Weathering steel, best-known under the trademark COR-TEN® steel, is a group of steel alloys which were developed to obviate the need for painting, and form a stable rust-like appearance if exposed to the weather for several years. COR-TEN® steel's unique look and naturally oxidizing finish make it especially desirable for many architectural projects. Weathering steel has increased resistance to atmospheric corrosion when compared to other steels.
COR-TEN® resists the corrosive effects of rain, snow, ice, fog, and other meteorological conditions by forming a coating of dark brown oxidation over the metal, which inhibits deeper penetration and negates the need for painting and costly rust-prevention maintenance over the years.
In simple terms the steel is allowed to rust and that rust forms a protective coating that slows the rate of future corrosion.