The final products of color coating production line, no matter what type: coating board, embossing board or laminating (film-pasting) board, must undergo surface purification treatment in the pretreatment stage of unit process section, namely degreasing treatment, chemical conversion film treatment and other two important stages. Among them, the effect of purification treatment has the following points.

(1) Improve the compatibility and compatibility between the coating and the steel substrate. In addition to degreasing, friction and other cleaning processes, surface treatment can also include chemical conversion treatment of the surface, i.e. coating chemical solution. After chemical conversion, a certain roughness layer can be formed on the surface, which enlarges the surface area of the contact between the coating and the substrate, and is very beneficial to the adhesion of the coating.

(2) Activate the surface of the substrate. Remove the rust-proof oil, dirt and rust stain coated in the production process of the substrate. The dirt can be divided into two categories: inorganic and organic. The organic dirt is mainly animal oil, vegetable oil and mineral oil. The inorganic dirt is metal oxide, salt, dust and Egyptian sand. In addition, there are organic and inorganic pollutants, such as abrasive debris, abrasive materials and so on. Animal and vegetable oils can be removed by alkali soap. Mineral oil pollution can not be removed by alkali soap. It needs to be removed by trichloroethylene, gasoline, petroleum solvent emulsifier, etc. Inorganic contaminants can be dissolved by acid or alkali, and removed by acid, alkali impregnation, chemical or electrolytic methods and mechanical grinding methods. The removal of inorganic and organic mixed wastes is difficult. In addition to chemical methods, electrolysis and mechanical abrasion should be combined to remove them. Their existence not only affects the appearance of the coating, but also seriously affects the adhesion between the coating and the substrate, which makes the film peel off. Therefore, surface treatment is usually carried out by friction before coating to improve the cleanliness and activation of the substrate surface.

(3) Enhance the adhesion of the coating to the surface of the object. After removing the rust-proof oil, rust spots and dust attached to the substrate surface, chemical coating can be carried out on the clean substrate surface to form a network chemical conversion film with CrC as the basic framework, so as to increase the adhesion between the substrate and the coating, improve the corrosion resistance of the substrate and the durability of the coating. In order to ensure that the chemical conversion coating with uniform structure and strong adhesion can be formed on the surface of the substrate during the chemical conversion treatment, so as to avoid the bubble phenomenon caused by oil pollution, air, moisture and so on.

(4) Ensure and improve the protective performance of the coating. In normal service environment, corrosive ions such as oxide scale and rust may penetrate into organic coatings and reach the surface of steel base. Because the electrode potential is about 0.15-0.26V higher than that of iron and steel, it becomes cathode, and steel itself becomes anode, which causes corrosion under film. With the continuous development of corrosion, the organic coatings will eventually appear drum. Bubble until destroyed and peeled. In order to improve the corrosion resistance of the organic coated steel plate, the surface treatment process was completed before the steel plate was coated, which greatly improved the corrosion resistance of the organic coated steel plate and prolonged the service life of the coating.