Abstract
A thick passive film showing interference colours has been obtained in sulphuric acid solution. The applied potential was modulated as a square wave at 5 Hz. The thickest coloured film, i.e. the fastest growth rate of film, was obtained at the condition where E L , the lower potential of square wave, was kept just above the corrosion potential of iron at which chromium is also in the active region, and E H , the higher potential of square wave, was kept just after the onset of the transpassive region of chromium. The deposited hydroxide, including mainly Cr 3+, formed at E L loses protons and grows as a passive film under the high field provided by E H . The coloured film thus formed contains many diffusion paths, which permit its further growth without any decrease in growth rate.
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