Introduction to the process of colored conductive

2022-08-13
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Aluminum and aluminum alloy color conductive oxidation process introduction

after aluminum and aluminum alloy are treated by the conductive oxidation process, the obtained oxide film still has excellent conductivity, which is its unique performance, and the protective and decorative performance of the film is also very good. The color of the film on the surface of pure aluminum is more elegant than the rainbow passivation film of zinc layer, with shallow and uniform fine grain color, which is a process with great application prospects and promotion value

aluminum and aluminum alloy conductors have come out of a great shape in the middle stage. The electro oxidation process is simple to operate and does not need special equipment. In recent years, the understanding that the conductive oxidation film is easy to absorb organic coatings and has a good adhesion has been further improved, so the application scope as a coating (electrophoresis, painting) substrate has also been gradually expanded

specific details that need to be paid attention to in the pretreatment process

aluminum materials are extremely unstable in the air and are easy to produce oxide films that are difficult to identify with the naked eye. Due to the different processing methods of aluminum parts, such as casting and forming, or directly cutting from rolled plates, or fine machining and forming, or forming through different processes and then heat treatment or welding, the workpiece surface will show different states and different degrees of dirt or traces. Therefore, in the pretreatment process, the pretreatment process method must be selected according to the actual situation of the workpiece surface

(1) problems needing attention in the pretreatment process of fine machined parts: Although the natural oxide film on the surface of fine machined parts is only initially formed and is easy to remove, it is heavy greasy, especially in and around the eyelet (added due to lubrication during machining). Such parts must be cleaned with organic solvents first. If they are directly washed with alkali, it is not only heavy greasy and difficult to remove, but also the fine machined surface cannot withstand long-term strong alkali corrosion, The result will also affect the roughness of the workpiece surface and the matching of tolerance, and may eventually become waste products

(2) problems needing attention in the pretreatment process of cast parts. Avoid that the experimental machine cannot operate normally with the software. Not all surfaces have been machined. The unmachined surface has an excessively thick oxide layer formed during the casting process, and some have sand layers. At this time, the original oxide film of this part should be removed first by machining or sandblasting, or processed after alkaline washing. Only in this way can the original oxide layer of the unmachined part be removed, It can also avoid the change of tolerance dimension of machining parts

(3) problems needing attention in the pretreatment process of workpiece after heat treatment or welding process: according to the process requirements, the workpiece needs to be cleaned with organic solvent before being transferred to heat treatment or welding process to remove the oil stain on the surface, but this is generally not possible at present, so a layer of burnt coking material with oil stain is formed on the surface of the workpiece, which is difficult to remove in organic solvent, and it will cause local corrosion if soaked in alkaline solution, Produce pitting or uneven, which seriously affects the product quality. The author uses the method of soaking in concentrated nitric acid to soften this layer of coking material. After the coking material is softened, it can be completely removed by slightly cleaning it in the alkaline solution

some specific methods of pretreatment are as follows

① organic solvent degreasing. If the oil stain is not too serious, it can be soaked in solvent for a short time; Those with serious oil stains should be wiped with cotton yarn dipped in solvent, or brushed with a bristle brush. Pay attention to safety during operation, and keep the residual solvent properly after use

② dry. No matter what kind of organic solvent cleaning method is used, the drying process must not be omitted, otherwise it will lose the significance of cleaning

③ binding. Aluminum wire should be used for binding, copper wire and galvanized iron wire are forbidden, and iron wire with zinc layer removed can be used

for the binding of a slightly larger single piece, the binding position should be considered, and it should be bound in the eyelet closest to the edge of the part as far as possible, so as to reduce the impact on the surface of the workpiece

different kinds of workpieces should not be tied together in a string, because the oxidation treatment time of aluminum materials with different components (brands) is different

pay attention to the direction of the bound workpiece when it is suspended, and avoid the cavitation caused by the downward concave part

alkali wash until the oil stain on the workpiece surface is removed

④ alkaline washing

⑤ circulating water flushing. It is best to wash with hot water after alkaline washing, which is conducive to cleaning the alkaline substances on the surface of the workpiece. For workpieces with blind holes and slits, strengthen the washing of this part, throw away the residual solution, and immediately turn nitric acid into light to avoid oxidation

⑥ nitric acid emits light. If dealing with miscellaneous aluminum and cast aluminum, 50ml/L hydrofluoric acid should also be added on the basis of this formula to accelerate the removal of insoluble substances adhered to the surface of aluminum parts during alkaline washing

technical requirements for oxidation film forming process

(1) oxidation. Solution formula and working conditions:

after pretreatment, it should be immediately transferred to the oxidation process, so as to prevent the quality of the oxide layer from being affected due to the natural oxide film generated by the workpiece stored in the atmosphere for too long. Although soaking in fresh water again is better than exposure to the atmosphere, it should not be soaked too long. If soaking in 3% dilute nitric acid, generally, it can continue to oxidize within 15 ~ 30min, but if soaking for too long, it will also affect the formation of the film, especially the old nitric acid containing impurities such as copper

the temperature of the solution in the oxidation process is a critical process condition. If the temperature of the solution is too high, the film-forming speed is accelerated, and the oxide film is prone to pulverization; The solution temperature is too low, the film forming speed is slow, the color of the generated film is light, and the adhesion is poor

in order to obtain basically the same color on the surface of the same type of aluminum, it should be treated at the same solution temperature for the same time

within a certain range, the temperature is inversely proportional to time, that is, the higher the solution temperature, the shorter the time required, and vice versa

the higher the purity of aluminum, the longer the oxidation treatment time is required. The oxidation treatment time is insufficient, and the generated oxide film is too light; The purity of aluminum is low, and the oxidation time is shortened, otherwise the oxide film will become obsolete, and even affect the conductivity of the film

in order to obtain uniform oxide film color, small parts can shake more in the solution during oxidation, and large parts can be treated by stirring the solution or static treatment (do not stir the solution, do not shake the workpiece), so as to prevent the uneven oxide film color caused by more exchanges between the edge of the workpiece and the solution than the center of the workpiece

