The chemical degreasing process involves immersing the pipes into a tank made of steel plates, with the inner walls lined with polyvinyl chloride (PVC) or polyethylene. A 12-15% nitric acid solution is used for chemical degreasing at a temperature of 40°C to 60°C for 2 to 4 hours. When immersing the galvanized steel pipes into the tank, attention should be paid to their placement to prevent trapped air inside the pipes. During the soaking process, the pipes should be moved up and down or flipped to continuously replace the solution within the inner cavity, enhancing the effectiveness.
If necessary, the pipes should be removed, rinsed with water, and then re-immersed. Chemical degreasing continues until the surface of the galvanized steel pipes is completely wetted by water. After rinsing with hot water, the degreased pipes are removed from the degreasing tank and immersed in a hot water tank at approximately 40-60°C for 5-20 minutes. The hot water tank is also made of steel plates with PVC or polyethylene lining, and the chloride ion content in the water should be less than 25ppm. The hot water-rinsed galvanized steel pipes are then rinsed with pressurized water, again ensuring the chloride ion content is below 25ppm. The tank immersion method is chosen for passivation, with the passivation solution formula and immersion time following relevant specifications.
The passivation tank is made of passivated steel plates with acid-resistant plastic lining on the inner walls. When immersing the pipes, care should be taken with their placement to prevent trapped air. During immersion, the pipes should be moved or flipped to continuously replace the solution within, enhancing the passivation effect. If necessary, the galvanized steel pipes can be removed, rinsed with water, and re-immersed. If the pipes are dry, the passivated galvanized steel pipes should be blown dry with clean compressed air or nitrogen, allowing sufficient time for natural passivation in the air.
Quality inspection follows, with self-inspection and mutual inspection conducted after passivation, and the pipes submitted to quality inspectors as required. For packaging and maintenance, if the outer surface of the galvanized steel pipes requires painting, it should be done according to shipping requirements. After inspection, the galvanized steel pipes are sealed with plastic plugs, wrapped with three-proof cloth for maintenance, and labeled accordingly.
