The principle of acid washing and passivation

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Austenitic stainless steels possess excellent corrosion resistance as well as good formability in both hot and cold conditions; for this reason, they are widely used in the manufacture of various pressure vessels that require corrosion protection. The passivation layer on the surface of austenitic stainless steels has a significant impact on their corrosion resistance. The passivation film of austenitic stainless steel is primarily obtained through acid pickling and passivation treatment of its surface. The principle of acid washing and passivation: Passivation refers to the phenomenon in which, after a metal is treated with an oxidizing medium, its corrosion rate decreases significantly compared to before treatment. Its passivation mechanism can be primarily explained by the thin film theory, which posits that passivation occurs as a result of the interaction between the metal and an oxidizing medium; during this interaction, a very thin, dense passivation film is formed on the metal surface, one that has good covering properties and can adhere firmly to the metal surface. This film exists as a separate phase, usually a compound of oxygen and metal. It serves to completely separate the metal from the corrosive medium, preventing direct contact between them and thus essentially stopping the dissolution of the metal. When austenitic stainless steel is exposed to oxidizing media, a passivation film that meets the aforementioned requirements can form on its surface; however, this passivation film is highly susceptible to degradation under the action of halide ions such as Cl-, Br-, and F-, which have an activating effect. This is also one of the reasons why, even for austenitic stainless steel pressure vessels that have undergone acid washing and passivation, if the water residues cannot be completely removed after a hydrostatic test, the Cl- content in the water should be kept below 25 ppm. Furthermore, not all oxide films of metals can be considered as passivation films; for example, the oxide film formed on carbon steel after high-temperature oxidation cannot serve as a passivation film because it fails to meet the requirement of adhering firmly to the metal surface. For austenitic stainless steels, solutions with strong oxidizing properties, primarily nitric acid, are generally used for treatment. To ensure the effectiveness of the passivation process, the surface to be passivated is first pickled before passivation. The entire processing procedure is known as acid washing and passivation, commonly referred to as acid washing and passivation.