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What is pickling and passivation of steel pipes?

Comprehensive pickling and passivation of stainless steel, removing various oil stains, rust, oxide skin, solder joints and other dirt. After treatment, the surface is uniformly silver white, greatly improving the corrosion resistance of stainless steel, suitable for various stainless steel parts, plates and equipment.

Easy to operate, convenient to use, economical and practical, with the addition of high-efficiency corrosion inhibitors to prevent metal corrosion and hydrogen embrittlement, and to suppress the generation of acid mist. Especially suitable for small and complex workpieces, not suitable for coating, superior to similar products on the market.

According to the severity of stainless steel material and oxide scale, the original solution can be used or diluted with water in a ratio of 1:1:1-4 before use; Ferrite, Martensite and austenitic stainless steel with low nickel content (such as 420.430. After dilution, austenitic stainless steel with high nickel content (such as 304), 321.316.316L, etc.) shall be soaked in stock solution; Generally, after normal temperature or heating to 50~60 ℃, soak for 3-20 minutes or longer (the specific time and temperature shall be determined by the user according to the trial situation) until the surface dirt is completely removed, evenly silvery white, forming a uniform and dense passive film. After treatment, take it out, wash it with clean water, and neutralize it with alkaline water or Limewater.

The necessity of stainless steel pickling and passivation
Stainless steel has good corrosion resistance, high-temperature oxidation resistance, good low-temperature performance, and good mechanical and R properties. Therefore, it is widely used in chemical, petroleum, power, Nuclear engineering, aerospace, marine, medicine, light industry, textile and other sectors. Its main purpose is to prevent corrosion and rust. 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 corroded. Acid pickling and passivation are commonly used in engineering to enhance the corrosion resistance of stainless steel. During the formation, assembly, welding, weld inspection (such as flaw detection, pressure test), and construction marking process of stainless steel equipment and components, surface oil stains, rust, non-metallic dirt, low melting point metal pollutants, paint, welding slag, and splashes can affect the surface quality of stainless steel equipment and components, damage the oxide film on their surface, reduce the comprehensive and local corrosiveness of steel (including pitting corrosion), gap corrosion), and even lead to stress corrosion cracking.
Cleaning the surface of stainless steel, pickling and passivation can not only improve corrosion resistance to a maximum extent, but also prevent product contamination and achieve aesthetic effects. GBl50-1998 “Steel Pressure Vessels” stipulates that the surface of containers made of stainless steel and composite steel plates should be pickled and passivated. This regulation is applicable to pressure vessels used in petrochemical industry. As these equipment are used in situations where they come into direct contact with corrosive media, it is necessary to propose acid pickling and passivation from the perspective of ensuring corrosion resistance and corrosion resistance. For other industrial sectors, if it is not for corrosion prevention, it is only based on the requirements of cleanliness and aesthetics, while stainless steel does not require pickling and passivation. But the welds of stainless steel equipment also require pickling and passivation For some chemical equipment with strict requirements for use, in addition to acid cleaning and passivation, high purity medium shall also be used for final fine cleaning or mechanical cleaning, finishing chemistry and Electropolishing.

Principles of Stainless Steel Pickling and Passivation
The corrosion resistance of stainless steel is mainly due to the fact that the surface is covered with an extremely thin (approximately 1) nm) dense passivation film, which isolates the corrosive medium and serves as the basic barrier for stainless steel protection. Stainless steel passivation has dynamic characteristics and should not be considered as a complete cessation of corrosion. Instead, a diffusion barrier layer should be formed, greatly reducing the anode reaction rate. Usually, when there is a reducing agent (such as chloride ions), the membrane tends to damage, and when there is an oxidizing agent (such as air), the membrane can be maintained or repaired.
Stainless steel workpieces placed in the air will form an oxide film, but their protection is not perfect. Usually, thorough cleaning is carried out first, including alkaline and acid washing, followed by passivation with an oxidant to ensure the integrity and stability of the passivation film. One of the purposes of pickling is to create favorable conditions for passivation treatment and ensure the formation of high-quality passivation films. Acid washing causes corrosion on the surface of stainless steel with an average thickness of 10m. The chemical activity of the acid solution causes the dissolution rate of the defect area to be higher than that of other parts of the surface. Therefore, acid washing can make the entire surface evenly balanced and remove some potential corrosion hazards. But more importantly, through acid pickling and passivation, iron and iron oxides dissolve more than chromium and chromium oxides, removing the poor chromium layer, resulting in rich chromium on the surface of stainless steel. The potential of the rich chromium passivation film can reach+1.0V (SCE), which is close to the potential of precious metals and improves the stability of corrosion resistance. Different passivation treatments can also affect the composition and structure of the film, thereby affecting its corrosion resistance. For example, through electrochemical modification treatment, the passivation film can have a multi-layer structure and form CrO3 or Cr2O3 in the barrier layer, or form a glass oxide film to enhance the corrosion resistance of stainless steel.

