Talking about the experience of electrolytic coloring and electrophoresis process of aluminum alloy extruded profiles

The coloring methods of aluminum alloy extruded profiles include anodic oxidation, electrolytic coloring and electrophoresis, which are widely used in construction and home decoration and decoration, and their market applications are constantly growing!

Production process and control points

1. Feeding worker: Before feeding the conductive rod, the conductive beam and the contact surface of the conductive rod should be cleaned with sand, and the conductive rod must be closely matched with the conductive beam to ensure sufficient and good conductivity.

2. Pay attention to the distance between the workpiece and the workpiece when feeding. Generally, the pigment distance is equal to the horizontal width of the workpiece. If the distance is too small, shielding will occur during the anodizing and coloring process, resulting in uneven film thickness and color inconsistency.

3. Pay attention to the inclination of the workpiece when loading, and it is not allowed to feed horizontally. Generally, the main rod is 10-15cm higher than the auxiliary rod, and the supporting workpiece is 2-3cm lower than the liquid level.

4. Degreasing (also known as degreasing): It is to remove the oil and dust left by the previous production process. Generally, we use sulfuric acid. There are also degreasing agents, alkaline ones. The general concentration is controlled at 150—160g/L. Controlled at room temperature.

5. After degreasing, two water washes are required. Process requirements; pH is greater than 4, water overflows, and room temperature control.

6. Alkaline washing: Alkaline corrosion is the most basic and important pretreatment method in the anodizing of aluminum plates and aluminum profiles for construction. Alkali corrosion results in a matt satin-finished corroded surface, which is the result of a high proportion of diffuse reflection. Electrochemical characteristics of intermetallic compound particles in aluminum alloy during alkali corrosion. Different intermetallic compounds, even the same compound with different particle sizes, will have completely different electrochemical characteristics. There are two mechanisms for the formation of surface corrosion pits due to the presence of the second phase of intermetallic compounds in aluminum alloys: the second phase acts as a cathode (such as Al3Fe) so that the aluminum matrix around the particles is preferentially dissolved; the second phase intermetallic compound particles are relative to aluminum The matrix is an anode (such as Mg2Si) and preferentially dissolves. The presence of the second-phase intermetallic compound in the aluminum alloy, regardless of whether it is a cathode or an anode relative to the aluminum matrix, will form corrosion pits on the surface during alkali corrosion, resulting in a dull diffuse scattering surface. The former forms corrosion pits due to the local dissolution of aluminum around the second phase; the latter forms corrosion pits due to the direct dissolution of intermediate compounds. Purpose; remove the natural oxide film on the surface of the workpiece, trim minor scratches, bruises and extrusion marks on the workpiece, and make the surface of the workpiece smooth. Process requirements; temperature 50-60 degrees Celsius, time; color material, 1-3 minutes, White material 3-5 minutes, corrosion sand surface, 10-15 minutes (according to the size of the sand surface). The bath concentration is adjusted according to the aluminum ion concentration to adjust the free alkali concentration. The workpiece out of the alkali washing tank must be completed within one minute. If the transfer time is too long, it will cause hanging marks on the surface of the workpiece.

7. After alkaline washing, after two washings, process requirements, water overflow, normal temperature control, the temperature of the washing tank is better controlled at 30 degrees Celsius,

8. Neutralization tank: also known as (light emitting tank), purpose; to remove the black ash left on the surface of the workpiece after alkali etching, because the alloy contains elements such as iron and copper, black ash will be produced after alkali etching. Process requirements; room temperature, time, 1-3 minutes. Concentration; H2SO4; 180-220g/L

9. Water washing: control at room temperature, drainage by overflow.

10. Oxidation tank: The purpose is to form a protective and decorative film on the surface of the workpiece. The oxide film is a dense and hollow type, with 76 billion holes per square centimeter. Process requirements: 202℃, current, 1.5A/dm2, time; according to the film later, film thickness = time current density coefficient. Oxidation tank should strictly control the temperature of the bath liquid. The film formed by excessively high temperature will become loose and cause hoarfrost. If the temperature is too low, the film formation speed is very slow, but the film is very dense. Therefore, the temperature of the bath liquid must be strictly controlled. Coloring has a key influence.

