Degreasing pickling passivation of seamless carbon steel pipe

For seamless carbon steel pipe, it should be degreased before pickling, because grease is insoluble in acid solution. If pickling first, neither the oil stain nor the rust covered by the oil stain can be removed.

Seamless steel pipe degreasing pickling method:

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

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°C to 80°C, and the soaking time is determined by the test method according to the amount of oil on the surface of the pipe, generally about 10min to 40min. The heating method can be steam. Degreased parts shall be inspected by any of the following methods. (1) Wipe the surface of the degreasing part with clean and dry white filter paper, and it is qualified if there is no oil stain on the paper. (2) Use 100% camphor solution below 1mm to check the condensed water and rinse the condensed water, and continue to rotate after passing the test. (3) Use ultraviolet light with a wavelength of 3200-3800 angstroms to check the surface of the degreasing part, and it is qualified if there is no oil stain fluorescence. After degreasing treatment, the oil stain 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 to use acid solution to remove scale and rust on the surface of cs seamless pipes. Scale, rust and other iron oxides (Fe3O4, Fe2O3, FeO, etc.) react with acid solution to form salts, which are dissolved in acid solution to remove. The most commonly used pickling is sulfuric acid and hydrochloric acid. The formula of the sulfuric acid solution is: the concentration of the sulfuric acid solution is 5% to 10%. The operating process requirements are: temperature 60°C-80°C, 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°C-50°C, soaking for 5min-20min. After pickling the pipeline, check the surface for derusting, and the exposed metallic luster is qualified.

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

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

6. Passivation
After pickling, washing, and rinsing, the metal surface is very clean, highly active, 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 passivation solution is: the concentration of NaNO2 is 8% to 10%; the concentration of NH4OH is 2%. The operating process requirements are: soak at room temperature for 10 minutes.

7. Water flush
Rinse with pure water with a pressure of 0.8MPa.

8. Dry
Drying must be done with clean, oil-free dry air or nitrogen.

9. Check
Wipe with white filter paper to check. If there are no traces of grease on the paper, it is acceptable. After passing the inspection, seal both ends of the tube with plastic sheeting and tape.

The inner wall of the carbon steel pipe treated by the above method is smooth and clean, and can be kept rust-free for about 2 months; if it is well preserved, the time can be extended. The above method can also be used for carbon steel pipe fittings.