As the bottom of coating (paint or powder spraying), phosphating film is the most widely used field. It improves the adhesion and resistance to moisture and water immersion of the paint layer and substantially prevents possible spread of corrosion. It cannot be assumed that sandblasting, shot peening and oil-free surfaces can be treated without phosphating. Although these clean surfaces have a good bonding force with the coating layer, once exposed to a corrosive environment, oxygen, water vapor, sulfur dioxide, etc. will penetrate into the substrate , due to the conductivity of the substrate and the capillary action between the coating and the substrate, the electrolyte is inevitably absorbed to form a micro-battery, electrochemical corrosion occurs and expands to the surroundings, and local coating damage caused by deformation and bumping will make the membrane Corrosion spreads faster and eventually causes bubbling of the coating.

　　After phosphating treatment, the corrosion process will be limited to the damaged part of the coating, because the rest of the coating is still firmly bonded to the substrate, and it is a non-conductive phosphating film, which cannot form a micro-battery, thereby inhibiting the corrosion under the film. corrosion.

Coating phosphating process

Coating phosphating is mainly thin-film phosphating, and the film thickness is generally 1~5um, which is most suitable for the bottom layer of paint. The thinner the film, the higher the mechanical deformation resistance of the paint film coated on the film, and the gloss and Adhesion performance is also the best.

(1) Zinc phosphating

The phosphating solution is generally composed of sodium nitrite or sodium chlorate as an accelerator, an acidic solution containing zinc dihydrogen phosphate and zinc nitrate, usually to make the phosphating film crystal fine, dense, good adhesion, good uniformity, and lower the phosphating temperature, but also add citric acid, tartaric acid, fluoride, heavy metal salts, and control the film weight at 1.5~2g/m2. Construction methods include: spraying, spraying plus immersion, and immersion.