Practical Application of Stainless Steel
In practical applications, steel resistant to weak corrosion medium is often called stainless steel, while steel resistant to chemical medium is called acid resistant steel. Due to the difference in chemical composition between the two, the former is not necessarily resistant to chemical medium corrosion, while the latter is generally stainless. The corrosion resistance of stainless steel depends on the alloy elements contained in the steel. Chromium is the basic element for stainless steel to obtain corrosion resistance. When the chromium content in the steel reaches about 12%, a thin oxide film (self passivation film) is formed on the steel surface due to the interaction of chromium and oxygen in the corrosion medium, which can prevent the steel substrate from further corrosion. In addition to chromium, commonly used alloy elements include nickel, molybdenum, titanium, niobium, copper, nitrogen, etc., to meet the requirements of various uses for stainless steel structure and properties. Stainless steel is generally divided into: ① ferritic stainless steel according to the matrix structure. It contains 12%~30% chromium. Its corrosion resistance, toughness and weldability increase with the increase of chromium content, and its chloride stress corrosion resistance is superior to other types of stainless steel. ② Austenitic stainless steel. Chromium content is more than 18%, and it also contains about 8% nickel and a small amount of molybdenum, titanium, nitrogen and other elements. It has good comprehensive performance and can resist corrosion of various mediums. ③ Austenitic ferritic duplex stainless steel. It has the advantages of both austenitic and ferritic stainless steels, and has superplasticity. ④ Martensitic stainless steel. High strength, but poor plasticity and weldability.