After the aluminum and aluminum alloys are treated by the conductive oxidation process, the obtained oxide film still has excellent conductive properties, which is its unique performance, and the protection and decorative properties of the film layer are also very good, and the film layer color ratio of the pure aluminum surface is higher than that of the aluminum oxide film. The iridescent passivation film on the zinc layer is more elegant and has a lighter and more uniform fine-grained color. It is a process with great application prospect and promotion value.
Aluminum and aluminum alloy conductive oxidation process is simple, no special equipment, in recent years about the conductive oxide film layer is easy to absorb organic coatings, good understanding of the bonding strength has been further improved, and thus used as a coating (electrophoresis, paint) substrate applications Also gradually expanded.
Specific details to be noted in the pretreatment process
The aluminum material is extremely unstable in the air, and it is easy to generate an oxide film that is difficult to recognize with the naked eye. Due to the different processing methods of aluminum parts, such as casting molding, direct shearing of sheet metal, or mechanical fine processing, or heat treatment or welding after forming by different processes, the surface of the workpiece will be presented. Different conditions, different levels of dirt or traces, for this purpose in the pre-treatment process must be based on the actual situation of the workpiece surface to select the pre-treatment process.
(1) Matters needing attention in the pre-processing process of the fine-worked parts: Although the natural oxide film on the surface of the fine-worked parts is only formed at the beginning, it is easy to remove, but the greasy weight, especially in and around the hole (because of the machining process) Lubrication needs to be added. This type of workpiece must be first cleaned with organic solvents. If it is washed directly with alkali it is not only difficult to remove the greasy weight, but also the fine processing surface cannot withstand long-term strong alkali corrosion. The result will also affect the surface of the workpiece. The combination of roughness and tolerances may eventually become a waste product.
(2) Problems that need to be paid attention to in the pretreatment process of the cast molding. Castings are not machined on all surfaces. Unpolished surfaces have excessive oxide layers formed during the casting process and some sand layers. This should be removed by machining or sandblasting. The original oxide film in this area, or after being processed by caustic washing, can only remove the original oxide layer of the unprocessed part, and can avoid the tolerance dimension change of the machined part.
(3) Problems requiring attention in the pre-treatment process of workpieces that have undergone heat treatment or welding process: according to the process requirements, the workpiece must be cleaned with organic solvents before it is transferred to the heat treatment or welding process, in addition to the net surface oil, but this is generally not possible at present. At one point, a layer of oil-sintered char is formed on the surface of the workpiece. This layer of char is difficult to remove in organic solvents. If it is soaked in alkaline solution, it will cause local corrosion, resulting in pitting or unevenness, which will seriously affect the product. quality. The author used a method of soaking in concentrated nitric acid to soften this layer of char, until the char was soft and then washed in the lye to clean it completely.
Some specific methods of preprocessing are as follows.
1 organic solvent degreasing. Less severe oil can be soaked in solvents for short periods of time; heavily soiled cotton swabs can be rubbed with solvent, or scrubbed with a brush. Pay attention to safety during operation, and keep the remaining solvent properly after use.
2 Let it 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 banding. The materials used for lashing should be Aluminum Wire, copper wire and galvanized wire should be disabled, and the wire with zinc layer removed.
Slightly larger one-piece lashings should consider the lashing position and tie them as far as possible to the hole near the edge of the part to reduce the effect on the surface of the workpiece.
Different types of workpieces should not be tied together in a string, because the oxidation treatment time of different components (grades) of aluminum is different.
Pay attention to the direction in which the banded workpiece is left unconnected, and avoid creating a gas trap when the recessed area is facing down.
Alkali wash to the surface of the workpiece until the oil pollution.
4 alkaline wash
5 cycles of water rinse. After washing with alkali, it is better to rinse with hot water first, which is good for cleaning alkaline substances on the surface of the workpiece. For workpieces with blind holes and slits, it is necessary to intensify the flushing of the parts and to remove the residual solution from them, and immediately transfer nitric acid to light to prevent oxidation.
6 Nitric acid light
If you deal with mixed aluminum, cast aluminum should also add 50mL / L of hydrofluoric acid on the basis of this formula, in order to accelerate the removal of insoluble matter adhered to the surface of the aluminum parts.