1.Stainless steel pickling and passivation method
The impregnation method is used for parts that can be placed in pickling or passivation tanks, but it is not suitable for long-term use of pickling solution in large equipment, with high production efficiency and low cost; The large volume equipment is filled with acid solution, and the immersion liquid consumption is too high.
Suitable for internal surface and local physical operations of large equipment. Poor working conditions and inability to recover acid solution.
The paste method is used on installation or maintenance sites, especially for manual operations in the welding department. The labor conditions are poor and the production cost is high.
The spray method is used on the installation site, with low liquid volume on the inner wall of large containers, low cost, and fast speed, but requires the configuration of a spray gun and a circulation system.
The circulation method is used for large-scale equipment, such as heat exchangers. The construction of tube and shell treatment is convenient, and the acid solution can be reused. It requires piping and pump connection to the circulation system.
Electrochemical methods can not only be used for parts, but also for surface treatment of on-site equipment. The technology is complex and requires a DC power supply or a potentiostat.
2.Pickling and passivation processes
Degreasing and cleaning of dirt → Washing the water purification section → Passivation → Washing with clean water → Blowing dry
3.Pretreatment before pickling and passivation
3.1 According to the requirements of the drawings and process documents, perform acid pickling and passivation pre-treatment on the stainless steel containers or parts after manufacturing.
3. Weld seam and welding slag on both sides. Clean up splashes, and use gasoline or cleaning agent to remove oil stains and other dirt on the surface of container processing parts.
3.3 When removing foreign objects on both sides of the weld seam, use a stainless steel wire brush, stainless steel shovel or grinding wheel to remove them, and rinse them with clean water (with a chloride ion content not exceeding 25mg/l).
When the oil stain is severe, use a 3-5% alkaline solution to remove the oil stain and rinse thoroughly with clean water.
3. Mechanical sand blasting can remove the oxide skin of stainless steel Hot working parts, and the sand must be pure silicon or aluminum oxide.
3.6 Develop safety measures for pickling and passivation, and determine necessary tools and labor protection equipment.
4.Acid pickling, passivation solution and paste formula
4.1 Acid washing solution formula: nitric acid (1). 42) 20%, hydrofluoric acid 5%, and the rest is water. The above is the volume percentage.
4.2 Acid cleaning cream formula: 20 milliliters of hydrochloric acid (ratio 1.19), 100 milliliters of water, 30 milliliters of nitric acid (ratio 1.42), and 150 grams of bentonite.
4. Passivation solution formula: nitric acid (ratio 1). 42) 5%, Potassium dichromate 4g, the rest is water. The above percentage of fallout, passivation temperature is room temperature.
4.4 Passivation paste formula: 30ml nitric acid (concentration 67%), 4g Potassium dichromate, add bentonite (100-200 mesh) and stir to paste.

5.Acid pickling and passivation operation
5.1 Only containers or components that have undergone pickling and passivation pre-treatment can undergo pickling and passivation.
5. 2 Acid pickling solution is mainly used for the overall treatment of small unprocessed parts, and can be sprayed. The solution temperature should be checked every 10 minutes at a temperature of 21-60 ℃ until a uniform white acid etching finish is present.
5.3 Pickling paste Pickling is mainly suitable for large containers or local processing. At room temperature, evenly clean the pickling paste on the equipment (about 2-3mm thick), leave it for one hour, and then gently brush with water or stainless steel wire brush until a uniform white acid etching finish appears.
5.4 Passivation solution is mainly suitable for the overall treatment of small containers or components, and can be immersed or sprayed. When the solution temperature is 48-60 ℃, check every 20 minutes, and when the solution temperature is 21-47 ℃, check every hour until a uniform passivation film is formed on the surface.
5.5 Passivation paste is mainly suitable for large containers or local processing. It is evenly applied to the surface of the pickled container (about 2-3mm) at room temperature and inspected for 1 hour until a uniform passivation film is formed on the surface.
5.6 Acid pickling and passivation containers or parts must be rinsed with clean water on the surface., Use acidic litmus test paper to test any part of the washed surface, in order to rinse the surface with water with a pH value between 6.5 and 7.5, and then wipe or blow dry with compressed air.
5.7. After pickling and passivation, it is prohibited to scratch the passivation film when handling, lifting, and storing containers and parts.

Post time: Aug-08-2023