Aluminum Anodizing and Surface Treatment Technology

Aluminum is popular material used for lots of daily products production such as 3C items, electric components, cookware and architecture parts etc. Thus the aluminum product surface treatment is very important technique. In this article we’ll briefly introduce the popular solution of aluminum anodizing and electrophoresis coating.

I – aluminum and alloy aluminum characteristics

1. Low density
The density of aluminum is about 2.7g/cm3, the second light metal which is only higher than magnesium in metal structure material, only 1/3 of iron or copper.

2. High ductility
Aluminum and its alloys have good ductility and can be made into various types, plates, foils, tubes and wires by pressure processing such as extrusion, rolling or drawing.

3. Easy to strengthen
Pure aluminum is not strong, but it can be easily strengthened by alloying and heat treatment to produce high-strength aluminum alloy, which is comparable in strength to alloy steel.

4. Good conduction
The aluminum electrical and thermal conductivity is only after silver, gold and copper. If copper have a relative conductivity of 100, then aluminum is 64 and iron is only 16. If calculated according to the conductivity of equal mass metals, aluminum is almost double that of copper.

5. Corrosion resistance
Aluminum and oxygen have a very high affinity. Under natural conditions, aluminum surface will form protective oxides, which have much better corrosion resistance than steel.

6. Easy to recycle
The melting temperature of aluminum is low, about 660 ° C, the waste is easy to regenerate and recovery rate is extremely high, and the energy consumption for recovery is only 3% of smelting.

7. Can be welded
The aluminum alloy can be welded by inert gas protection method. After welding, the mechanical properties are good, the corrosion resistance is good, the appearance is beautiful, be able to meet structural material requirements.

8. Easy surface treatment
Aluminum can be anodized coloring, with high hardness after treatment, good wear resistance, corrosion resistance and electrical insulation. It can also be used for electroplating, electrophoresis, spraying, etc. to further improve the decorative and protective properties.

II – surface mechanical pretreatment of aluminum

1. The purpose of mechanical pretreatment

• Provide good appearance conditions and improve the quality of surface finishes;
• Improve product quality;
• Reduce the impact of welding;
• Produce a decorative effect;
• Get a clean surface.

2. Common methods of mechanical pretreatment
Commonly used mechanical pretreatment methods include polishing, sand blasting, brushing, and rolling. The specific pretreatment to be used depends on the type of product, the production method, the initial state of the surface, and the final finishing level.

3. Principle and purpose of mechanical polishing
The high-speed rotating polishing wheel rubs against the workpiece to generate high temperature, which is plastic deformation of the metal surface, thereby flattening the convex and concave points on metal surface, at the same time, the extremely thin oxide film on metal surface generated instantaneously under the oxidation of the surrounding atmosphere is repeatedly ground. Thus become more and more bright. The main purpose is to remove burrs, scratches, corrosion spots, blisters, pores, etc. on the workpiece surface.

The slight unevenness on workpiece surface is further removed, so that it has a higher gloss until the mirror effect.

4. Principle and purpose of sand blasting
Dry sand or other abrasive particles are sprayed onto the aluminum product surface with purified compressed air to remove surface defects and present a uniform matte surface. The main purpose is to remove burrs on surface, casting slag and other defects and dirt; improve its mechanical properties and achieve a uniform surface matting effect.

5. The principle and purpose of brushing
The brushing is to remove burrs and dirt on the surface by rotation of brush wheel. The main purpose is wire brushing for decoration.

6. The principle and function of rolling
Rolling is to place the workpiece into a drum containing abrasive and chemical solution. By rotating the drum, the workpiece and the abrasive are rubbed together to achieve the polishing effect.

III – the chemical pretreatment of aluminum

1. Definition and purpose of chemical pretreatment
The process of pretreating on aluminum surface with a chemical solution or a solvent can effectively remove oil stains, contaminants, and natural oxide films on the surface, so can obtain a clean and even surface.

