All metals react with oxygen in the atmosphere to form an oxide film on the surface. Unfortunately, the iron oxide formed on ordinary carbon steel continues to oxidize, allowing the rust to expand and eventually form holes. Carbon steel surfaces can be protected by electroplating with paint or oxidation-resistant metals (such as zinc, nickel, and chromium), but, as we all know, this protection is only a thin film. If the protective layer is damaged, the steel underneath begins to rust.
The corrosion resistance of stainless steel depends on chromium, but because chromium is one of the components of steel, the protection methods are different.
When the added amount of chromium reaches 10.5%, the atmospheric corrosion resistance of steel increases significantly. However, when the chromium content is higher, although the corrosion resistance can still be improved, it is not obvious. The reason is that alloying steel with chromium changes the type of surface oxide to one similar to that formed on pure chromium metal. This tightly adherent chromium-rich oxide protects the surface from further oxidation. This kind of extremely thin oxide layer, and the natural luster of the steel surface can be seen through it, giving the stainless steel a unique surface. Moreover, if the surface layer is damaged, the exposed steel surface will react with the atmosphere to repair itself and re-form the "passivation film" to continue to protect.
Therefore, all stainless steels have a common characteristic, that is, the chromium content is above 10.5%.
There are usually only six types of steel relevant to building construction applications. They all contain 17 to 22% chromium, and the better steel types also contain nickel. Adding molybdenum can further improve the atmospheric corrosion resistance, especially the corrosion resistance of chloride-containing atmosphere.

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