Why 304 stainless steel is corrosion-resistant

All metals react with oxygen in the atmosphere and form oxide films outside. Unfortunately, the iron oxide formed on the common carbon steel continues to oxidize, causing the corrosion to expand and eventually form pores. Paint or oxidation-resistant metals (e.g., zinc, nickel, and chromium) can be used to coat carbon steel, but, as is known, this protection is only a thin film. If the protective layer is damaged, the steel below will begin to rust.

Corrosion resistance of 304 stainless steel depends on chromium, but because chromium is one of the components of steel, the protection is not the same.

The atmospheric corrosion resistance of steel increases obviously when the addition of chromium reaches 10.5%, but it is not obvious when the chromium content is higher. The reason is that when the steel is alloyed with chromium, the type of oxide outside is changed to be similar to that formed on pure chromium metal. This carefully adhered chromium rich oxide protects the outside and prevents further oxidation. This oxide layer is very thin, through which it can see the natural luster of the steel outside, so that 304 stainless steel has a unique outside. Moreover, if the surface layer is destroyed, the exposed 304 stainless steel will be self-repaired by the atmospheric response, and the "passivation film" will be formed again, which will continue to protect the surface.