2. Chemical pretreatment common process
Commonly used chemical pretreatment methods include degreasing, caustic washing, ash removal, fluoride sand surface treatment, and water washing. Depending on the use of the aluminum to be treated, different chemical pretreatment processes can be used for surface quality requirements.

3. Principle and function of degreasing
The oil will be hydrolyzed in the acidic degreasing solution to produce glycerin and corresponding higher fatty acids. With the aid of a small amount of wetting agent and emulsifier, the oil is more soluble and improves the degreasing effect. Degreasing treatment can remove the grease and dust on the aluminum surface, so that the subsequent alkali washing is relatively uniform.

4. Principle and function of alkaline washing
The aluminum material is placed in a strong alkaline solution containing sodium hydroxide as a main component to carry out an etching reaction to further remove surface contamination, completely remove the natural oxide film on the aluminum surface, and reveal a pure metal substrate for subsequent anoddize oxidation treatment.

5. Principle and function of ash removal
After alkali washing, product surface tends to adhere to a metal compound which is insoluble in the alkali washing bath and its alkali washing product. They are a layer of taupe or grayish black hanging ash. The purpose of ash removal is to remove this layer of ash that is insoluble in lye to prevent contamination of the bath in the subsequent anodization process.

6. Principle and function of fluoride sand surface treatment
Fluoride sand surface treatment is an acid etching process that uses fluoride ions to produce highly uniform, high-density pitting corrosion on the surface of aluminum. The purpose is to eliminate the extrusion marks on the surface of the product and produce a flat surface. However, due to serious environmental pollution problems in the fluoride sand surface treatment process, it has not been promoted.

IV – aluminum (electric) chemical polishing and chemical conversion

1. The use of chemical polishing or electrochemical polishing
Chemical polishing is a high-grade finishing treatment method, which can remove slight mold marks and scratches on the surface of aluminum products, remove frictional stripes, heat deformation layers, oxide films, etc. which may be formed in mechanical polishing, so that the rough surface tends to be smooth. Achieve a mirror-like surface that enhances the decorative effect.

2. The principle of chemical polishing
The chemical polishing is achieved by controlling the selective dissolution of the surface of the aluminum material, so that the microscopic convex portion of the aluminum surface is preferentially dissolved compared with the concave portion of the aluminum material, thereby achieving the purpose of smoothing the surface. The principle of electrification polishing is tip discharge, and other chemical polishing is similar.

3. The use of chemical conversion
Chemical conversion is mainly used to protect aluminum and its alloys from corrosion. It can be directly used as a coating or as a bottom layer of organic polymer, which not only solves the adhesion of coating to aluminum, but also improves the corrosion resistance of organic polymer coating.

4. Principles of chemical conversion
In the chemical treatment solution, the surface of the metal aluminum reacts with the chemical oxidant in the solution to form a chemical conversion film. The common chemical conversion is divided into chemical oxidation treatment, chromate treatment, phosphoric chromate treatment and chromium-free chemical conversion.

5. Introduction of chemical conversion
Aluminum can obtain a dense protective chemical oxide film in boiling water. This method is called chemical oxidation treatment, but it is not mass-produced due to film formation speed and performance. The chromate formed by chromate treatment is currently corrosion resistant. Excellent aluminum chemical conversion film, which is not only used for spraying the bottom layer but also directly used as the final coating of aluminum alloy, but its disadvantage is serious environmental pollution.

Phosphochromate treatment can meet the bottom layer of spray coating and trivalent chromium is non-toxic, currently used more in 3C products; chromium-free chemical conversion currently industrial production mainly uses fluorine complexes containing titanium or (and) zirconium The chromium-free treatment of the material requires no strict chemical pretreatment, and the non-chromium film is colorless and transparent. We cannot determine the actual chemical conversion by observation.

The effect is therefore more dependent on the strict control of reliable processes. In summary, the chemical conversion commonly used in 3C products is phosphorochromate treatment.

