All metals react with oxygen in the atmosphere and form an oxide film on the surface. Unfortunately, the iron oxide formed on ordinary carbon steel continues to oxidize, causing corrosion to continue to expand and eventually form voids. Coatings can be made by coating or oxidizing metals (e.g., zinc, nickel, and chromium) to ensure the surface of carbon steel, but, as everyone knows, this kind of protection is only a film. If the protective layer is damaged, the steel 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 addition of chromium reached more than 11.7%, the atmospheric corrosion resistance of steel increased remarkably, but the corrosion resistance could be improved although the chromium content was higher, but it was not obvious. The reason is that alloying of steel with chromium changes the type of surface oxide to a surface oxide similar to that of pure chromium metal. This close adhesion chromium oxide protection surface, prevent further oxidation. This oxide layer is extremely thin, through it can see the natural luster of the steel surface, make the stainless steel have a unique surface. Moreover, if the surface is damaged, the exposed steel surface will react with the atmosphere to repair itself and re form the oxide passivation film, which will continue to act as a protective agent.
Therefore, all stainless steel elements have a common characteristic, that is, chromium content is above 10.5%.