The process of acid pickling and passivation for 304 precision stainless steel tubes

1. Chemical degreasing

For chemical degreasing of stainless steel pipes, the method usually adopted is immersion in a tank. The degreasing tank should be made of steel plates, and polyethylene or PVC can be laid inside. Then, 12% to 15% nitric acid is used, with the temperature controlled between 40℃ and 60℃, and the duration is 2 to 4 hours. It should be noted here that when the precision stainless steel pipes are immersed in the tank, the placement position is very important. To avoid leaving air inside the pipes. During the degreasing process, the pipes should be turned 360° to ensure that the solution inside the pipes is constantly replaced, in order to improve the effect. Only when the surface of the precision pipe is completely wetted can the purpose of chemical degreasing be achieved. 

2. Hot water rinsing

After the chemical oil removal process, the pipe fittings are taken out of the oil removal tank and then immersed in a hot water tank at a temperature of 40°C to 60°C for rinsing. The rinsing time is 5 to 20 minutes. 

3. Rinse with water

Take out the stainless steel pipe that has been rinsed with hot water, and then rinse it with pressurized water (pressure P ≥ 0.1 Mpa). 

4. Passivation

The passivation process is carried out using the tank-in-tank bubble method. The passivation tank is made of steel plates and the inner wall is covered with anti-acid plastic. The temperature of the passivation tank is controlled at 20-25℃, and the steel plates are immersed in a 10%-20% volume concentration nitric acid solution for 1 hour. Additionally, depending on the shape of the pipe, methods such as immersion, coating, or spraying can be used. For the passivation solution, the HNO3 concentration should be controlled between 20% and 50%. According to electrochemical tests, a HNO3 concentration less than 20% results in unstable passivation film quality and is prone to pitting corrosion, but the HNO3 concentration should not exceed 50% to prevent excessive passivation. Finally, the surface of the passivated precision stainless steel pipe should have a uniform silver-white protective film. 

5. Drying

After the precision tubes have undergone passivation treatment, they should be promptly dried using clean compressed air or nitrogen. Moreover, there must be sufficient time (at least 2 hours) for them to naturally undergo passivation in the air. 

6. Inspection

After the precision stainless steel tubes have undergone passivation treatment, self-inspection and mutual inspection should be conducted, and then submitted to the quality inspector for acceptance according to the requirements. Since chemical inspection may damage the passivation film of the product, it is usually carried out on the sample. The method is as follows: 

(Copper sulfate titration test): Add 8g of CuSO4 + 500mL of water + 2-3mL of H2SO4 solution onto the sample surface and keep it in a wet state. If no copper precipitates within 6 minutes, it is considered qualified. 

(2) High-speed cyanide titration test: 2 mL of HCl + 1 mL of H2SO4 + 1 gram of K3Fe(CN)6 + 97 mL of water solution is dropped onto the sample surface. The quality of the passivation film is determined by the number and duration of the blue spots formed. 

7. Packaging

If the outer surface of the pipe needs to be treated, it should be done in accordance with the painting requirements; after passing the inspection, the precision stainless steel pipe should be sealed with a plastic plug, wrapped with three-proof fabric for protection, and labeled.