V – Anodizing of aluminum alloy

1. Definition of anodizing
Anodizing is an electrolytic oxidation in which the surface of an aluminum alloy is usually converted into an oxide film that is protective, decorative, and other functionas.

2. Classification of anodized film
The oxide film is divided into two categories: a barrier-type oxide film and a porous oxide film. The barrier-type oxide film is a dense non-porous thin oxide film which is close to the metal surface, and the thickness is generally not more than 0.1 μm depending on the applied voltage. The porous oxide film is composed of a barrier film and a porous layer, and the thickness of the barrier layer is related to the applied voltage, and the thickness of the porous layer depends on the amount of electricity passed. Porous oxide film is most common used.

3. Characteristics of anodized film

• a. The structure of the oxide film is a porous honeycomb structure, and the porosity of the film has a good adsorption capacity, and the bottom layer which can be used as a coating layer can also be dyed to improve the decorative effect of the metal.
• b. The hardness of the oxide film is high, the hardness of the anodized film is very high, and its hardness is about 196-490 HV, because the high hardness determines the wear resistance of the oxide film is very good.
• c. Corrosion resistance of oxide film, aluminum oxide film is stable in air and soil, and has strong binding force with matrix. Under normal circumstances, it will be dyed and sealed or sprayed after oxidation, which further enhances corrosion resistance. .
• d. The bonding force of the oxide film, the bonding strength of the oxide oxide film to the base metal is very strong, and it is difficult to separate them by mechanical means. Even if the film layer is bent with the metal, the film layer remains well bonded to the base metal, but is oxidized. The film has small plasticity and large brittleness. When the film layer is subjected to large impact load and bending deformation, cracks may occur, so the oxide film is not easy to be mechanically used, can be used as the bottom layer of the painting.
• e. Insulation of oxide film, high impedance of aluminum anodized film, low thermal conductivity, thermal stability up to 1500 degrees, thermal conductivity 0.419W/(m•K)—1.26W/(m• K). It can be used as an insulating layer of a dielectric layer or an electrical product of an electrolytic capacitor.

VI – aluminum alloy oxide film formation process

1. The first stage of anodizing
During the formation phase of the non-porous layer, the ab segment, the voltage at the start of the energization start (few to tens of seconds) increases sharply, reaches the critical voltage, and the maximum value of the voltage indicates that a continuous, non-porous film is formed on the surface of the anode. Floor. The non-porous layer has a large resistance, which hinders the continued thickening of the film. The thickness of the non-porous layer is proportional to the formation voltage, and the dissolution rate of the oxide film in the electrolyte is inversely proportional. The thickness is about 0.01 to 0.1 μm.

2. The second stage of anodizing
In the formation stage of the porous layer, in the bc section, the hole is first dissolved in the thinnest part of the film, and the electrolyte can pass through the holes to reach the fresh surface of the aluminum, and the electrochemical reaction is continued, the electric resistance is reduced, and the voltage is varied. The decrease (10-15% of the highest value) occurs and a porous layer appears on the film.

3. The third stage of anodizing
The porous layer is thickened, in the cd section, when the voltage rises smoothly and slowly, when the non-porous layer is continuously dissolved into a porous layer, the new non-porous layer is growing, so that the porous layer is continuously thickened, when the formation speed When the dissolution rate reaches a dynamic equilibrium, the thickness of the film does not increase, and the reaction should stop.

VII – aluminum alloy anodizing process

1. Common processes of anodizing
Common processes for anodizing aluminum alloys include: sulfuric acid anodizing process, chromic acid anodizing process, oxalic acid anodizing process and phosphoric acid anodizing process. The most common is anodizing sulfuric acid.

2. Sulfuric acid anodizing
At present, the widely used anodizing solution is sulfuric acid anodizing. Compared with other methods, it has great advantages in production cost, oxide film characteristics and performance. It has low cost, good transparency of film, corrosion resistance and friction resistance. Good character, easy coloring, etc. It uses a dilute sulfuric acid as an electrolyte to anodize the product, and the thickness of the film can reach 5um-20um.