658. Technical requirements for oxide film formation
(1) Oxidation. Solution formulation and working conditions:
After the pretreatment, it is immediately transferred to the oxidation process to prevent the formation of a natural oxide film due to the workpiece being left in the atmosphere for too long and affect the quality of the oxide layer. Re-immersion in fresh water is better than exposure to the atmosphere, but it should not be soaked too long, if soaked in 3% dilute nitric acid is generally soaked l5 ~ 30min can still continue to oxidize, but if the time is too long for the film Generation also has an effect, especially old nitric acid containing impurities such as copper.
The temperature of the solution in the oxidation process is a critical process condition. The solution temperature is too high, the film formation rate is accelerated, and the oxide film is prone to powdering. When the solution temperature is too low, the film formation rate is slow, and the resulting film is light in color tone. Poor efforts.
In order to obtain the same color of the surface of the same model aluminum, the same time should be treated at the same solution temperature.
In a certain range, the temperature is inversely proportional to time, that is, the higher the solution temperature, the shorter the required time, and vice versa.
The higher the purity of the aluminum material, the longer the oxidation treatment time required. Oxidation treatment time is insufficient, the resulting oxide film is too light; aluminum purity is low, oxidation time is shortened, otherwise the oxide film is obsolete, and even affect the conductivity of the film.
In order to obtain a uniform oxide film color, the small pieces may be shaken in the solution during oxidation, and the large pieces may be stirred or statically treated (without agitating the solution or shaking the workpiece) to prevent the peripheral parts of the workpiece from being exchanged with the solution. The center part of the workpiece has many uneven oxide film colors.
(2) flush with water. For parts with blind holes and slits, it is necessary to intensify the rinsing of these parts and to reject the residual solution inside to prevent the oxide solution from flowing out of the oxidized surface.
(3) Self-test. After the workpiece is rinsed with circulating water, it should be self-inspected. If it is found to be defective, it can be removed from the lye and re-oxidized after leaving. If it is dried and then retired and returned for repair, it is difficult to remove and it is easier to damage the substrate.
(4) Drying. Drying is the key to maintenance quality. The oxidizer needs to shake off the free water on the surface of the workpiece before drying and then expose it to sunlight. Can also be baked at 45 ~ 50 °C drying conditions, the temperature can not be too high, so as not to scorch, aging, cracks, looks old and outdated.
659. Large area of ​​chlorination
(1) Overall treatment. According to the size of the outer edge of the oxidation part (appropriately to relax the margin), a box is enclosed by a wooden strip or brick, and a plastic cloth is laid inside the box to form a concave pool. If the height is treated with a plate-shaped part, then there is 100 mm. About the right and left, when the workpiece is shaken up and down in this tank, an oxide film can be formed on the surface.
(2) Partial processing. Sub-site treatment means that the workpiece is rapidly changed or rotated successively at different locations in the tank (pool), and the operation method of gradually forming and thickening the oxide film is performed after the entire surface of the workpiece is brought into contact with the solution for multiple times.
By adopting the above two methods, large-scale plating tanks can be exempted, a large amount of solution can be prepared, and the waste caused by the use for a long time can be reduced, and the production area in the workshop can also be avoided.
660. Three possible causes of color unevenness in oxide films
(1) When the workpiece area is too large, the oscillation in the groove is too large during operation, and the contact, renewing, and exchanging of the edge and the center portion with the solution are too different, resulting in inconsistent color of the oxide film. .
Preventive method: The amplitude of workpiece swing during oxidation is small, static treatment can also be, but when the solution temperature is too low, it is easy to appear map-like piebald, it seems unnatural.
(2) Partial clad aluminum layer was destroyed and was cut off during processing of aluminum-coated parts. The outer layer of aluminum was high-quality aluminum, and the inner layer of the coated aluminum was hetero-aluminum. The difference between the two aluminum species was large, so the oxidation occurred. Good hurricane like spots. This phenomenon is often not understood by customers. Manufacturers should do more explanation work and explain the reasons for this so as to avoid misunderstandings.
(3) Process Operation Problems
1 The alkali etching treatment of the workpiece was not complete, and the original oxide film and dirt could not be removed at the local part;
2 After the alkali etching, the light treatment was not performed immediately, and the surface of the workpiece was still alkaline;
3 The workpiece touched the foreign material during the transfer process.
When there are uneven film colors, we must look for the reasons in many ways and take targeted measures to solve them.
