How is the anodising of aluminium carried out

The basic definition and importance of anodizing.

Anodizing is a process that uses the principles of electrolysis, where a metal workpiece acts as the anode under specific voltage and electrolyte conditions, generating an oxide film on its surface through the action of electrical current.

Unlike natural oxidation, this process forms a uniform oxide layer with controllable thickness.

The importance of anodizing:

1. Enhancing corrosion resistance:

• The oxide layer can effectively block corrosive agents such as air, moisture, and salts.
• It is particularly suitable for industries with high corrosion resistance requirements, such as aviation, construction, and automotive sectors.

2. Enhancing surface hardness and wear resistance:

• The hardness of the aluminium oxide film is high (up to HV500–1000), significantly extending the service life of components.

3. Enhancing decorative properties:

• Its porous structure allows dye absorption, producing diverse colours after treatment.
• People often use it in architectural decoration, household items, and electronics enclosures.

4. Improving insulation properties:

• The oxide film is an effective insulator, used in insulation layers and capacitors in electronics.

5. Providing a good base for coating:

• The oxide layer improves coating adhesion, serving as a base for painting and electroplating.

Principle of anodizing

Anodizing (Anodic Oxidation) is a technique that forms an oxide film on the surface of a metal through an electrochemical process, primarily applied to valve metals such as aluminium, magnesium, and titanium. Its principle is based on an electrolytic oxidation reaction.

Explained in detail as follows:

1. Basic principle

In anodizing, the metal workpiece acts as the anode. Applying current in an electrolyte triggers surface oxidation, forming a dense oxide film. The core reactions include:

• Anodic reaction: The metal (such as aluminium) loses electrons, is oxidised into metal ions, and combines with oxygen to form an oxide. (2Al+3H2​O→Al2​O3​+6H++6e−)
• Cathodic reaction: Hydrogen ions in the electrolyte (such as sulphuric acid) are reduced to form hydrogen gas. (2H++2e−→H2​↑)

2. Formation process of the oxide film

• Initial stage: A thin natural oxide film forms on the metal surface (such as the native aluminium oxide film, approximately 4–5 nm thick).
• Driven by the electric field: Under the applied voltage, oxygen ions (O²⁻) in the electrolyte combine with metal ions, and the oxide film gradually thickens.
• Competing reactions: As the oxide film grows, the electrolyte partially dissolves the film (e.g. sulphuric acid dissolving aluminium oxide), resulting in a porous structure.
  • Barrier layer: A dense thin layer closely attached to the metal (thickness ≈ applied voltage × 1 nm/V).
  • Porous layer: The outer layer consists of a honeycomb-like porous structure, with the pore diameter and density depending on the type of electrolyte (such as sulphuric acid, oxalic acid, etc.).

3. Characteristics of the oxide film

• High porosity (porous), making it suitable for dyeing or sealing treatments.
• The film thickness is controllable, typically ranging from 5 to 30 μm, with hard anodising achieving thicknesses of over 50 μm.
• It has high adhesion, forming a strong bond with the metal substrate.
• The colour is natural (initially grey-white, transparent, or slightly yellow), and can be further treated with dyeing.

Influence of key process parameters

The results ofanodisingg are influenced by several process parameters. Adjusting these parameters can control the thickness, hardness, appearance, corrosion resistance, and other characteristics of the oxide film.

The following are the key parameters that influence the anodizing process and their effects:

Voltage

• Effect: Voltage is one of the most critical parameters in the anodizing process. The magnitude of the voltage directly affects the growth rate of the oxide film and the structural properties of the film.
  • Low voltage (approximately 10-20 V): Produces a thinner, smoother oxide film, with the film colour typically being lighter. This is suitable for applications where high aesthetic requirements are needed.
  • High voltage (approximately 100 V or higher): This results in a thicker oxide film with higher hardness, but the appearance of the film may become rough, and overheating may occur more easily.
• Voltage control: High voltage is beneficial for generating a hard oxide film, while low voltage creates a transparent and colourful oxide film, making it suitable for dyeing.

Current Density

• Effect: Current density (the current intensity per unit area) affects the thickness, uniformity, and quality of the film. Current density is typically related to voltage, but an increase in current density accelerates the formation of the oxide film.
  • High current density: It accelerates the formation of the film, which may lead to uneven surface, increased porosity, and the generation of excess gas, affecting the quality of the film.
  • Low current density: The film is more uniform and of higher quality, but the formation of the film is slower.

