Weathering steel, often referred to by the generic trademark CorTen steel, is a group of steel alloys developed to eliminate the need for painting and form a stable, ruddy appearance after a curing period by exposure to weather. In architectural applications it is used most often as wall and/or roof cladding.
Weathering refers to the chemical composition of these steels, allowing them increased resistance to atmospheric corrosion compared to other steels. This is because the steel forms a protective layer, known as patina, on its surface. This patina develops and regenerates continuously when subjected to the elements, thus forming the protective coating.
The material has a certain pedigree, having been successfully used on notable architectural landmarks over several decades including the John Deere World Headquarters (architect Eero Saarinen, 1964), the U.S. Steel Tower (Harrison, Abramovitz & Abbe, 1971), Odense University (Holscher and Vesterholt 1971-1976), the Australian Centre for Contemporary Art (Wood Marsh, 1998), and Barclays Center Arena (SHoP Architects, 2012).
As can be expected, advantages of the material include its excellent durability and longevity together with extremely low maintenance. The material conveys an avant, contemporary aesthetic, one which is often heightened – or downplayed – by deliberate juxtaposition with traditional materials.
Like any other architectural material, weathering steel has certain limitations which must be accommodated in order to optimize its full potential. For example, it is not rustproof in itself – if water is allowed to accumulate in pockets, those areas will experience corrosion. It is not recommended in near-shore environments where, due to airborne sea salts, the protective patina may not stabilize properly. Finally, unless precautions are taken, the initial weathering of the material (known as “bleeding” or “runoff”) can discolor adjacent surfaces.
Weathering steel is a distinctive material capable of presenting striking variegated effects in subtle chocolates, ochers, and siennas. In the way that it patinas, weathering steel is a living material, changing quickly and then more slowly over time. Those working with the material must take care in its design, detailing, and specification. Considered carefully, the effects can be sublime, creating memorable buildings in any era.