661. Caused by excessive accumulation of aluminum ions in the alkaline solution
A reader calls to ask the workpiece after the alkaline etching is difficult to obtain the reasons for the conductive oxide film, after the elimination of many factors difficult to form a conductive oxide film, taking into account the existence of excessive aluminum ions in the alkaline solution, the other said that the alkaline solution Very thick. However, the alkali corrosion rate is not fast. At that time, the author suggested replacing the alkaline solution because the alkali solution will accumulate too much aluminum ions after it is used for a long time. The aluminum ions are difficult to elute on the surface of the workpiece, which affects the contact between the surface of the aluminum part and the conductive oxide solution. Formation of oxide film. Another suggestion is that if the alkaline etching solution is replaced unconditionally at the time, the alkali-etched workpiece can be rinsed in hot water immediately after rinsing with hot water, and then light is emitted from concentrated nitric acid containing hydrofluoric acid, and then fully electrically-conductive after being rinsed. Oxidation treatment. After the reader came to the phone that after alkaline etching with hot water washing effect is very good.
The author's experience is that after washing in hot water, the hot water quickly leaves the hot water and is immediately immersed in running water to prevent the formation of the conductive oxide film due to oxidation after the workpiece is dried.
662. Four possible causes of poor adhesion of oxide film:
(1) Oxide film is too thick (oxidation time is too long);
(2) The concentration of oxidation solution is too large;
(3) The temperature of the oxidation solution is too high;
(4) The oxide film is not aged.
The operator can adjust according to the above four factors that affect the adhesion of the oxide film.
663. There are two main reasons for the formation of oxide films in the pores of the oxidation member and its surroundings.
(1) Incomplete flushing of the workpiece after caustic washing. If the lye that enters the pores during alkaline washing fails to be washed cleanly, the lye will flow out of the pores after the oxidation treatment, causing the oxide film around the pores to be corroded.
(2) There is butter around the hole of the workpiece. Aluminium tapping screw hole is very astringent, the operator often to apply butter to lubricate, if the lack of emulsifier alkali in the alkaline wash, butter is difficult to get rid of.
Solution:
(1) Wash it with gasoline before caustic washing, emulsifier should be added in caustic solution;
(2) Wash the workpiece after caustic washing.
664. Oxide film poor conductivity
Cause: The oxidation time is too long and the oxide film is too thick. According to the process requirements of 30 ~ 60s operation, the obtained oxide film is light rainbow, the film conductivity is good, basically no resistance can be measured, if the oxidation time is too long, the film thickness increases, not only will affect the conductivity of the film Performance, the film will be khaki, look old.
Preventive methods: The operating time should be strictly controlled.
665. Four points to note in post-processing operations
(1) Wash with hot water. The aim of hot water washing is to age the film. However, the water temperature and time must be strictly controlled. The water temperature is too high and the film thickness is reduced. If the processing time is too long, similar problems mentioned above may occur. The appropriate temperature and time are:
Temperature 40 ~ 50 °C time 0.5 ~ 1MIN
(2) Drying. It is better to dry and dry naturally, and the parts that have been washed with hot water are slung on the rack so that the free surface of the working surface flows downward in the vertical direction. The water droplets flowing to the lower corners are sucked off with a towel, and the color of the film dried in this way is not affected and appears natural.
(3) Aging. The aging method can be determined according to the weather conditions. In the summer with sunlight, it can be exposed to sunlight, and it can be baked in an overcast or rainy day. The process conditions are as follows:
Temperature 40 ~ 50 °C time 10 ~ 15min
(4) Rework of defective parts. Unqualified conductive oxide film should be singled out before the drying and aging process, because the film is difficult to remove after drying and aging and will affect the roughness of the workpiece surface. The author of this problem has carried out some groping in the process, after a variety of methods and found that using the following methods works well, the method is simple, without affecting the surface quality of the workpiece, the specific process is as follows. First, the unqualified workpiece is clamped on the aluminum anodizing jig, then anodized in the sulfuric acid solution of aluminum in anodizing method for 2 ~ 3min, until the film layer is soft, shedding, and then slightly washed by alkali solution and nitric acid After the light is released, the conductive anodization can be resumed.
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