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, causing the corrosion to expand and finally forming holes. The surface of carbon steel can be ensured by electroplating with paint or oxidation-resistant metals (e.g. zinc, nickel, chromium), but as is known, this protection is only a thin film. If the protective layer is damaged, the steel below will start 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 chromium content reaches 10.5%, the atmospheric corrosion resistance of steel increases significantly, but when the chromium content is higher, although the corrosion resistance can still be improved, it is not obvious. The reason is that when the steel is alloyed with chromium, the type of surface oxide is changed to the surface oxide similar to that formed on pure chromium metal. This tightly adherent chromium rich oxide protects the surface from further oxidation. This kind of oxide layer is very thin, through which we can see the natural luster of the steel surface, so that stainless steel has a unique surface. Moreover, if the surface layer is damaged, the exposed steel surface will react with the atmosphere for self repair, and the “passivation film” will be formed again to continue to play a protective role.
Therefore, all stainless steels have a common characteristic, that is, the chromium content is more than 10.5%.