What is pickled stainless steel sheet?
Stainless steel will appear black and yellow oxide scales during processing. In order to improve the appearance and corrosion resistance of stainless steel, the processed stainless steel must be pickled and passivated. Remove the oxide scale produced after welding and high-temperature processing to make the silver bright and shiny, and form an oxide film with chromium as the main substance on the treated surface, which will not produce secondary oxygen corrosion and achieve the purpose of passivation , Thereby improving the surface anti-corrosion quality of stainless steel products and prolonging the service life of the equipment.
The main purpose of pickling is to remove the oxide film on the surface of stainless steel. After pickling the surface of stainless steel, a dense oxide film is formed. This process is called pickling passivation. This film is the key for stainless steel to not rust and corrode in general media. Therefore, stainless steel does not have to be pickled. It is only needed when there is oxide scale on the surface.
Stainless steel pickling is often associated with passivation. Generally, stainless steel that needs to be pickled must be passivated, because there is no passivation film formed on the surface after pickling or the thickness of the passivation film is very thin, and it has no effect. Increase the follow-up passivation process to form and perfect a passivation film on the surface of the stainless steel. For example, annealing with protective gas such as pure hydrogen, ammonia decomposition and other atmosphere protection does not require pickling.
Stainless steel pickling is generally treated with pickling passivation paste and pickling passivation liquid. The pickling passivation paste is the simultaneous pickling and passivation, which is completed in one step, which changes the traditional pickling and passivation process. Simple, convenient construction, low cost. Suitable for large-area painting operations. The pickling passivation solution is suitable for the immersion operation of small workpieces.
The necessity of stainless steel pickling passivation: The corrosion resistance of stainless steel mainly depends on the surface passivation film. If the film is incomplete or defective, the stainless steel will still be corroded. In engineering, pickling and passivation treatment is usually carried out, so that the corrosion resistance of stainless steel can be exerted even more. In the process of forming, assembling, welding, welding seam inspection (such as flaw detection, pressure test) and construction marking of stainless steel equipment and components, surface oil stains, rust, non-metallic dirt, low-melting metal contaminants, paint, and welding Slag and splashes, etc., these substances affect the surface quality of stainless steel equipment and components, destroy the oxide film on the surface, reduce the overall corrosion resistance of steel and local corrosion resistance (including pitting corrosion, crevice corrosion), and even cause Stress corrosion cracking.
For nuclear engineering, certain chemical equipment and other applications that have strict requirements, in addition to pickling passivation, high-purity media must be used for final fine cleaning or mechanical, chemical and electrolytic polishing and other finishing treatments.
The principle of stainless steel pickling: because the surface of stainless steel is corroded by an average of 10um thick surface through pickling, the chemical activity of acid makes the dissolution rate of defective parts higher than other parts on the surface, so pickling can make the entire surface Due to the uniform balance, some hidden dangers that were easy to cause corrosion have been removed. But more importantly, through pickling passivation, the iron and iron oxides are dissolved preferentially than the chromium and chromium oxides, and the chromium-poor layer is removed, resulting in the enrichment of chromium on the surface of the stainless steel. This chromium-rich passivation film The potential can reach +1.0V (SCE), which is close to the potential of precious metals, which improves the stability of corrosion resistance. Different passivation treatments will also affect the composition and structure of the film, thereby affecting the rust resistance. For example, through electrochemical modification treatment, the passivation film can have a multilayer structure, forming CrO3 or Cr2O3 on the barrier layer, or forming a glass state The oxide film makes the stainless steel exert the greatest corrosion resistance.
Stainless steel pickling is suitable for 200 series, 300 series, 400 series stainless steel, and is widely used in the surface treatment of stainless steel equipment, pressure vessels and engineering components in the chemical, pharmaceutical, papermaking, food, aviation, nuclear industry, construction, etc.
The stainless steel pickling test standard is: the international general stainless steel blue spot test liquid test.
Note for stainless steel pickling: Safety work must be done during pickling, acid-resistant clothing, energy gloves, masks and goggles must be worn. When pickling in the container, you must wear a qualified gas mask and perform it under the supervision of a blogger. At the same time, the pickling time should not exceed 24 hours.