Colour tone and chromaticity in a coloured film on stainless steel by alternating current electrolysis method

Abstract

A coloured film was formed on SUS 304 stainless steel in an aqueous solution containing 2.5 M CrO 3 and 5.0 M H 2SO 4 by the alternating current electrolysis method. Quantitative measurement of the colour tone and colour difference were conducted to evaluate the controllability and reproducibility of the colouring process. The chromaticity of a coloured film formed at 80°C appeared much higher in degree than that of films formed at 72 and 65°C. A cathodic peak potential, E,, as measured on specimens increased with the colouring time. A different colour tone of the coloured film was obtained at specific E cp used during the colouring process. E cp values around 420, 750, 850 and 950 mV displayed colour tones of blue, blue-golden, golden and purple respectively at 80°C. A desired colour tone of the films was achieved by measuring the E cp or by controlling the alternating current cycles (N). Either method exhibited an excellent reproducibility, with the colour difference (Δ E*ab) within 0.9. Although the values of E cp and N were dependent on the colouring temperature, adjustment of the colour tone was available through use of either E cp-controlled or N-controlled methods to overcome the problem of temperature drift, and thus a constant colour tone was assured.