Chromium is the basic element for obtaining corrosion resistance of stainless steel (all stainless steel elements have a common characteristic, that is, the chromium content is above 10.5%). When the chromium content in steel reaches about 12%, chromium and The action of oxygen in the corrosive medium forms a very thin oxide film (self-passivation film) on the steel surface, which prevents further corrosion of the steel matrix. In addition to chromium, commonly used alloying elements are nickel, molybdenum, titanium, niobium, copper, nitrogen, etc., to meet the requirements of stainless steel structure and performance for various purposes.
Stainless steel is usually divided into matrix structures:
1. Ferritic stainless steel. Containing 12% to 30% chromium. Its corrosion resistance, toughness and weldability increase with the increase of chromium content, and its resistance to chloride stress corrosion is better than other types of stainless steel.
2. Austenitic stainless steel. Containing more than 18% chromium, it also contains about 8% nickel and a small amount of elements such as molybdenum, titanium and nitrogen. Good overall performance, resistant to a variety of media corrosion.
3. Austenitic - ferritic duplex stainless steel. It combines the advantages of austenitic and ferritic stainless steels and has superplasticity.
4. Martensitic stainless steel. High strength, but poor plasticity and weldability.