No matter what method is used to process aluminum and products, there will be dirt and defects on the surface to varying degrees. Such as dust, metal oxides (natural or high temperature formed aluminum oxide film), residual oil, asphalt marks, manual handling fingerprints (mainly fatty acids and nitrogen compounds), welding flux and corrosive salts, metal burrs, minor scratches and so on. Therefore, before oxidation treatment, the surface of the product is cleaned by chemical and physical methods to expose the pure metal matrix, so as to facilitate the smooth oxidation and coloring, so as to obtain an artificial film that is firmly combined with the matrix, meets the requirements of color and thickness, and has the best corrosion resistance, wear resistance, weather resistance and other good properties.
(1) degreasing
The surface degreasing of aluminum and aluminum alloys includes organic solvent degreasing, surfactant degreasing, alkaline solution degreasing, acidic solution degreasing, electrolytic degreasing and emulsification degreasing.
(2) alkali etching agent
Alkali corrosion is the process of surface cleaning of aluminum products in a solution of sodium hydroxide with or without the addition of other substances, which is also commonly known as alkali corrosion or alkali washing. Its function is as a supplementary treatment of the product after degreasing by some degreasing methods, in order to further clean up the oil stolen goods attached to the surface; Remove the natural oxide film and slight scratches on the surface of the product. Thus, the product exposes the pure metal matrix, which is conducive to the formation of anode film and obtain a higher quality film layer. In addition, by changing the composition of the solution, temperature, treatment time and other operating conditions, different states of the etched surface such as smooth or satin matt or gloss can be obtained. The basic composition of the etching solution is sodium hydroxide, in addition to the addition of regulating agents (NaF, sodium nitrate), scale inhibitors, (gluconate, enanthate, tartrate, gum Arabic, dextrin, etc.), polyvalent chelating agents (polyphosphates), decontamination agents.
(3) Neutralization and water cleaning
The gray or black hanging ash attached to the surface of aluminum products after corrosion washing is insoluble in cold or hot water, but it can be dissolved in acidic solution, so the products etched by hot alkali solution have to be acid-soaked cleaning aimed at removing the hanging ash and residual lye to expose the bright basic metal surface, this process is called neutralization, gloss or light treatment. The process is the product in 300-400g/L nitric acid (1420kg/ cubic meter) solution, at room temperature soaking, soaking time varies with the metal composition, the general soaking time is 3-5 minutes. The hanging ash on aluminum alloy products containing silicon or manganese can be treated at room temperature for 5-15 seconds with a mixture of nitric acid and hydrofluoric acid with a volume ratio of 3:1. Neutralization can also be carried out at room temperature in a solution containing 300-400g/L nitric acid and 5-15g/L chromium oxide or 100g/L chromium oxide plus sulfuric acid (1840kg/ m3) 10ml/L solution. The purpose of water cleaning between each process is to completely remove the residual liquid on the surface of the product and the reaction products soluble in water, so that the tank liquid of the next process is not polluted, and ensure the treatment efficiency and quality. Most of the cleaning uses a cold water cleaning. However, the products after alkali corrosion are generally cleaned with hot water followed by cold water. The temperature of the hot water is 40-60 degrees. The neutralized products can be oxidized by water cleaning, so this cleaning should be particularly careful to prevent the clean surface from being contaminated. Otherwise, the effective treatment of the previous several processes may be wasted due to improper cleaning at the end. After neutralization and water cleaning, the product should be oxidized with the top. The time to stay in the air should not be too long, such as staying 30-40 minutes, the product needs to be re-washed and neutralized.
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