FORMATION PROCESS OF COLORED FILMS ON SUS304 STAINLESS-STEEL WITH THE SQUARE-WAVE POTENTIAL PULSE METHOD

A thick film with interference color could be obtained on the surface of SUS304 stainless steel with the square wave potential pulse method in 5 kmol . m-3 H2SO4 solution of 323 K with no addition of chromium ions. The higher potential, E(H), and the lower potential, E(L), of the square wave potential pulse were controlled in the range between the active peak potential and the transpassive potential of chromium. A thick colored film was obtained at the condition that E(L) is kept in the transient region from the active peak to the passive range of chromium and E(H) in the passive range of iron and chromium. The deposited hydroxide film including Fe2+ and Cr3+ formed at E(L) losses protons and grows under the high field provided by E(H). The colored film thus formed has many diffusion paths, which permit the further growth of the film without any decrease in the growth rate. Therefore, the thickness of the film, i.e. the color, could be easily controlled by the polarization time and / or potential.