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4/1/2006
Several architects have asked a question about Cor-TenTM steel recently. Interesting. Firstly, the term “Cor-tenTM” steel is a brand name. The generic term is "weathering" steel. As you know, the beauty of weathering steel is basically that its surface oxidizes quickly and creates a protective layer over time, and hence no paint is needed and maintenance is reduced.
Weathering steel is usually specified as CSA G40.21-350 A grade plate. More commonly in bridge applications, CSA G40.21-350 AT is used to get better behaviour at low temperatures, which is generally not necessary for a building application. Cor-tenTM steel is in fact a trademark of US Steel. It was patented in 1933. However, any steel mill can produce either Cor-tenTM steel by getting the rights to the specific "recipe", or provide its own version, a bit like a "President's Choice" version. Algoma told me they can produce both for instance. In each case, the mill has to adhere to a recognized material standard (CSA G40.21 350A or ASTM A606 Grade E).
It is important to understand how weathering steel works before you specify it for an architectural application. For instance, I would not recommend it for the specific application you described to me. Weathering steel has the unique characteristic such that, under proper conditions, it corrodes to form a dense and tightly adhering oxide barrier that seals out the atmosphere and retards further corrosion. This is in contrast to other steels that form a coarse, porous and flaky oxide that allows the atmosphere to continue penetrating the steel. The oxidized layer on weathering steel in many climates does not consume a significant amount of steel in its formation. However, climate is important – the oxide layer will form provided there are wet/dry cycles.
Words of caution: runoff of water from upper portions of a structure tend to produce long-lasting streaks or other patterns of redder oxide on lower portions. So, special attention must be paid to the drainage of storm water (or condensate) to prevent staining of surrounding structures, sidewalks, and other surfaces. So far, we have talked about plate. Weathering steel is also available in sheets, for roofing and cladding. However, they were not meant for architectural applications. Weathering steel must be kept free from debris such as leaves, pine needles, etc. These waste products retard the wet/dry cycle necessary for weathering steel and corrosion is accelerated. Also, in an accelerated environment, loss of material may be more significant, and could cause perforation of very thin sheets. In addition, for green building design, one should know that a thin weathering steel roof has low solar reflectivity, i.e. it is a "hot roof".
In terms of availability, few steel service centres will stock a large inventory of weathering steel because of its specific bridge application. However, they will order it from the mill on request. Unfortunately, when you want to use steel for an exterior wall element, that usually represents low tonnage for a service centre. But do call the two service centres I provided, as they will give you the direct answer. If you are really in a bind, you might want to consider contacting a steel bridge fabricator, as some of them stock CSA G40.21-350 A and 350 AT grades in their own yard! Weathering steel is not readily available in W shapes and HSS from Canadian sources.
You stated the example of the new McGill School of Music. The architects were careful not to have any important architectural elements underneath the exterior wall element as it develops its natural coating. You do realize it will not stay that colour. After about two years, which is about the time it takes to develop the oxide skin, the colour is going to be much darker and reddish brown. I can imagine architects would love to be able to choose the colour from a catalog but that is not possible as we are dealing with "living steel"! In wetter climates the color of weathering steel will generally have an overall redder cast relative to those exposed in drier climates. However, one can be most certain that the final colour will have a rich dark earthy tone and will be low maintenance, durable and beautiful, provided one is careful about the details.
An early example of the use of weathering steel in a building is the John Deere World Headquarters in the US. It was built in 1964 and was the first use of Cor-tenTM steel in an architectural application. All structural steel is Cor-tenTM steel, inside and outside the building! A real case history with expressive steel detailing. The building has received a number of awards, including the American Institute of Architects 25 Year Award. However, remember such use is an exception. For more details on the John Deere & Company World Headquarters: www.deere.com/en_US/attractions/worldhq/index.html.
In a nutshell, weathering steel is not normally used for architectural applications. It does take two years to stabilize and will create dark streaks, even if in small quantities, during the oxidization process. It is not appropriate for green roofs. However, it is low maintenance, no paint is required, and properly used "vertically", it can add a distinct and green character to your project.
This question appeared in the "Ask Dr. Sylvie" column of Advantage Steel No.25, Spring 2006
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