Producing a thin coloured film on stainless steels – a review. Part 2: non-electrochemical and laser processes

The high lustre and desirable aesthetics of stainless steels have resulted in their application for decorative purposes in a variety of industries including homeware and automotive, but primarily in architecture. It is possible to further enhance the aesthetics of stainless steel by imparting a thin coloured film on the surface that does not impair any inherent desirable properties, including corrosion resistance. There are two different types of thin coloured films, iridescent and non-iridescent. Iridescent colour can visually change depending on the angle from which it is viewed and is a consequence of a phenomenon known as thin film interference. Non-iridescent colour is created by the absorbance of light waves of all but a given wavelength, the colour is intrinsic to the thin film and does not change with viewing angle. For the purpose of this review, colouring techniques have categorised into three groups, electrochemical, non-electrochemical and laser colouring. Laser colouring can be utilised as either an electrochemical or non-electrochemical process and is particularly suited to colouring small areas and intricate shapes. Non-electrochemical techniques, such as physical vapour deposition (PVD) and thermal tinting, avoid the use of toxic chemicals, however, they present other challenges, including energy consumption and scrappage rate. The use of non-electrochemical and laser techniques to colour stainless steel can negate the use of toxic hexavalent chromium; however, they are yet to be widely adopted by industry for this purpose. Therefore, it is important to consider each technique to understand why this is the case. The present Part 2 of the review discusses non-electrochemical and laser processes used to produce coloured films on stainless steel, whereas Part 1 discussed the electrochemical processes.