11. After oxidation, it is washed with water for two times, controlled at room temperature, PH>4,

12. Coloring tank; this tank can be made of champagne and imitated steel. Bronze, pearl black and other colors, the main ingredient is stannous sulfate. Nickel sulfate, sulfuric acid. additive. The daily management method is to add a small amount according to the production consumption. Note that the valve cannot be opened too much when adding water to prevent the hydrolysis of divalent tin. The tank level should be lower than the oxidation level. The voltage is generally controlled at 14-16 volts. Temperature, imitation steel; 16℃; incense, bronze, pearl black; 18-25℃ champagne. Among electrolytic colored aluminum profiles, light colors such as champagne account for about half. In the early days, tin salt or tin-nickel salt was used. In the past two years, single nickel salt has gradually gained the upper hand due to its stable color tone. Champagne has a large difference in color depth and tone, which is not only caused by the difference between nickel salt and tin salt, but also related to many factors such as alloy state, power supply equipment, process parameters, bath composition and factory water quality. Therefore, it is difficult for different manufacturers to produce exactly the same champagne color and imitation stainless steel color. The key points of the process for producing champagne products are as follows:

1) Reasonable binding material, generally the groove is upward, and the decorative surface is upward or facing the electrode. Insufficient inclination of the binding material will cause color difference between the upper and lower surfaces and air bubbles on the lower surface. Insufficient electrode spacing will cause chromatic aberration in the vertical plane. Poor electrical contact when binding the material will cause the color difference between the upper profile of a hanging material and the fading of the binding material.

2) The counter electrode in the coloring tank beyond the hanging area should be removed or covered with a plastic plate to prevent the surrounding color from deepening.

3) When the material is hung into the coloring tank, first do not power on and soak for 1~2min, which is conducive to the deposition of colored particles into the bottom of the hole, so that the color is uniform and not easy to fade.

4) When using light colors such as champagne, the voltage rise speed should be fast (5~10s). Since the total coloring time of light colors is short, the low voltage time of the voltage rise section is too long, which will cause the color of the profile groove to be too light.

5) The light-colored voltage should not be lower than the bronze-colored voltage. The coloring voltage is low, the color dispersion is poor, and the color is easily faded.

6) The concentration of free sulfuric acid should be higher to improve the conductivity of the bath solution and make the color dispersion better. Although the degree of sulfuric acid is high when it is light-colored, it will not corrode the surface of the oxide film due to the short time.

7) After the coloring is finished, the power should be washed quickly, the transfer speed is slow, and the dark or light color band of the profile will appear.

8) Among the additives colored by tin salt, the complexing agent is different, and the background color of the color is different.

9) Pay attention to the influence of cold sealing on the light color.

13. Washing tank: normal temperature control, overflow drainage.

14. Electricity gold slot:

The main components of the first process are selenium dioxide, copper sulfate, sulfuric acid, and pure water. The temperature is controlled at 25 ~ 28 ℃ during production, and the cover is usually protected. Golden yellow (with potassium manganate as the main salt). It is easier to control the production of manganate with golden yellow color. If electrophoresis is subsequently carried out, the production will be more difficult.

The key points of the second process using manganate coloring production process are as follows:

1) Potassium manganate is a strong oxidant, and the bath solution itself consumes quickly. The free sulfuric acid concentration in the tank solution is high, the golden yellow is brighter, but the consumption is also greater.

2) The golden yellow quality of potassium manganate has a great relationship with the process control of anodizing and sealing. The film thickness should be 12~15μm. The film is too thin and the color is not bright red. The sealing quality is not good and the color will fade after light.

3) The ratio of colored electrode area should be greater than 1:1. Loose plates, breakages and small electrode ratios will affect the dark color or cause "small white spots" on the surface and "white circles" in the middle of the hanging material.

4) When producing ordinary golden yellow, the pore expansion method of oxide film can be used to deepen the color, such as soaking in oxidation tank for 2~4min, soaking in acidic water tank for 10~20min, or soaking in coloring tank for 3~5min. However, the sealing quality must be strictly guaranteed at this time.

5) When producing electrophoretic golden yellow, the hole expansion method cannot be used to deepen the color. On the contrary, it is necessary to appropriately reduce the temperature of the oxidation tank and the landing tank to speed up the transfer speed, and even appropriately reduce the curing temperature of the electrophoretic layer to avoid defects such as small white spots in coloration, curing and fading.

15. Washing: Controlled at room temperature, drained by overflow.

16. Titanium tank: Main ingredients: potassium permanganate and sulfuric acid. "Titanium Gold" color (with selenate as the main salt). This is also a kind of golden yellow, but it is quite different from the golden yellow of manganate, and is closer to the golden hue, so it is also called "K gold" color. Our country is generally called "titanium gold" color, in fact, it has nothing to do with titanium, and it is also called rose gold abroad. Electrophoretic titanium gold also has the problem of fading during curing, so the temperature difference of the curing furnace is required to be within ±5℃, and the temperature should be appropriately lowered.

17. Water washing: control at room temperature, drainage by overflow.

18. Sealing tank: main ingredients: nickel acetate, this tank controls PH, nickel ion, temperature; 25~28℃; time: 1um/1.5 minutes.