The adsorption is good, colorless and transparent, the process is simple, and operation is convenient.

3. Chromic acid anodizing
The film obtained by chromic acid anodizing is thinner, only 2-5um, which can maintain the original precision and surface roughness of the workpiece; the porosity is low and difficult to dye, and it can be used without sealing; the film is soft and the wear resistance is poor. However, the elasticity is good; the corrosion resistance is strong, the solubility of chromium to aluminum is small, and the residual liquid in the pinhole and the gap is less corroded to the components, and is suitable for structural parts such as castings, and the process is popularly used in military application. At the same time, the quality of the part can be inspected, and the brown electrolyte will flow out at the crack, which is obvious.

4. Oxalic acid anodizing
Oxalic acid has low solubility in the oxide film of aluminum, so the porosity of the oxide film is low, and the wear resistance and electrical insulation of the film are better than that of the sulfate film; however, the oxidation cost of oxalic acid is 3 to 5 times higher than that of sulfuric acid; Oxalic acid can be reacted in both cathode and anode, resulting in poor electrolyte stability; the color of the oxalate oxide film is easy to change with the process conditions, resulting in color difference, so the application of the process is limited. However, oxalic acid can be used as a sulfuric acid oxidation additive.

5. Phosphoric anodizing
The oxide film is dissolved in the phosphoric acid electrolyte larger than sulfuric acid, so the oxide film is thin (only 3μm) and the pore diameter is large. Because the phosphoric acid film has strong water resistance, it can prevent the adhesive from aging due to hydration, so that the bonding strength of the adhesive is better, so it is mainly used for the surface treatment of printed metal plates and the pretreatment of aluminum workpiece bonding.

VIII – aluminum alloy hard anodizing

1. Characteristics of hard oxide film
Hard anodizing of aluminum alloy has the following characteristics compared with ordinary oxide film: oxide film is relatively thick (generally thickness is not less than 25um), hardness is relatively high (more than 350HV), wear resistance is good, void ratio is low, and breakdown resistance is resistant. The voltage is higher and the surface flatness may appear a little worse.

2. Process characteristics of hard anodizing
There is no essential difference in the principles, equipment, processes, and testing of hard anodization and ordinary oxidation. Hard oxidation seeks to reduce the solubility of the oxide film. The main features are:

• a. The bath temperature is low (common solution about 20 degrees or so, hard solution below 5 degrees), and the oxide film formed by low temperature is generally high in hardness.
• b. Low bath concentration (common sulfuric acid concentration 20%, hard way 15% or less), low concentration and low membrane solubility
• c. Organic acid is added to the bath, and oxalic acid or tartaric acid is added to the sulfuric acid.
• d. The applied voltage and current are high (normal current 1.5A/dm2, voltage below 18V, hard current 2~5A/dm2, voltage above 25V. Up to 100V)
• e. The applied voltage should adopt the method of gradually increasing the voltage. Because of its high voltage and high current, the processing time is long and the energy consumption is large. At the same time, hard oxidation often uses pulse power or special waveform power.

3. Casting aluminum alloy hard anodizing
Cast aluminum alloy usually needs hard anodizing to improve its performance. Cast aluminum alloy is commonly used aluminum/silicon alloy and aluminum/copper alloy. Aluminum silicon has good casting performance and wear resistance and is used in large quantities. Parts and components, sometimes added with copper and magnesium to improve mechanical properties and heat resistance. Aluminum-copper system is also a commonly used casting alloy, mainly used to withstand large movements or sand castings with uncomplicated loads and shapes. Casting aluminum alloys need to improve the electrolyte and power supply waveforms due to the inclusion of non-metal elements. The electrolyte can generally be added with certain metal salts or organic acids, sulfuric acid-oxalic acid-tartaric acid solution, sulfuric acid-dry oil solution in the sulfuric acid; Generally changed to AC and DC superposition, asymmetrical current, pulse current, etc., wherein the pulse effect is better.

