Protective layer of rust
Weathering steel is a special type of weather-resistant metal with virtually the same characteristics as regular steel. Put outside, weathering steel however reacts to erosion by gradually covering itself with a protective layer of rust, also called platinum layer. As a result, further erosion due to rain or fire is prevented and the weathering steel can safely be left outside. You should be aware of stains: if weathering steel is left outside for a long time, rain combined with rust can cause stains. Weathering steels are high strength, low alloy, weldable structural steels that possess good weather resistance in many atmospheric conditions without the need for protective coatings. They contain up to 2.5 percent alloying elements, e.g. chromium, copper and nickel. On exposure to air, a protective rust patina forms that adheres to the surface of the steel. This layer causes the rate of corrosion to slow so that after two to five years, corrosion almost ceases.
Wheatering steel hardly need any maintenance.