Why Anodising Matters for Aluminium Profiles
Aluminium offers excellent physical properties – it’s lightweight, strong, and provides good thermal conductivity and machinability. However, the natural oxide layer on its surface is extremely thin (around 2–3 nanometres), which makes it vulnerable to corrosion, discolouration, and surface damage over time, especially in humid or chemically aggressive environments.
Anodising is a surface treatment process that strengthens aluminium by creating an artificial oxide layer (aluminium oxide, Al₂O₃) through electrolysis. This anodised layer offers several key benefits:
- Improved Surface Hardness
The anodised layer can reach a hardness of over HV300, making it suitable for applications that require wear resistance. - Better Corrosion Resistance
With proper sealing, the anodised layer forms a barrier that protects against moisture, acids, and alkalis. - Enhanced Adhesion
Moreover, it provides a better base for subsequent processes such as painting, powder coating, or screen printing. - Controlled Appearance
Its porous structure allows the anodised surface to be dyed or coloured electrolytically, offering a wide range of decorative finishes.
Industries widely use anodising for applications ranging from industrial equipment housings and architectural components to electronic heat sinks and railway system parts – anywhere enhanced surface performance is required.
For customers, anodising not only improves the overall performance of aluminium profiles, but also plays a vital role in ensuring product durability and consistent appearance.
Understanding the Principle of Anodising
Anodising is an electrochemical process that forms a stable aluminium oxide (Al₂O₃) layer on the surface of aluminium parts. The process applies electrolytic oxidation, with the following core principles:
1. Electrolytic System Setup
In anodising, the aluminium profile is connected as the anode (positive electrode) and immersed in a specific electrolyte – typically a diluted sulphuric acid solution. A cathode (usually made of lead or stainless steel) completes the circuit. Once direct current (DC) is applied, electrochemical reactions begin on the surface of the aluminium.
2. Formation of the Oxide Layer
During electrolysis, the following reaction occurs at the anode:
Anode Reaction:
2Al + 3H₂O → Al₂O₃ + 6H⁺ + 6e⁻
The resulting aluminium oxide layer has a two-part structure:
- Dense Barrier Layer:
Tightly bonded to the aluminium substrate, this compact layer offers fundamental protection. - Porous Outer Layer:
With pore sizes between 20–100 nm, this layer allows for post-treatment processes such as colouring or sealing.
3. Factors Influencing the Oxide Layer
The properties of the anodised film – such as thickness, hardness, and porosity – depend on several process parameters, including:
- Concentration of the electrolyte (e.g., sulphuric acid content)
- Voltage or current density applied
- Duration of the anodising process
- Electrolyte temperature
- Standard anodising: around 20 ± 2°C
- Hard anodising: typically −5 to 5°C
4. Process Control and Variation
Different anodising methods – such as standard anodising, hard anodising, or coloured anodising – can be selected depending on the application. Typical film thickness ranges from 5–25 μm, while hard anodised layers can reach 40–70 μm, making them ideal for parts that require exceptional wear resistance.
Standard Anodising Process for Aluminium Profiles
The anodising of aluminium profiles typically involves four key stages: pre-treatment, electrolytic oxidation, colouring (optional), and sealing. Each step plays a critical role in determining the final quality of the oxide layer, colour consistency, and corrosion resistance.
At our factory in Conco, we operate a dedicated anodising production line with tanks measuring 6.5 metres in length and 1.2 metres in depth, capable of handling a wide range of large aluminium profiles. By integrating the entire process in-house, we can precisely control the thickness of the oxide layer, ensure consistent colour uniformity, and maintain high-quality sealing — all while significantly reducing lead times and avoiding the uncertainties associated with outsourcing.
Pre-treatment Stage
The aim of pre-treatment is to remove oils, natural oxide, and machining residues from the aluminium surface, ensuring that the anodised film forms evenly and adheres well.
