THE ELECTROCHEMICAL CONDITIONS FOR COLORED FILM FORMATION ON TYPE-304 STAINLESS-STEEL WITH SQUARE-WAVE POLARIZATION

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.c. the fastest growth rate of film, was obtained at the condition A here 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 Cr3+, 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.