Email:jessy@jshengda.comWuxi Hengda Liquid-Purification Chemical Plant

Wuxi Hengda Liquid-Purification Chemical Plant

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The Principle Of Stainless Steel Pickling Passivation

The corrosion resistance of stainless steel is mainly due to the thin (approximately 1nm) dense passivation film covering the surface of stainless steel,this passivation film is separated from the corrosive medium and is a basic barrier for stainless steel protection.Stainless steel passivation has dynamic characteristics.It should not be seen as corrosion completely stopped but as a barrier of diffusion which makes the anode reaction rate greatly reduced.Usually in the case of a reducing agent (such as chloride), the film tends to damage the film, while in the presence of the oxidant (such as air), it can maintain or repair the film.


It forms an oxide film when the stainless steel workpieces are placed in the air,but the protection for this membrane is not perfect.It is usually necessary to thoroughly clean, including alkaline wash and pickling, and then use oxidant passivation to ensure the integrity and stability of the passivation film.


One of the purposes of pickling is to create favorable conditions for passivation treatment to ensure the formation of high quality passivation film. An average of 10um layer on the stainless steel surface will be corroded by pickling.The chemical activity of the acid solution causes the dissolution rate of the defect site to be more higher than other parts of the surface.As a result, pickling can evenly balance the entire surface, and some of the hidden dangers of corrosion are removed.


But more importantly,the iron and iron oxides are dissolved earlier than the chromium and chromium oxides by pickling passivation,at the same time poor chromate layer is removed and resulting in chromium enrichment on the stainless steel surface.This kind of rich chromium passivation film has a potential of +1.0V(SCE), which is close to the potential of precious metal and improves the stability of corrosion resistance.


Different passivation treatments also affect the composition and structure of the film, thus affecting the non-rust.Such as through electrochemical modification, the passivation film has a multilayer structure and form CrO3,Cr2O3 or glassy oxide file in the barrier layer, so that stainless steel can exert the greatest corrosion resistance.


Domestic and foreign scholars have done a lot of research on the formation of stainless steel passivation film.The phenomenon of stainless steel passivation is due to the adsorption of surface layer and water molecule after dissolving, it forms oxides and hydroxide under the catalysis of oxidants, and reacts with the CR,Ni,Mo elements that formed stainless steel,finally forms a stable phase film, prevents damage and corrosion of the film.

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