Corrosion resistance of stainless steel was mainly due to the surface covered with a layer of very thin (about 1nm) and compact passive film, this layer of film isolated corrosive media, it is the basic protective barrier of stainless steel. Stainless steel passivation has dynamic characteristics, it should not be regarded as complete cessation of corrosion, but the formation of diffusion barrier which greatly reduced the anodic reaction. Usually in a reducing agent (such as chloride) cases tended to undermine the membrane, and in the oxidizer (eg air) exists to maintain or repair the membrane.

Stainless steel workpiece placed in the air will formed oxide film, but the protective film is inadequate. In order to ensure the integrity and intrinsic stability of the passive film, we usually have to carry out thorough cleaning, including alkaline cleaning and pickling, and then oxidizing passivation. One purpose of pickling is to create conditions for the passivation, so as to ensure the quality of forming passive film.

Because an average of 10um thick layer of stainless steel surface is corroded away by pickling, chemical activity of acid makes rate of dissolution of the defect parts is higher than other parts on the surface. Therefore pickling can make the whole surface becomes uniform balance, a number of original hidden dangers which are easy to cause corrosion have been cleared up. But more importantly, by pickling passivation, iron of iron oxides is priority dissolved compared with chromium and chromium oxides, and removed the depleted chromium layer, resulting in enrichment of chromium formed in stainless steel surface, this chromium passivation membrane potential reached up to +1.0 V (SCE), closed to the precious metal's potential and improved the corrosion resistance stability. Different passivation will affect different membrane composition and structure, thus affect the stainless property. Such as the passive film will has a multilayer structure through electrochemical treatment, formed CrO3 or Cr2O3 in the barrier layer or formed glassy oxide film to maximize the corrosion resistance of stainless steel.