Specifying coloured stainless steel finishes and their applications

Introduction

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 coloured stainless steel flat products.

The source of much of the information in this article is:

Colouring Stainless Steel by Euro Inox.

Producing chemically coloured finishes on stainless steel

Colouring of stainless steel is normally restricted to sheet products, and although it is possible to colour fabricated components. Only the finest quality stainless steel sheet can be successfully chemically coloured on a production basis.

Chemical colouring of stainless steel sheet uses a mixture of chromic and sulphuric acids that 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 on the operation’s parameters and a considerable operator skill to get the required colour consistency. A far less frequently produced ‘blackened’ finish can be done by immersion in a fused sodium dichromate salt bath at around 400 C. This will not be discussed further in this article.

The colours 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 steel surface. The sequence of colours formed as the film grows in thickness ranges through bronze, blue, black, charcoal, gold, red-violet and green. With care, intermediate colours are possible. The finished thickness of the passive film ranges from 0.02 microns to produce a bronze colour effect, to 0.36 microns to produce a green colour effect.

The range of colours commercially available on austenitic 1.4301, (304), and 1.4401, (316), grades, includes blue, black, bronze, gold, green and red/violet. A charcoal colour effect can be produced by treating a satin polished sheet to the same conditions that normally produce a blue on non-polished sheet surfaces. The range of colours possible on ferritic grade 1.4016, (430), material is generally more restricted.

A wide range of pre-finished surfaces can be coloured. These include flat mechanically ground, polished (satin) or blasted finishes or roll-patterned surfaces. Colouring can also be combined with acid etched patterns to provide an even wider range of textures, lustres and reflectivity effects

Comparison with anodised, painted and oxidised surfaces

Unlike anodising, as used widely on aluminium products, the process is not an electrochemical one and in contrast to painting, there are no pigments or colouring chemicals used. Perhaps more important the process does not rely on heat to oxidise the surface, like the temper colours on heating at temperatures between 300 and 600 C. In contrast to these oxidised surfaces, chemically coloured stainless steel retains the aqueous, (and atmospheric), corrosion resistance of the un-coloured surface.

Surface Blackening

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.

PVD Coatings (Physical Vapour Deposition)

This involves depositing a very thin layer of ceramic material (typically 0.3 micron). This allows the substrate texture to show through. A range of colours is possible with this process – gold, bronze, blue, black and red.

Specifying coloured stainless steel products

Table 6 of BS EN 10088-2 uses special finish 2L for coloured flat products. This restricts the starting material to cold rolled only, (hot rolled finishes are not suitable for a good standard of coloured finish), and only one side is required to meet the agreed colour tone and surface quality. The standard does not specify specific colours. This is to be agreed between the specifier and the supplier.
Suppliers of these finished sheet products produce handy swatch samples to aide the finish selection process.

The BSSA Buyers Guide can be used to find member companies who can help with the specification and supply of coloured finishes. At the ‘Member Products and Services’ page type ‘coloured’ in the ‘Find What’ box with the ‘Product Supplier’ button selected. Click on the ‘Search’ button with ‘Sheet-coloured-Flat Products’ highlighted to view a contacts list.

Applications for coloured stainless steel sheets

A particular attraction of chemically coloured stainless steel is that it appears to change colour 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 coloured sheets and panels include architectural external cladding, (facades, columns, roofing etc.), internal cladding in low traffic areas, signs, shop display panels, sculptures etc.
Coloured stainless steel is difficult to repair if scratched which is why it is best suited to these applications where scratches and abrasion are relatively unlikely. Cold forming of coloured sheet during fabrication is feasible however, provided the surfaces are suitably protected from scratch damage.

Joining by heat process, including welding is not advisable as changes to the original colour are very difficult to repair or blend-in. Joining by cold processes such as adhesive bonding and mechanical fixing are however suitable.

It is important that the steel sheets are sourced from the same mill supplier and batch run to obtain consistency of overall appearance.