(2) circulating water flushing. For workpieces with blind holes and slits, it is necessary to strengthen the washing of these parts, and throw away the residual solution inside, so as to prevent the oxidation solution from flowing out and the oxidation surface from being damaged

(3) self test. The workpiece should be self inspected immediately after being washed with circulating water. If any defect is found, it can be removed in the alkaline solution and re oxidized after light. If it is removed and repaired after drying, it is difficult to remove and it is easy to damage the matrix

(4) drying. Drying is the key to maintain quality. Before drying, the oxidized parts need to throw away the free water on the surface of the workpiece, and then expose to the sun. It can also be baked and dried at 45 ~ 50 ℃, and the temperature should not be too high, so as to avoid scorching, aging, cracks and old appearance color

chlorination of large-area parts

(1) overall treatment. According to the size of the outer edge of the oxidized parts (appropriately relax the allowance), form a frame with wooden strips or bricks, and lay plastic cloth in the frame to form a concave pool. If the plate-shaped parts are treated, its height is about 100mm. During operation, as long as the workpiece is shaken up and down in this pool, the oxide film can be formed on its surface

(2) segment bit processing. Bit by bit processing refers to the operation method that the workpiece changes or rotates rapidly in different parts successively in the groove (pool), and finally makes the whole workpiece surface contact with the solution for many times to gradually form and thicken the oxide film

using the above two methods can avoid making large-scale plating bath and preparing a large amount of solution, reduce the waste caused by long-term little use, and also avoid occupying the production area in the workshop

three possible reasons for uneven color of oxide film

(1) the area of the workpiece is too large, the swing in the groove is too large during operation, and the contact, renewal and exchange between the edge and the center and the solution are too different, resulting in inconsistent color of oxide film

prevention method: the swing range of the workpiece during oxidation should be small, and static treatment is also OK, but when the solution temperature is too low, it is easy to appear map like spots, which is unnatural

(2) during the processing of aluminum clad parts, some aluminum clad layers are damaged and cut off. The outer aluminum clad is high-quality aluminum, and the inner aluminum clad is miscellaneous aluminum. There is a large difference between the two kinds of aluminum, so "vitiligo" like spots appear after oxidation. Customers often don't understand this phenomenon very well. Manufacturers should do more explanation to explain the reason, so as not to cause misunderstanding

(3) process operation problems

① the alkali corrosion treatment of the workpiece is not complete, and the original oxide film and dirt at some parts are not removed

② the workpiece surface is still alkaline without light treatment immediately after alkali corrosion

③ the workpiece has been in contact with foreign matters during transmission

in case of uneven film color, we should find the reasons from many aspects and take targeted measures to solve it

the fault is caused by the high accumulation of aluminum ions in the alkali etching solution

a reader called to inquire about the reason why it is difficult to obtain the conductive oxide film after alkali etching. After eliminating many factors that are difficult to form the conductive oxide film, considering whether there is high aluminum ions in the alkali etching solution, the other party said that the alkali etching solution is very thick. But the rate of alkaline corrosion is not fast. At that time, the author suggested replacing the alkaline etching solution, because the alkaline etching solution will accumulate too much aluminum ions after being used for too long, which is difficult to elute on the surface of the workpiece, thus affecting the contact between the surface of the aluminum workpiece and the conductive oxidation solution, thus affecting the formation of the oxide film. Another suggestion is that if the alkali etching solution is unconditionally replaced at that time, the alkali etched workpiece can be rinsed in flowing water immediately after rinsing with hot water, and then shine in concentrated nitric acid containing hydrofluoric acid, and then conduct conductive oxidation treatment after full rinsing. Later, for the reader, washing and ironing with hot water after alkaline corrosion is very effective

the author's experience is that after washing and ironing in hot water, quickly leave the hot water and immediately immerse in running water to prevent the formation of conductive oxide film due to oxidation of the workpiece after drying

four possible reasons for poor adhesion of oxide film:

(1) too thick oxide film (too long oxidation time)

(2) the concentration of oxidation solution is too high

(3) the temperature of oxidation solution is too high

(4) the oxide film has not been aged

the operator can adjust according to the above four factors that affect the adhesion of oxide film

there are two main reasons why it is difficult to form oxide film around the eyelet of the oxidized part

(1) the workpiece is not washed thoroughly after alkaline washing. If the alkaline solution entering the eyelet during alkaline washing is not washed thoroughly, the alkaline solution will flow out of the eyelet after oxidation treatment, resulting in corrosion of the oxide film around the eyelet

(2) there is butter around the hole of the workpiece. Aluminum tapping is very astringent, and the operator often lubricates it with butter. If there is a lack of emulsifier in the alkaline solution during alkaline washing, the butter is difficult to remove

solution:

(1) wash it with gasoline before alkaline washing, and emulsifier should be added to the alkaline washing solution

(2) the workpiece should be washed clean after alkaline washing

poor conductivity of oxide film

reason: oxidation time is too long and oxide film is too thick. According to the operation of 30 ~ 60s required by the process, the obtained oxide film is light rainbow, and

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