Pure iron is not easy to rust, but ordinary steel, usually containing copper, carbon and other impurities, the activity of these impurities, lower than the iron in water with iron to form the air in the galvanic cell (i.e., an electrical current through the REDOX reaction of a device), separate oxidation and reduction reaction, and provide the conditions for the steel corrosion form quickly objectively. Corrosion of steel rust is a loose porous structure in which many microcracks connect the pores. In this way, the rust ACTS like a sponge and continues to absorb moisture from the air, allowing the steel to rust further until it is completely worn through.
Weathering steel is different from ordinary steel, weathering steel generally has a layer of transparent colorless matte sealing protection on the surface, and ensure that the sealing layer does not change the color and texture of rust steel plate itself, sealing paint and weather resistant sealant can be compatible. At the beginning, it also rusts on the surface like ordinary steel. This process is even faster than normal steel because of its high alloy strength. But because the lattice inside weathering steel is more complex, a dark and dense rust layer will grow under the loose rust on the surface. This rust layer consists of -α-FeOOH of nanoparticles. In this dense rust layer, nickel atoms take the place of some iron atoms, making the rust layer cationic selectivity and inhibiting the corrosive anion penetration.
It is this dense layer of rust that prevents weathering steel from being corroded on the surface, but not on the inside. In fact, as long as you carefully distinguish, you can see that the surface of weathering steel and ordinary rust
is not the same: weathering steel rust is uniform dense, close to the surface of the steel to protect the steel; Rust, on the other hand, is mottled, porous, and will drop slag when touched. Instead of protecting steel, such rust sucks water and oxygen to the surface.