The corrosion resistance of stainless steel relies heavily on the surface passivation film, if the film is incomplete or defective, the stainless steel will still be corroded. Engineering is usually carried out pickling passivation treatment, so that the corrosion resistance potential of stainless steel to play a greater role.
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 corrosive media isolation, is the basic barrier to stainless steel protection. Stainless steel passivation has dynamic characteristics and should not be seen as a complete cessation of corrosion, but rather the formation of a barrier layer for diffusion, so that the anodic reaction rate is greatly reduced. It usually tends to destroy the film in the presence of reducing agents (e.g. chloride ions), while being able to maintain or repair the film in the presence of oxidising agents (e.g. air).
Stainless steel workpieces left in the air will form an oxide film, but this film is not sufficiently protective. Usually a thorough cleaning, including alkaline and pickling, followed by passivation with an oxidising agent is required to ensure the integrity and stability of the passivation film. One of the purposes of pickling is to create favourable conditions for passivation treatment to ensure the formation of a high quality passivation film. Because the surface of the stainless steel surface through pickling an average of 10μm thick layer of the surface was corroded away, the chemical activity of the acid makes the defective parts of the dissolution rate than other parts of the surface, so pickling can make the entire surface tends to be uniformly balanced, some of the original easy to cause corrosion of the hidden problems were removed.
But more importantly, by pickling passivation, so that iron and iron oxides than chromium and chromium oxides preferentially dissolved, removed the poor chromium layer, resulting in chromium enrichment in the stainless steel surface, the potential of this chromium-rich passivation film up to +1.0V (SCE), close to the potential of precious metals, to improve the stability of corrosion resistance. Different passivation treatments can also affect the composition and structure of the film and thus the stainless steel. For example, through electrochemical modification treatment, the passivation film can be made to have a multi-layer structure, forming CrO3 or Cr2O3 in the barrier layer, or forming a glassy oxide film so that the stainless steel can be used for maximum corrosion resistance.
