(1) Uniform corrosion. Chromium is the easiest element to passivate. In the atmosphere, iron-chromium alloys with a chromium content of more than 12% can be self-passivated. In the oxidizing medium, the chromium content can be passivated if the content is more than 17%. In some highly corrosive medium, high chromium and adding molybdenum, nickel, copper and other elements can obtain good corrosion resistance.
(2) Intergranular corrosion. Ferritic stainless steels and austenitic stainless steels are subject to intergranular corrosion, but sensitization treatment and heat treatment to avoid this corrosion are just the opposite. Ferritic stainless steel is prone to intergranular corrosion when rapidly cooled from above 925°C, and the state (sensitized state) that is prone to intergranular corrosion can be eliminated after a short period of tempering at 650-815°C. The intergranular corrosion of ferritic steel is also the result of chromium depletion caused by carbide precipitation. Therefore, reducing the carbon and nitrogen content in the steel and adding elements such as titanium and niobium can reduce the susceptibility to intergranular corrosion.
(3) Pitting and crevice corrosion. Chromium and molybdenum are the most effective elements to improve the resistance of stainless steel to pitting and crevice corrosion. As the chromium content increases, the chromium content in the oxide film also increases, and the chemical stability of the film increases. Molybdenum is adsorbed on the active metal surface in the form of MoO4, inhibits the dissolution of the metal, promotes repassivation, and prevents film damage. Therefore, high-chromium, molybdenum ferritic stainless steel has excellent resistance to pitting and crevice corrosion.
(4) Resistance to stress corrosion cracking. Due to the characteristics of the structure, ferritic stainless steel is resistant to corrosion in the medium where austenitic stainless steel produces stress corrosion cracking.