Weathering steels form a compact rust layer during long-term exposure to the atmosphere.
The compact rust layer gradually becomes denser over time and eventually protects the steel from corrosive environment & thus corrosion is inhibited by the corrosion product. Minor damage to the rust heals itself; therefore maintenance is greatly reduced. Due to such beneficial property, weathering steels have been used for many structural and engineering applications.
It is well known that the protective rust layer of weathering steel consists of two layers; a compact, dense, amorphous and adherent inner layer, enriched with some alloying elements such as Cr, Cu, Ni, Mn, and P and a loose crystalline outer layer distributed with cracks and pores.