Weathering steel
[5] 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 hopper cars, for the handling of heavy bulk loads including coal, metal ores, other mineral products and grain.
[7] Railroad passenger cars were also being built with Cor-Ten, albeit painted, by Pullman-Standard for the Southern Pacific from 1936,[8] continuing through commuter coaches for the Rock Island Line in 1949.
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.
It is very widely used in marine transportation, in the construction of intermodal containers[23] as well as visible sheet piling along recently widened sections of London's M25 motorway.
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 Denmark, all masts for supporting the catenary on electrified railways are made of weathering steel for aesthetic reasons.
[25] 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.
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.
Weathering steel is sensitive to humid subtropical climates, and in such environments it is possible that the protective patina may not stabilize but instead continue to corrode.
For example, the former Omni Coliseum, built in 1972 in Atlanta, never stopped rusting, and eventually large holes appeared in the structure.
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.
Rainwater runoff from the slowly rusting steel stained the numerous large windows and increased maintenance costs.
[29] 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 initial weathering of the material resulted in a discoloration, known as "bleeding" or "runoff", of the surrounding city sidewalks and nearby buildings.
