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Optimization of sealing process and maintenance of sealing slot during anodizing of aluminum profile

September 27, 2019

The factors affecting the sealing quality of aluminum at room temperature are as follows: nickel salt, fluoride ion, sealing additive and its content in sealing solution. These three factors are the key factors to determine the sealing quality of aluminum. The pH value of solution, temperature and sealing time are the important factors affecting the sealing quality of aluminum profile. Improving the cleanliness of the bath solution and reducing the content of impurities are the important guarantee of aluminum sealing quality. The experimental results show that it is very important to ensure the suitable production conditions of aluminum sealing quality at room temperature.

Optimization of sealing process

In general industry, the following production processes are adopted: room temperature degreasing, washing, alkali etching, washing, light washing, anodizing, secondary washing, sealing and secondary washing. Test of experimental materials and instruments. The aluminum alloy material used for inspection is 6063 type material, The reagents and drugs used are all used in general factories, and the sealing additive is prepared by the author's experimental study.


1. Sulfuric acid concentration optimization: Production practice shows that when other technological conditions remain unchanged, with the increase of sulfuric acid concentration, the dissolution of sulfuric acid on the oxide film in the process of anodic oxidation is obviously strengthened, the taper of the oxide film is increased, and the sealing becomes difficult, so the sealing time needs to be prolonged appropriately. Generally, the concentration of sulphuric acid over 185g/L has a significant impact on the staining test of thick film, but the solubility of sulphuric acid is too low and the coloring performance of anodic oxide film is not good. Therefore, in actual production, it is necessary to weigh and control in a reasonable range.


2.Optimization of solubility of nickel ion: sealing is realized by entering the pores of oxidation film and hydrolyzing precipitation by nickel ion. The mechanism of rapid sealing of Ni-F system at room temperature is dissolution-deposition reaction, and the reaction product (plug substance) is mainly composed of N i (OH), A1OO H and AlF3. It is the comprehensive result of the hydrolytic deposition of metal salts, the hydration reaction and the formation of chemical conversion film. The filling rate of nickel ion directly affects the sealing speed, and its content has a great influence on the sealing quality. The higher the solubility of nickel ion is, the better the sealing quality is. However, the effect of sealing at room temperature on the color difference of billet extrusion is very limited, the content of high oxide film background color is blue, the color difference is obvious. Therefore, it is appropriate to control the concentration of nickel ion at 1.0 g/L and 1.3 g/L, and the sealing quality can be guaranteed when the nickel ion is above 0.9g/L in practical production.


3. Optimization of the solubility of fluoride ions: fluoride ions play a neutralizing role in the oxidation film, and the surface of the film is electrically negative, which is beneficial to the diffusion and hydrolysis of nickel ions into the pores. On the other hand, because of the small radius of fluoride ion, it can be adsorbed in the membrane pore and react with the oxide film of the pore wall to form fluoro-aluminum complex, which leads to the accumulation of aluminum ion and the increase of pH in the pore, which is beneficial to the migration and hydrolysis of Ni to the inner surface of the membrane pore. The fluoride ion concentration is 0.3% 0.6 g/L, too high is easy to start powder, too low, the sealing is bad. At the same time, in the process of sealing, the consumption rate of fluoride ion is faster than that of nickel ion, so it needs to be replenished frequently, but it is not recommended to add ammonia fluoride or ammonium hydrogen fluoride alone, which is easy to cause the aging of the bath, and the PH fluctuation of the bath is too large. Third, the free fluorine ions added to the bath are very limited due to the complexation reaction between additives and nickel ions.


4. Optimization of PH value: increasing the pH value of bath can effectively reduce the consumption of fluoride ion, promote the absorption of nickel ion and improve the sealing quality. However, when the pH value is too high, the nickel salt is easy to hydrolyze, the bath is turbid seriously, the profile surface is easy to produce white ash, and when the pH value of the bath is too low, it is not enough to cause nickel salt hydrolysis and can not achieve the sealing effect. According to the production practice, the sealing effect of the new slot PH is generally better at 5.8-6.2, and the old slot PH should be controlled at 6.0-6.5.


5. Optimization of sealing temperature: when the temperature is too low, the reaction between F and oxide film is weak, which is not enough to cause nickel salt solution, which affects the sealing effect. The sealing tank at room temperature usually needs to be heated at a certain temperature in winter. Under normal conditions, the higher the sealing temperature, the shorter the sealing time. If the content of fluoride ion in the old tank is lower than that in the 0.3g/L tank, it often needs to be heated to more than 30 ℃ to seal the hole normally. According to the production practice, poor sealing occurs when the sealing temperature is lower than 25 ℃ or higher than 35 ℃.


