Aluminum is one of the most often used materials in all industries. This is because it has high strength, small gravity, outstanding ductility, excellent electrical conductivity, strong corrosion resistance, and superior physical and chemical properties.
A dense oxide film is formed spontaneously on aluminum under natural conditions, and its thickness is generally below 5nm. Although the natural oxide film on the surface of aluminum alloy can be automatically repaired immediately after being destroyed, its corrosion resistance and wear resistance are limited because of its thin thickness.
In order to meet the requirements of modern industry, the aluminum alloy must be properly surface processed, and anodizing is the most commonly used surface process method for aluminum. It can be used to prevent the corrosion of products or achieve the dual purpose of protection and decoration.
There are many methods for anodizing aluminum. By selecting different types of electrolytes with different concentrations and controlling the process conditions during oxidation, anodized films with other properties and thicknesses of about tens to hundreds of microns can be obtained.
Aluminum is anodized, corrosion-resistant, wear-resistant and decorative properties can be significantly improved and produce rich and changeable colors.
What is Anodized Aluminum
Anodized aluminum refers to placing aluminum in a specific electrolyte with aluminum parts as the anode and stainless steel or lead as the cathode. After power on, an oxide film will form on the surface of the aluminum parts. It is a method to obtain the corrosion resistance and decorative surface of aluminum products, and it is widely used in industry.
The characteristics of anodized aluminum
• Prevent corrosion of aluminum products
Because the anodized coating is stable in the atmosphere, the oxide layer on the aluminum surface can be a protective layer.
- The oxide layer formed by anodizing aluminum in a chromic acid solution is dense and corrosion-resistant;
- The oxide layer formed by the sulfuric acid solution has larger pores, but its film layer is thicker, and its adsorption capacity is strong if it is properly filled, sealed, and corrosion-resistant.
• Decorative products
For most aluminum products that require surface finishing, after chemical or electrochemical polishing, anodized with sulfuric acid solution can obtain a transparent oxide film.
This kind of oxide film can absorb many organic and inorganic dyes, so it has bright colors. This color film is both an anti-corrosion layer and a decorative layer.
• As a hard wear layer
Through hard anodizing of aluminum products, a hard and thick Al2O3 film can be formed on the surface. This film not only has high hardness and thickness, but also low roughness. In sulfuric acid or oxalic acid solution, a thick and hard oxide coating can also be formed on aluminum products by anodizing.
The porous thick oxide film can store lubricating oil, so it can be effectively applied to aluminum products that work under friction, such as engine cylinders and pistons in vehicles and tractors after anodizing, which can significantly improve their wear resistance.
• As an electrical insulating layer
Aluminum is a good electrical conductor, and the oxide coating obtained by anodized aluminum products has a greater resistance. The insulation breakdown voltage can reach 30~200V/um, the room temperature volume resistivity is 109Ω/cm3, and it can reach 1013Ω/cm3 at 250℃.
Therefore, it has some influence on enhancing the electrical insulation of certain products. We can use anodizing to prepare the capacitor’s dielectric layer, or aluminum oxide can be used to produce an insulating film.
• As the bottom layer of the painting
The anodic oxide film may be used as the bottom layer of paint and other organic films because of its porosity and strong adsorption capacity. This allows the paint and organic film to strongly connect with the product, boosting its corrosion resistance.
• As the bottom layer of electroplating
Before the aluminum products are electroplated, the bottom layer must be applied to them before they can be electroplated. There are many methods for applying the bottom layer on the surface of the substrate. In addition to electro-galvanizing, dipping zinc, and electroless nickel plating, anodizing is also one of the important methods.
Types of Anodizing Aluminum
According to electrolyte: sulfuric acid anodization, oxalic acid anodization, chromic acid anodization, phosphoric acid anodization and mixed acid anodization;
According to the function of the oxide film, it can be divided into: wear-resistant film, corrosion-resistant film, glued film, insulating film, porcelain film, decorative film, etc.
- Type I: It uses chromic acid to form a thin and pliable anodized coating on an aluminum part
- Type II: It uses sulfuric acid to create a thicker anodized coating on an aluminum part, making parts look good.
- Type III: It is for those parts that need extreme hardness surfaces. It also uses sulfuric acid to form a thicker anodized coating.
The commonly used processes of aluminum anodization are: sulfuric acid anodization process, chromic acid anodization process, oxalic acid anodization process and phosphoric acid anodization process.
The appearance and characteristics of the resultant oxide coating vary significantly depending on the electrolyte utilized. In real manufacturing, the best anodizing method must be chosen based on the intended usage.
1. Sulfuric Anodizing
Sulfuric acid anodizing is the most common anodizing technique.
Sulfuric acid anodizing has low cost, simple process, short time, easy production and operation, high transparency of the paint film, good candle resistance and abrasion resistance. Compared with other acid anodizing, it has great advantages over other acid anodizing.
Due to the low current density of sulfuric acid AC anodizing and the poor quality of the oxide film, DC sulfuric acid anodizing is mostly used.
The technological process of sulfuric acid anodizing is: mechanical polishing → degreasing → secondary cleaning → chemical or electrolytic polishing → secondary cleaning → anodizing → secondary cleaning → preparation for dyeing.
2. Chromic Anodizing
The film obtained by chromic acid anodization is relatively thin, generally only 2-5μm in thickness, which can maintain the original precision and surface roughness of the workpiece. The film is soft, and its wear resistance is not as good as that of sulfuric acid oxide film, but it has good elasticity. In addition, the film is opaque, has low porosity, and is difficult to dye. It can be used directly without sealing.
The solubility of chromic acid solution to aluminum alloy is low, so that the residual solution in pinholes and crevices has little effect on the corrosion of parts. It is used for surface treatment such as castings, riveting parts and mechanical processing.
3. Oxalic Anodizing
Oxalic acid is less soluble in aluminium and aluminium alloys, which results in a lower porosity of the oxide layer and greater corrosion resistance, wear resistance, and electrical insulation than sulfuric acid film. However, the price of oxalic acid anodization is considerable, often 3-5 times that of sulfuric acid anodization; in addition, the colour of the oxalic acid oxide film is very susceptible to change depending on the parameters of the process, resulting in colour variation in the final product. As a result, there are certain limitations on how the procedure may be applied. It is often only utilised in rare circumstances, such as when creating electrical insulation layers.
4. Phosphoric Anodizing
The phosphoric acid film is thin (about 3 m thick) and has a high pore size because the oxide layer is more soluble in the phosphoric acid electrolyte than in sulfuric acid. The phosphoric acid film is primarily used for the surface treatment of printed metal plates and the pretreatment of the adhesive bonding of aluminium workpieces due to its strong water resistance, which can prevent the adhesive from ageing due to hydration and improve the bonding force of the adhesive.
The anodized film has steadily evolved from a traditional acid anodized film to a mixed acid anodized film as a result of advancements in science and technology, environmental protection, and the need to increase performance or lower manufacturing costs.
Anodizing is currently the most widely used surface finishing technology for aluminum and aluminum alloys. It gives aluminum and aluminum alloys better corrosion resistance, wear resistance, decoration and electrical insulation. With the further deepening of understanding, the application of aluminum alloy anodizing technology in functional devices will be more extensive.
If you want to learn more about how anodizing can help improve your prototypes and parts, please contact RpProto’s team of experts. Our experts will provide advice on rapid prototyping and small batch manufacturing, helping you find a solution that fits your budget, time to market and expected results.