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Weathering steel, often called corten steel (or its trademarked name, COR-TEN) is a group of that form a stable external layer of rust that eliminates the need for painting.
U.S. Steel (USS) holds the Trademark on the name COR-TEN. The name COR-TEN refers to the two distinguishing properties of this type of steel: corrosion resistance and tensile strength. Although USS sold its discrete plate business to International Steel Group (now ArcelorMittal) in 2003, it makes COR-TEN branded material in strip mill plate and sheet forms.
The original COR-TEN received the standard designation A242 (COR-TEN A) from the ASTM International standards group. Newer ASTM grades are A588 (COR-TEN B) and A606 for thin sheet. All of the alloys are in common production and use.
The surface oxidation generally takes six months to develop, although surface treatments can accelerate this to as little as one hour.
In 1933 the United States Steel Corporation decided to commercialize the results of their studies and patented a steel with exceptional mechanical resistance, primarily for use in railroad , for the handling of heavy bulk cargo including coal, metal , other mineral products and grain. The controlled corrosion for which this material is now best known was a welcome benefit discovered soon after, prompting USS to apply the trademarked name Cor-Ten. Because of its inherent toughness, this steel is still used extensively for bulk transport, intermodal shipping containers and bulk storage.
Railroad passenger cars were also being built with Cor-Ten, albeit painted, by Pullman-Standard for the Southern Pacific from 1936,“The Sunbeams are Hustlers,” page 38 Trains magazine, January 1950 continuing through commuter coaches for the Rock Island Line in 1949.“Railroad News and Editorial Comment,” Trains magazine, January 1950
In 1964, the Moorestown Interchange was built over New Jersey Turnpike at milepost 37.02. This overpass is believed to be the first highway structure application of weathering steel. Other states including Iowa, Ohio, and Michigan followed soon after. Those were followed by University of York Footbridge in the United Kingdom in 1967. Since then, the practice of using weathering steel in bridges has expanded to many countries.
The corrosion-retarding effect of the protective layer is produced by the particular distribution and concentration of alloying elements in it. It is not yet clear how exactly the patina formation differs from usual rusting, but it's established that drying of the wetted surface is necessary and that copper is the most important alloying element.
The layer protecting the surface develops and regenerates continuously when subjected to the influence of the weather. In other words, the steel is allowed to rust in order to form the protective coating.
| + Weathering steel grade chemical composition (excl. iron) by weight (%) |
| 0.65 |
| 0.40 |
The mechanical properties of weathering steels depend on which alloy and how thick the material is.
Since the 1960s, artists and architects have not only valued weathering steel for its durability, but also embraced the oxidation process as an aesthetic and symbolic element. Sculptors such as Eduardo Chillida, Bernard Venet, and Mauro Staccioli have created monumental works in Corten steel where the evolving patina is integral to the artwork’s meaning.
Art historians and critics have highlighted this reinterpretation of rust as an aesthetic feature . Robert Pincus-Witten described the role of natural processes, including oxidation, in the emergence of Postminimalism. Germano Celant, theorist of Arte Povera, emphasized how artists such as Jannis Kounellis and Giovanni Anselmo integrated oxidation-prone materials to underscore themes of change and entropy. Florence de Meredieu argued that rust acts both as a bearer of memory and as a transformative process that confers individuality and historical resonance on materials.
Beyond sculpture and architecture, weathering steel oxidation has also been incorporated into installations and photography. Artists such as Leonardo Drew and Mark Dion use rusted elements to explore decay, memory, and material culture, while the industrial typologies of Bernd and Hilla Becher influenced subsequent generations of artists who turned rust into a metaphor for impermanence.
It is very widely used in marine transportation, in the construction of intermodal containers, and visible sheet piling along recently widened sections of London's M25 motorway.
The first use of weathering steel for architectural applications was the John Deere World Headquarters in Moline, Illinois. The building was designed by architect Eero Saarinen, and completed in 1964. The main buildings of Odense University (built 1971–1976), designed by Knud Holscher and Jørgen Vesterholt, are clad in weathering steel, earning them the nickname Rustenborg (Danish for "rusty fortress"). In 1977, Robert Indiana created a Hebrew version of the Love sculpture made from weathering steel using the four-letter word ahava (אהבה, "love" in Hebrew) for the Israel Museum Art Garden in Jerusalem, Israel. In Denmark, all masts for supporting the catenary on Overhead line are made of weathering steel for aesthetic reasons.
Weathering steel was used in 1971 for the Highliner electric cars built by the St. Louis Car Company for Illinois Central Railroad. The use of weathering steel was seen as a cost-cutting move in comparison with the contemporary railcar standard of stainless steel. A subsequent order in 1979 was built to similar specs, including weathering steel bodies, by Bombardier. The cars were painted, a standard practice for weathering steel railcars. The durability of weathering steel did not live up to expectations, with rust holes appearing in the railcars. Painting may have contributed to the problem, as painted weathering steel is no more corrosion-resistant than conventional steel, because the protective patina will not form in time to prevent corrosion over a localized area of attack such as a small paint failure. These cars were retired by 2016.
Weathering steel was used to build the exterior of Barclays Center, made up of 12,000 pre-weathered steel panels engineered by ASI Limited & SHoP Construction. The New York Times says of the material: "While it can look suspiciously unfinished to the casual observer, it has many fans in the world of art and architecture." In 2015, a new building for the KTH Royal Institute of Technology School of Architecture was completed on its campus. The use of weathering steel helped the futuristic shapes of the facade fit in well with its much older surroundings and in 2015 it was awarded the Kasper Salin Prize.
The rate at which some weathering steels form the desired patina varies strongly with the presence of atmospheric pollutants which catalyze corrosion. While the process is generally successful in large urban centers, the weathering rate is much slower in more rural environments. Uris Hall, a social sciences building on Cornell University's main campus in Ithaca, a small city in Upstate New York, did not achieve the predicted surface finish on its Bethlehem Steel Mayari-R weathering steel framing within the predicted time. Rainwater runoff from the slowly rusting steel stained the numerous large windows and increased maintenance costs. Corrosion without the formation of a protective layer apparently led to the need for emergency structural reinforcement and galvanizing in 1974, less than two years after opening.
The U.S. Steel Tower in Pittsburgh, was constructed by U.S. Steel in part to showcase COR-TEN steel. The initial weathering of the material resulted in a discoloration, known as "bleeding" or "runoff", of the surrounding city sidewalks and nearby buildings. A cleanup effort was orchestrated by the corporation once weathering was complete to clean the markings. A few of the nearby sidewalks were left uncleaned, and remain a rust color. This problem has been reduced in newer formulations of weathering steel. Staining can be prevented if the structure can be designed so that water does not drain from the steel onto concrete where stains would be visible.
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