Stainless steel workpiece placed in the air will form an oxide film, but this film is not perfect protection. Usually, thorough cleaning, including alkaline washing and pickling, and then passivation with oxidizer, can ensure the integrity and stability of passivation film. One of the purposes of pickling is to create favorable conditions for passivation and ensure the formation of high quality passivation film. Because through pickling stainless steel surface has an average of 10μm thick layer of surface corrosion off, the chemical activity of acid makes the defect part of the dissolution rate is higher than other parts of the surface, so pickling can make the whole surface tends to be uniform balance, some of the original easy to cause corrosion of the hidden danger is removed.

But more importantly, through pickling passivation, iron and iron oxides are preferentially dissolved than chromium and chromium oxides, and the chromium poor layer is removed, resulting in chromium enrichment on the surface of stainless steel. The potential of this chromium rich passivation film can reach +1.0V(SCE), close to the potential of precious metals, and improve the stability of corrosion resistance. Different passivation treatment will also affect the composition and structure of the film, thus affecting the rust resistance, such as through electrochemical modification, can make the passivation film has a multi-layer structure, forming CrO3 or Cr2O3 in the blocking layer, or forming glassy oxide film, so that the stainless steel can play the greatest corrosion resistance.