Pure iron does not rust easily, but ordinary steel usually contains impurities such as copper and carbon. These impurities are less reactive than iron and form a primary cell (that is, a device that generates an electric current by redox) with iron in aqueous air, separating the oxidation and reduction reactions and subjecting the steel to corrosion.
The rust produced by steel corrosion problems is a rust with a loose, porous structure in which one of many microcracks interconnect the pores. In this way, rust is like a sponge that continues to absorb moisture from the air, further rusting the steel until it passes through completely.
Weathering steel is different from ordinary steel. At first, it will rust on the surface like ordinary steel. Due to its high degree of alloying, this development process is even faster than ordinary steel. But due to the more complex lattice structure within weathering steel, a dense, deep black layer of rust grows underneath the loose surface rust. This rust layer consists of α-FeOOH nanomaterial particles. In this process, nickel atoms replace some of the iron atoms in the dense rust layer, making the layer selective for different cations and inhibiting corrosive and anionic penetration.
It is this dense rust layer, making the surface of the weathering steel rust, but the internal will not continue to rust. In fact, it is as long as we can carefully distinguish between the surface of the weathering steel and ordinary rust can be clearly seen are not the same: weathering steel rust flat and dense, attached to the surface of the steel in order to protect our steel, while the rust is mottled, loose, porous, a touch will melt. Instead of protecting the steel, such rust draws water and oxygen to the steel surface.