6. Optimization of sealing time: the beginning of sealing is at the mouth of the oxide film, with the increase of time, gradually penetrate into the interior of the film to complete the sealing, in the whole sealing process, the sealing speed in the first half of the time is fast, and the sealing speed in the second half of the time is slow. According to the production practice, the sealing time of white material, sandblasting material and color material is quite different, and the sealing time can not be allocated in proportion according to the film thickness, especially the ancient copper material and black material, although 3-8UM colored particles will be deposited at the bottom of the oxidation film. However, too short sealing time will cause the danger of decolorization viscose. The sealing time is easy to ash when the sealing time is a little longer, which requires the workshop operator to decide the sealing time according to the actual situation. Due to the development of modern additives, sealing agents have appeared in the market for a long time (30 minutes) without ash, which is very beneficial to automatic production.

7. Optimization of aging process: during the sealing process, the root of the oxide film is not completely closed and needs to be aged for a period of time. Under the normal technological conditions, the natural aging after sealing can reach the national standard within 1-3 days, but the thermal cracking resistance of the natural sealing film is not good, and some of the thick film materials will appear obvious cracks after exposure to the sun for a period of time. The thermal cracking property of oxide film can be significantly improved by thermal pure water aging, the thermal pure water conductivity is not more than 50, the temperature is 60-80 ℃, the time is 1 um/min. the hot water washing temperature has an obvious effect on the surface quality, and the excessive aging temperature will cause ash.

Sealing slot maintenance

1.The profile must be cleaned and controlled thoroughly before sealing. In the production process, if the cleaning is not clean or the control liquid is not thorough, impurity ions will continue to be brought into the production process. The accumulation of impurity ions will affect the stability of the sealing solution and cause cross pollution, resulting in the decrease of the activity of F ions. It is not conducive to the hydrolysis of metal ions into the micropores. Conditional workshop can add a pure water wash in front of the sealing tank, which can effectively reduce the impurity ion.


2.The sealing tank is sensitive to water quality, the tank liquid should be supplemented with pure water as far as possible, and it is not recommended to use groundwater distribution tank and cleaning tank before sealing. The impurity cation in the water quality will consume fluorine ion greatly, and the tap water treated by alum will be used in some workshops. It will also cause the proportion of fluorine and nickel ions in the sealing slot to be out of balance. Because the water quality is not guaranteed and the fluoride ion content in the tank is on the low side, many workshops use increasing the tank temperature to increase the fluoride ion activity to achieve the purpose of sealing, but the comprehensive cost is often a lot higher.


3. Proper circulation can effectively alleviate the aging of sealing slot. The sealing at room temperature has a good retention to the color of electrolytic coloring, but the fluoride ion inevitably reacts with the particles in the coloring film. If the output of the coloring material is large, the yellow-red colloidal precipitation will be produced in the sealing tank, and in serious cases, the color of the bath will turn yellow and red. At this time, properly reducing the sealing slot PH and then opening the filter can effectively reduce the phenomenon of sealing and gray of the coloring material.


4. Matters needing attention when adding medicine. The filling effect of the medicine adding bucket with stirring is the best. The sealing agent can be dissolved in the tank and stirred evenly, and the tank can be sealed normally after the liquid circulates evenly. If there is no stirring bucket, the small package sealing agent can be dissolved in the simple drug adding container, stirred evenly, and then evenly sprinkled on the groove surface with a spoon, and the sealing agent can be dissolved in the process of natural sedimentation. Generally speaking, it is best not to work within half an hour. The uneven dissolution of the agent will result in different sealing effects in different parts of the same branch. In addition, it is not recommended to use compressed air stirring tank liquid, the dust, oil pollution and solid impurity precipitation in the workshop will cause secondary pollution of the tank liquid due to severe air stirring.


5. To strengthen the inspection of the slot surface, if there are aluminum parts falling into the sealing groove, it must be salvaged at the first time. The aluminum parts will greatly consume the fluoride ion in the bath solution and cause poor sealing. At the same time, the oily foam on the trough surface should be fished out in time, and the foam will cover the drug particles which are not dissolved in time in the process of adding medicine, resulting in drug waste.


6. When PH is low, it is suggested that dilute ammonia water should be used to adjust PH:. When adjusting, the dilute ammonia water should be sprinkled evenly on the trough surface, and the blue nickel precipitation can be effectively reduced by gently stirring the trough surface while sprinkling. The regulation of tablet alkali is not recommended, mainly because the alkalinity of tablet is too large, which is easy to cause precipitation. If the bath PH deviates greatly from the process, it is recommended to adjust it many times. The fluctuation of PH is too large, which is easy to cause poor sealing.


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