated with substrates of reduced purity.