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The following shows the non-uniform dyeing of the film during the anodization of sulfuric acid. Analyze the causes of these phenomena and take effective preventive measures.
In fact, the color unevenness of the aluminum alloy oxide film will present a variety of phenomena (such as some of the faults introduced earlier). How to ensure the uniformity of the oxide layer dyeing, due to the color uniformity of the aluminum sulfate anodized film mainly depends on the composition of the aluminum alloy and the anodizing process conditions, such as temperature, current density, fixtures, water quality, process protection, etc. . It is necessary to start from the finer points in the actual process to effectively take measures to obtain an oxide film layer with uniform color and excellent performance.
1. Select the appropriate pretreatment method for different aluminum alloys, such as casting, rolling or machining forming or heat treatment welding process of aluminum alloy parts, according to the actual situation to choose appropriate pre-treatment methods (including degreasing, light, etc.). For example, the non-machined surface of cast aluminum alloy parts should be sandblasted or shot blasted to remove the original oxide film and sticky sand. For aluminum alloys with high silicon content (cast aluminum), they should be etched and activated in a mixed solution containing about 5% of hydrofluoric acid and nitric acid in order to maintain a good activated surface and ensure the quality of the anodized film.
Aluminum alloys, bare aluminum and pure aluminum parts of different materials, or aluminum and aluminum alloy parts of different sizes and sizes are generally not suitable for oxidation treatment in the same tank.
2. The use of suitable fixtures for mounting fixture materials must ensure good electrical conductivity. Generally, pure aluminum or aluminum alloy rods with higher specifications are selected to ensure a certain degree of elasticity and strength. And according to the need to determine whether the need for heat treatment. Used special or general-purpose fixtures, such as those used for anodization, must be re-used and their surface oxide layer must be removed to ensure good contact. Fixtures should not only ensure enough conductive contact area, but also minimize the imprint of the fixture, and at the same time ensure the smooth discharge of gas during the oxidation process, avoid the formation of airbags at certain oxidation sites, and cause the oxide film layer to be too thin or have no oxide film layer.
3. The temperature of the solution during the anodization process is strictly controlled. It is known from the film formation process of the anodic oxidation that as the temperature of the anodic oxidation increases, the color of the film layer gradually becomes deeper, and the thickness of the film layer gradually becomes thinner, mainly because the anodic oxide film has Insulation, when the oxide film is formed, the resistance is increased accordingly. After these resistors are energized, a voltage drop occurs. This causes a large amount of electrical energy to be converted into heat energy, which increases the temperature of the oxidizing solution and accelerates the dissolution of the film. The higher the temperature of the oxidation solution, the stronger the dissolution, so the thickness of the film will gradually decrease as the temperature of the oxidation solution increases. When the temperature of the oxidizing solution is low, the formed oxide film is dense, the porosity is small, and it is difficult to be colored. As the oxidation temperature increases, the oxide film gradually becomes loose, the porosity of the film gradually increases, and the color of the film increases with temperature. The increase gradually deepens. Therefore, the temperature of the oxidation solution is an important factor in determining the degree of densification of the oxide film, and is also an important factor in determining the color uniformity of the oxide film.
Sulfuric acid anodizing solution temperature must be strictly controlled, according to different aluminum alloys choose different temperature range, for hard aluminum, super hard aluminum and pure aluminum, the best solution temperature range is 15 °C ~ 25 °C; for rust prevention Aluminum alloy parts, the best temperature range is l0 °C ~ l8 °C. In the sulfuric acid anodizing process, compressed air is used for stirring and equipped with a refrigeration device. In the absence of a cooling device, 1.5% to 2% of glycerol or organic carboxylic acid such as oxalic acid, lactic acid, etc. can be added to the sulfuric acid solution so that the anodic oxidation concentration temperature can exceed 30° C. to avoid or reduce the oxide film. Loose or powdered. Some oxidation processes and production practices prove that the addition of an appropriate amount of organic carboxylic acid or glycerin in the anodizing solution of sulfuric acid can effectively reduce the influence of the reaction heat effect, and increase the temperature of the anodizing solution without decreasing the thickness and hardness of the oxide film. Allow the upper limit. In addition, under the condition of controlling the temperature of the oxidizing solution, it is also necessary to pay attention to the effective control of the anode current density.
