The electrochemical colouring of austenitic stainless steel in sodium molybdate and other environmentally benign solutions

In the 1970’s, the International Nickel Company Ltd (INCO) developed an electrochemical process for colouring stainless steel, often referred to as the INCO process, which utilises chromic acid to produce a thin interference oxide film on the surface of the stainless steel. The iridescent colour is produced by a phenomenon known as thin film interference and therefore the colour is dependent on film thickness. The use of hexavalent chromium (Cr6+) has been subject to scrutiny from the EU in the Registration, Evaluation, Authorisation and restriction of Chemicals (REACh) regulations and the Restriction of Hazardous Substances (RoHS) directive. Restricted use and an eventual ban on the use of Cr6+ for this application has resulted in the need to find a replacement for the INCO process. Although other methods of colouring stainless steel do exist and are reviewed in this thesis, there is currently no industrially established direct replacement.In the present work, several new electrochemical processes for colouring 304 stainless steel have been developed which utilise sodium molybdate and sodium tungstate based colouring solutions and avoid the use of hexavalent chromium. The processes utilise simple immersion, galvanostatic and galvanodynamic techniques to achieve colour. Using the alternative processes, it was possible to produce a range of interference colours on type 304 stainless steel, some of which have not been previously achievable with the INCO process. The desirable mechanical properties produced via the INCO process were difficult to replicate, however, an alternative process did yield equivalent wear performance. The use of advanced analytical techniques, such as transmission electron microscopy (TEM) and X-ray photoelectron spectroscopy (XPS), enabled the characterisation of film chemistry and morphology, using INCO samples coloured by an industrial supplier as a base for comparison. A galvanodynamic sodium molybdate based process yielded the most promising colours in terms of range and aesthetics and produced the best wear results. However, providing that as part of further investigation and development, a greater range of colours is achieved, it is thought that a sodium tungstate based process is the most likely to be a suitable replacement for the INCO process. This is due to similarities in interference film composition with the INCO process and promising preliminary results showing uniform colour, good wear resistance and visually stable colour in ambient, indoor conditions over two years.