• importx posted an update 1 week, 4 days ago

    The anodic oxidation of aluminum uses aluminum or aluminum alloy as the anode, and a lead plate as the cathode to electrolyze in the electrolyte to form an oxide film layer on the surface. After anodizing, the surface of aluminum can form an oxide film with a thickness of several to several hundred microns. The surface of this oxide film is porous and honeycomb. Compared with the natural oxide film of aluminum alloy, its corrosion resistance, abrasion resistance and decoration are significantly improved. With different electrolytes and process conditions, anodized films with different properties can be obtained.

    The main process of anodizing include: degreasing and degreasing-alkali corrosion-polishing-anodizing-dyeing-sealing treatment.

    The purpose of alkali corrosion is to remove the dense but uneven oxide film on the surface of aluminum alloys. For high-silicon aluminum alloys, a mixed solution of HN03 and HF is used, and other aluminum alloys use an alkaline bath mainly composed of NaOH solution.

    The purpose of polishing is to obtain a bright substrate surface, which is divided into mechanical polishing, electrolytic polishing, and chemical polishing. Mechanical polishing is the use of mechanical methods to polish bright surfaces. Electrolytic polishing is the use of electric current to cause the aluminum alloy to undergo an electrochemical reaction, and the uneven parts of the aluminum alloy surface dissolve to varying degrees, so that the surface of the aluminum piece has a smooth mirror effect. Chemical polishing involves immersing parts in a chemical solution and reacting to obtain a bright surface, which is divided into acid polishing and alkaline polishing.

    Dyeing is to make use of the porous structure and strong adsorption performance of the anodized film. The anodized aluminum parts are dipped in a solution of organic or inorganic dyes, and the oxide film pinholes absorb the dye and color it. There are also electrolytic coloring and overall coloring.

    Due to the high porosity and adsorption of the oxide film formed by aluminum and aluminum alloy, it is easy to be polluted. Especially in corrosive environment, the corrosive medium easily enters the pores of the film layer and causes corrosion. After the oxide film is dyed, it should be sealed to increase the color’s light resistance and corrosion resistance. In industrial production, the anodized film is generally closed, regardless of whether it is dyed or not. Hot water sealing and steam sealing are generally used. The principle is that the aluminum oxide on the surface of the oxide film and the pore wall undergoes a hydration reaction in hot water to generate hydrated alumina, which increases the original oxide film volume by 33 ~ 100%. The expansion of the oxide film volume significantly reduces the film pores, thereby achieving the purpose of sealing the pores. The reaction formula is:

    Al2O3 + nH2O = Al2O3 • nH2O

    In addition, various chemical solutions can be used for sealing, such as potassium dichromate solution. The anodic oxide film after sealing is light yellow and has particularly high corrosion resistance.

    The electrolyte used in anodizing is mainly sulfuric acid, oxalic acid, and chromic acid. Among them, sulfuric acid anodizing treatment is most widely used. Unless otherwise specified, it generally refers to sulfuric acid anodization.

    Sulfuric acid anodizing has the characteristics of low tank liquid cost, simple composition, easy operation and maintenance, high transparency of the oxide film, and high colorability, so it is widely used. The operating conditions of sulfuric acid anodizing are:

        H2SO4 (volume) 10% 30%

        Temperature 18 22

        Al / g.L-1 20

        Current density / 0.6 3

        Time / min 10 60

    The characteristics of the oxalic acid oxide film are similar to those of the sulfuric acid oxide film, the porosity is lower than that of the sulfuric acid oxide film, and the corrosion resistance and hardness are higher than those of the sulfuric acid oxide film. The cost and operating voltage of oxalic acid are higher than that of sulfuric acid. Some alloys have darker oxalic acid oxide films. Both oxalic acid and sulfuric acid anodizing require good cooling systems.

    Chromic acid anodic oxide film is particularly resistant to corrosion. It is mainly used in the dry aircraft manufacturing industry. Chromic acid anodic oxide film and paint have strong adhesion. It is also used as the bottom layer of paint. The chromic acid anodic oxide film is gray and opaque and generally not used for decoration.

    In order to increase the hardness and thickness of the anodic oxide film, the temperature of the sulfuric acid oxidation bath is reduced to 0 ° C, and the current density is increased to 2.7 to 4.0 A / dm2, and a “hard oxide film” of 25 to 50 μm can be obtained. With oxalic acid and a small amount of sulfuric acid, a hard oxide film can be obtained at 5 to 15 ° C. Some patents use hard anodizing to optimize the concentration of sulfuric acid, add organic acids or other additives.

    The thickness of the anodic oxide film is generally 5-20μm, the hardest anodic oxide film can reach 60-250μm, the hardness is generally HV240-500, the melting point is as high as 2050 , and the short-term resistance to high temperature of 1500-2000 , and the thermal conductivity is low. The resistivity is good, and the anodized film is very brittle. As the thickness increases, the brittleness increases, so the hard anodized film cannot withstand impact, bending and deformation. The corrosion resistance of the hard anodized film is also better than that of the ordinary anodized film.

    Matexcel conducts surface hardening treatment for all kinds of metals, including steel, aluminum, titanium, superalloy, composite, etc. Our scientists are experienced with all technologies, and provide relevant supporting research directions and batch production capability of surface hardening metallic products.

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