The rust layer that forms on ordinary hot rolled carbon steel is porous and detaches from the surface after a certain time, and the corrosion cycle commences again. The corrosion rate progresses as a series of incremental curves approximating to a straight line, the slope of which depends on the aggressiveness of the environment. With weathering steel, the rusting process is initiated in the same way, but the specific alloying elements in the steel produce a stable rust layer that adheres to the base metal, and is much less porous. This rust ‘patina’ develops under conditions of alternate wetting and drying to produce a protective barrier that impedes further access of oxygen, moisture, and pollutants, significantly reducing further metal loss. Note that the surface must be able to dry for the patina to form. If it remains constantly moist, weathering steel will continue to corrode. Similarly, if the environment is heavily contaminated with chlorides (e.g., structures near the ocean or areas subjected to deicing salts that remain damp), the patina will not properly form. But if the environment is suitable, weathering steel results in a much lower corrosion rate than would be found on ordinary structural steel.