The orange-brown finished colour in A606-4 comes mainly from the copper content. With 5% copper in the alloy mix, the copper immediately comes to the top as the patina process begins. Additionally, the copper along with manganese, silicon and nickel content in A606-4 creates that protective layer as the material continues to patina. Standard carbon steel will rust but it will not have the beautiful colors that come from A606-4.
COR-TEN steel gets its properties from a careful manipulation of the alloying elements added to steels during the production process. It has a combination of Cr (chromium), Cu (copper), Si (silicon), and P (phosphorus), the amounts depending on the properties required. COR-TEN steel works by controlling the rate at which O2 (oxygen) in the atmosphere can react with the surface of the steel. The rusting of a steel takes place in the presence of air and water,
resulting in the product of corrosion which is the 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 delaminates from the surface of the steel, exposing the surface and causing more damage.