A New Process for Colouring Stainless Steel

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Abstract
A process has been developed for colouring stainless steel. It produces attractive, bright colours on the surface whilst still retaining the characteristic lustre of the metal. The process is short in duration and easily operated in equipment available to the metal finisher and uses only inxepensive chemicals. It consists of two stages, the first being an immersion treatment in a hot chromic/sulphuric acid solution, in which a range of colours is developed depending on the duration of the treatment. In the second stage the coloured steel is cathodically treated in a chromic/sulphuric acid solution, of different composition from the colouring solution, in order to improve the wear resistance of the coloured film. The coloured steel may be subjected to metal forming operations without detriment to the film. The colours produced are generally uniform over the whole surface treated and the cathodic process shows excellent throwing power. The coloured films show excellent colour retention on external exposure and the corrosion resistance of treated panels has been shown to be satisfactory in both atmospheric and accelerated laboratory corrosion tests. The coloured steel also shows excellent heat resistance up to 200°C and in boiling water.

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