The corrosion resistance of stainless steel mainly depends on the surface passivation film, if the film is incomplete or defective, stainless steel will still be corrode. Engineering usually pickling passivation treatment, so that the corrosion resistance potential of stainless steel to play a greater.
The corrosion resistance of stainless steel is mainly due to the surface covered with a very thin (about 1nm) dense passivation film, this film 1N corrosion medium isolation, is the basic barrier of stainless steel protection. Stainless steel passivation has dynamic characteristics, should not be regarded as corrosion completely stopped, but the formation of diffusion barrier layer, so that the anode reaction speed is greatly reduced. Generally tends to destroy the membrane in the presence of reducing agents (e.g., chloride ions), while maintaining or repairing the membrane in the presence of oxidizing agents (e.g., air).
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.
