Nowadays, metal technology is more popular. Many machines on the market will have large pieces of metal. Anodizing, one of the metal surface treatment processes, is very commonly used and can be used to prevent corrosion of products or to achieve the dual function of protective decoration.

01

anodic oxidation

Anodic oxidation is known as the "universal" technology for aluminum alloy surface treatment. Anodic oxidation treatment of aluminum and aluminum alloys is a process in which aluminum or aluminum alloy products are placed in an electrolyte solution as an anode, and the aluminum oxide film is formed on the surface by electrolysis. The thickness of anodic oxide film of aluminum and its alloys can reach tens to hundreds of microns, which not only has good mechanical properties and corrosion resistance, wear resistance, weather resistance, but also has strong adsorption performance. Aluminum products after anodic oxidation can be electrolytic coloring, with the growth of color time, the color from light to dark, can produce a variety of colors, using a variety of coloring methods can be beautiful decorative appearance.

02

Process properties

1. Porosity-the oxide film has a porous honeycomb structure, and the porosity of the film layer depends on the type of electrolyte and the process conditions of oxidation. The porous structure of the oxide film can make the film show good adsorption ability to various organic substances, resins, wax, inorganic substances, dyes and paints. It can be used as the bottom layer of the coating layer, and the oxide film can be dyed into various colors to improve the decorative effect of the metal.

2. Wear resistance-aluminum oxide film has high hardness, which can improve the wear resistance of the metal surface. When the film absorbs the lubricant, it can further improve its wear resistance.

3. Corrosion resistance-aluminum oxide film is very stable in the atmosphere, so it has good corrosion resistance. Its corrosion resistance is related to the thickness of the film, composition, porosity, composition of the substrate material and the integrity of the structure. In order to improve the corrosion resistance of the film, the film layer after anodic oxidation is usually sealed or painted.

4. Electrical insulation-anodic oxide film has high insulation resistance and breakdown voltage, can be used as the dielectric layer of electrolytic capacitors or the insulating layer of electrical products.

 

5. Thermal insulation-aluminum oxide film is a good thermal insulation layer, and its stability can reach 1500 ℃. Therefore, parts working at instantaneous high temperature can prevent the melting of aluminum due to the existence of the oxide film.

6. Binding force-the binding force of the anodic oxide film and the base metal is very strong, and it is difficult to separate them by mechanical methods. Even if the film layer bends with the base until it breaks, the film layer and the base metal still maintain a good bond.

03

formation mechanism

The electrolyte used in the anodic oxidation of aluminum and its alloys is generally an acidic solution of medium solubility, and lead, as the cathode, only plays a conductive role. When aluminum and its alloys are anodized

The formation and dissolution of the oxide film at the same time, the initial oxidation, the formation rate of the film is greater than the dissolution rate, the thickness of the film increases; with the increase of the thickness, the resistance also increases, the result of the growth rate of the film slows down, until the film dissolution rate is equal, the thickness of the film is a certain value. In addition, the generation law of oxide film can be illustrated by the voltage time curve of anodic oxidation.

The entire anodizing voltage-time curve is roughly divided into three sections:

First stage A: Non-porous layer formation. In section AB of the curve, the voltage increases sharply from zero to the maximum value in the first few seconds to tens of seconds of power-on, which is called the critical voltage. It is shown that a continuous, non-porous film layer is formed on the anode surface. The appearance of this membrane hinders the further thickening of the membrane layer. The thickness of the non-porous layer is proportional to the formation voltage and inversely proportional to the dissolution rate of the oxide film in the electrolyte.

Second stage B: Porous layer formation. The curve bc section, after the voltage reaches the maximum value, begins to decline, and its decline is 10% ~ 15% of the maximum value. It is shown that the non-porous film begins to be dissolved by the electrolyte and a porous layer appears.

Third stage C: The porous layer is thickened. In the cd section of the curve, after about 20s of oxidation, the voltage begins to enter a steady and slow rising phase. It is shown that the non-porous layer is continuously dissolved to form a porous layer, while a new non-porous layer is growing, that is to say, the porous layer is continuously thickening, and the process of membrane formation and dissolution is carried out at the bottom of each membrane cell. When the formation rate and dissolution rate of the film reach dynamic equilibrium, even if the oxidation time is prolonged, the thickness of the oxide film will not increase again, and the anodic oxidation process should be stopped at this time.

04

Detailed explanation of process

There are many methods of anodic oxidation of aluminum and its alloys. Here, the commonly used sulfuric acid anodic oxidation, chromic acid anodic oxidation and oxalic acid anodic oxidation are mainly introduced. Other anodizing methods for aluminum and its alloys include hard anodizing and porcelain anodizing.

