THE DEVELOPMENT OF CHEMICAL CONVERSION COATINGS ON ALUMINUM

The growth of the chemical conversion coatings on aluminium is strongl y dependent upon the purity of the metal. For specimens up to 99.99% p urity, the aluminium surface is not homogeneous. Flaws are always pres ent in the thin oxide layer covering the aluminium surface; such flaws are sited above the grain boundaries and cellular boundaries associat ed with impurity segregation in the aluminium substrate. These flaws a re of the residual type and provide easy paths for electronic conducti on in an otherwise insulating oxide layer. Consequently, coating growt h (i.e. the reduction of the dichromate species to hydrated chromium o xide) occurs preferentially along the grain boundaries or cellular bou ndaries of the aluminium substrate. However, for aluminium specimens o f 99.9996% purity or higher, where the cellular structure is absent, t he population density of the residual-type flaws in the oxide layer is considerably reduced. Consequently, coating growth proceeds by the tu nnelling of electrons through the thin, insulating, passive oxide laye r to produce a coating of more uniform appearance than associated with substrates of reduced purity.