By: Catherine Houska, TMR Consulting
Stainless steel offers a broader range of color possibilities than for other architectural metals. This is the second installment of a two-part article on coloring methods for stainless steel. Part 2 features the nickel oxide, plating and painting coloring techniques. Part 1 featured the electrochemical and ceramic coloring process.
These coloring options provide a broad range of opaque colors and provide considerable design flexibility. The service life of these coloring processes varies considerably. Some will last the life of the building, if they are used appropriately and well maintained, while others are limited by the coating life in the service environment. The higher corrosion rates of other metals can result in premature color loss and aesthetic failure. Stainless steel’s superior corrosion resistance makes a more durable option.
Nickel Oxide Ceramic Coatings
This proprietary coloring process was developed in Japan to obtain the appearance of weathered copper while offering the higher strength, improved corrosion resistance, wind uplift resistance, and increased fire resistance of stainless steel. It also eliminates potential problems with discoloration of surrounding materials and environmental concerns that can be caused by copper run-off.
This pre-weathered nickel oxide ceramic coating process produces dull green and green-gray colors. Like the ceramic coatings discussed in Part 1 of this two-part article, this is a stable, durable finish that will not change color over time. This finish is less scratch resistant than the other ceramic coatings and it is also not repairable. It should not be used in locations where scratching or exposure to wind blown abrasives, such as sand, are likely. The color does not change when viewed from different angles but there will be a slight color lightening at sharp bends.
This finish is appropriate for exterior roofing and wall panel applications and many interior applications. Figure 1 shows a building in Japan with a green nickel-oxide coated stainless steel roof. Because the coating is nickel and nickel is an alloying element in the stainless steels to which it is applied, the coloring process does not limit the recyclability of the stainless steel.
|Figure 1: Nickel oxide coated stainless steel roof. Photo courtesy of Nippon Metal Industry.|
Terne and Tin/Zinc Coatings
Terne metal (80 percent lead, 20 percent tin) and zinc/tin-plated coatings on stainless steel have primarily been used for roofing. There have also been some exterior wall panel applications. Traditional lead-containing terne coatings are no longer available because of environmental concerns. Tin/zinc coatings were developed to replace lead/tin coatings. The coating enhances the contractor’s ability to solder roofing panels. Figure 2 shows a terne-coated stainless steel roof and elevated wall panels of the student union buildings at Carnegie Mellon University.
|Figure 2: Terne-coated stainless steel roof and elevated wall panels on the Carnegie Mellon University student union building, Photo courtesy of Nickel Institute, Catherine Houska photographer.|
These metal coatings weather to a medium to dark gray tone but the color is dependant on the environment. Scratching or abrasion can easily damage the coating. These coatings can be applied to either Type 304 or Type 316 stainless steel. Although the coating will corrode first to protect the stainless steel, it should not be considered a substitute for specifying a more corrosion resistant stainless steel in more aggressive environments.
If a custom painted stainless steel roof is desired, tin/zinc coatings can be painted using manufacturer recommended primers and paints. This is generally an easier field painting process than painting bare stainless steel.
Zinc coatings are not desirable when recycling stainless steel and this should be considered if environmental friendliness is important. Capturing zinc roof run-off can also be a consideration in fragile environments.
Plating with other Metals
Stainless steel can be plated with other metals including gold, copper alloys, and other metals. Copper-plated stainless steel has had limited use in roofing applications; it combines the strength of stainless steel with the color of copper. Gold plating has been used for some applications but the same color is more cost effectively achieved using ceramic coatings. Gold is soft and easily abraded and should not be used in applications where damage is a concern.
Stainless steel is coil or resin paint coated in the same manner as other metals except that it is done for aesthetic reasons rather than corrosion protection. Coil-coated stainless steel is particularly popular in Japan but it is also used in Europe. North American applications have been more limited but the product is available. Suppliers generally stock the most popular colors but custom paint colors are available if multiple coils are purchased. Figure 3 shows the custom green coil coated roof of the airport in Kuala Lumpur, Malaysia.
|Figure 3: Coil coated stainless on the roof of Kuala Lumpur International Airport, Photo courtesy of Outokumpu.|
Powder coating stainless steel is also an option. This technique produces a durable finish and has been used for reflective panels in corrosive tunnel environments and for adding recessed color to embossed stainless steel. Figure 4 shows powder-coated stainless steel tunnel reflector panels that replaced badly corroded enameled carbon steel.
|Figure 4: Powdered coated stainless tunnel panels. Photo courtesy of the Nickel Institute.|
The paint systems applied to stainless steel are identical to those applied to other metals but some processing modifications are needed to obtain sufficient paint adherence. Resin-based paints provide a shorter service life than other coloring methods for stainless steel. Like all paint systems, they will fade over time and will eventually fail. Paint systems generally last longer on stainless steel substrates because peeling due to corrosion under the paint is not an issue. Service life is instead determined by the performance of the paint system.
Eventually the stainless steel will have to be repainted based on the manufacturers’ recommendations. Alternatively, the residual paint can be removed and the surface left bare if the stainless steel provides adequate corrosion resistance for the environment.
Stainless steel can also be painted after installation but the paint system’s service life is generally shorter than that of a factory applied coating. The surface finish must be clean, dry, and rough enough for proper adherence. An appropriate primer and paint system should be selected with the assistance of a paint system supplier. The No. 2B or No. 2D mill finishes and rougher polished finishes, such as a No. 4, have been painted successfully.
Ceramic coated, plated, and painted stainless steel offer a variety of opaque color options. Suppliers can provide samples and work with designers to obtain unique finishes. The durability and recyclability of these stainless steel finishes varies considerably and should be taken into consideration during selection. The appearance of colored stainless steel surface finishes is not defined by standards, so a finish sample should be used as a visual standard. It is important to select an appropriate stainless steel for the service environment or the finish will eventually deteriorate due to corrosion. Additional information on stainless steel selection is available from the Nickel Institute at www.stainlessarchitecture.org. The Specialty Steel Industry of North America (SSINA) has a free brochure on special finishes for stainless steel which can be downloaded from their Web site at www.ssina.com.
The author would like to acknowledge the assistance and support of the Nickel Institute in preparation of this article. Without their support, this article would not have been possible.
Catherine Houska, CSI, is a metallurgical engineer with an MBA in industrial marketing. She is an internationally recognized expert in architectural metal applications and is senior development manager at TMR Consulting, a consulting firm in Pittsburgh, Pa. She can be reached by e-mail at email@example.com.