After the conductive oxidation process of aluminum and aluminum alloy, the obtained oxide film still has excellent conductivity, which is its unique performance, and the protective and decorative performance of the film is also very good. The color of the film on the surface of pure aluminum is more elegant than that of the rainbow passivation film of zinc layer, with shallow and uniform fine grain color. It is a process with great application prospect and popularization value.
Aluminum materials are extremely unstable in the air and are easy to produce oxide films that are difficult to identify with the naked eye. Due to the different processing methods of aluminum parts, such as casting molding, or direct cutting of rolled plates, or fine machining molding, or forming through different processes and then heat treatment or welding, the workpiece surface will show different states and different degrees of dirt or traces. Therefore, in the pretreatment process, the pretreatment process method must be selected according to the actual situation of the workpiece surface.
(1) Problems needing attention in the pretreatment process of fine machined parts: Although the natural oxide film on the surface of fine machined parts is initially formed and easy to remove, it is heavy and greasy, especially in and around the hole (added due to lubrication during machining). Such workpieces must be cleaned with organic solvent first. If they are washed directly with alkali, it is not only heavy and greasy, but also the fine machined surface can not withstand strong alkali corrosion for a long time, The result will also affect the roughness of the workpiece surface and the matching of tolerance, and may eventually become a waste.
(2) Problems needing attention in the pretreatment process of cast parts. Not all surfaces of cast molded parts are machined, and the unmachined surface has a thick oxide layer formed in the casting process, and some are also sandwiched with sand layer. At this time, the original oxide film of this part should be removed by machining or sand blasting, or processed after alkali washing. Only in this way can the original oxide layer of the unmachined part be removed and the change of the tolerance dimension of the machined part be avoided.
(3) Problems needing attention in the pretreatment process of workpiece after heat treatment or welding process: according to the process requirements, the workpiece needs to be cleaned with organic solvent before being transferred to heat treatment or welding process to remove the oil stain on the surface, but this is generally not possible at present, so a layer of oily sintered coking material is formed on the surface of the workpiece, which is difficult to remove in organic solvent, and local corrosion will be caused if soaked in alkali solution, Produce pitting or unevenness, which seriously affects the product quality. The author uses the method of soaking in concentrated nitric acid to soften this layer of coking material. After the coking material is soft, it can be completely removed by slightly cleaning in the alkali solution.