19. Water washing: control at room temperature, drainage by overflow.

20. Common water washing: control at room temperature, drain by overflow, and clean up regularly.

21. Transfer tank: drain water in overflow mode and clean it regularly.

22. Pure water washing: control PH and conductivity.

23. Hot water washing: temperature 60~70℃, time 3~5 minutes. The main function of hot pure water washing is to expand the anodic oxide film of aluminum alloy material to facilitate thorough cleaning of the workpiece, avoid contamination of the electrophoresis bath by impurity ions, especially sulfate ions, in the pretreatment process, and at the same time, it has a certain sealing effect on the anodic oxide film to improve The corrosion resistance of the workpiece, the main parameters that need to be controlled in this process are temperature, conductivity, PH value, and mechanical impurities

24. Pure water washing: The purpose of pure water washing is to continue to clean the workpiece, prevent impurities from entering the electrophoresis tank, and at the same time, return the temperature of the workpiece to room temperature, and prevent the workpiece from entering the electrophoresis tank at a high temperature to accelerate the electrophoresis bath liquid. The main control parameters of this process are Electrical conductivity, PH value of mechanical impurities.

25. Electrophoresis tank: anodic electrophoretic coating is a new type of coating that was put into industrial application in the 1960s. The film-forming substance is an anionic polymer with carboxyl groups, which is neutralized with alkaline substances to give it ionic and water-soluble properties. Dispersed in water, the commonly used neutralizers are mainly organic amines and inorganic bases. The formation of the electrophoretic coating film goes through the four processes of electrophoresis, electrolysis, electrodeposition, and electroosmosis, forming a dense coating film with extremely low moisture content and high insulation on the surface of the aluminum alloy. The characteristics of anodic electrophoretic coatings are: uniform coating film, smooth, flat, strong adhesion, uniform coating film can be formed in irregular places such as edge weld grooves, and good anti-corrosion performance.

The quality of electrophoretic coatings mainly depends on the stability of the coating, the penetration of the coating and the performance of the Coulomb efficiency coating film.　　 Coulombic efficiency: refers to the coating film equivalent of electrophoretic coatings that can be deposited and deposited during electrodeposition of a unit of electricity consumption. It represents the current efficiency or energy consumption level of coatings during deposition. Generally, the coulombic efficiency of anodic electrophoretic coatings is 15-20 mg/lun. Electrophoretic coatings: according to the different types of resins, it can be divided into stages of maleated oil and maleated oil-induced anodic electrodeposition coatings, alkyd acid anodic electrodeposition coatings, epoxy ester anodic electrodeposition coatings, pure phenolic resin anodic electrodeposition coatings, polybutylene Acrylic anode electrophoretic paint and acrylic anode paint, etc. Acrylic anodic electrophoretic coating: The base material of this type of coating is copolymerized with methacrylate, carboxyl-containing acrylate and acrylic monomer to form a resin with hydroxyl and hydroxyl groups with large relative molecular mass and narrow relative molecular mass distribution. Low amines are neutralized and water soluble. Use water-soluble suitable amino resin as crosslinking agent. During electrophoretic coating, it can deposit and precipitate in proportion. During drying and curing, the hydroxyl group of acrylic acid crosslinks with amino resin to form an electrophoretic coating film with excellent weather resistance and corrosion resistance, which is used for decoration and protection of the surface of aluminum alloy materials. Then the surface of the painted workpiece must be pretreated before electrophoretic coating to remove oil on the surface of the workpiece and form a dense conversion film to improve the corrosion resistance and bonding force of the coating, and then electrophoretic coating is carried out to deposit a layer on the surface of the workpiece. Defect-free electrophoretic coating with uniform layer can complete the electrophoretic coating process after baking. The main factors affecting electrophoretic coating under the same pretreatment process conditions, the key to determining the quality of electrophoretic coating is to control the parameters of the electrophoresis bath, which mainly include solid content, pH value, electrophoresis temperature, conductivity, electrophoresis voltage, and electrophoresis time , The ratio between the distance between the electrodes and the area of the anode and cathode, etc. The solid composition of the bath liquid is one of the most important process parameters in electrophoretic coating. It is closely related to the quality of the electrophoretic coating. Generally, if a low solid composition electrophoretic liquid is used, the loss of the bath liquid brought out by the quilt coating workpiece is small, and the electroosmosis High performance, low water consumption for washing, but too low solid content will cause the coating to become thinner and easy to produce pinholes, while too high solid content will cause the coating to be rough, orange peel and other defects, so the electrophoresis bath The solid content should be kept within an appropriate range. PH value: The pH value of the electrophoresis bath is an essential parameter to ensure the water solubility of the electrophoresis resin to obtain a high-quality electrophoretic coating. If the pH value is too low, the depth of the electrophoresis resin will be poor and the electrophoresis solution will become turbid or even use the right resin. Precipitated from the electrophoresis solution, and cannot be electrophoresed or the coating becomes rough. Basic knowledge of electrophoresis coating