4. Strictly control the anode current density As the anode current density increases, the color of the oxide film layer gradually becomes deeper, and the thickness of the film layer first becomes thicker and then becomes thinner. The reason is that with the increase of the anode current density, the pores of the oxide film are enlarged and easy to be colored; and with the decrease of the anode current density, the growth rate of the film is reduced, and the film layer is dense, so as the current density increases, the film The layer color gradually darkens. Under the same conditions, the anode current density is increased, the formation rate of the oxide film is faster, the anodization time can be shortened, the amount of chemical dissolution of the film layer can be reduced, the obtained film layer is hard, and the wear resistance is good. However, the anode current density cannot be raised too high. Otherwise, due to Joule heat, the thermal effect in the film pores increases, the local temperature rises significantly, the dissolution of the oxide film is accelerated, the film formation rate decreases, and the parts are easily burned. eclipse. In other words, the anode current density is increased, and the formation rate of the oxide film can be accelerated within a certain range, but after a certain value, the film formation rate is decreased. Anodic current density directly affects the uniformity of the film color, according to different aluminum alloys choose different anode current density, for all kinds of pure aluminum, aluminum parts, the best current density is 1.0A/dm2 ~ 1.5A / dm2; For cast aluminum, wrought aluminum parts, the best current density of 1.5A/dm2 ~ 2.5A/dm2; for hard aluminum and superhard lead parts, the best current density l. 3A/ dm2 ~ 1.5A/dm2; For rust-proof aluminum alloy parts, the best current density is 1.0A/dm2 ~ 1.3A/dm2.
5. To ensure the quality of water used in the anodizing process The quality of the water used for the anodizing solution of sulfuric acid and the content of harmful impurities in the solution must be strictly controlled. The anodizing solution for the preparation of sulfuric acid must use demineralized water, deionized water or distilled water, and must not use tap water containing Ca2+, Mg2+, SiO32-, and Cl-. Under normal circumstances, when the Cl- concentration in the water reaches 25 mg/L, it will have an adverse effect on the anodizing treatment of the aluminum alloy. Cl- ions (including other halogens) can destroy the oxide film formation and even cannot form oxide films at all. Sulfuric acid anodizing washing water and closed water should also use deionized water or distilled water, require water [Cl] ≤ 15mg/L, total mineral ≤ 50mg/L.
6. To strengthen the protection between processes is generally believed that aluminum alloy will not "rust", in fact, aluminum alloy contains a variety of alloying elements, such as Cl- and other active anions in the medium, coupled with large production volume, long production cycle, If no effective protective measures are taken during this period, corrosion spots appear on the local surface and quickly develop into the depths. It is obvious after alkali corrosion that eventually leads to small "pit spots" or small "pits" on the surface of the oxide film. In order to avoid the occurrence of small hole corrosion, first of all, in the process, especially after the completion of finishing, the surface of the part must be coated with rust-proof oil; Secondly, during the turnover of parts, it is forbidden to use "naked hands" to take it, and at the same time pay attention to Anti-rust and moisture-proof.
