Anodic electrophoretic coatings are a variety of coatings put into industrial application in the 1970s, the film-forming substance is an anionic polymer with carboxyl groups, under the action of direct current electric field, the negatively charged resin is coated with pigments, and together swim towards the anode and deposited on the anode surface.
Due to the shortcomings of anodic oxidation of anodic electrophoretic coatings, corrosion will occur on metal surfaces (except aluminum and inactive metals), affecting the gloss of the coating, and is generally used as a primer. However, for some metals (such as aluminum profiles), the use of anode electrophoresis coatings is better, which is not achieved by other surface treatment methods, so anode electrophoresis coatings are still widely used.
In the process of use of anode electrophoretic coatings, there are many factors that affect the quality of the paint film, such as pretreatment mismatch, workpiece pocket liquid, bacteria, etc., as follows:
Substrate corrosion occurs in the reaction mechanism of anode electrophoresis:
Electrolysis: Anode: Me →Me2+ + 2e- , 2H2O → O2↑ +4H+ + 4e- cathode (plate): 2H 2O+2e- → H2↑+ 2OH- Electrodeposition:RCOO_ (water soluble) + H+ → RCOOH↓ (insoluble) Me2+ + 2RCOO- → (RCOO)2 During the Me↓ (resin deposition) cathodic electrophoresis reaction, the surface of the substrate is a strong alkaline interface, which is very corrosive to the substrate (except aluminum and reactive metals) and the pretreatment film. However, the surface of the electrophoresis substrate in the anode electrophoresis process is a strong acidic interface, which will corrode the phosphating film and the substrate, affecting the adhesion of the paint film, and because the metal ions are deposited in the paint film, the anti-corrosion performance of the paint film will be reduced, so the anti-corrosion performance of the anode electrophoresis coating is not good, and it is not recommended to match the phosphating pretreatment with poor acid resistance.
Workpiece pocket fluid
Anode electrophoretic coatings are neutralized with alkaline substances to impart ionicity, solubilization and dispersion in water. Alkaline neutralizers are commonly used organic amines and inorganic bases, such as ethanolamine, triethanolamine, triethylamine, KOH, etc., such resins and yan (filler) materials, various types of additives to form a stable water dispersion system, so the anode electrophoresis solution is generally weak alkaline. At present, most of the pretreatment film on the market is a weakly acidic system, if the film pretreatment cleaning is not cleaned by the workpiece into the anode electrophoresis tank, the pH of the tank will decrease, destroying the stable water dispersion system of the anode electrophoresis fluid, the resin hydrophilicity will decrease, and the stability of the tank will also decrease. The lower the pH, the more severe the trend, with insoluble particles produced by the electrophoresis bath. Flocculation, delamination, precipitation, and even tank scrap. Therefore, it is necessary to eliminate the liquid situation of the workpiece, or choose alkaline membrane pretreatment. Ensure that the workpiece is cleaned before entering the groove, and strictly control the pH of the anode tank fluid within the process range.
Bacteria will destroy the stability of the bath, resulting in the aging of the tank, paint film accumulation and foaming and other drawbacks. In the daily maintenance of the anode electrophoresis coating line, it is necessary to often detect the indicators of bacteria and carry out sterilization treatment regularly. The pure water system is equipped with a sterilization device to prevent bacteria from being carried into the electrophoresis tank from pure water. For the choice of fungicide, because the anode system is a weak alkaline system, the general selection of neutral or alkaline fungicides, strong acidic fungicides (especially casson) added to the electrophoresis tank will appear flocculation, paint film accumulation and other adverse conditions, so it is not recommended to use. After the tank is sterilized, the groove is turned over, and the filter bag is changed frequently to avoid particles in the paint film.