Discuss the effectiveness of chemical cleaning techniques in removing rust from heat exchanger tube bundles

Thread Content

Generally, nitric acid cleaning is used to remove scale from the inner wall of the tube bundles in cold exchange equipment, and excellent results have been achieved. However, using nitric acid to remove the rust layer from the outer wall of the tube bundle does not yield satisfactory results. Because the outer surface of the tubes without anti-corrosion measures is covered with numerous corrosion products, this not only affects heat transfer efficiency but also accelerates the corrosion of the metal surface. Although high-pressure water jet technology is used to extract the core during maintenance, the results are not satisfactory. To address this issue, X-ray diffraction analysis was conducted on the corrosion products on the outer surface of the tube; the main components were FeS2, Fe3O4, and Fe2O3. Comparative tests show that using a hydrochloric acid solution with an appropriate corrosion inhibitor yields the best results, while also being the least costly method. An aqueous hydrochloric acid solution has a dissolving effect on iron oxides; the mechanism is as follows: FeO + 2HCl → FeCl2 + 2H2O; Fe2O3 + 6HCl → FeCl3 + 3H2O; Fe3O4 + 8HCl → FeCl2 + 2FeCl3 + 4H2O; Fe + HCl → FeCl2 + H2↑. Hydrochloric acid is an inorganic acid commonly used for the chemical cleaning of metal equipment. It exhibits a high dissolution rate and capacity for various iron oxides; the resulting salts are readily soluble, the process is simple and safe, and the equipment remains in excellent condition after cleaning. Chemical cleaning was carried out on the outer wall of the cooler tube bundle that was severely corroded. The main methods are as follows: equipment pretreatment—oil and fat removal—acid washing—rinse—passivation. Cleaning improves the service life of the tube bundle by allowing the onset of corrosion beneath the scale to be delayed.