7. For secondary heat treatment of special aluminum alloy parts For special parts due to uneven heat treatment of materials, according to the actual production, secondary heat treatment (quenching + artificial aging) procedures are added after roughing is completed, and quenching transfer is strictly controlled during quenching. Time and quenching cooling rate. These two factors have a great influence on the mechanical properties and corrosion resistance of quenched materials. It is generally required that the transfer time of the superhard aluminum alloy does not exceed 15 s. Otherwise, due to the decrease in the temperature of the material, a large number of strengthening phases such as T.sub.3 are precipitated from the grains and the strength and corrosion resistance after quenching are reduced. The influence of quenching cooling rate and transfer time is as great as that. The cooling rate during quenching must ensure that the supersaturated solid solution is fixed. It plays a decisive role in the performance of the alloy. In order to ensure the proper mechanical properties and corrosion resistance after quenching, a high quenching cooling rate must be adopted. For different aluminum alloys, there are different requirements for the quenching cooling rate. Generally, the quenching speed of the super hard aluminum alloy must not be lower than 170°C/s, otherwise the ideal quenching effect is not obtained.
8. Periodically clean impurities in the electrolyte. Anodic oxidation The main impurities in the solution are Al3+, Cu2+, Fe2+, Cl-, F-, NO3-, etc. When the content of anion impurities such as C1-, F-, and NO3- is high, the anode The porosity of the oxide film greatly increases, and the oxidized surface becomes rough and loose. The allowable content of these impurities in the oxidation solution is [Cl]<0.059/L, [F]<0.019/L, and the like. The Al3+ in the oxidizing solution mainly comes from the dissolution of the workpiece during the oxidation process. When [A13+]> 209/L in the solution, the oxide film shows white spots or massive white spots on the surface of the oxide film, and the adsorption performance of the oxide film decreases, resulting in difficulty in dyeing. When the [Cu2+] content reaches 0.029/L, dark stripes and black spots appear on the oxide film. To exclude Cu2+, it is possible to pass direct current, the anode current density is controlled at about 0.1A/dm2 to 0.2A/dm2, and the metal copper in the solution is precipitated at the cathode. When the content of Cu2+ is high, the lead plate can be directly placed in the electrolyte, and the lead plate will soon be replaced with a layer of metallic copper. Using this method can remove higher levels of copper ions, and then power treatment, can completely remove the Cu2 +. If the impurity content exceeds the allowable value, the anodizing solution may be partially or completely replaced.
In fact, the color unevenness of the aluminum alloy oxide film will present a variety of phenomena (such as some of the faults introduced earlier). How to ensure the uniformity of the oxide layer dyeing, due to the color uniformity of the aluminum sulfate anodized film mainly depends on the composition of the aluminum alloy and the anodizing process conditions, such as temperature, current density, fixtures, water quality, process protection, etc. . It is necessary to start from the finer points in the actual process to effectively take measures to obtain an oxide film layer with uniform color and excellent performance.
1. Select the appropriate pretreatment method for different aluminum alloys, such as casting, rolling or machining forming or heat treatment welding process of aluminum alloy parts, according to the actual situation to choose appropriate pre-treatment methods (including degreasing, light, etc.). For example, the non-machined surface of cast aluminum alloy parts should be sandblasted or shot blasted to remove the original oxide film and sticky sand. For aluminum alloys with high silicon content (cast aluminum), they should be etched and activated in a mixed solution containing about 5% of hydrofluoric acid and nitric acid in order to maintain a good activated surface and ensure the quality of the anodized film.
Aluminum alloys, bare aluminum and pure aluminum parts of different materials, or aluminum and aluminum alloy parts of different sizes and sizes are generally not suitable for oxidation treatment in the same tank.
2. The use of suitable fixtures for mounting fixture materials must ensure good electrical conductivity. Generally, pure aluminum or aluminum alloy rods with higher specifications are selected to ensure a certain degree of elasticity and strength. And according to the need to determine whether the need for heat treatment. Used special or general-purpose fixtures, such as those used for anodization, must be re-used and their surface oxide layer must be removed to ensure good contact. Fixtures should not only ensure enough conductive contact area, but also minimize the imprint of the fixture, and at the same time ensure the smooth discharge of gas during the oxidation process, avoid the formation of airbags at certain oxidation sites, and cause the oxide film layer to be too thin or have no oxide film layer.
