Colour has opened new fields of application for stainless steel, in architectural exteriors and interiors, consumer goods, and solar energy.
Decorative appearance and ease of handling and maintenance weigh heavily in favour of coloured stainless steel in markets long the domain of other materials.
These requirements are met by the Polispectral process used to colour stainless steel, based on Inco Limited patents and development by Poligrat Inox-Colour GmbH, Wallduern, West Germany. It produces colours by dipping stainless steel into hot chromic acid-sulphuric acid solutions which cause the formation of a thin transparent layer of chromium oxides, the thickness depending on exposure time.
Interference effects of incident light produce characteristic colours depending on the layer thickness, similar to the effect of an oil droplet on water.
Thickness of the chromium oxide layer, the interference film, is usually 0,05-to-0,3 μm. Desired film thickness is achieved by comparing the electro-chemical potential difference between the stainless steel surface being coloured and a reference electrode which is simultaneously placed in the bath.
Immediately following the colouring process, the interference film is soft, porous, sensitive and unsuitable for practical use. In subsequent treatment, chromium oxides are cathodically precipitated in porous locations in the film which in turn is densified and hardened by filling the pores. Then a zero-current bath renders the surface resistant to soiling and waterproofs it.
Minimum contents of 50 per cent iron and 13 per cent chromium are required to colour stainless steels by the Polispectral process. A chromium content below 17 per cent generally allows darker colouration only. With more than 17 per cent chromium content, the entire colour spectrum is attainable in ferritic and austenitic materials.
The most spectacular colouration is achieved on austenitic materials with electro-polished surfaces.
Following are maximum levels of other alloy components: 12 per cent nickel, 10 per cent manganese, three per cent each for niobium, molybdenum and copper, two per cent silicon and 0,12 per cent carbon.
Particularly well suited for colouration are large parts such as sheets and cross-sectional profiles.
Uniform colouration of small parts is attained when all pieces in the bath are from the same charge and have been subjected to the same surface treatment. Mass-produced parts – screws, nuts and bolts, discs, rivets and springs – can be coloured brown or black, and with high-quality results.
The Polispectral process produces this sequence of colours with increasing film thickness: brown, violet, cobalt, azure, steel blue, olive, gold, Bordeaux red, turquoise and May green.
Grey to black colouration is also achieved on ground austenitic and ferritic surfaces, and hardenable martensitic steels.
Standard colours offered are brown, cobalt, steel blue, and gold.
The effect of coloured stainless steel is heavily influenced by the spectral wavelength of incident light – far more strongly than pigmented coatings – and this imparts a liveliness and adaptation to its surroundings.
Corrosion resistance of coloured stainless steel is determined by, and generally higher than, that of the uncoated material.
Coloured austenitic stainless steel surfaces are not affected by temperatures up to 200 °C, although short-term exposure to temperature up to 300 °C will not usually have deleterious effects.
Being devoid of resins, dyes and pigments, Polispectral coloured surfaces are insensitive to light, weather and aging. Extended exposure to harsh conditions does not lead to cracking or peeling. Testing for 15 years under a variety of atmospheric conditions yielded no visible changes. But abrasive conditions that would affect uncoloured stainless will also affect the coloured surfaces.
Coloured stainless steel, with suitable surface protection, can be deformed by bending, edging, pressing, cold working, rolling and deep drawing. Mechanical fasteners may be used without restriction.
If it is to be welded, the rules applying to the uncoloured base material should be followed. Because the heat-affected region will be discoloured, and subsequent recolouration impossible, weld seams should be avoided, placed in inconspicuous locations, or arranged symmetrically and polished to yield clean edges, thus allowing for decorative contrast between coloured and cleanly polished regions that then act as décor stripes.
Applications in the automobile industry include interior and exterior paneling in buses, hub caps, screws and window wipers.
In solar energy technology, coloured stainless steel is used as a highly-efficient and durable absorber material – it has a high selectivity for sunlight, especially in the region of dark blue colour tones on electroplated surfaces. About 90 per cent of the incident light in the visible spectrum is absorbed and converted to heat energy while the infra-red radiation emission factor is only about 0,12 per cent, thus substantially reducing heat losses due to radiation.
Selectively-coated coloured stainless steel by the Polispectral process is suitable for the manufacture of solar energy collectors based on widely differing principles of function. Material therefore exists for the solar architecture field which meets aesthetic and energy requirements without restricting design of panels and roofs.
And a highly promising field of application for selectively-coated stainless is the manufacture of solar power-based drying equipment for agricultural fodder and grains.
Coloured stainless steel is registering increased sales in specific applications and constantly finding new outlets.