Electrolytic castings should be protected from concentrating guides and burrs before oxidation to prevent current concentration.

IX – aluminum alloy micro-arc oxidation (MAO)

1. Principle of micro-arc oxidation technology:
Micro-arc oxidation, also known as micro-plasma surface ceramization technology, refers to the use of arc discharge to enhance and activate the reaction occurring on the anode on the basis of ordinary anodization, thereby using aluminum, titanium, magnesium metal and its alloys as materials. The method of forming a high-quality reinforced ceramic film on the surface of the workpiece is to apply a voltage to the workpiece by using a dedicated micro-arc oxidation power source to make the surface of the workpiece metal interacts with the electrolyte solution to form a micro-arc discharge on the surface of the workpiece. Under the action of high temperature and electric field, the metal surface forms a ceramic film to achieve the surface strengthening of the workpiece.

2. Characteristics of micro-arc oxidation

• a. greatly improve the surface hardness of the material (HV> 1200), exceeding the hardness of high carbon steel, high alloy steel and high speed tool steel after heat treatment;
• b. Good wear resistance;
• c. Good heat resistance and corrosion resistance (CASS salt spray test > 480h), which fundamentally overcomes the shortcomings of aluminum, magnesium and titanium alloy materials in application, so the technology has broad application prospects;
• d. Has good insulation performance, insulation resistance up to 100MΩ.
• e. The process is stable and reliable, the equipment is simple. The reaction is carried out at room temperature, and the operation is convenient and easy to grasp.
• f. The matrix grows in-situ ceramic film, the bonding is firm, and the ceramic film is dense and uniform.

3. Application of micro-arc oxidation
Micro-arc oxidation is a new surface treatment technology for aluminum alloys. It combines the ceramic properties of alumina with the metallic properties of aluminum alloys to provide better physicochemical properties on the surface of aluminum alloys. However, due to technical and economic reasons, it is currently not widely used in China. However, due to the special properties of oxide film, it can be applied in many fields, including aero-engine engines, petrochemical industry, textile industry and electronics industry.

4. Insufficient micro-arc oxidation
Micro-arc oxidation can cause spark discharge and spark corrosion, which makes the surface of the product rough, and the rough layer is worn away during use, resulting in waste. The higher energy consumption is five times that of ordinary oxidation.

X – Electrolytic coloring of aluminum oxide film

1. Common coloring process of aluminum alloy oxide film:

The commonly used coloring processes for aluminum alloys can be broadly classified into three categories:

• a. The overall coloring method includes two kinds of natural coloring and electrolytic coloring. Natural coloring means that the anodizing process oxidizes the added components (Si, Fe, Mn, etc.) in the aluminum alloy, and the coloring of the oxide film occurs. Electrolytic coloring refers to coloring of an oxide film caused by changes in electrolyte composition and electrolysis conditions.
• b. Dyeing method: an oxide film dyed with an inorganic pigment or an organic dye based on a primary oxide film.
• c. Electrolytic coloring method: based on a primary oxide film, a method of electrolytically coloring with a direct current or alternating current in a solution containing a metal salt, the weathering resistance, light resistance and service life of electrolytic coloring are better than the dyeing method, and the cost thereof is much lower. In the overall coloring method, it is currently widely used in the coloring of architectural aluminum profiles. Industrialized electrolytic coloring baths at home and abroad are basically nickel salts and tin salts.

There are two types of solutions (including tin-nickel mixed salt), and the colors are generally from pale to deep bronze.

