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Measures to reduce or avoid anodized failure

October 31, 2019

The quality of anodizing film of aluminum alloy sulfuric acid is good or bad, and the corrosion protection performance mainly depends on the composition of aluminum alloy, the thickness of film and the technological conditions of anodizing treatment. Such as temperature, current density, water quality and filling and sealing process after anodizing, etc. In order to reduce or avoid anodizing failure and improve product quality, effective measures should be taken.

The main results are as follows:

(1) for different aluminum alloys, such as casting molding, Calendering molding or machining molding or heat treatment welding, the appropriate pretreatment method should be selected according to the actual situation. For example, the cast aluminum alloy surface, its non-machined surface should generally use sandblasting or shot peening to remove its original oxide film, sand sticking and so on. Aluminum alloys with high silicon content (especially cast aluminum) should be etched and activated by nitric acid mixed acid solution containing about 5% hydrofluoric acid in order to effectively maintain a good activated surface and ensure the quality of oxide film. Aluminum alloys of different materials, bare aluminum and pure parts or aluminum and aluminum alloy parts of different sizes and specifications are generally not suitable for oxidation treatment in the same slot.

For aluminum alloy assemblies with lap, spot welding or riveting, sulfuric acid anodizing process is generally not allowed for aluminum alloy parts which are easy to form airbags in the process of anodizing.

(2) the installation and hanging fixture material must ensure good electrical conductivity, generally choose hard aluminum alloy bar, the plate should have a certain degree of elasticity and strength. Copper or copper alloy material should be selected for pull hook. If the used special or general fixture is used again during anodizing, the oxide film on its surface must be removed completely to ensure good contact. The fixture should not only ensure enough conductive contact area, but also reduce the fixture imprint as much as possible. If the contact surface is too small, it will cause burning erosion of anodized parts.

(3) the temperature of sulfuric acid anodizing solution must be strictly controlled, and the optimum temperature range is 15 ℃ and 25 ℃. Compressed air stirring should be used in the process of sulfuric acid anodization, and refrigeration equipment should be equipped. In the case of no refrigeration device, the addition of 1.5% to 2.0% malonic acid, oxalic acid, lactic acid and other carboxylic acids in sulfuric acid electrolyte can make the temperature range of anodizing solution exceed 35 ℃ and avoid or reduce the loosening or pulverization of oxide film. Some process experiments and production practices have proved that the addition of appropriate amount of carboxylic acid or glycerol to sulfuric acid anodized electrolyte can effectively reduce the adverse effects of reaction heat effect. Under the condition of not reducing the thickness and hardness of the oxide film, the allowable upper limit of the temperature of the anodized electrolyte can be increased, and the production efficiency can be improved on the premise of ensuring the quality. In addition, under the condition of constant temperature, attention should also be paid to the effective control of anode current density in order to better ensure the quality of oxide film.

(4) the water quality used in sulfuric acid anodizing electrolyte and the harmful impurities in the electrolyte must be strictly controlled. The preparation of sulfuric acid anodizing solution is not suitable for tap water, especially turbid tap water with high content of Ca2+,Mg2+,SiO32- and Cl-. In general, when the concentration of Cl- in water reaches 25mg/L, it will have a harmful effect on the anodizing treatment of aluminum alloy. Cl- (including other halogen elements) can destroy the formation of oxide film, or even can not form oxide film at all. Softened water, deionized water or distilled water should be selected for sulfuric acid anodizing. Ccl- ≤ 15 mg/L, total minerals ≤ 50mg/L in electrolyte. In the process of anodizing sulfuric acid solution, oil foam and suspended impurities will be produced, which should be eliminated regularly. Other harmful impurities in sulfuric acid positive plate oxidation solution are Cu2+,Fe3+,Al3+ and so on. If the impurity content exceeds the allowable content, it will have a harmful effect, some or all of the sulfuric acid solution can be replaced in order to effectively ensure the quality of sulfuric acid anodizing of aluminum alloy. Sulfuric acid anodizing treatment of aluminum alloy is a widely used and mature anti-corrosion protection decoration treatment process. As long as the process conditions are strictly carried out and carefully operated, the quality of sulfuric acid anodizing film can be guaranteed.

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