Pickling stainless steel is the process of removing the surface layer of metal or cleaning metal surfaces of corrosion products, oxides, rust, and scale — especially where the chromium content at the steel’s surface has been reduced.
Pickling is most effective when using strong oxidizing acids, such as nitric acid or hydrofluoric acid. Pickling of alloy steels is carried out in mixtures of properly selected acids of different concentrations, sometimes with the addition of salts to enhance the process. It can be performed using pickling baths or, locally, with pastes and gels. Pickling stainless steel ensures uniform oxidation across the entire surface (forming an oxide layer). To further stabilize this coating, the treated part can additionally undergo passivation.
Passivation of metals is the process by which some metals, in the presence of oxoacids or their salts, enter a passive state, reaching a higher standard electrode potential. This is due to the formation of a tightly adhering oxide layer on the metal's surface, which protects it from further oxidation. Passivation can occur under the influence of oxidizers (e.g. atmospheric oxygen) or through electrochemical processes.
As a result of passivation, a thin oxide layer is created on the metal surface that exhibits high resistance to chemical agents and is passive toward atmospheric conditions. This protective oxide layer can form naturally on a pickled surface or can be created artificially through anodic oxidation using oxidizing acids. Passivation is a fundamental means of protecting corrosion-resistant steel.
Restoring corrosion resistance to stainless steel through the oxidizing action of acids can be achieved via immersion in passivation baths or by local application with spray systems.
