(1) improve the thermodynamic stability of stainless steel in environmental medium. The passivated stainless steel is in a positive position in the metal potential sequence, i.e. it is close to precious metal and has stable chemical properties, while the non passivated stainless steel is in an active state and has a negative potential, similar to general steel.
(2) prevent the local corrosion of stainless steel. All kinds of corrosion, including pitting corrosion, intergranular corrosion, wear corrosion and corrosion fatigue, are related to the surface state of stainless steel. Passivation can eliminate the initiation sources of corrosion and make the potential of critical point corrosion positive.
(3) passivation can make the surface of stainless steel clean enough. Passivation can remove the metal pollutants on the surface layer of stainless steel, as well as the impurities embedded in stainless steel, such as copper, zinc, cadmium, lead, low melting metal and free iron, so as to enrich and stabilize the chromium and nickel contained on the surface. The pollution of these metals can easily lead to corrosion damage of stainless steel.
(4) eliminate the oxide of hot working on stainless steel surface. Use the nitric acid solution containing hydrofluoric acid in the passivation process, which has the function of etching oxide and passivating surface.
(5) passivation, as a post-treatment, requires various pretreatment of stainless steel, including sand blasting, shot peening, electrochemical polishing and chemical polishing to improve the surface condition of stainless steel before passivation.