What is the principle that stainless steel does not rust?

All metals react with oxygen in the atmosphere to form an oxide film on the surface. Unfortunately, the iron oxide that forms on ordinary carbon steel continues to oxidize, allowing the corrosion to expand and eventually form holes. The surface of carbon steel can be secured by electroplating with paint or oxidization-resistant metals such as zinc, nickel and chromium, but, as is known, this protection is only a thin film. If the protective layer is broken, the steel underneath begins to rust.

The corrosion resistance of stainless steel depends on chromium, but because chromium is a component of steel, protection methods vary.

The atmospheric corrosion resistance of the steel increases significantly when the chromium content reaches 10.5%, but it does not increase significantly when the chromium content is higher. The reason is that alloying steel with chromium changes the type of surface oxides that form on pure chromium metals. This tightly adhering chrome-rich oxide protects the surface from further oxidation. This oxide layer is extremely thin, through which the natural sheen of the steel surface can be seen, giving stainless steel a unique surface. Moreover, if the surface layer is damaged, the exposed steel surface reacts with the atmosphere to repair itself, reforming this oxide "passivation film" that continues to protect it.

Therefore, all stainless steel elements have a common characteristic, that is, the chromium content is above 10.5%.

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