COR-TEN: The origins of weathering steel
Weathering steels were first developed by US Steel Corporation back in the 1930s. They were looking for a steel alloy for their ore carrying hopper carts they would use to transfer the iron ore and coal to the furnace. They needed a stronger, more durable steel alloy that had exceptional hardness and did not need to be painted. It was only later they realized this alloy had interesting corrosion resistant traits that could be marketed in other industries, so it trademarked the name COR-TEN® for this corrosion-resistant steel alloy. COR stands for corrosion resistance and TEN stands for tensile strength. (Today, while the terms Corten and COR-TEN are used interchangeably, COR-TEN® is a trademarked product of US Steel.)
The steel was not introduced as an architectural metal, however, until the 1950s. In 1964, the John Deere Headquarters, in Moline, Illinois, opened and was one of the first major architectural structures to be clad in weathering steel. Eero Saarinen designed the John Deere complex but, unfortunately, he died before seeing it completed.
William Hewitt, then president of the John Deere company, said:
“The seven buildings should be thoroughly modern in concept but should not give the effect of being especially sophisticated or glossy. Instead, they should be more ‘down to earth’ and rugged.
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Richard Serra Sculpture |
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Saarinen chose this special weathering steel, Corten, to age and provide the earth tones and the “down to earth” feel his client desired. He knew the metal would age gracefully and create a deep, natural tone as the thickened oxide formed. The deep rich surface oxide exhibits a natural brownish-red tone that is both durable and stable.
Many artists have also incorporated weathering steel into their work, the most well-known being Richard Serra. Serra has used weathering steel plates on numerous massive sculptures found around the world.
The great minimalist artist Donald Judd also used weathering steel in his works. Judd would often preweather large plates and then allow them to continue to oxidize outdoors in Marfa, Texas. He also created smaller weathering steel wall units where he worked the surface oxide to create a fine velvet-like appearance.
Easy to work with, weldable, and now with appealing surface finishing, weathering steel is giving new value and inspiration to the designer and artist.
What is weathering steel?
In the context of art and architecture, steels are rarely used uncoated because of their tendency to absorb moisture and develop the porous and friable oxide commonly called rust. Coatings can consist of a sacrificial zinc, as in the galvanizing process where the steel is electroplated
or immersed into a molten bath of zinc. Other metal coatings in common use are the aluminum–zinc coatings and Galfan™, a combination of zinc and rare earth metals. These are all considered sacrificial coatings of other metals that involve the protection of the base steel by offering up their electrons as well as acting as barrier coatings.
In addition to the preweathering rust effects, there are also treatments that induce a blue or black protective coating on the steel, such as Oscura blackened steel. These involve the integration of metal salts onto the surface of the steel. There is some diffusion of the iron into these coatings to create the dark, sulfide, phosphate or selenide salt. Not as protective as the sacrificial coating of zinc in the galvanized process, these coatings are often applied to smaller features, pieces and areas.
Weathering steel, with its copper-bearing properties, forms its own distinctive oxide barrier over the surface. This barrier grows outward and inward from the surface. Unlike common rust, the oxide formed on weathering steel is a thick, impervious ferro-oxyhydroxide that will afford the base metal extensive resistance to corrosion. Below are some of the most common coatings applied when necessary for internal and external design, in art and architecture.
Surface Coatings on Steel Used in Art and Architecture
High-strength, low-alloy weathering steels are designed to develop a special thick iron oxide capable of slowing down the effects of atmospheric corrosion much like the green patina that develops on copper surfaces exposed to the atmosphere. This oxide grows on exposure to moisture and air.
As with all uncoated steels, the oxide develops when the surface is exposed to moisture and oxygen. For the weathering steels, however, the difference lies in the nature of the oxide that develops over time.
The oxide that gives weathering steel its protective ability is ferric oxyhydroxide, FeO(OH) an oxygenated version of ferric oxide — essentially, a mixture of oxides and hydroxide. This attractive, orange to rich purple brown oxide acts as a protective barrier to the base metal. When correctly formed, this tough layer of oxide develops into a surface that resists atmospheric degradation in outdoor environments. Once the oxide develops correctly, further changes are very slow in most environmental exposures.
Source: Zahner
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