- Degreasing
An alkaline degreaser is used to clean the surface, removing oils, fingerprints, and other contaminants. - Etching (Chemical Polishing)
A sodium hydroxide-based solution etches the surface to improve brightness and surface uniformity. - Neutralising
Diluted nitric acid neutralises any remaining alkali and removes residues from the etching stage, helping to prevent spotting during anodising.
Note:
Etching strength and time should be adjusted based on the intended use of the aluminium profile.
Anodising Stage
Electrolysis forms a dense oxide layer on the aluminium surface — a critical step in the anodising process.
- Electrolyte
A 15–20% sulphuric acid solution is commonly used, with temperature maintained between 18–22°C. - Voltage and Current Density
These are set depending on the desired film thickness – typically 12–18V and 1–2A/dm². - Processing Time
Anodising usually lasts between 15–40 minutes. Longer times result in thicker oxide layers, which offer better hardness and corrosion resistance but may produce a slightly rougher surface. Parameters should be chosen based on product requirements.
Colouring (Optional)
After anodising, technicians can apply dyeing or electrolytic colouring to the porous oxide layer, depending on customer requirements.
- Organic Dyeing
Dyes are absorbed into the pores of the film to produce a wide range of colours. This method is often used for home appliance housings or decorative panels. - Electrolytic Colouring
Metal salts are deposited in the pores, giving a metallic finish. Common finishes include black, champagne, and gold.
Sealing
Sealing is the final and essential step in anodising. It closes the pores of the oxide layer, significantly improving corrosion resistance and stain resistance.
Common sealing methods include:
- Hot Water Sealing
Immersion in deionised water at 60–98°C causes the oxide to hydrate. This is a cost-effective method but highly sensitive to water quality. - Nickel Salt Sealing
Nickel salts are added to increase sealing efficiency and film stability, making it suitable for industrial-grade products. - Organic Sealing
Used mainly for dyed products, this improves colour fastness and resistance to fading.
Key Quality Control Points in Aluminium Anodising
During the anodising of aluminium profiles, stable process control and systematic quality checks are essential to ensure the performance and consistency of the oxide layer. Below are the key aspects of quality control:
1. Anodic Film Thickness Measurement
The thickness of the anodised layer directly affects corrosion resistance, wear resistance, and visual uniformity. Common methods for measuring film thickness include:
- Eddy Current Thickness Gauge
A non-contact method ideal for quick checks on the production floor. - Microscopic Cross-sectioning
A lab-based method for detailed, accurate analysis. - Weighing Method (Mass Loss Technique)
Used for verifying hard anodising samples.
Typical film thickness ranges:
- Decorative anodising: 5–15 μm
- Industrial functional anodising: 15–25 μm
- Hard anodising: 40–70 μm
Colour Consistency and Shade Control
For coloured anodised products, especially in batch production, colour uniformity is a major concern. Quality control measures include:
- Monitoring temperature and concentration of the electrolyte and dye baths.
- Maintaining consistent oxidation time, voltage, and current density.
- Comparing colour under standard lighting conditions, either visually or with instruments.
Tip: Use the same batch of aluminium and chemicals for each production run to minimise variation in reaction behaviour.
Adhesion Testing
The bond strength between the oxide layer and the base metal is crucial for any subsequent surface treatment, such as painting or silk screening. Common test methods include:
- Cross-cut Tape Test
Used to assess the film’s adhesion strength. - Rubbing/Friction Test
Measures the durability of the layer under mechanical stress.
Surface Defect Inspection
- Visual + Light Box Inspection
Identifies surface issues such as scratches, stains, pitting, or uneven colouring.
Moreover, in-house processing enables us to offer greater flexibility in customisation and enforce stricter quality control. Our quality inspection team conducts film thickness measurements and corrosion resistance sampling on every batch to ensure that all anodised products consistently meet high standards.
Through this streamlined process, we provide our clients with more reliable delivery schedules and improved product consistency, making us a trusted partner in architectural cladding, industrial equipment, and high-end electronic enclosures.