Stainless steel can be coloured either by the application of paint or by chemical treatments. Both are durable in normal service, but damage can be difficult to repair, especially for finishes produced by chemical treatment. Paint systems rely upon introducing a second layer of material onto the surface of the stainless steel, whereas chemical systems rely upon altering the thickness and nature of the passive film. This article describes the specification and applications for chemically colored stainless steel sheet products.
Coloring of stainless steel is normally restricted to sheet products, although it is possible to color fabricated components. Only the finest quality stainless steel sheet can be successfully chemically colored on a production basis.
Chemical coloring of stainless steel sheet uses a mixture of chromic and sulfuric acids to develop the thickness of the naturally occurring passive film on the steel surface, depending on the immersion time. It is a two-stage immersion process that requires close control of the operation's parameters and considerable operator skill to achieve the required color consistency. A far less frequently produced 'blackened' finish can be achieved by immersion in a fused sodium dichromate salt bath at around 400°C. This will not be discussed further in this article.
The colors produced are the result of light wave interference effects between light reflected from the surface of the thickened oxide passive layer and light reflected from the interface between the passive layer and the steel surface. The sequence of colors formed as the film grows in thickness ranges through bronze, blue, black, charcoal, gold, red-violet, and green. With care, intermediate colors are possible. The finished thickness of the passive film ranges from 0.02 microns to produce a bronze color effect, to 0.36 microns to produce a green color effect.
The range of colors commercially available on austenitic 1.4301 (304) and 1.4401 (316) grades includes blue, black, bronze, gold, green, and red/violet. A charcoal color effect can be produced by treating a satin polished sheet under the same conditions that normally produce a blue on non-polished sheet surfaces. The range of colors possible on ferritic grade 1.4016 (430) material is generally more restricted.
A wide range of pre-finished surfaces can be colored. These include flat mechanically ground, polished (satin) or blasted finishes, or roll-patterned surfaces. Coloring can also be combined with acid-etched patterns to provide an even wider range of textures, lusters, and reflectivity effects.
Unlike anodizing, which is widely used on aluminum products, the process for coloring stainless steel is not an electrochemical one. In contrast to painting, there are no pigments or coloring chemicals used. Perhaps more importantly, the process does not rely on heat to oxidize the surface, unlike temper colors formed by heating at temperatures between 300 and 600°C. In contrast to these oxidized surfaces, chemically colored stainless steel retains the aqueous (and atmospheric) corrosion resistance of the uncolored surface.
This is a chemical process involving the use of molten sodium dichromate. Immersion time controls the degree of "blackness" achieved. The oxide film thus formed is quite adherent, allowing reasonable formability.
This process involves depositing a very thin layer of ceramic material (typically 0.3 microns). This allows the substrate texture to show through. A range of colors is possible with this process, including gold, bronze, blue, black, and red.
A particular attraction of chemically colored stainless steel is that it appears to change color under different shades and angles of artificial and natural light. It is important, however, to ensure that a good match is achieved between sheets intended for multiple panel features. Applications for these colored sheets and panels include architectural external cladding (facades, columns, roofing, etc.), internal cladding in low traffic areas, signs, shop display panels, sculptures, etc.
Colored stainless steel is difficult to repair if scratched, which is why it is best suited to applications where scratches and abrasion are relatively unlikely. Cold forming of colored sheets during fabrication is feasible, provided the surfaces are suitably protected from scratch damage.
Joining by heat processes, including welding, is not advisable as changes to the original color are very difficult to repair or blend in. Joining by cold processes such as adhesive bonding and mechanical fixing, however, is suitable.
It is important that the steel sheets are sourced from the same mill supplier and batch run to obtain consistency of overall appearance.
