Stainless steel refers to steel that is resistant to weak corrosive media such as air, steam, and water and chemical erosion media such as acids, alkalis, and salts, also known as stainless acid-resistant steel. In practical applications, steel that is resistant to weak corrosive media is often called stainless steel, while steel that is resistant to chemical media is called acid-resistant steel. Due to the difference in chemical composition between the two, the former is not necessarily resistant to chemical media corrosion, while the latter is generally rust-resistant. The corrosion resistance of stainless steel depends on the alloy elements contained in the steel.

Usually, according to the metallographic structure, ordinary stainless steel is divided into three categories: austenitic stainless steel, ferritic stainless steel, and martensitic stainless steel. Based on these three basic metallographic structures, duplex steel, precipitation hardening stainless steel, and high-alloy steel with an iron content of less than 50% have been derived for specific needs and purposes.

1. Austenitic stainless steel. The matrix is mainly austenite (CY phase) with a face-centered cubic crystal structure, non-magnetic, and is mainly strengthened by cold working (and may cause certain magnetism). The American Iron and Steel Association uses 200 and 300 series numbers, such as 304.

2. Ferritic stainless steel. The matrix is mainly ferrite (a phase) with a body-centered cubic crystal structure, magnetic, generally cannot be hardened by heat treatment, but can be slightly strengthened by cold working. The American Iron and Steel Association uses 430 and 446 as labels.

3. Martensitic stainless steel. The matrix is martensitic (body-centered cubic or cubic), magnetic, and the mechanical properties of stainless steel can be adjusted by heat treatment. The American Iron and Steel Association uses 410, 420 and 440 digital markings. Martensite has an austenite structure at high temperature. When cooled to room temperature at an appropriate speed, the austenite structure can be transformed into martensite (i.e. hardened).

4. Austenite-ferrite (duplex) stainless steel. The matrix has both austenite and ferrite phases, in which the content of the lesser phase matrix is generally greater than 15%. It is magnetic and can be strengthened by cold working. 329 is a typical duplex stainless steel. Compared with austenitic stainless steel, duplex steel has high strength, resistance to intergranular corrosion, chloride stress corrosion and pitting corrosion. The corrosion resistance is significantly improved.

5. Precipitation hardening stainless steel. The matrix is austenite or martensite, and the stainless steel can be hardened by precipitation hardening. The American Iron and Steel Association uses 600 series numbers, such as 630, which is 17-4PH.

Generally speaking, except for alloys, austenitic stainless steel has excellent corrosion resistance. In a less corrosive environment, ferritic stainless steel can be used. In a mildly corrosive environment, if the material is required to have high strength or high hardness, martensitic stainless steel and precipitation hardening stainless steel can be used.