Ordinary carbon steel and the oxygen in the atmosphere form an oxide film on the metal surface, and then continue to oxidize, causing the rust to continue to expand and form a "thousand-feu cake"-style corrosion product until it wears out. The stainlessness of stainless steel is related to the chromium content in the steel. When the chromium content in the steel reaches 12%, it comes into contact with the atmosphere and produces a passivation film (Cr2O3) on the surface of the stainless steel. It is a dense chromium-rich oxide, which effectively Protects stainless steel surfaces, especially from further re-oxidation. This oxide film is extremely thin (only a few microns), and the natural luster of the steel surface can be seen through it. It is the unique surface of stainless steel. If the surface passivation film is once destroyed, the chromium in the steel and the oxygen in the atmosphere will regenerate the passivation film and continue to play a protective role.

Certain local corrosions may also occur in the special environments encountered by stainless steel, such as pitting corrosion (pitting corrosion), intergranular corrosion, stress corrosion, galvanic corrosion, etc. To overcome these corrosions, molybdenum, nitrogen, and titanium are added to the steel respectively. , or niobium and other elements, and new varieties such as low carbon, ultra-low carbon, and duplex stainless steel have been developed to improve the corrosion resistance of stainless steel.

Overview of choosing stainless steel: It must have sufficient corrosion resistance during its service life without wasting too much alloy. When excellent corrosion resistance is not required, a relatively low-cost type 201 (1Cr17Mn6Ni5N),Type 430 (1Cr17) ​​is the most appropriate corrosion resistance grade. Although its corrosion resistance is lower than that of 304 steel, its corrosion resistance is sufficient to meet the requirements of food, fresh water and non-marine environments. For those applications that require only extremely low corrosion resistance, type 405 stainless steel (0Cr13Al) can be selected, which is resistant to soap, alcohol, crude oil, gasoline and other weakly corrosive environments. For example, stainless steel water tanks have been widely used in water supply and drainage in construction projects. In the past, austenitic stainless steel (304 or 316) was commonly used. Later, manufacturers used ferritic stainless steel (444), which is cheap, high-quality, and excellent in chloride ion TV.