Why does aluminum veneer turn yellow?

Occasionally, it is found that aluminum veneer aluminum material turns yellow. What is the reason? Jinluzhu brand aluminum veneer analyzes the following four situations based on experience.　　1, the appearance of yellowing caused by the electrophoretic paint itself 　　anodic electrophoretic paint is mainly composed of acrylic resin and amine-based resin. During the baking process of the electrophoretic profile, the resin produces a cross-linking reaction, resulting in a flat and transparent coating film. However, due to the immature production process of some electrophoretic paint manufacturers, they may use poorer quality chemical raw materials to reduce costs, and then make their curing scales narrower. If the baking is a little lacking, the hardness of the paint film is not good; if the baking is a little too over, the paint film will turn yellow, which will bring certain difficulties to the production management. 2. Yellowing appearance caused by poor conduction during oxidation. When the profile and the conductive rod are not touched well, the resistance at the contact will increase greatly, the end of the profile will heat up, and the oxide film will be formed too fast, accompanied by the appearance of roasting, and even show an oxide film的粉化. At this time, the oxide film is a little dirty, and the color is yellowish. If it is produced by electrophoresis, it will show a very significant yellowish appearance. This yellowing phenomenon usually only has a few in a row, and it is basically present at the end of the profile. Therefore, measures must be adopted to ensure that the profile and the conductive rod touch excellently.　　3, yellowing appearance caused by incomplete washing before electrophoresis 　　The oxide film is honeycomb, and its porous layout determines that sulfuric acid will remain in the pores of the oxide film. Generally, if the profile used for electrophoresis is not completely washed with water, it is very capable of showing the appearance of yellowing. Regarding the appearance of this kind of yellowing, it is generally believed that the acid radicals in the pores of the oxide film react with the electrophoretic paint and then cause the yellowing of the electrophoretic paint film. I do not agree with this knowledge, because this yellowing is not caused by the paint film. Yellowing, but the yellowing of the oxide film. The normal oxide film is clear and transparent. If there are more sulfate radicals remaining in the oxide film holes, the oxide film will react with sulfate radicals under high temperature conditions, and then the clear and transparent oxide film will become dirty and transparent. Decrease; together with the high transparency of the electrophoretic paint film and the high reflectivity of light, this shortcoming is further enlarged, which constitutes the so-called yellowing. Therefore, several washings before electrophoresis are very critical, not only to ensure the quality of the washing water, but also to ensure the washing temperature and time. 4. The yellowing appearance of the oxidation tank liquid being polluted by nitric acid. In order to achieve a better ash removal effect, it is understandable to increase a certain amount of nitric acid in the neutralization tank, but if the water washing control after neutralization is not good, the nitric acid will be When brought to the oxidation tank, when the nitrate in the oxidation tank reaches a certain concentration, it will have a certain influence on the oxidation formation, and even cause the yellowing of the electrophoretic profile. During the oxidation process, the nitrate that enters the oxide film holes will have an etching effect on the oxide film, corrode the barrier layer of the oxide film, make the oxide film holes deeper, and then change the layout of the film holes. This corrosion has two effects on the oxide film: 1. The barrier layer of the oxide film becomes thinner, and the adhesion to the aluminum substrate becomes worse, and the adhesion of the oxide film is reduced. 2. Under normal washing conditions, it is difficult to remove the sulfuric acid radical in the membrane pores. The electrophoretic profiles produced under this condition will have the same yellowing appearance. How to avoid this yellowing appearance? Add a neutralizer in the pure water tank before the scalding tank, adjust the PH value to 8~9.5, wash for 2~3 minutes, and neutralize the sulfate in the oxide film holes with amine root After electrophoresis is carried out, the yellowing phenomenon will not appear. ————The article comes from the Internet. If there is any infringement, please contact to delete it.
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