What is Anodized Aluminum


Aluminum is the second largest category of metal materials used in metal materials after steel. It has properties of small gravity, high specific strength, excellent ductility, good electrical conductivity, strong corrosion resistance, and excellent 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, aluminum alloy must be properly surface processed, and anodizing is the most commonly used surface process method for aluminum and aluminum alloys. It can be used to prevent 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 different properties and thicknesses of about tens to hundreds of microns can be obtained.

Aluminum is anodized, corrosion resistance, wear resistance and decorative properties can be significantly improved, and can 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.

anodized Aluminum

The characteristics of anodized aluminum

• Prevent corrosion of aluminum products

Since the film layer obtained by anodizing has sufficient stability in the atmosphere, the oxide film on the aluminum surface can be used as a protective layer.
• The oxide film obtained by anodizing aluminum in a chromic acid solution is dense and has good corrosion resistance;
• The oxide film obtained from the sulfuric acid solution has larger pores than the former, but its film layer is thicker and its adsorption capacity is strong. If it is properly filled and sealed, its corrosion resistance is also very good.

• 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 kinds of organic dyes and inorganic dyes, so it has a variety of 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 thick and hard Al2O3 film can be obtained on the surface. This film not only has high hardness and thickness, but also low roughness. In sulfuric acid or oxalic acid solution, a hard and thick oxide film can also be obtained 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 the engine cylinders and pistons of automobiles and tractors after being anodized, which can greatly improve their wear resistance.

• As an electrical insulating layer

Aluminum is a good electrical conductor, and the oxide film 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 a certain effect on improving the electrical insulation of certain products. Anodizing can be used to prepare the dielectric layer of the capacitor, or aluminum oxide can be used to prepare an insulating layer on its surface.

• As the bottom layer of the painting

Due to the porosity and good adsorption capacity of the anodic oxide film, it can be used as the bottom layer of paint and other organic films, so that the paint film and organic film can be firmly combined with the product, thereby increasing 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.

Anodized parts

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 :             Basic type,  using chromic acid to produce a thin and ductile anodized layer on an aluminum part
  • Type II :           Utilizes sulfuric acid to produce a thicker anodized layer on a part, making it for a variety of visual effects
  • Type III:         Known as “hard anodizing”.  Also uses sulfuric acid, but produces a thicker anodized layer, is for surfaces that need to be extra hard

Anodizing process

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.
When different electrolytes are used, the resulting oxide film has great differences in appearance and properties. In actual production, it is necessary to select a suitable anodizing process according to the purpose of use.


1. Sulfuric acid anodizing

The widely used anodizing process is sulfuric acid anodizing.

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 obvious advantages in all aspects.
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 polishing or electrolytic polishing → secondary cleaning → anodizing → secondary cleaning → preparation for dyeing.

2. Chromic acid 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 acid anodizing

Because the solubility of oxalic acid to aluminum and aluminum alloys is small, the porosity of the oxide film is low, so the corrosion resistance, wear resistance and electrical insulation of the film are better than that of sulfuric acid film. However, the cost of oxalic acid anodization is high, generally 3-5 times that of sulfuric acid anodization;
Moreover, the color of the oxalic acid oxide film is easy to change with the process conditions, causing the product to produce color difference. Therefore, the process is subject to certain restrictions in application. Generally, it is only used under special requirements, such as making electrical insulation layers.

4. Phosphoric acid anodizing

Since the oxide film is more soluble in phosphoric acid electrolyte than sulfuric acid, the phosphoric acid film is thin (about 3μm thick) and has a large pore size. Because the phosphoric acid film has strong water resistance, it can prevent the adhesive from aging due to hydration and make the bonding force of the adhesive better, so it is mainly used for the surface treatment of printed metal plates and the pretreatment of the adhesive bonding of aluminum workpieces.

With the development of science and technology, in order to follow the concept of environmental protection or to improve the performance of a certain aspect of the anodized film, or to reduce production costs, the anodized film has gradually developed from a traditional acid anodized film to a mixed acid anodized film. membrane.


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.

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