Electrolytic Colouring of Anodized Aluminium with Nickel Sulphate

SummarySome basic experiments on electrolytic colouring of anodized aluminium with nickel sulphate are reported. Anodic oxide films on aluminium formed in a sulphuric acid bath are cathodically polarised in a mixed solution of nickel sulphate with boric acid by the potential sweep method. Three peaks of cathodic current are observed. The current peak at -4 V is due to reduction of H+ ions, and the cause of the current peak at -18 V is reduction of Ni2+ ions. Cathodic polarisation behaviour is influenced by the thickness of both the barrier layer and the porous layer, and complex effects of these were also observed. In the case of electrolytic colouring using a c, deformed current waveforms were observed on an oscilloscope screen, consisting of a Faradaic current and the capacitance current due to the oxide film. Electrolytic colouring originates from the metallic colloids in the pores of the oxide film. Blue, green or red coloured film can be obtained with nickel sulphate by controlling the dispersion of metallic colloids in the oxide film.