Why doesn't stainless steel rust?
The core reason why stainless steel is "stainless" is that it contains over 10.5% chromium (Cr). This special chemical composition allows it to form an invisible, extremely thin, dense, and transparent "protective film" on the surface.
Formation and self-healing of membranes: the mystery of stainless steel's "stainless" properties
When oxygen comes into contact with the surface of stainless steel containing sufficient chromium, a reaction occurs and a thin film of chromium oxide (Cr ₂ O3) is formed. This film has several characteristics:
Extremely thin
only equivalent to the thickness of a few atoms (about 1-5 nanometers), completely invisible to the naked eye.
Dense structure
It can tightly adhere to the surface of steel without peeling off like ordinary rust, thus isolating the internal metal from external water, oxygen, and corrosive substances.
Strong stability
Once this film is formed, its chemical properties are very stable and it will not continue to react in water or air.
The most unique thing is that this protective film has a super strong self-healing ability. If the film is scratched or damaged, the metal chromium exposed to the air will immediately react with oxygen to regenerate a new protective film, continuing to protect the internal metal from corrosion. In contrast, the rust (iron oxide) on the surface of ordinary carbon steel is loose and constantly oxidizes, leading to "rust penetration".
Summary
The "stainless" of stainless steel is an active protection based on its chemical composition, relying on the thin and tough chromium oxide protective film on the surface and its self-healing ability.
It should be remembered that stainless steel ≠ never rusts. It is not easy to rust, rather than absolutely rust free. Its corrosion resistance is closely related to the material grade, usage environment, surface condition, and proper maintenance. Only by understanding these points correctly can we better choose and use stainless steel.
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