In the 1930s, the United States Steel Corporation developed COR-TEN Steel, to control corrosion. The development of COR-TEN was a welcome by-product of the need for a tough steel capable of withstanding the rigors of America’s burgeoning marshaling yards and collieries.
The civil engineering applications that appeared in the early 1960s 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.