Stainless steel pickling passivation principle

Stainless steel corrosion resistance can be mainly due to the surface covered with a layer of very thin (about 1nm) dense passivation film, the film 1n corrosion medium isolation, stainless steel protection is the basic barrier. Stainless steel passivation has a dynamic character and should not be considered as complete stop of corrosion but rather as a diffusion barrier that greatly reduces the anode reaction rate. Often, the membrane tends to break down in the presence of a reducing agent (such as chloride), while maintaining or repairing the membrane in the presence of an oxidizing agent (such as air). Stainless steel workpiece placed in the air will form an oxide film, but the protective film is not perfect. Usually first to conduct a thorough cleaning, including alkaline washing and pickling, and then oxidized passivation, in order to ensure the integrity and stability of passivation film. One of the purposes of pickling is to create favorable conditions for passivation to ensure a good passivation film. Because the stainless steel surface by pickling an average of 10μm thick layer of surface corrosion, acid chemical activity makes the defect site dissolution rate higher than other parts of the surface, so the pickling can make the entire surface tends to be evenly balanced, some of the original Easily lead to corrosion of the hidden dangers have been removed. But more importantly, by the pickling passivation, the oxides of iron and iron are preferentially dissolved than the oxides of chromium and chromium, the chromium-depleted layer is removed, resulting in the enrichment of chromium on the stainless steel surface. This chromium-rich passive film The potential of up to + 1.0V (SCE), close to the potential of precious metals, improve the stability of anti-corrosion. Different passivation treatment will also affect the composition and structure of the membrane, thus affecting the stainless steel, such as through electrochemical modification, the passivation film can have a multi-layer structure, the formation of barrier CrO3 or Cr2O3, or the formation of the glassy state Oxide film, so that stainless steel can maximize corrosion resistance. Domestic and foreign scholars have done a lot of research on the formation of stainless steel passivation film. In recent years, Beijing Normal University of 316L steel passive film photoelectron spectroscopy (xps) as an example to briefly [1]. Stainless steel passivation is the surface layer for some reason the dissolution of water molecules and adsorption, the oxidant under the catalytic action, the formation of oxides and hydroxides, and composition of stainless steel cr, Ni, Mo element conversion reaction, the final formation of stable Of the phase film, preventing the destruction of the film and the occurrence of corrosion.