Preparation and analysis of films on aluminium by high voltage anodization in phosphoric acid and sodium tungstate solution

A high voltage anodic film on aluminium was prepared in a mixed electrolyte of phosphoric acid and sodium tungstate. The properties, structure, morphology and chemical composition of the film were investigated. It was found that the elements in the film were O, Al, P and W and the main chemical compositions of the film were aluminium oxides with some phosphates and tungstates. Compared with conventional anodic films, the high voltage anodic film showed good hardness and excellent corrosion and heat resistance. Scanning electron microscopy indicated that the film could be divided into two layers, a porous surface layer and a compact underlayer. Many cracks were observed in both layers, which are significantly different from those of conventional anodic films.