Other material is commonly added during the process. Stainless steel has added chromium and molybdenum, for example, and weathering steel has a combination of chromium, copper, silicon and phosphorus, the amounts of which are dependent on the exact attributes required.
Weather-resistant steel works by controlling the rate at which oxygen in the atmosphere can react with the surface of the metal. Iron and steel both rust in the presence of air and water, resulting in corrosion (rust / iron oxide). Non-weathering resistant 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 will delaminate from the surface of the metal, exposing the surface and causing more damage. Rusting rates seen on a graph would appear as a series of curves approximating to a straight line.