Why can EN 10025 S355J0W weathering steel rust without corrosion?

EN 10025 S355J0W weathering steel is different from ordinary steel. At the beginning, it will also rust on the surface like ordinary steel. Due to the high degree of alloying, this process is even faster than ordinary steel. However, due to the more complex lattice inside the EN 10025 S355J0W weathering steel, a dark black dense rust layer is formed under the loose rust on the surface. This rust layer consists of nanoparticles of α-FeOOH. In this uniform rust layer, the nickel atom replaces the position of a part of the iron atom, so that the rust layer has cation selectivity, and the corrosive anion permeation is suppressed.

(The left is EN 10025 S355J0W weathering steel weathering steel, the right is rust, you can clearly see the difference between the two)

It is this dense layer of rust that makes theEN 10025 S355J0W weathering steel rust, but the interior will not continue to rust. In fact, as long as you carefully distinguish it, you can see that the surface of EN 10025 S355J0W weathering steel is not the same as ordinary rust: EN 10025 S355J0W weathering steel rust is uniform and dense, and the steel surface protects the steel; Mottled, loose and porous, will slag when touched. Instead of protecting the steel, such rust "leads the wolf into the room" and sucks water and oxygen onto the steel surface.Why can

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