History of Cor-ten steel
  • 12 May 2017

In the 1930s, the United States Steel Corporation developed Cor-Ten, primarily for use in railway coal wagons. The controlled corrosion that is a feature of the material was a welcome by-product of the need for a tough steel capable of withstanding the rigorous of America's burgeoning marshalling yards and collieries. Because of its inherent toughness, weathering steel (the generic name for Cor-Ten, along with weather-resisting steel) is used extensively for ISO shipping containers.

The civil engineering applications that appeared in the early 1960s like above picture made direct use of the improved resistance to corrosion, and it would not be long before the applications in architecture would become apparent. Cor-Ten gets its properties from a careful manipulation of the alloying elements added to steels during the production process. All steel produced by the primary route (in other words, from iron ore as opposed to scrap) comes into being when the iron smelted in blast furnaces is reduced in a converter. The carbon content is lowered and the resultant iron, now steel, is less brittle and has a higher capacity for loading than before.  Other material is commonly added during the process. Weathering steel has a combination of chromium, copper, silicon and phosphorus, the amounts depending on the exact attributes required.

Weather-resistant steel works by controlling the rate at which oxygen in the atmosphere can react with the surface of the metal. Iron and steel both rust in the presence of air and water, resulting in the product of corrosion - rust, iron oxide. Non-weather-resisting steels have a relatively porous oxide layer, which can hold moisture and promote further corrosion. After a certain time (dependent on conditions), this rust layer will delaminate from the surface of the metal, exposing the surface and causing more damage. Rusting rates seen on a graph would appear as a series of curves approximating to a straight line.

Cor-Ten exhibits superior corrosion resistance over regular carbon steel as a result of the development of a protective oxide film on the metals surface that slows down further corrosion. Their yield strength allows cost reduction through the ability to design lighter sections into structures. These steels were designed, primarily to be used in unpainted applications where a reduction in maintenance costs, such as painting, were desired. Weathering steels are now being used in a variety of applications, including bridges, rail cars, transmission towers, chimneys and shipbuilding. It is also becoming increasingly popular with sculptors and as an architectural feature like shipping container building.

Cor-Ten is the primary brand name for corrosion resistant products that were developed by United States Steel Corp. Cor-Ten has subsequently been licensed to be produced by other steel producers. There are basically two types of Cor-Ten that are most prevalent, Cor-Ten A (generally up to 12mm thick) and Cor-Ten B (generally 15mm thick and above).

The comparison of Cor-Ten to the ASTM grades is loosely stated as Cor-Ten A is equivalent to ASTM A242 and Cor-Ten B is equivalent to ASTM A588 Grade A.  Cor-Ten A and B both meet and/or exceed the requirements of ASTM A606 Type 4.

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