[China Aluminum] In the electroplating production line, a series of pretreatment before the workpiece is plated in order to get a better coating, then the chemical polishing is also very important in the entire pretreatment, let's look at the electroplating below Pre-treatment chemical polishing. Chemical polishing refers to polishing the workpiece in a suitable solution without using an external power source and relying on chemical etching. During the chemical polishing process, due to the formation of a non-uniform passivation film on the microscopic surface of the metal, or due to the formation of a viscous mucus film similar to that formed during the electropolishing process, the dissolution rate of the surface microscopic projections is significantly greater than that of the microscopic depressions. Into the part, thus reducing the surface of the part of the micro-roughness, so that the surface of the part is relatively bright and flat. Chemical polishing is widely used in the polishing of stainless steel, copper and aluminum alloys, and is also used to decorate some parts. Chemical polishing can be used as a pre-plating treatment process, and can also be used directly after polishing with the necessary protective measures. Compared with electropolishing, chemical polishing does not require a power supply and a conductive hanger, and it can polish parts of various sizes with complex shapes, resulting in high production efficiency. The disadvantage is that the solution has a short service life, the concentration adjustment and regeneration are difficult, and some harmful gases are usually precipitated. The polishing quality of chemical polishing is also worse than that of electropolishing. This is mainly due to the fact that in chemical polishing, due to the non-uniform mass of the material, local electric potential may be varied, resulting in localized cathode and anode regions, and the formation of partial short-circuit microbatteries and anodes. Local dissolution occurs. In electropolishing, the local cathode region can be completely eliminated due to the effect of the applied potential, so that a full electrolysis is performed, and thus the effect is better. (1) The composition of polishing liquid and other influencing factors. 1 Slurry composition: In order to ensure the chemical polishing effect, the metal surface must be dissolved and the aforementioned liquid film or solid film formed on the surface. The basic composition of a chemical polishing fluid generally includes an etchant, an oxidant, an additive, and water. Among them, the corrosive agent is the main component, which mainly dissolves the workpiece in the solution; the oxidant and the additive can inhibit the corrosion process and make the reaction proceed in the direction favorable for polishing; the water regulates the concentration of the solution and facilitates the diffusion of the reaction product. The components used for the dissolution of metals are generally acids, of which H2S04, HN03, HC1, and H3PO are more commonly used. , HF and other strong acids, but also for the amphoteric metal such as aluminum, NaOH can also be used in these acids because of high concentrations of phosphoric acid and sulfuric acid have a higher viscosity, can form a liquid film diffusion layer, so this component has two functions . This is also the reason why the phosphoric acid and sulfuric acid are mainly used in the composition of the chemical polishing liquid. In order to increase the viscosity and make the diffusion layer easier to form, additives such as gelatin or glycerin that can increase the viscosity can also be added. In order to promote the formation of a solid film, it is necessary to add a strong oxidant mainly consisting of nitric acid or chromic acid. 2 Polishing time: There is a better polishing time range for chemical polishing. If the time is too short, only a pearly surface with no gloss can be obtained; if it is too long, not only the loss of the solution will increase, but also stains or spots will appear on the processed surface. This time range is affected by factors such as the material, the composition of the polishing liquid, and the polishing temperature, and is usually difficult to predict and can only be determined experimentally. Hydrogen is often generated simultaneously in the chemical polishing, which is a problem that must be paid attention to when polishing materials having hydrogen embrittlement susceptibility. In addition, the polishing liquid temperature is up to 100~ At 200CC, annealing can also occur. In order to reduce the effects of hydrogen embrittlement and annealing, it is necessary to select the shortest possible polishing time within a suitable temperature range. 3 Polishing temperature: When chemical polishing, the dissolution rate increases significantly with the temperature of the polishing liquid. In addition, oxidation of strongly oxidizing acids at high temperatures can become significant. In chemical polishing, since the dissolution and oxidation of these acids occur at the same time, the polishing liquid is heated to a relatively high temperature for polishing. Metals that require temperature for polishing include steel, nickel, lead, etc. If the temperature is lower than a certain value, it will lose smooth corrosion surface, so there is a critical point for forming a shiny surface, in the temperature range above the critical point Polishing effect is better. This temperature range is different depending on the composition of the liquid. If it is higher than this temperature range, pitting, partial stains or spots will be formed, and the entire polishing effect will be reduced. In addition, the higher the temperature, the greater the loss of dissolution of the material. (2) Chemical polishing of copper and its alloys. Copper and single-phase copper alloys can be chemically polished in a phosphoric acid nitric acid monoacetic acid or sulfuric acid mononitrate-chromic acid type solution. Nitric acid is often added during use. When the nitrogen dioxide precipitates during polishing, the surface of the part is dark yellow. When you can, you can add nitric acid by 1/3 of the amount. To prevent excess water from entering the tank, the parts should be dry and then polished. (3) Chemical polishing of aluminum and aluminum alloys. In order to obtain a bright surface, the nitric acid content of the polishing liquid must be strictly controlled. If the nitric acid content is too low, the polishing speed is slow and the gloss of the surface after polishing is poor; if the nitric acid content is too high, squamous corrosion tends to occur. When the concentration of phosphoric acid is low, a bright surface cannot be obtained. In order to prevent the solution from being diluted, the surface of the part before polishing should be dried. Acetic acid and sulfuric acid can suppress pitting corrosion and make the polished surface uniform and delicate. Ammonium sulfate and urea can reduce the precipitation of nitrogen oxides and help improve the quality of polishing. A small amount of copper ions can prevent over-etching, thereby improving the uniformity of the polished surface, but too high levels can reduce the reflective ability of the polished surface. Chromium anhydride can improve the quality of polishing of aluminum-zinc-copper alloys. Aluminum alloys with high strength containing zinc and copper wires are difficult to obtain a bright surface in a polishing solution that does not contain chromic anhydride. The chemically-polished parts should generally be impregnated at 400-500 g/L nitric acid or 100-200 g/L chromic anhydride solution at room temperature for several seconds to several tens of seconds to remove the surface contact copper.