COR-TEN steel, sometimes written as "Corten steel," is the registered trademark of a group of weathering steel alloys, all of which feature an outer layer that naturally rusts and protects the core from corrosion. The metal does not need painting, and forms a uniform rust-like appearance when exposed to the elements.
COR-TEN steel has increased resistance to corrosion as compared to other steels. It forms an oxidized top layer characterized by its dark brown color that prevents rust from penetrating to the base metal. The steel saves users money on rust prevention and maintenance as these services are not needed.
As of 2014, U.S. Steel provides three types of weathering steel, designated A 242, A 588 and A 606-4 by the ASTM International standards group. Type A 242, or COR-TEN A, is the original alloy, and type A 588, or COR-TEN B, is a newer one. Type A 606-4 is thin sheet.
The manufacturer provides all alloys in a rust-free form and they begin to rust naturally after installation upon exposure to the elements. Since the alloys require climates with alternating wet and dry periods to form a proper protective layer, they are not suitable for use in marine areas, or places with high humidity, constant fog or long periods of rain.