3. The temperature of the solution during the anodization process is strictly controlled. It is known from the film formation process of the anodic oxidation that as the temperature of the anodic oxidation increases, the color of the film layer gradually becomes deeper, and the thickness of the film layer gradually becomes thinner, mainly because the anodic oxide film has Insulation, when the oxide film is formed, the resistance is increased accordingly. After these resistors are energized, a voltage drop occurs. This causes a large amount of electrical energy to be converted into heat energy, which increases the temperature of the oxidizing solution and accelerates the dissolution of the film. The higher the temperature of the oxidation solution, the stronger the dissolution, so the thickness of the film will gradually decrease as the temperature of the oxidation solution increases. When the temperature of the oxidizing solution is low, the formed oxide film is dense, the porosity is small, and it is difficult to be colored. As the oxidation temperature increases, the oxide film gradually becomes loose, the porosity of the film gradually increases, and the color of the film increases with temperature. The increase gradually deepens. Therefore, the temperature of the oxidation solution is an important factor in determining the degree of densification of the oxide film, and is also an important factor in determining the color uniformity of the oxide film.
Sulfuric acid anodizing solution temperature must be strictly controlled, according to different aluminum alloys choose different temperature range, for hard aluminum, super hard aluminum and pure aluminum, the best solution temperature range is 15 °C ~ 25 °C; for rust prevention Aluminum alloy parts, the best temperature range is l0 °C ~ l8 °C. In the sulfuric acid anodizing process, compressed air is used for stirring and equipped with a refrigeration device. In the absence of a cooling device, 1.5% to 2% of glycerol or organic carboxylic acid such as oxalic acid, lactic acid, etc. can be added to the sulfuric acid solution so that the anodic oxidation concentration temperature can exceed 30° C. to avoid or reduce the oxide film. Loose or powdered. Some oxidation processes and production practices prove that the addition of an appropriate amount of organic carboxylic acid or glycerin in the anodizing solution of sulfuric acid can effectively reduce the influence of the reaction heat effect, and increase the temperature of the anodizing solution without decreasing the thickness and hardness of the oxide film. Allow the upper limit. In addition, under the condition of controlling the temperature of the oxidizing solution, it is also necessary to pay attention to the effective control of the anode current density.
4. Strictly control the anode current density As the anode current density increases, the color of the oxide film layer gradually becomes deeper, and the thickness of the film layer first becomes thicker and then becomes thinner. The reason is that with the increase of the anode current density, the pores of the oxide film are enlarged and easy to be colored; and with the decrease of the anode current density, the growth rate of the film is reduced, and the film layer is dense, so as the current density increases, the film The layer color gradually darkens. Under the same conditions, the anode current density is increased, the formation rate of the oxide film is faster, the anodization time can be shortened, the amount of chemical dissolution of the film layer can be reduced, the obtained film layer is hard, and the wear resistance is good. However, the anode current density cannot be raised too high. Otherwise, due to Joule heat, the thermal effect in the film pores increases, the local temperature rises significantly, the dissolution of the oxide film is accelerated, the film formation rate decreases, and the parts are easily burned. eclipse. In other words, the anode current density is increased, and the formation rate of the oxide film can be accelerated within a certain range, but after a certain value, the film formation rate is decreased. Anodic current density directly affects the uniformity of the film color, according to different aluminum alloys choose different anode current density, for all kinds of pure aluminum, aluminum parts, the best current density is 1.0A/dm2 ~ 1.5A / dm2; For cast aluminum, wrought aluminum parts, the best current density of 1.5A/dm2 ~ 2.5A/dm2; for hard aluminum and superhard lead parts, the best current density l. 3A/ dm2 ~ 1.5A/dm2; For rust-proof aluminum alloy parts, the best current density is 1.0A/dm2 ~ 1.3A/dm2.
