The Bars used by 3B are ‘Cor-Ten‘ steel.
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 rigours 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 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.