The main process parameters that affect electrophoretic coating

1. Voltage

 Electrophoretic coating adopts constant voltage method, the equipment is relatively simple and easy to control. The voltage has a great influence on the paint film; the higher the voltage, the thicker the electrophoretic paint film, which can improve the coating ability and shorten the construction time for the parts that are difficult to paint. However, if the voltage is too high, the surface of the paint film will be rough, and the phenomenon of "orange peel" will easily occur after drying. The voltage is too low, the electrolysis reaction is slow, the paint film is thin and uniform, and the swimming penetration is poor. The choice of voltage is determined by the type of paint and construction requirements. In general, the voltage is inversely proportional to the solid content of the paint and the paint temperature, and directly proportional to the distance between the two poles. The surface of steel is 40～70V, the surface of aluminum and aluminum alloy can be 60～100V, and the surface of galvanized parts is 70～85V.

2. Electrophoresis time

 The thickness of the paint film increases with the prolongation of the electrophoresis time, but when the paint film reaches a certain thickness, continuing to extend the time will not increase the thickness, but will aggravate the side reaction; on the contrary, if the electrophoresis time is too short, the coating is too thin. The electrophoresis time should be based on the voltage used, and under the condition of ensuring the quality of the coating, the shorter the better. Generally, the electrophoresis time for workpieces is 1 to 3 minutes, and for large workpieces, it is 3 to 4 minutes. If the surface geometry of the coated object is complicated, the voltage and time can be increased appropriately.

3. Paint temperature

 The coating temperature is high, the film formation rate is fast, but the appearance of the paint film is rough, which will also cause the coating to deteriorate; the temperature is low, the amount of electrodeposition is small, the film formation is slow, and the coating film is thin and dense. During the construction process, as part of the electrical energy is converted into heat energy during electrodeposition, the mechanical friction in the circulation system generates heat, which will cause the temperature of the paint to rise. Generally, the temperature of the paint solution is controlled at 15-30°C in some aspects.

4. The solid content and pigment-to-base ratio of the coating

 The solid content of commercially available electrophoretic coatings is generally about 50%. During construction, distilled water is required to control the solid content of the coating at 10% to 15%. The solid content is too low, the hiding power of the paint film is not good, the pigment is easy to precipitate, and the stability of the paint is poor. If the solid content is too high, the viscosity will increase, which will cause the paint film to be rough and loose, and the adhesion will be poor. Generally, the ratio of pigment to base is about 1:2, and the ratio of pigment to base of high-gloss electrophoretic coatings can be controlled at 1:4. As the amount of paint in the paint will gradually decrease in actual operation, it is necessary to add paint with high pigment content at any time to adjust.

5. PH value of paint

 The pH value of the electrophoretic coating directly affects the stability of the bath. If the pH value is too high, the newly deposited film will dissolve again, the paint film will become thinner, and the film will peel off after washing after electrophoresis. If the PH value is too low, the surface gloss of the workpiece is inconsistent, the stability of the paint solution is not good, the dissolved resin will be precipitated, the surface of the paint film is rough, and the adhesion is reduced. Generally, the PH value should be controlled between 7.5 and 8.5 during the construction process. In the construction project, due to continuous electrophoresis, cationic ammonium compounds accumulate in the paint, leading to an increase in the PH value. Can be used to supplement Add low-pH stock solution, replace the cathode cover with distilled water, use ion exchange resin to remove ammonium ions, and use anode cover to lower the pH. If the PH value is too low, add ethanol ammonium to adjust.

6. Paint resistance

  The impurity ions brought by the coated object into the electrophoresis tank from the previous process cause a decrease in the resistance value of the paint, which leads to the appearance of roughness and unevenness of the paint film and pinholes. In the painting construction, the paint needs to be purified. In order to obtain high-quality coating film, cathode mask equipment can be used to remove ammonium, calcium, magnesium and other impurity positive ions.

7. The distance between the workpiece and the cathode

The distance is close and the deposition efficiency is high. However, if the distance is too close, the paint film will be too thick, resulting in sagging, orange peel and other ills. Generally, the distance is not less than 20cm. For large-scale and complex-shaped workpieces, when a very thick coating film has been deposited on the outside and the internal coating film is still thin, an auxiliary cathode should be added at a location far away from the cathode.