5. To ensure the quality of water used in the anodizing process The quality of the water used for the anodizing solution of sulfuric acid and the content of harmful impurities in the solution must be strictly controlled. The anodizing solution for the preparation of sulfuric acid must use demineralized water, deionized water or distilled water, and must not use tap water containing Ca2+, Mg2+, SiO32-, and Cl-. Under normal circumstances, when the Cl- concentration in the water reaches 25 mg/L, it will have an adverse effect on the anodizing treatment of the aluminum alloy. Cl- ions (including other halogens) can destroy the oxide film formation and even cannot form oxide films at all. Sulfuric acid anodizing washing water and closed water should also use deionized water or distilled water, require water [Cl] ≤ 15mg/L, total mineral ≤ 50mg/L.
6. To strengthen the protection between processes is generally believed that aluminum alloy will not "rust", in fact, aluminum alloy contains a variety of alloying elements, such as Cl- and other active anions in the medium, coupled with large production volume, long production cycle, If no effective protective measures are taken during this period, corrosion spots appear on the local surface and quickly develop into the depths. It is obvious after alkali corrosion that eventually leads to small "pit spots" or small "pits" on the surface of the oxide film. In order to avoid the occurrence of small hole corrosion, first of all, in the process, especially after the completion of finishing, the surface of the part must be coated with rust-proof oil; Secondly, during the turnover of parts, it is forbidden to use "naked hands" to take it, and at the same time pay attention to Anti-rust and moisture-proof.
7. For secondary heat treatment of special aluminum alloy parts For special parts due to uneven heat treatment of materials, according to the actual production, secondary heat treatment (quenching + artificial aging) procedures are added after roughing is completed, and quenching transfer is strictly controlled during quenching. Time and quenching cooling rate. These two factors have a great influence on the mechanical properties and corrosion resistance of quenched materials. It is generally required that the transfer time of the superhard aluminum alloy does not exceed 15 s. Otherwise, due to the decrease in the temperature of the material, a large number of strengthening phases such as T.sub.3 are precipitated from the grains and the strength and corrosion resistance after quenching are reduced. The influence of quenching cooling rate and transfer time is as great as that. The cooling rate during quenching must ensure that the supersaturated solid solution is fixed. It plays a decisive role in the performance of the alloy. In order to ensure the proper mechanical properties and corrosion resistance after quenching, a high quenching cooling rate must be adopted. For different aluminum alloys, there are different requirements for the quenching cooling rate. Generally, the quenching speed of the super hard aluminum alloy must not be lower than 170°C/s, otherwise the ideal quenching effect is not obtained.
8. Periodically clean impurities in the electrolyte. Anodic oxidation The main impurities in the solution are Al3+, Cu2+, Fe2+, Cl-, F-, NO3-, etc. When the content of anion impurities such as C1-, F-, and NO3- is high, the anode The porosity of the oxide film greatly increases, and the oxidized surface becomes rough and loose. The allowable content of these impurities in the oxidation solution is [Cl]<0.059/L, [F]<0.019/L, and the like. The Al3+ in the oxidizing solution mainly comes from the dissolution of the workpiece during the oxidation process. When [A13+]> 209/L in the solution, the oxide film shows white spots or massive white spots on the surface of the oxide film, and the adsorption performance of the oxide film decreases, resulting in difficulty in dyeing. When the [Cu2+] content reaches 0.029/L, dark stripes and black spots appear on the oxide film. To exclude Cu2+, it is possible to pass direct current, the anode current density is controlled at about 0.1A/dm2 to 0.2A/dm2, and the metal copper in the solution is precipitated at the cathode. When the content of Cu2+ is high, the lead plate can be directly placed in the electrolyte, and the lead plate will soon be replaced with a layer of metallic copper. Using this method can remove higher levels of copper ions, and then power treatment, can completely remove the Cu2 +. If the impurity content exceeds the allowable value, the anodizing solution may be partially or completely replaced.
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