1. Polishing Solution Composition: To ensure the effectiveness of chemical polishing, the metal surface must be dissolved, forming the aforementioned liquid or solid film. The basic components of a chemical polishing solution generally include a corrosive agent, an oxidizing agent, additives, and water. The corrosive agent is the main component, primarily dissolving the workpiece in the solution; oxidizing agents and additives inhibit the corrosion process, directing the reaction in a direction favorable to polishing; water regulates the solution concentration, facilitating the diffusion of reaction products.
The components used for dissolving the metal are generally acids, with strong acids such as H₂SO₄, HNO₃, HCl, H₃PO₄, and HF being commonly used. For amphoteric metals like aluminum, NaOH can also be used. In these acids, high concentrations of phosphoric acid and sulfuric acid have high viscosity, forming a liquid film diffusion layer; therefore, this component serves two functions. This is why phosphoric acid and sulfuric acid are mainly used in the composition of chemical polishing solutions. To increase viscosity and facilitate diffusion layer formation, additives such as gelatin or glycerol can be added. To promote the formation of a solid film, strong oxidizing agents, primarily nitric acid or chromic acid, are added.
2. Polishing Time: Chemical polishing has an optimal polishing time range. If the time is too short, only a dull, pear-skin-like surface can be obtained; if the time is too long, not only will the solution loss increase, but stains or spots will also appear on the processed surface. This time range is affected by factors such as the material, the composition of the polishing solution, and the polishing temperature, and is usually difficult to predict, so it can only be determined experimentally. Hydrogen gas is often generated during chemical polishing, which is a problem that must be considered when polishing materials that are sensitive to hydrogen embrittlement. In addition, when the polishing solution temperature is as high as 100~200CC, annealing will also occur. In order to minimize the effects of hydrogen embrittlement and annealing, the shortest possible polishing time must be selected within the optimal temperature range.
3. Polishing Temperature: In chemical polishing, the dissolution rate increases significantly with increasing polishing solution temperature. Furthermore, the oxidation effect of strong oxidizing acids becomes very significant at high temperatures. In chemical polishing, because the dissolution and oxidation effects of these acids occur simultaneously, the polishing solution is usually heated to a relatively high temperature for polishing.
Metals requiring temperature for polishing include steel, nickel, and lead. If the temperature is below a certain value, a smooth, corroded surface will be lost. Therefore, there exists a critical point for forming a glossy surface; polishing results are better within the temperature range above this critical point. However, this temperature range varies depending on the composition of the liquid. If the temperature is above this range, pitting, localized stains, or spots will form, reducing the overall polishing effect. In addition, the higher the temperature, the greater the material dissolution loss.