[China Aluminum Industry Network] Anodic oxidation process conditions are closely related to the quality of the oxide film, because the process conditions are based on different process recipes, after a series of experiments, before the anodization, the specified process The conditions should all be known and should be done strictly in accordance with the process requirements. One of the more basic and sensitive factors is the control range of the solution temperature, voltage and current density, the anodizing time, the solution stirring method, the current density of the bath volume, the ratio of the bath volume to the anodized area, etc. . If there are deviations during the execution of these process conditions, the quality of the anodized film will be significantly affected. When the deviation is too large, the parts may be scrapped, resulting in economic losses.
When the process conditions exceed the control range, the degree of influence on the quality of the anodized film, the different symptoms, and the corrective methods are discussed separately in the following questions.
How to control the voltage when anodizing?
The voltage adjustment depends on the temperature of the solution. When the solution temperature is lower, the upper limit voltage should be used. This is because the oxide film obtained when the solution temperature is low is dense and the resistance of the oxide film is large. To increase the oxide film, it is necessary to adopt a higher voltage, otherwise it is difficult to obtain normal oxidation. Film quality. When the temperature of the solution is high, the voltage is reduced. Otherwise, the resulting oxide film is loose and the film solution is too fast, making it difficult to obtain a desired thickness of the oxide film.
For example, in a unit without a cooling device, the temperature of the solution in summer will be close to the limit temperature. If it is still necessary to continue working, the voltage may not exceed 12V. The temperature of the solution in winter is lower than the lower limit of the limit temperature, when the voltage rises to a high value, such as 18V.
Anodizing is an exothermic reaction. When the workload is full, the temperature of the solution will gradually rise. Therefore, it must be tested at any time to provide the basis for adjusting the voltage. If the temperature continues to increase, it is difficult to ensure quality even if the voltage is below the specification. At this point, production should be stopped. Take appropriate measures to reduce the temperature, and then proceed with the process requirements.
How to control the current density when anodizing?
Under normal temperature conditions (about 20°C), in addition to the special process recipes, the current density of the anodized aluminum and its alloys is generally controlled between 1 and 1.5 A/dm2.
Select according to the temperature of the solution, the concentration of the solution, the shape of the part, and other relevant process conditions.
Under possible conditions, an appropriate increase in current density is favorable for accelerating the generation rate of the film, shortening the anodizing time, increasing the porosity of the film, and improving the coloring effect. However, when the current density continues to increase, the effect of Joule heating will increase during the anodization process, the thermal effect in the film pores will increase, and the local temperature rise will be significant. As a result, the dissolution rate of the oxide film will increase, and the deposition rate will decrease. The pieces will also cause uneven current distribution and affect the coloring effect. A loose oxide film that may be easily wiped off may also appear on the surface of the article, or the film may be brittle, cracked, or appear white marks. In severe cases, ablation products may also be caused.
Choosing an appropriate current density can accelerate the growth rate of the film within a certain range, but when it exceeds a certain value, the film formation rate decreases.
According to the above rules, in order to ensure product quality and increase production efficiency, the following methods can be taken.
When the cooling conditions are good and the solution can meet the strong stirring, the upper limit of the current density can be used to improve the working efficiency.
In the absence of cooling device and no strong stirring, although the temperature of the solution is moderate at the time, the current density must be properly controlled to prevent quality problems due to excessive heating during the anodizing process. In severe cases, it may also cause artifacts. Ablation. The more effective method at this time is to reduce the volume current density.
The correct estimation of the surface area of ​​anodized parts is also an important condition for reasonable control of current density and should be considered.
The surface of the anodized part in the deep part should be delivered with the same current density as other surfaces.
Several major factors that affect the quality of the anodized film
1 Current density: Within a certain limit, the current density is increased, the film growth rate is increased, the oxidation time is shortened, the porosity of the resulting film is large, and the film is easy to be colored, and the hardness and wear resistance are increased; if the current density is too high, it may be due to The effect of Joule heating causes the surface of the part to overheat and the temperature of the local solution to rise. The dissolution rate of the film increases, and the possibility of burning the part may occur. When the current density is too low, the film growth rate is slow, but the resulting film is denser and harder. Reduced wear resistance.
Aluminium oxidation, used for protection, decoration and pure decoration processing, use the upper limit of the allowable concentration, that is, 20% sulfuric acid as the electrolyte.
2 Oxidation time: The choice of oxidation time depends on the electrolyte concentration, temperature, anode current density, and desired film thickness. Under the same conditions, when the current density is constant, the growth rate of the film is proportional to the oxidation time; however, when the film grows to a certain thickness, the film resistance increases, affecting the conductivity, and the film dissolution rate increases due to temperature rise. Therefore, the growth rate of the film will gradually decrease, and will not increase until later.
3 sulfuric acid concentration: usually 15% - 20%. As the concentration increases, the dissolution rate of the membrane increases, the growth rate of the membrane decreases, the porosity of the membrane is high, the adsorption force is strong, the elasticity is good, the dyeing property is good (easy to dye and dark color), but the hardness and wear resistance are slightly poor; When the concentration of sulfuric acid is reduced, the growth rate of the oxide film is accelerated, the porosity of the film is small, the hardness is high, and the wear resistance is good.
4 electrolyte temperature: electrolyte temperature has a great impact on the quality of the oxide film. As the temperature rises, the dissolution rate of the film increases and the film thickness decreases. When the temperature is 22 - 30 °C, the resulting film is soft, good adsorption capacity, but the wear resistance is rather poor; when the temperature is greater than 30 °C, the film becomes loose and uneven, sometimes even not continuous, And the hardness is low, thus losing the use value; When the temperature is between 10 - 20 °C, the resulting oxide film is porous, strong adsorption capacity, and full of elasticity, suitable for dyeing, but the hardness of the film is low, and the wear resistance is poor; Below 10°C, the thickness of the oxide film increases, the hardness is high, and the wear resistance is good, but the porosity is low. Therefore, the temperature of the electrolyte must be strictly controlled during production. To make a thick and hard oxide film, the operating temperature must be reduced. Compressed air is used during the oxidation process and the temperature is relatively low. Hard oxidation is usually performed at about zero degrees.
5 Stirring and moving: It can promote the convection of the electrolyte, strengthen the cooling effect, ensure the uniformity of the solution temperature, and will not cause the quality of the oxide film to decrease due to the local temperature rise of the metal.
6 impurities in the electrolyte: Possible impurities in the electrolyte used in the aluminum anode oxidation are Clˉ, Fˉ, NO3ˉ, Cu2+, Al3+, Fe2+ and so on. Among them, Clˉ, Fˉ, and NO3ˉ increase the porosity of the film, and the surface is rough and loose. If its content exceeds the limit value, it will even cause corrosive perforation of parts (Clˉ should be less than 0.05g/L, Fˉ should be less than 0.01g/L); when the content of Al3+ in the electrolyte exceeds a certain value, the surface of the workpiece often appears white. Spot or patchy white block, and the film adsorption performance decreased, difficult dyeing (Al3 + should be less than 20g / L); When the Cu 2+ content of 0.02g / L, the oxide film will appear dark stripes or black spots; Si2+ often in suspension The state exists in the electrolyte, causing the electrolyte to be slightly turbid, and adsorbed on the film as a brown powder.
7 Aluminum Alloy Composition: In general, other elements in the aluminum metal degrade the quality of the film, and the resulting oxide film is not thick as obtained on pure aluminum, and the hardness is also low. Aluminum alloys of different compositions undergo anodizing treatment Be careful not to go with the slot.
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