(1) Uniform corrosion. Chromium is the easiest element to passivate. In the atmospheric environment, iron-chromium alloys with a chromium content of more than 12% can be self-passivated. In an oxidizing medium, the chromium content can be passivated when the content of chromium is more than 17%. In some highly aggressive medium, high chromium and molybdenum, nickel, copper and other elements can obtain good corrosion resistance.
(2) Intergranular corrosion. Ferritic stainless steels suffer from intergranular corrosion like austenitic stainless steels, but sensitization treatments and heat treatments to avoid this corrosion are just the opposite. Ferritic stainless steel is prone to intergranular corrosion when quenched above 925°C, and the state (sensitized state) susceptible to intergranular corrosion can be eliminated by short-term tempering at 650~815°C. The intergranular corrosion of ferritic steel is also the result of poor chromium due to 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 corrosion and crevice corrosion. Chromium and molybdenum are the most effective elements to improve the pitting and crevice corrosion resistance of stainless steel. When 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, which inhibits the dissolution of the metal, promotes repassivation, and prevents the destruction of the film. Therefore, high-chromium and molybdenum ferritic stainless steel has excellent resistance to pitting and crevice corrosion.
(4) Stress corrosion cracking resistance. Due to the characteristics of the structure, ferritic stainless steel is corrosion-resistant in the medium where austenitic stainless steel produces stress corrosion cracking.
