Corten steel is a high-grade of structural steel which, thanks to the content of elements such as copper, chromium, nickel and phosphorus, it shows increased resistance to corrosion. Appropriate selection of alloying elements causes so-called copper oxides which are released on the surface of corten steel creating the desired patina - commonly reminiscent of rust. However, this coating is fully intentional, durable and does not progress into the material, thus there is no loss of sheet thickness, and there is no flaking, although as corten steel weathers it will emit a residue/run-off that can stain surfaces the corten steel product is placed on.