1. Sulfuric acid anodic oxidation

Aluminum and its alloys are anodized in a dilute sulfuric acid electrolyte by direct or alternating current. A colorless transparent oxide film with a thickness of 5 μm to 20 μm and good adsorption properties can be obtained. The process is simple, the solution is stable and the operation is convenient. The following table shows the process specifications for sulfuric acid anodizing.

1) Mass concentration of sulfuric acid

The mass concentration of sulfuric acid is high, the chemical dissolution rate of the film is accelerated, the resulting film is thin and soft, with many voids, strong adsorption and good dyeing performance; reducing the mass concentration of sulfuric acid, the oxide film grows faster, while the porosity is low, the hardness is high, and the wear resistance and light reflection are good.

2) Effect of temperature

The temperature of the electrolyte has a great influence on the quality of the oxide film. When the temperature is between 10 ℃ and 20 ℃, the oxide film is porous, has good adsorption performance, and is elastic, suitable for dyeing, but the hardness of the film is low. If the temperature is higher than 26°C, the oxide film becomes loose and the hardness is low. When the temperature is lower than 10 ℃, the thickness of the oxide film increases, the hardness is high, and the wear resistance is good, but the void ratio is low. Therefore, the temperature of the electrolyte must be strictly controlled during production.

3) Effect of current density

Increasing the current density, the growth rate of the film is accelerated, the oxidation time can be shortened, the amount of chemical dissolution of the film is reduced, the film is hard, and the wear resistance is good. However, if the current density is too high, the dissolution of the film layer will increase due to the influence of Joule heat, resulting in a decrease in the growth rate of the film. The current density is too low and the oxidation time is very long, which makes the film loose and the hardness is reduced.

4) Effect of time

The anodizing time can be determined according to the mass concentration of the electrolyte, the temperature, the current density and the required film thickness. Under the same conditions, with the extension of time, the thickness of the oxide film increases, and the gap increases. But after reaching a certain thickness, the growth rate will slow down, and finally no longer increase.

5) Effect of stirring

Stirring can promote the convection of the solution, so that the temperature is uniform, and the quality of the oxide film will not be reduced due to the local heating of the metal. Mixing equipment has air compressor and water pump.

6) Effect of alloy composition

The composition of aluminum alloy has a very important influence on the quality, thickness and color of the film. In general, other elements in aluminum alloy reduce the quality of the film. For Al-Mg alloys, when the mass fraction of magnesium exceeds 5% and the alloy structure is non-uniform, appropriate heat treatment must be adopted to homogenize the alloy, otherwise the transparency of the oxide film will be affected. For Al-Mg-Si alloys, with the increase of silicon content, the color of the film changes from colorless transparent to gray, purple and finally to black, making it difficult to obtain a uniform film. For Al-Cu-Mg-Mn alloys, copper makes the hardness of the film layer decrease, the porosity increases, the film layer is loose, and the quality decreases. Under the same oxidation conditions, the oxide film obtained on pure aluminum is the thickest, the hardness is the highest, and the corrosion resistance is the best.

2. chromic acid anodic oxidation

The thickness of the oxide film obtained by chromic acid anodic oxidation is 2 μm ~ 5 μm, the porosity is low, the film is soft, and the wear resistance is poor. Because the dissolution of aluminum is less, the parts can still maintain the original accuracy and surface roughness after the formation of oxide film, so the process is suitable for precision parts. The following table is the chromic acid anodizing process specification.

1) Mass concentration of chromic anhydride

Chromic anhydride content is too high or too low, the oxidation ability is reduced, but slightly higher is allowed. An electrolyte with too low a chromic anhydride content is unstable. It will cause the film quality to decrease.

2) Impurities

Chloride ion, sulfate ion and trivalent chromium ion in chromic acid anodic oxidation electrolyte are harmful impurities. Chloride ions will cause the etching of parts; the increase in the number of sulfate ions will make the oxide film from transparent to opaque, and shorten the service life of chromic acid solution; too much trivalent chromium ions will make the oxide film dark and dull.

3) Voltage

Within 15min from the beginning of anodic oxidation, the voltage is gradually increased from 0V to 40V, and each increase is not more than 5V to keep the current within the specified range. When the cell voltage reaches 40V, it is kept until the end of oxidation.

3. Oxalic acid anodic oxidation

Oxalic acid is a weak acid, the corrosion of aluminum and aluminum alloy is small, so the oxidation film obtained by anodic oxidation of oxalic acid has high hardness, thick film, up to 60μm, good corrosion resistance and good electrical insulation properties. With the different alloy elements and content of aluminum, the film can be a variety of bright colors, the following table is the oxalic acid anodic oxidation process specification.

Oxalic acid anodic oxidation electrolyte is very sensitive to chloride ions, the mass concentration of more than 0.04g/L, the film will appear corrosion spots. The mass concentration of trivalent aluminum ions is also not allowed to exceed 3g/L.