2. Principle of electrolytic coloring

The regular and controllable micropores of the porous anodized film deposit very fine metal and/or oxide particles at the bottom of the pore by electrolytic coloring, and different colors can be obtained due to the scattering effect of light. The depth of the color is related to the amount of deposited particles, that is, to the coloring time and applied voltage. In general, electrolytic coloring colors are similar from champagne, light to dark bronze color is until black, and the color tone is not completely the same, which is related to the size distribution of the precipitated particles. At present, electrolytic coloring is only available in bronze, black, gold, and red.

3. Electrolytic coloring application

Sn salt and Sn-Ni mixed salt are the main coloring methods in China and Europe. The salt is SnSO4, which is precipitated by anodizing by Sn2+ electrolysis in the anodized micropores; but the stability of Sn2+ is easily oxidized to have no coloring ability. Sn4+, so the key to tin salt coloring is that the bath composition and tin salt stability are the key to this process. Tin salts are not sensitive to impurities and coloration uniformity is good, there is not much pollution to the water. Ni salt electrolysis coloring is more common in Japan. He is often used in light color (like stainless steel, light champagne). His coloring speed is fast, the bath stability is good, but it is sensitive to impurities. At present, the impurity equipment is mature, but one-time investment is big.

XI – Dyeing of aluminum oxide film

1. Definition of aluminum oxide film dyeing

The dyeing method is to immerse the aluminum alloy immediately after oxidation in a solution containing a dye, and the pores of the oxide film are dyed with various colors by adsorbing the dye. This process is fast in color, bright in color, and easy to operate, but it needs to be sealed after dyeing.

2. Requirements for dyeing on oxide film

• a. The oxide film obtained by aluminum in a sulfuric acid solution is colorless and porous, and is most suitable for dyeing. The oxalic acid oxide film itself can only be dyed dark in yellow, the porosity of the chromic acid film is low, the film itself is gray, and it can only be dyed dark.
• b. The oxide film must have a certain thickness, the minimum requirement is greater than 7um, and the thin oxide film can only be dyed with a very light color.
• c. The oxide film should have certain loose pores and adsorptivity, so the hard oxide film and the conventional oxide film of chromic acid are not suitable for dyeing.
• d. The oxide film should be complete and uniform, and should not have scratches, blisters, pitting corrosion and other defects.
• e. The film layer itself has a suitable color, and there is no difference in metallographic structure, such as grain size or severe segregation, etc.

3. Dyeing mechanism of oxide film

• a. Organic dyeing mechanism: based on the adsorption theory of matter, divided into physical adsorption and chemical adsorption; physical adsorption refers to the adsorption of molecules or ions by electrostatic force; chemical force (covalent bond generated by reaction, hydrogen bonding, chelation) The key adsorption method is called chemical adsorption. Physical adsorption is expected to be low temperature, high temperature is easy to desorb; chemical adsorption is carried out at a certain temperature, generally dyeing two adsorptions are mainly carried out by chemical adsorption, so they are carried out at a medium temperature.
• b. Inorganic dyeing mechanism: usually carried out at room temperature, the workpiece is first immersed in an inorganic salt solution in a certain order, and then immersed in another inorganic salt solution to chemically react these inorganic substances in the pores of the membrane. A water-soluble, colored compound that fills the pores of the oxide film and seals the pores of the membrane (in some cases, the sealing process is omitted). Inorganic dye color range is limited, the color is not bright enough, but the temperature and light fastness are excellent.

4. Fading of unqualified dyed film

After the dyeing, it was found that the poor use of 27% (mass fraction) of nitric acid or 5 ml/l of sulfuric acid was removed at 25 degrees.

XII – aluminum alloy oxide film sealing

1. Definition of sealing of aluminum oxide film

The physical or chemical treatment of the oxide film after the anodization of the aluminum to reduce the porosity and adsorption capacity of the oxide film, so as to seal the dye in the micropores, and at the same time improve the corrosion resistance and wear resistance of the film. In the construction industry, the sealing of oxide film in the world basically adopts three processes of high temperature steam method, cold sealing hole and electrophoretic coating method, but the current medium temperature sealing hole is more and more popular. From the principle of sealing, there are three main categories: hydration reaction, inorganic material filling or organic material filling.

