In the process of leaving the factory or sticking aluminum plates, there are sometimes black spots on them. What's the matter with this situation? Let's study it. The first is the material of aluminum plate. The aluminum plate upper cover produced by aluminum plate manufacturers adopts LF2 1 aluminum plate with a thickness of 0.5 mm.. There are many parts of this type of aluminum plate with the same material, similar shape and the same technology, but the parts with similar shapes processed at the same time do not have this kind of fault. And this is not a failure caused by raw material defects. Then, in terms of processing technology, the general process route of silver plating on the upper cover is: machining parts-pretreatment-zinc dipping-zinc electroplating-copper electroplating-silver electroplating-painting-dip coating 823- warehousing. In the process of aluminum plate electroplating, it is possible that a certain production link of electroplating changed at first, which led to the failure. However, after investigation, it was found that similar parts processed in the same period did not have this kind of fault without any change in production conditions. Therefore, the factors that change the electroplating production conditions are excluded. According to previous production experience, the most common cause of discoloration of silver-plated parts is the influence of sulfur on silver-plated layer. It has also happened before that silver-plated parts were not packaged and sealed in time and left in the electroplating factory for a long time, which led to the yellowing of the silver-plated layer. However, after silver plating, all the defective parts are packaged in time and stored in a dry container, which shows that this is not affected by the air atmosphere of the factory building. In terms of spray painting process, it is found that the spray painting process of the upper cover has batch repair records. The most important reason is that the gap is not blocked, allowing paint mist to enter and adhere to the surface of the silver plating layer, which cannot be removed. In order to prevent this kind of fault, it is suggested to seal the gap with 3M adhesive tape during processing to prevent paint mist from entering. Through this elimination method, the root of the problem was found, and immediate measures were taken to prevent the recurrence of hidden dangers, so that the surface of aluminum plate was as bright as new and the factory yield was greatly improved.