Pure iron is not easy to rust, but ordinary steel usually contains impurities such as copper and carbon. These impurities have lower activity than iron, and form a galvanic cell with iron in water-containing air (that is, a current generated by redox reaction. Plant) to separate oxidation and reduction reactions and provide a “highway” for steel corrosion.
The rust produced by the corrosion of steel is a loose and porous structure, and there are many micro cracks connecting the pores with each other. In this way, rust, like a sponge, can continue to absorb moisture in the air, allowing the steel to rust further until it is completely rusted through.
Weathering steel is different from ordinary steel. At the beginning, it will also rust on the surface like ordinary steel. Due to its high degree of alloying, this process is even faster than ordinary steel. However, due to the more complicated internal lattice of weathering steel, a dense dark rust layer will grow under the loose rust on the surface. This rust layer is composed of nanoparticles of α-FeOOH. In this uniform dense rust layer, nickel atoms replace part of the iron atoms, making the rust layer cation selective and inhibiting the penetration of corrosive anions.
It is this dense rust layer that makes the weathering steel rusted on the surface, but it will not be rusted inside. In fact, as long as it is carefully distinguished, it can be seen that the surface of weathering steel is not the same as ordinary rust: the rust of weathering steel is uniform and dense, and the steel surface is close to the steel to protect the steel; while rust is mottled, loose and porous The slag will fall off when touched. This kind of rust can not only protect the steel, but "leads the wolf into the room", sucking water and oxygen to the surface of the steel.