2. Heat sealing process

• a. boiling water sealing: in pure water close to the boiling point (temperature above 95 degrees, deionized water), the amorphous alumina is converted into hydrated alumina by hydration reaction of alumina, because the hydrated alumina is more than the original The volume is 30% larger, and the volume expansion causes the microporous filling of the oxide film to be closed.
• b. High-temperature steam sealing: the principle is the same as boiling water sealing. The advantages are: fast speed, low water quality dependence, less white ash and less risk of fading. The equipment needs to be sealed to ensure the temperature and humidity. The general temperature is 115~120 degrees, the pressure is 0.7~1atm, and the cost is high!

3. Cold sealing hole process

Cold sealing hole is the most common and most basic sealing technology in China. The operating temperature is 20~25, the time and heat sealing hole ratio are shortened by half, and it is sealed by the deposited filler in the micro hole. The most mature The process is a cold sealing process in which nickel fluoride is the main component. After the cold sealing hole is completed, it should be treated by hot water aging (60~80 degrees deionized hot water, 10~15 minutes) to avoid poduct micro crack with high temperature.

4. Medium temperature sealing process

Inorganic salt medium temperature sealing technology has been developed for the defects of heat sealing and cold sealing processes, mainly including chromate sealing, silicate sealing and acetate sealing.

• a. Chromate sealing: can provide good anti-corrosion effect, especially for die-casting aluminum alloy and high copper aluminum alloy (pH 6.32~6.64, about 10min)
• b. Silicate sealing: due to the frequent occurrence of white ash or discoloration after silicate sealing, the current process is not used unless special needs
• c. Nickel acetate sealing: The sealing quality is better, and it is used more in North America. In addition to the organic dyeing of small parts in China, the other is basically not used.

XIII – Electrophoretic coating of aluminum oxide film

1. Definition of electrophoretic coating

Electrolytic coating particles in solution are formed by coating under the action of direct current. The electrophoresis (ED) coating of aluminum is generally performed by anodic electrophoresis. Electrophoresis is a low-pollution, low-energy process. It has the characteristics of flat coating, good water and chemical resistance, and is easy to automate. It is suitable for the coating of complex shapes with edge edges or holes.

2. Electrophoretic coating process principle

Electrophoretic coating is divided into anodic electrophoresis and cathodic electrophoresis. The water-soluble resin coated by anodic electrophoresis is a high-valent acid carboxylate, generally an ammonium carboxylate salt. The electrophoretic coating can be ionized into a doped colloidal particle in an acid or alkaline solution and dispersed in water. Under the action of direct current, the charged resin colloid will adhere to a resin mold on the metal surface. The electrophoretic coating of aluminum alloy oxide film is main component is a water-soluble acrylic polymer compound and is a translucent emulsion liquid. The electrophoretic coating process is an electrochemical process, which mainly includes four processes of electrophoresis, electrodeposition, electroosmosis and electrolysis.

3. Aluminum alloy electrophoresis process

The typical electrophoresis process after aluminum oxidation is: feeding – degreasing – washing – alkali etching – washing (2 times) – removing ash – washing – anodizing – washing (2 times) – electrolysis Coloring – Washing – Hot Pure Washing – High Purity Washing – Draining – Electrophoretic Coating – RO1 Circulating Washing – RO2 Circulating Washing – Draining – Bake Curing – Cooling – Unload.

4. Characteristics of electrophoretic coating

Advantages: high degree of automation of coating process, high paint recovery rate, high coating efficiency, uniform film thickness, can reduce unnecessary waste, easy to control the bathing conditions, easy to control and manage film thickness, uniform swimming permeability, and internal The rust prevention of the board does not cause undesirable phenomena such as leakage coating and flow marks.

Disadvantages: The equipment one-time investment is big. The object to be coated must be electrically exchanged and the color is difficult.