Studies on electrolytically coloured aluminium

Attractive coloured finishes can be imparted to aluminium, by anodising it and subsequently subjecting it to ac electrolysis in metal salt solutions. Conventional process involves H2S04 anodising, subsequent H3P04 anodising and ac electrolysis. Alternately one can use a two step process13; omitting H3P04 anodising. A new two step process involving H3PO4 anodising and ac electrolysis is described.13; It is shown here that the colour of electrolytically coloured aluminium is due to optical interference. Optical models for the two step and three step processes are proposed to explain the colours