The function and basic principle of chemical polishing on aluminum alloy surface

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The aluminum alloy workpiece after mechanical polishing has obtained bright surface, but if the workpiece after mechanical polishing is anodized directly, it is only a smooth surface and can not get the film with high reflective coefficient, so the workpiece after mechanically polished must be chemically polished or electropolishing. The crystal deformable layer formed on the workpiece surface during mechanical polishing is used to obtain a bright and delicate surface.

Chemical polishing can be considered as a chemical etching process under a specific condition. The result is that a smooth and bright surface is needed, but not all aluminum alloy materials can get this effect. In general, the quality of chemical polishing varies with the components of aluminum alloy, and the polishing effect of aluminum alloy containing copper and zinc is poor, while high silicon aluminum is not suitable for chemical polishing. The higher the purity of aluminum is, the better the polishing effect and the higher the reflectance of the polishing.            There are three applications in chemical polishing: one is the processing of the aluminum alloy after sanding or drawing with the aim of polishing the surface of the alloy; the two is a mechanically polished workpiece with two chemical grinding to eliminate the polishing texture, and the smooth and smooth surface is obtained. In the ideal state, chemical polishing can be compared to the electrolytic polishing. The more smooth and bright mirror effect; three is the processing of aluminum alloy workpiece before the texture etching or before anodizing to obtain a basic luminosity (or the bottom light). In these three applications, chemical polishing after mechanical polishing is the most stringent and difficult to control.

Chemical polishing is achieved by polishing solution on the micro concave surface of the workpiece formation and dissolution rate of different film to achieve the purpose of polishing. In order to minimize the dissolution rate of the aluminum alloy in the polishing process, the formation rate of the film is required to be greater than the dissolution rate, which is achieved by increasing the viscosity of the polishing solution and by the oxidizing agent or other film forming additives in the solution. There is no authoritative explanation for the principle of chemical polishing. There are two views.

First, the oxide film or replacement layer is formed by controlling the diffusion of the workpiece during polishing, and the dissolution rate of the metal is inhibited to achieve the purpose of grinding. Of course, no matter whether the film or replacement layer can grow without restriction, it will also be dissolved at the same time of growth. The concave or convex surface of the workpiece surface, the concave film or replacement layer will give priority to one step, and at the same time be thicker. The convex film or replacement layer will lag behind and form thin at the same time. This is due to the concentrated current on the convex surface, the large activity and the larger dissolution rate than the concave surface. The corrosion rate of the concave convex surface of the aluminum alloy is poor, and the chemical polishing is the purpose of smoothing the surface of the aluminum alloy by using the corrosion rate to complete the leveling effect on the surface of the aluminum alloy. The polishing solution related to this principle is high in concentration, such as the commonly used tri acid, diacid polishing, and so on. Because of the high viscosity of this kind of polishing solution and the effect of adding material, the diffusion rate is very low.

The two is to use the low concentration of chemical polishing solution to achieve the low dissolution rate of aluminum workpiece surface and produce grinding effect. The polishing solution used in this method is usually made up of dilute nitric acid and phosphoric acid, and chromic anhydride provides oxidant, and hydrogen peroxide is also an oxidizing agent. The polishing principle is basically the same as above, which is the purpose of smoothing and brightening the aluminum alloy workpiece after polishing through the dissolution rate of the polishing solution to the concave convex surface of the aluminum alloy.

Chemical polishing is very uniform on the leveling of the metal surface, and the surface after chemical polishing has a better durability than the surface luminance of the mechanically polished surface.

Chemical polishing of aluminum alloys can be divided into two types: alkaline polishing and acid polishing. The main raw materials for acid chemical polishing are phosphoric acid, sulfuric acid, nitric acid, acetic acid, ammonium hydrogen fluoride, etc. Many kinds of formulations can be made up of these basic materials according to the different processing purposes. In chemical polishing, the only ingredients made up of these basic materials are not well satisfied with the production requirements, and some additives are also needed to be added to the polishing solution to improve the gloss and smoothness of the polishing solution. These added substances can be classified into two categories: inorganic salt and organic matter two. The most commonly used inorganic salts are silver, copper, nickel and chromium salts. Organic compounds include glycerol, oxalic acid, citric acid and amino acids.


Post time: Apr-23-2018