Anodizing | Details that need to be done
Release time:
2020-05-09 17:03
The difference of aluminum composition and the content of impurities will directly affect the appearance quality of anodic oxide film.When the alloy composition of aluminum is certain, the main factors affecting the appearance quality are harmful impurities, such as copper, silicon, chromium, iron, zinc, manganese and other metal elements. The less the content of these metal elements in aluminum, the better the brightness of the anodized finish; on the contrary, the worse.
Various grades of high-purity aluminum and pure aluminum, the transparency of the oxide film and smooth, brightness with the increase of impurity content gradually deteriorated.
The growth rate of the oxide film is closely related to the concentration of sulfuric acid in the electrolyte. The thickening process of the film depends on the ratio of the dissolution and growth rate of the film: usually with the increase of the concentration of sulfuric acid, the dissolution rate of the oxide film also increases; on the contrary, the concentration decreases, the dissolution rate also decreases. Therefore, the use of dilute sulfuric acid is beneficial to the growth of the film.
It is basically the same as the effect of the change of sulfuric acid concentration. As the solution temperature increases, the dissolution rate of the oxide film increases, the growth rate of the film decreases, and the thickness of the oxide film must decrease.
At the same time, the change of temperature will have a serious impact on the thickness and wear resistance of the oxide film. Generally, the oxide film obtained when the temperature is controlled at 18-20 ℃ is porous, has good adsorption performance, elasticity, and good salt spray resistance. Therefore, the temperature is high in summer, and the temperature should be controlled.
When the aluminum parts are electrified and oxidized, a thin and dense oxide film is generated on the surface of the aluminum parts at the beginning. With the increase of resistance, the voltage rises sharply, and the anode current density decreases gradually. When the voltage continues to rise to a certain value, the oxide film begins to be electrocuted at the weaker part due to the dissolution of the electrolyte, which promotes the passage of current and the oxidation process continues.
When the alloy contains copper, silicon and other elements, with the oxidation process, also due to the anodic dissolution in the electrolyte, the alloying elements Cu2 , Si2 continue to gather. When the Cu2 content reaches 0.02g/L, dark stripes or black spots will appear on the oxide film.
The possible impurities in the electrolyte are Cl-, F-, NO3-and Al3, Cu2, Fe2 plasma. When the content of Cl-,F-, NO3-and other anionic impurities is high, the porosity of the oxide film increases greatly, and the surface of the oxide film becomes rough and loose. The allowable contents of these impurities in the electrolyte are Cl- <0.05g/L and F- <0.01g/L. When this limit is exceeded, the surface of the product will be perforated and scrapped. These anionic impurities come from the water source in the electrolyte preparation and cleaning process, so the water quality must be strictly controlled.
During anodic oxidation, the turbidity of the electrolyte has a great influence on the surface brightness of the oxide film.
Generally, the sulfuric acid oxide film is transparent, and its main component is Al2O3. Porous oxide film has great adsorption performance, the use of this feature of aluminum and aluminum alloy surface for a variety of color pattern decoration. If a variety of opaque solid turbidity is contained in the electrolyte, it is also absorbed and filled into the film hole, which will reduce the transparency of the oxide film and block the light reflection rate of the film, thus affecting the brightness of the oxide film.
Turbidity comes from poor pretreatment of aluminum parts and unclean cleaning water quality, or impurities are not easy to precipitate at the bottom of the cylinder due to the severe negative and negative reactions and the convection of the solution, and are dispersed and suspended in the electrolyte. The transparency of the electrolyte is poor, even opaque and has a certain color. Therefore, for those aluminum products with high appearance requirements, the electrolyte must be continuously filtered during the oxidation process.
Due to the porous structure and strong adsorption performance of the anodic oxide film, the surface is easy to be polluted, especially in a corrosive environment, the corrosion medium into the hole is easy to cause corrosion.
Therefore, the anodic oxidation of the film, whether colored or not, need to be closed to improve the corrosion resistance of the oxide film, insulation and wear resistance, and reduce its adsorption of impurities or oil.
Oxidation, electrolyte, impurity, anode, surface, speed, content, influence, dissolution
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