Weathering steel, often referred to by the genericised trademark COR-TEN steel and sometimes written without the hyphen as corten steel, is a group of steel alloys which were developed to eliminate the need for painting, and form a stable rust-like appearance after several years' exposure to weather. U.S. Steel (USS) holds the registered 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 still sells 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 alloys are in common production and use. The surface oxidation of weathering steel takes six months, but surface treatments can accelerate the oxidation to as little as one hour. History The history of weathering steels began in the US in 1910s, when steels alloyed with different amounts of copper were exposed to the elements; the research continued into the 1920s and ca. 1926 it was discovered that phosphorus content also helps with the corrosion resistance. 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. 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 and storage containers. Railroad passenger cars were also being built with Cor-Ten, albeit painted, by Pullman-Standard for the Southern Pacific from 1936, continuing through commuter coaches for the Rock Island Line in 1949. 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 York University Footbridge in the United Kingdom in 1967. Since then, the practice of using weather steel in bridges has expanded to many countries. Properties Weathering refers to the chemical composition