Integral colour anodizing of aluminium and aluminium alloys in molybdate containing acidic baths

Various aspects of production of colour integrally during anodizing of high purity aluminium and a number of aluminium alloys have been studied in a bath containing ammonium paramolybdate, an organic acid and a small quantity of an inorganic acid. Different shades of blue, green, grey and black colours have been obtained. The colours were brighter if the samples were either electrically or chemically polished before anodizing in this bath. Metallographic examination of transverse sections of the anodic coatings showed that the coatings were uniform. The inter-dependence of different parameters, such as current density, bath temperature and anodizing time on each other, have been studied during anodizing at a constant voltage. A possible mechanism of the production of colour integrally during anodizing has been discussed. (Shri T.L. Sharma and Dr. Y.N. Trehan, Scientists, National Metallurgical Laboratory)