Producing a thin coloured film on stainless steels – a review. Part 1: electrochemical processes

Stainless steels are often used for decorative purposes, primarily in the architectural industry, but also in homeware and the automotive industry, among others. It is possible to impart colour on articles to enhance aesthetics without impairing the desirable properties of the stainless steel itself. The colour can be non-iridescent and created by the absorbance of light waves of all but a given wavelength, or iridescent and created by a phenomenon known as thin-film interference. Colouring methods have been categorised into three groups, electrochemical, non-electrochemical and laser colouring. Laser colouring can be utilised as either an electrochemical or non-electrochemical process. The colouring technique most widely adopted by the industry is a 2-stage electrochemical process, known as the INCO process, which utilises chromic-sulphuric acid baths to produce a thin interference film on the stainless steel surface. Alternatives developed by investigators regularly cite the toxicity of hexavalent chromium as motivation for their work and use the INCO process as a benchmark. Authors claim that alternative techniques match or exceed many of the properties achieved through colouring using hexavalent chromium, however, none are yet widely adopted by industry for the purpose of colouring stainless steel. It is therefore important to consider each technique to understand why this is the case. Part 1 of this review discusses the electrochemical processes utilised to produce coloured films on stainless steel, and Part 2 discusses non-electrochemical and laser processes.