When weathering steel is exposed to the ambient atmosphere it develops an initial layer of iron oxide in the same way as carbon steel. The rate of oxidisation depends on how much oxygen, moisture, and atmospheric contaminants can access the surface of the metal. In the initial stages, a complex mix of iron oxides covers the surface to create a layer of rust. As the process progresses, the rust layer forms a barrier against the corrosive agents and the rate of corrosion slows. On a low alloy carbon steel, the iron oxide layer is porous. Over time that layer detaches from the surface of the metal and the corrosion process starts again. The oxidation rate progresses in increments which depend on the chemical and mechanical aggressiveness of the environment. It can end with the complete destruction of the metal.