The use of metal materials in various products is increasing as they better reflect product quality and brand value. Among the many metal materials, aluminum is widely used by various manufacturers due to its advantages such as easy processing, good visual effects, and rich surface treatment methods. This article shares relevant knowledge about the anodizing process of aluminum and aluminum alloys for reference.
- Anodizing is a method of forming an oxide film on the surface of a metal in an appropriate electrolyte by using the metal as the anode and applying an external current. By selecting different types and concentrations of electrolytes, and controlling the process conditions during oxidation, anodized films with different properties and thicknesses ranging from tens to hundreds of microns can be obtained. (The thickness of the natural oxide film layer of aluminum is 0.010μm~0.015μm).
- The porous oxide film has a porous honeycomb-like 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 allows the film layer to exhibit good adsorption ability for various organic materials, resins, waxes, inorganic substances, dyes, and paints, and can be used as a bottom layer for coating, or the oxide film can be dyed into various colors to improve the decoration effect of the metal.
The wear-resistant aluminum oxide film has a high hardness, which can improve the wear resistance of the metal surface. After the film layer adsorbs lubricants, its wear resistance can be further improved.
The corrosion-resistant aluminum oxide film is stable in the atmosphere and therefore has good corrosion resistance. Its corrosion resistance is related to the thickness, composition, porosity, matrix material composition, and structural integrity of the film layer. To improve the corrosion resistance of the film, the anodized film layer is usually sealed or painted.
The electrically insulating anodized film has high insulation resistance and breakdown voltage, and can be used as the dielectric layer of electrolytic capacitors or the insulation layer of electrical products.
The insulating aluminum oxide film is a good insulation layer, and its stability can reach 1500℃. Therefore, for parts that work under instant high temperature, the existence of the oxide film can prevent aluminum from melting.
The bonding force of the anodized film with the matrix metal is strong, and it is difficult to separate them by mechanical means. Even if the film layer is bent with the matrix until it breaks, the film layer and the matrix metal still maintain a good bond.
- Formation mechanism:
- The electrolyte used for the anodizing of aluminum and its alloys is generally an acidic solution with medium solubility, and lead serves only as a conductor.
The generation and dissolution of the oxide film occur simultaneously. In the initial stage of oxidation, the growth rate of the film is greater than the dissolution rate, and the thickness of the film layer continues to increase. With the increase of thickness, its resistance also increases, which results in the growth rate of the film slowing down until it is equal to the dissolution rate, and the thickness of the film reaches a certain value. In addition, the generation law of the oxide film can also be explained by the voltage-time curve of anodizing. The entire anodizing process can be divided into three stages: growth, equilibrium, and breakdown. The thickness and properties of the film are determined by the anodizing voltage, current density, time, and the composition of the electrolyte.
Overall, anodizing can significantly improve the properties of aluminum and aluminum alloys, making them more suitable for various applications.
Methods for anodizing aluminum and its alloys are varied. This article mainly introduces the commonly used methods of sulfuric acid anodizing, chromic acid anodizing, and oxalic acid anodizing. Other anodizing methods for aluminum and its alloys include hard anodizing and ceramic anodizing.
- Sulfuric acid anodizing: aluminum and its alloys are anodized by direct current or alternating current in dilute sulfuric acid electrolyte. This method produces a 5μm to 20μm thick, good adsorption, colorless, and transparent oxide film. This method has a simple process, a stable solution, and is easy to operate.
- The quality concentration of sulfuric acid affects the thickness and hardness of the oxide film. A higher quality concentration leads to a thinner and softer film with more gaps and stronger adsorption and dyeing properties. Lower quality concentration leads to a faster growth rate of the oxide film with lower porosity, higher hardness, and good wear resistance and reflectivity.
- Temperature has a significant impact on the quality of the oxide film. When the temperature is between 10℃ and 20℃, the resulting oxide film is porous, has good adsorption properties, and is elastic, making it suitable for dyeing. However, the film is less hard and less wear-resistant. At temperatures higher than 26℃, the oxide film becomes loose and less hard. At temperatures below 10℃, the thickness of the oxide film increases, and the hardness and wear resistance increase, but the porosity is lower. Therefore, it is necessary to strictly control the temperature of the electrolyte during production.
- Increasing the current density can accelerate the growth rate of the oxide film, shorten the oxidation time, reduce the chemical dissolution of the film, and increase the hardness and wear resistance. However, if the current density is too high, the heat generated can increase the dissolution of the film, leading to a decrease in the growth rate. If the current density is too low, the oxidation time is prolonged, resulting in a loose film layer and reduced hardness.
- The oxidation time can be determined based on the quality concentration of the electrolyte, temperature, current density, and desired film thickness. With the same conditions, as the time increases, the thickness and porosity of the oxide film increase. However, once a certain thickness is reached, the growth rate slows down and does not increase further.
- Stirring promotes solution convection, makes the temperature uniform, and avoids the decrease in the quality of the oxide film due to local metal heating. Stirring equipment includes air compressors and water pumps.