Electrolyte Type & Concentration

• Effect: The type and concentration of the electrolyte have a significant impact on the performance and colour of the oxide film.
  • Sulphuric acid solution: A commonly used electrolyte, capable of producing a transparent oxide film, suitable for general anodizing and dyeing.
  • Oxalic acid solution: It can produce a smoother, transparent film with lower porosity, making it suitable for high-strength applications.
  • Chromic acid solution: Used for hard anodizing, it can produce a thicker, harder oxide film, making it suitable for applications with high wear resistance requirements.
  • Electrolyte concentration: The concentration of the electrolyte determines the efficiency of current flow through the solution. Higher concentrations can accelerate the oxidation reaction, but may also lead to uneven film formation.

Temperature

• Effect: The temperature of the electrolyte affects the growth rate of the oxide film and the properties of the film.
  • Low temperature: Typically operated between 0°C and 5°C, it produces a denser film, suitable for applications with high corrosion resistance requirements.
  • Medium to high temperature: Excessively high temperatures (e.g., 20°C to 30°C) accelerate the oxidation reaction, increasing the film thickness, but may cause the surface of the film to become irregular and reduce its corrosion resistance.
  • Heat sealing: High temperatures (around 95°C) during the sealing process help improve the stability and impermeability of the film.

Oxidation Time

• Effect: The anodizing time directly determines the thickness of the oxide film. Longer times result in thicker films, but may also affect the uniformity and surface quality of the film.
  • Short anodizing time: Produces a thinner film, suitable for applications with lower requirements for film thickness.
  • Long anodizing time: Produces a thicker film, suitable for applications with higher requirements for corrosion resistance and wear resistance. However, excessively long durations may lead to an uneven film structure.

pH of Electrolyte

• Effect: The pH value of the electrolyte directly influences the structure and quality of the oxide film.
  • Acidic electrolytes (ph < 4): Commonly used solutions for anodising (such as sulphuric acid), where the acidic environment helps dissolve aluminium ions and allows the formation of a porous oxide film.
  • Neutral or alkaline electrolytes (pH > 7): Suitable for certain specialised anodizing processes, particularly for different metals or applications requiring high corrosion resistance and high electrical insulation.

Cathode Material & Arrangement

• Effect: The material and configuration of the cathode affect the distribution of the electric field during the anodizing process, indirectly influencing the uniformity of the film. (Common cathode materials: Stainless steel, lead plates, graphite, etc.)
• Configuration: The cathode should cover as much of the surface of the electrolytic cell as possible to ensure uniform current distribution. Improper positioning or configuration of the cathode may result in localised weak spots or unevenness in the oxide film.

Sealing Process

• Effect: Sealing treatment is used to enhance the stability and corrosion resistance of the oxide film. During the sealing process, the pores of the oxide film are closed, preventing harmful substances from penetrating.
  • Hot sealing: Typically performed in water at temperatures between 90°C and 98°C, this process closes the pores of the film through a hydration reaction.
  • Chemical sealing: Using chemical agents (such as chromate or aluminium salt solutions) to seal the film, which enhances its corrosion resistance.

Dyeing Process

• Effect: Dyeing is typically carried out in the pores on the surface of the oxide film. Controlling the choice of dye and the dyeing time can adjust the colour of the film.
  • Types of dyes: Organic or inorganic dyes can be used. The dyeing effect depends on the properties, concentration, temperature, and duration of the dyeing process.
  • Dyeing temperature: High-temperature dyeing can accelerate the dyeing process, but it may affect the structure of the film.

Summary

In summary, this article has provided an initial understanding of the importance and basic principles of anodizing. We have used some fundamental formulas to roughly estimate the anodizing process and explored how several key process parameters can influence the final product. Overall, the process parameters of anodizing have a profound impact on the performance of the resulting oxide film. By optimising these parameters, it is possible to precisely control the film’s thickness, hardness, colour, and other functional properties to meet the demands of various applications. Whether in aerospace, automotive, electronic devices, or decorative fields, anodizing plays a vital role in enhancing both the surface performance and visual appeal of metals.

At the same time, our company, Conco, operates multiple anodising dye tanks in a variety of colours: one silver tank, two black tanks (one using imported dye and one using domestic dye), two grey tanks (each with a different shade), one gold tank, one antique bronze tank, and one teal-gold tank.

Each dye tank is paired with a dedicated rinsing tank and a sealing tank. The anodising tanks are 6.5 metres in length and 1.2 metres deep. Additionally, we have tanks for red, blue, green, and orange, each 1 metre in length, also equipped with one rinsing tank and one sealing tank.

The detailed production process and types of anodising.