Molybdenum and its applications
Colored stainless steel: specification & problem avoidance
Coloring of stainless steel is not a new concept. Any color can be obtained but only the surface color is changed – the base metal is inherently silver. Coatings and surface chemical treatments are the most common coloring methods. These methods can achieve grey and black colors but there are also other options, which will be addressed separately. The underlying finish affects both electrochemical and Physically Vapor Deposited (PVD) color, so both finishing steps must be carefully specified.
Alloy specification should be conservative. Superficial corrosion staining, which can often be removed from uncolored mechanical finishes without causing permanent damage, destroys PVD and electrochemical color and requires panel replacement. If the location has salt (coastal or deicing) or higher pollution exposure, the minimum specification should be Type 316/316L or alloys with equivalent or higher corrosion resistance. One illustration of this is a colored residential roof application described in the article Stainless Steel for Severe Coastal Environments.
Abrasion also removes surface color, so it is important to determine if wind-blown abrasives will be present (e.g. sand storms) and whether accidental or deliberate scratching is likely, before selecting a colored finish. An appropriate cleaning regime should be obtained from the finish supplier.
Some types of fabrication must be done before coloring. For example, welding melts the stainless steel and destroys color. Fabrication plans should be discussed with the finish supplier during design.
Any color is achievable. The color is translucent, so the substrate finish affects color, and appearance will vary with the lighting and angle at which the installation is observed. There are spectacular early projects where the original appearance of colored stainless steel has remained unchanged for decades – like Tokyo’s Reiyukai Shakaden Temple (completed 1975).
The Wales Millennium Centre in Cardiff, UK, an opera house completed in 2004, is in a coastal location. Champagne-colored Type 316 stainless steel was used for the roof and façade. Photo credit Rimex.
Artist Daniel Mihalyo was commissioned by Amazon to design a sculpture for their new Seattle headquarters. The electrochemically colored woven Type 316 stainless steel installation was inspired not the least by the company’s method of delivery – stacked shipping boxes. Small custom shape runs are feasible using this coloring method. The artist worked with Millennium Forms on the project. Photo credit Daniel Mihalyo
It is possible to achieve coloring by applying chemicals until the desired color change is achieved without an electrochemical bath, in an older and less expensive process. Electrochemical coloring was developed to create a more stable, durable and abrasion-resistant finish and should be preferred for longer-term projects over simply applying chemicals.
Opaque and Translucent Paints & Enameling
Paint and enamel are the oldest coloring methods for stainless steel, with the first large curtain wall installations appearing in the early 1950’s. Standard coating systems developed for metal are used on stainless steel.
To avoid delamination, stainless steel’s protective passive film must be removed immediately before applying the coating. This is done with chemicals, abrasives or by using an etchant primer. Unlike other metals, stainless steel lasts 100 years or more. The alloy itself should therefore be corrosion resistant enough for the environment should the owner choose to revert to bare metal after the coating system fails. In a location with salt or higher pollution levels, Type 316 or a similar stainless steel should be used.
Kuala Lumpur International Airport is in a very corrosive tropical coastal and industrial environment. The roof is Type 316 with a custom paint color. Photo credit Outokumpu.
PVD colored stainless steel sheet is available in a wide range of colors. This process applies a titanium and nitrogen-based ceramic coating to the surface. Process and alloying variations are used to obtain a wide range of colors.
It is commonly used to color window glass; faucets and hardware; consumer products; and jewelry. It is more scratch resistant than electrochemical coloring but it can be damaged. PVD can be applied over any stainless steel. Supplier vetting is very important, particularly for exterior applications, since quality issues can lead to color change (darkening) and delamination. There are however many examples of excellent long-term performance.
This Japanese building entrance used PVD sputtered bronze color over a non-directional vibration finish for the entrance door framing. Photo credit Tsukiboshi Art Co., Ltd.