- The composition of aluminum alloys has a significant impact on the quality, thickness, and color of the film. In general, other elements in aluminum alloys reduce the quality of the film. For Al-Mg series alloys, if the mass fraction of magnesium exceeds 5% and the alloy structure is non-uniform, appropriate heat treatment is necessary to homogenize the alloy. Otherwise, the transparency of the oxide film will be affected. For Al-Mg-Si series alloys, as the silicon content increases, the color of the film changes from colorless and transparent to gray, purple, and finally black, making it difficult to obtain a uniform color film. For Al-Cu-Mg alloys, the copper content should be less than 2%, as a higher content can lead to poor adhesion between the oxide film and the substrate.
After anodizing aluminum and its alloys, a porous oxide film is formed on the surface. By coloring and sealing treatment, various colors can be obtained, and the corrosion and wear resistance of the film can be improved.
- Coloring of oxide film
- Inorganic pigment coloring
- The mechanism of inorganic pigment coloring is mainly physical adsorption, in which the molecules of the inorganic pigment are adsorbed on the surface of the micropores of the film layer and filled in. The coloring tone of this method is not bright, and the adhesion with the matrix is poor, but it has good resistance to sunlight. The following table shows the process specification of inorganic pigment coloring. It can be seen from the table that the dyes used for inorganic pigment coloring are divided into two types, and the metal after anodization should be immersed alternately in two solutions until the quantity of the reaction product (pigment) of the two salts in the oxide film meets the required tone.
- Organic dye coloring
- The mechanism of organic dye coloring is more complicated, and it is generally believed that it involves both physical adsorption and chemical reactions. There are three ways for organic dye molecules to chemically bind to alumina: forming a covalent bond between alumina and the sulfonic group on the dye molecule; forming a hydrogen bond between alumina and the phenolic group on the dye molecule; forming a complex between alumina and the dye molecule. The color of organic dye coloring is bright and the color range is wide, but the sunlight resistance is poor. The following table shows the process specification of organic dye coloring. The water used to prepare the dyeing solution should preferably be distilled or deionized water instead of tap water, because the ions such as calcium and magnesium in tap water can complex with the dye molecules to form complexes, which will cause the dyeing solution to be scrapped.
- Electrolytic coloring
- Electrolytic coloring is a method in which anodized aluminum and its alloys are placed in an electrolyte solution containing a metal salt for electrolysis. Through electrochemical reaction, heavy metal ions entering the micropores of the oxide film are reduced to metal atoms, which are deposited on the pore-free layer at the bottom of the pore and colored. The colored oxide film obtained by the electrolytic coloring process has the advantages of good wear resistance, sunlight resistance, heat resistance, corrosion resistance, and stable and persistent color, and it has been widely used in aluminum profiles for building decoration. The following table shows the process specification of electrolytic coloring. The higher the voltage and the longer the time used in the electrolytic coloring process, the deeper the color.
After anodizing aluminum and its alloys, regardless of whether they are colored or not, they need to be closed in a timely manner to fix the dye in the micropores, prevent bleeding, and improve the wear resistance, weather resistance, corrosion resistance, and insulation of the film. The methods of closure include hot water closure, water vapor closure, chromate closure, hydrolysis closure, and filling closure.
- The principle of hot water closure is to use the hydration of amorphous Al₂O₃, in which n is 1 or 3. When Al₂O₃ hydrates into monohydrate (Al₂O₃·H₂O) or trihydrate (Al₂O₃·3H₂O), its volume increases almost 100%. Due to the hydration of Al₂O₃ on the surface and pore wall of the oxide film, the pore of the film is closed. The hot water closure process uses a water temperature of 90℃100℃, a pH value of 65, and a time of 15min~30min. The water used for closure must be distilled or deionized water, not tap water, which will reduce the transparency and color of the oxide film. The principle of water vapor closure is the same as that of hot water closure, but the effect is much better, albeit at a higher cost.
- The chromate closure method is carried out in a potassium dichromate solution with strong oxidizing properties and at a higher temperature. When anodized aluminum enters the solution, the aluminum oxide on the surface and pore wall of the oxide film reacts with the potassium dichromate in the aqueous solution to generate basic aluminum chromate and basic chromic acid aluminum, which precipitate and seal the micropores of the oxide film along with the monohydrate and trihydrate of aluminum oxide and water molecules. The formula and process conditions of the closure solution are as follows: the treated oxide film is yellow and has good corrosion resistance. It is suitable for sealing aluminum alloy anodized for protection, but not for sealing colored oxide films for decoration.
- In the hydrolysis closure method, when a very dilute solution of nickel salt or cobalt salt is adsorbed on the oxide film, the following hydrolysis reaction occurs: the resulting nickel hydroxide or cobalt hydroxide deposits in the micropores of the oxide film and seals the pores. Because a small amount of nickel hydroxide and cobalt hydroxide are almost colorless, this method is particularly suitable for closing colored oxide films. The table below shows the specifications for commonly used hydrolysis salt closure processes.
- In addition to the closure methods described above, the anodized oxide film can also be closed using organic substances such as transparent varnish, melted paraffin wax, various resins, and drying oils.