Metal surface pretreatment refers to the removal of various foreign matters such as oil, rust, dust, etc. attached to the surface of the object before painting, and the formation of a conversion film, providing a good substrate suitable for painting requirements, ensuring that the film has good adhesion and Corrosion resistance process. The main technological processes are: degreasing, pickling, surface adjustment, phosphating (or passivation), deionized water washing, etc. Multi-stage water washing is also set between each process as required. Degreasing is the process of removing various grease, dust, sediment, metal powder and hand sweat from the surface of the workpiece by chemical or electrochemical methods; the process of removing oxide scale or rust on the surface of steel substrates by acid cleaning and rust removal; the process of surface adjustment Use mechanical or physical and chemical means to eliminate the uneven surface caused by alkaline degreasing or rust removal, and improve the adaptability of the workpiece surface to the phosphating solution; phosphating uses a solution containing phosphoric acid or phosphate to form an insoluble solution on the surface of the base metal Phosphate film process; passivation uses hexavalent chromium or trivalent chromium chemical acid to form a chromate passivation film on the surface of the base metal to improve the corrosion resistance of the base metal; phosphate film passivation uses chemical The method seals the phosphating film, reduces the porosity of the phosphating film, and improves the corrosion resistance. The three main functions of metal surface pretreatment are ① to provide a clean surface; ② to significantly improve the adhesion of the coating film; ③ to double the corrosion resistance of the coating film.