All metals react with oxygen in the atmosphere to form an oxide film on the surface. Unfortunately, the formation of iron oxide on ordinary carbon steel continues to oxidize, so that corrosion continues to expand, the ultimate formation of holes. The surface of the carbon steel can be secured by painting with paint or oxidation resistant metals (eg zinc, nickel and chromium), but, as is known, this protection is only a film. If the protective layer is destroyed, the following steel will begin to rust.

The corrosion resistance of stainless steel depends on chromium, but because the chromium is one of the components of steel, so the protection method is not the same.

When the amount of chromium added reaches 10.5%, the corrosion resistance of steel is significantly increased, but the chromium content is higher, although it can improve the corrosion resistance, but not obvious. The reason is that when the steel is alloyed with chromium, the surface oxide type is changed to a surface oxide similar to that formed on pure chromium metal. This tightly adhered chromium-rich oxide protects the surface from further oxidation. This oxide layer is extremely thin, through which it can see the natural gloss of the steel surface, so that stainless steel has a unique surface. Moreover, if the surface damage, the exposed steel surface and the atmosphere will be self-repair, re-formation of this "passivation film", continue to play a protective role.

Therefore, all stainless steel has a common characteristic, that is, chromium content of 10.5% or more.

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