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Degreasing and Pickling Passivation of Carbon Steel Pipes

Date:2022-04-27    keywords: carbon steel pipe degreasing, pipe pickling passivation

For carbon steel pipe (CS pipe), it should be degreased first and then pickled, because grease is insoluble in acid solution. If pickling is performed first, neither the oil stains nor the rust covered by the oil stains can be removed. 


Degreasing pickling method for carbon seamless steel pipe:

At present, the tank immersion method is often used in construction, and the liquid level exceeds the outer surface of the pipe by more than 50mm during immersion. Since the oxygen pipeline needs to be treated by degreasing, pickling and passivation, four tanks are generally made according to the actual diameter of the pipe used. That is, degreasing tank, pickling tank, neutralization tank, passivation tank. The oxygen pipeline degreasing and pickling process is: degreasing→water washing→pickling→water washing→neutralization→passivation→water washing→drying.


carbon steel seamless pipe


1. Degreasing
The formula of the degreasing solution is as follows: each 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 ℃ ~ 80 ℃, the soaking time is determined by the test method according to the amount of oil on the surface of the pipe, generally about 10min ~ 40min. The heating method can be steam. Degreasing parts shall be inspected by any of the following methods. (1) Wipe the surface of the degreased part with clean and dry white filter paper, if there is no trace of grease on the paper, it is qualified. (2) Use 100% camphor below 1mm to check the condensate to flush the condensate, and it is qualified to keep rotating. (3) Check the surface of the degreasing part with ultraviolet rays with a wavelength of 3200-3800 angstroms, and the grease-free fluorescence is qualified. After degreasing treatment, the oil pollution on the pipe surface 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 cs seamless pipes. The oxides of iron (Fe3O4, Fe2O3, FeO, etc.) such as scale and rust react 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℃~80℃, soaking for 5min~20min. The formula of the hydrochloric acid solution is: the concentration of the hydrochloric acid solution is 5% to 20%. The operating process requirements are: temperature 20℃~50℃, soaking for 5min~20min. After the pipeline is pickled, check that the surface rust is removed, and the exposed metallic luster is qualified.

4. Water flush
Rinse with clean water with a pressure of 0.8MPa.

5. Neutralize
The formula of the neutralizing solution is: NH4OH diluted to a solution with a pH value of 10-11. The operating process requirements are: soaking at room temperature for 3 minutes.

6. Passivation
After pickling, water rinsing, and rinsing, the metal surface is very clean, very activated, and easily corroded. Therefore, passivation treatment must be performed immediately to form a protective film on the cleaned metal surface to slow down corrosion. The passivation solution formula is: NaNO2 concentration is 8% to 10%; NH4OH concentration is 2%. The operating process requirements are: immersion at room temperature for 10 minutes.

7. Water flush
Rinse 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 two ends of the pipe with plastic cloth and tape.

The inner wall of the carbon steel pipe treated by the above method is clean and bright, and can be kept for about 2 months without rusting; if it is well preserved, the time can be extended. The above method can also be used for carbon steel pipe fittings.

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