Coloring ferritic stainless steel by an electrochemical–photochemical process under visible light illumination

Colored oxide films have been grown at room temperature on ferritic 430 stainless steel samples by a two-step photocatalytic coloring method. This coloration process consists of treating a stainless steel sample in a sulphochromic electrolyte solution during a series of cyclic (triangular wave) galvanodynamic polarization scans (electrochemical step), followed by a period of further chemical reaction in the same electrolyte solution at open-circuit potential and under visible light illumination (photochemical step). The electrochemical step produces thin, passive oxide films that are translucent to light interference, whereas the photochemical step produces interference-colored chromium-rich oxide films whose thicknesses depend on the immersion time in the electrolyte solution. SEM-EDS and Raman spectroscopy analyses revealed that both the composition and the structure of the colored films also change with the duration of the immersion time.