For carbon steel pipes (cs pipe), it should be degreased first and then pickled, because grease is insoluble in acid. If pickled first, neither the oil stain nor the rust covered by the oil stain can be removed.
Degreasing and pickling method for seamless tubes: At present, the tank soaking method is often used in construction, and the liquid level exceeds the outer surface of the pipe by more than 50mm during soaking. Since the oxygen pipeline needs to be degreased, pickled and passivated, generally 4 tanks are made according to the actual pipe diameter. That is, degreasing tank, pickling tank, neutralization tank, passivation tank. Oxygen pipeline degreasing and pickling process is: degreasing→water flushing→pickling→water flushing→neutralization→passivation→water flushing→drying.
1. Degreasing
The formula of the degreasing liquid is: every liter of the solution contains 20g-30g of sodium hydroxide, 35g-50g of sodium nitrate, and 3g-5g of sodium silicate. The operating process requirements are: the liquid temperature is 70°C to 80°C, and the soaking time is determined by the test method according to the amount of oil stain on the surface of the pipe, generally about 10min to 40min. The heating method can adopt the steam method. Degreased parts should be inspected by any of the following methods. (1) Wipe the surface of the degreased part with clean and dry white filter paper, and it is qualified if there is no trace of grease on the paper. (2) Use 100% camphor below 1mm to check the steam to purge the condensate, and it is qualified to keep rotating. (3) Use ultraviolet light with a wavelength of 3200 to 3800 Angstroms to inspect the surface of the degreased part, and if there is no grease and fluorescence, it is qualified. After degreasing treatment, the oil stains on the surface of the pipeline can be removed.
2. Water flush
Rinse with clean water with a pressure of 0.8MPa.
3. Pickling
Pickling is the use of acid solution to remove scale and rust on the surface of seamless steel tubes. Iron oxides (Fe3O4, Fe2O3, FeO, etc.) such as scale and rust react chemically with the acid solution to form salts that are dissolved in the acid solution and removed. The most commonly used pickling is sulfuric acid and hydrochloric acid. The formula of sulfuric acid solution is: the concentration of sulfuric acid solution is 5% to 10%. The operating process requirements are: temperature 60°C-80°C, soaking for 5min-20min. The formula of hydrochloric acid solution is: the concentration of hydrochloric acid solution is 5% to 20%. The operating process requirements are: temperature 20°C-50°C, soaking for 5min-20min. After the pipeline is pickled, it is qualified to check that the surface rust is removed and the metallic luster is exposed.
4. Water flush
Rinse it with clean water with a pressure of 0.8MPa.
5. Neutralization
The formula of the neutralizing solution is: a solution diluted with NH4OH to a pH value of 10-11. The operating process requirements are: soaking at room temperature for 3 minutes.
6. Passivation
After pipe pickling, washing with water, and rinsing, the metal surface is very clean, very activated, and easily corroded. Therefore, passivation treatment must be carried out immediately to form a protective film on the cleaned metal surface to slow down corrosion. The formula of the passivation solution is: the concentration of NaNO2 is 8% to 10%; the concentration of NH4OH is 2%. The operating process requirements are: soaking at room temperature for 10 minutes.
7. Water flush
Rinse it with purified water with a pressure of 0.8MPa.
8. Dry
Drying must be done with clean, oil-free dry air or nitrogen.
9. Check
Wipe and check with white filter paper. If there is no trace of grease on the paper, it is qualified. After passing the inspection, seal the nozzles at both ends of the tube with plastic cloth and tape.
The inner wall of the carbon steel pipe treated according to the above method is clean and bright, and can be kept rust-free for about 2 months; if it is well preserved, the time can also be extended. The above method can also be used for carbon steel pipe fittings.
As a manufacturer of stainless steel and carbon steel pipes, Permanent Steel Manufacturing Co., Ltd. specializes in the production and supply of seamless steel pipes: including thin-wall/thick-wall seamless pipes, precision seamless pipes, small-diameter/large-diameter seamless pipes, and various specifications and materials , large inventory, rich experience, can provide customized services according to customer requirements.
