Weathering steel is different from ordinary steel in that, in the beginning, it rusts on the surface just like ordinary steel. Due to its higher degree of alloying, this process is even faster than for ordinary steel. However, due to the more complex internal lattice of weathering steel, a dense dark black rust layer grows under the loose rust on the surface. In this dense layer, nickel atoms take the place of some of the iron atoms, making the layer cation-selective and resisting the passage of corrosive anions.
It is this dense layer of rust, so that although the surface of the weathering steel rust, but the internal will not be continued to rust. In fact, as long as carefully distinguish, you can see that the surface of the weathering steel and ordinary rust is not the same: weathering steel rust is uniformly dense, close to the surface of the steel to protect the steel; while the rust is mottled, loose and porous, a touch will fall slag.
