Corrosion resistance of ferritic stainless steel

(1) Uniform corrosion. Chromium is the easiest element to passivate. In the atmospheric environment, iron chromium alloys with chromium content above 12% can self passivate. In oxidizing media, passivation can be achieved when the chromium content is above 17%. In some highly corrosive media, high chromium combined with elements such as molybdenum, nickel, and copper can achieve good corrosion resistance.
(2) Intergranular corrosion. Both ferritic stainless steel and austenitic stainless steel undergo intergranular corrosion, but sensitization treatment and heat treatment to avoid such corrosion are exactly the opposite. Ferritic stainless steel is prone to intergranular corrosion when rapidly cooled above 925 ℃, and the state (sensitized state) that is prone to intergranular corrosion can be eliminated by short-term tempering at 650-815 ℃. The intergranular corrosion of ferritic steel is also the result of chromium deficiency caused by carbide precipitation. So reducing the carbon and nitrogen content in steel and adding elements such as titanium and niobium can reduce the sensitivity 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 adsorbs on the active metal surface in the form of MoO4, inhibiting metal dissolution, promoting re passivation, and preventing film damage. Therefore, high chromium and molybdenum ferritic stainless steel has excellent resistance to pitting and crevice corrosion.
(4) Resistance to stress corrosion cracking. Due to the characteristics of its organizational structure, ferritic stainless steel is corrosion-resistant in the medium where stress corrosion cracking occurs in austenitic stainless steel.