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.

Contact us to discuss your requirements of colored stainless steel sheet products. Our experienced sales team can help you identify the options that best suit your needs.

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.

Related Links

1. Suppliers of Coloured Finishes

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May. 05, 2023

After development in recent years, colored stainless steel has become an increasingly popular choice for household appliances and kitchen equipment. But did you know it's safe to use colored stainless steel in your home or business? Let's take a look at the safety of colored stainless steel and what to keep in mind when deciding whether to use this material in your in your home or business.

Introduction of the colored stainless steel sheet.

Colored stainless steel sheet is made by adding a layer of material film on ordinary stainless steel. Austenite is the most suitable coloring material for colored stainless steel, and a satisfactory color table can be obtained. The PVD coating is harder than the stainless steel surface itself and offers greater scratch and abrasion resistance. This makes it more resistant to stains and scratches, while colored stainless steel allows for more design options as you can choose from a variety of colors. Colored stainless steel sheet are often used in building exterior wall decoration and interior decoration, household appliances, cooking utensils, kitchen equipment and bathroom utensils, etc.

The good news is that colored stainless steel sheet is just as safe as regular stainless steel. However, there are some basic things to consider when using it in your daily life.

How to choose colored stainless steel sheet?

When purchasing anything in stainless steel, the first thing to do is to choose a high-quality material. Inferior materials may contain metal or chemical impurities, which may endanger the health of the body after ingestion. Also, make sure the manufacturer has taken all necessary scientific steps to guarantee that the coloring process does not introduce any chemicals that could harm your health. Finally, always follow the cleaning instructions that came with the product to ensure that your stained stainless steel will stay safe and hygienic over time.

Benefits of the colored stainless steel sheet.

There are many benefits to choosing colored stainless steel in daily life. Aside from safety concerns. For those new to colored stainless steel material, it can help create a unique look for any space, and even use colored stainless steel fixtures as accent elements throughout the room. It's also easy to clean and maintain compared to other materials like wood or metal, so you won't need to spend extra time or money on it. Lastly, its durability makes it ideal for kitchens, especially where food is a regular occurrence. It won't scratch easily, and it can withstand frequent cleaning without losing its luster or color.

Any material used in daily life must first be safe, and fortunately, colored stainless steel meets this requirement. It is as safe as regular stainless steel and has other added benefits such as ease of maintenance and aesthetics. With these characteristics in mind, colored stainless steel can be a great choice for those looking for something unique and modern!

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