05

Coloring and closing

After anodic oxidation treatment, a layer of porous oxide film is formed on the surface of aluminum and its alloys. After coloring and sealing treatment, various colors can be obtained, and the corrosion resistance and wear resistance of the film can be improved.

1. Coloring of oxide film

1) inorganic pigment coloring

The coloring mechanism of inorganic pigment is mainly physical adsorption, that is, the inorganic pigment molecules are adsorbed on the surface of the micropores of the film layer to fill. The coloring color is not bright, the binding force with the matrix is poor, but the sun is better. The following table is the process specification for the coloring of inorganic pigments. As can be seen from the table, the dyes used for coloring inorganic pigments are divided into two types, and the anodized metal is alternately impregnated in the two solutions until the amount of the reaction product (pigment) of the two salts in the oxide film meets the desired color tone.

 

2) organic dye coloring

The coloring mechanism of organic dyes is more complex, and it is generally believed that there are physical adsorption and chemical reactions. Organic dye molecules and aluminum oxide chemical combination of ways: aluminum oxide and dye molecules on the sulfo group to form a covalent bond; aluminum oxide and dye molecules on the phenol group to form a hydrogen bond; aluminum oxide and dye molecules to form a complex. Organic dyes are colored brightly, with a wide range of colors, but with poor light fastness. The following table is the process specification for organic dye coloring. The water for preparing the dyeing solution is preferably distilled water or deionized water instead of tap water, because calcium and magnesium plasma in tap water will complex with dye molecules to form complexes, making the dyeing solution useless.

3) Electrolytic coloring

Electrolytic coloring is the anodic oxidation of aluminum and its alloys into the electrolyte containing metal salts for electrolysis, through the electrochemical reaction, so that into the oxide film micropores in the heavy metal ions reduced to metal atoms, deposited on the pore bottom of the non-porous layer and coloring. The color oxide film obtained by electrolytic coloring process has the advantages of good wear resistance, light resistance, heat resistance, corrosion resistance and stable and lasting color, and has been widely used in architectural decoration aluminum profiles. The following table is the process specification for electrolytic coloring. The higher the voltage used for electrolytic coloring, the longer the time and the darker the color.

2. Closed treatment

Aluminum and its alloys by anodic oxidation, whether or not coloring are required to be closed in a timely manner, the purpose is to fix the dye in the micropores, to prevent exudation, while improving the film's wear resistance, sun resistance, corrosion resistance and insulation. Closed methods include hot water closed method, water vapor closed method, dichromate closed method, hydrolysis closed method and filling closed method.

1) Hot water sealing method

The principle of hot water sealing method is to use the hydration of amorphous Al₂ O₂

Where n is 1 or 3,Al₂ O₂ is hydrated as a hydrated alumina (Al₂ O · H ₂ O); When alumina trihydrate (Al₂ O · 3H O) is generated, its volume increases by almost 100%. As a result of the hydration of the oxide film surface and the pore wall, the volume increases and the membrane pores are closed. The hot water sealing process is hot water temperature 90 ℃ ~ 100 ℃,pH value 6~7.5, time 15min ~ 30min. Closed water must be distilled water or deionized water, not tap water, otherwise it will reduce the transparency and color of the oxide film. The principle of the water vapor closure method is the same as that of the hot water closure method, but the effect is much better, but the cost is higher.

2) Dichromate sealing method

This method is carried out in a highly oxidizing potassium dichromate solution and at a relatively high temperature. When the anodized aluminum part enters the solution, the following chemical reactions occur between the oxide film and the aluminum oxide on the pore wall and the potassium dichromate in the aqueous solution:

The resulting basic aluminum chromate and basic aluminum dichromate precipitates and hot water molecules together with the alumina monohydrate and alumina trihydrate generated from alumina closed the micropores of the oxide film. The formulation and process conditions of the sealing solution are as follows:

The oxide film treated by this method is yellow and has good corrosion resistance. It is suitable for the sealing of aluminum alloy after anodizing for the purpose of protection, and is not suitable for the sealing of colored oxide film for the purpose of decoration.

3) Hydrolysis sealing method

After the extremely dilute solution of nickel salt and cobalt salt is adsorbed by the oxide film, the following hydrolysis reaction occurs:

The resulting nickel hydroxide or cobalt hydroxide is deposited in the micropores of the oxide film to close the pores. Because a small amount of nickel hydroxide and cobalt hydroxide are almost colorless, this method is particularly suitable for the sealing treatment of colored oxide films. The following table is a common hydrolysate salt closure process specification.

4) Filling and closing method

In addition to the above-described sealing method, the anodized film may be sealed with organic substances such as transparent varnish, molten paraffin, various resins and drying oils.