The previous article provided an overview of the key parameters and fundamental principles of anodising. In this article, we will delve deeper into the specific steps involved in Conco’s anodising process.
A Detailed Explanation of the Anodising Process
Conco divides its anodising process into five main stages. I’ve included below our detailed explanation.
Pre-treatment process
The pre-treatment process before anodising has a decisive impact on the quality of the final coating. It mainly includes the following steps:
Degreasing (Oil Removal)
- Purpose: To remove oil stains, fingerprints, machining residues, and other contaminants from the workpiece surface, ensuring the uniformity of subsequent treatments.
- Method: Chemical cleaning typically uses alkaline degreasers (such as sodium carbonate or sodium phosphate solutions), with ultrasonic cleaning employed when necessary to enhance the effect.
Water rinse
- Purpose: To remove any residual degreasing solution and prevent contamination of the next treatment bath.
- Method: Typically, rinsing with flowing clean water ensures that the workpieces are free from contaminants.
Pickling (Oxide Removal)
- Purpose: To remove the natural oxide film and corrosion products from the aluminium surface, achieving a clean and active surface.
- Method: A mixture of nitric acid, sulphuric acid, or phosphoric acid typically treats the workpieces for a short duration to avoid excessive corrosion.
Neutralisation (Optional)
- Purpose: To neutralise any residual pH after pickling or alkaline cleaning, ensuring surface stability.
- Method: They use a weak acid or weak alkali solution, such as diluted nitric acid or ammonia solution.
Anodising process
Electrolyte preparation
- The commonly used electrolyte for anodising is a sulphuric acid solution (with a concentration of 15% to 20% sulphuric acid), although variations in the process employ oxalic acid, chromic acid, or phosphoric acid.
- They control the temperature between 15°C and 25°C to ensure a uniform and dense coating.
Electrolytic reaction
- They connect the workpiece to the anode (+), while they typically use lead plates or stainless steel for the cathode.
- They apply direct current, controlling the current density between 1 and 2 A/dm².
- Reaction time: 15 to 60 minutes, depending on the required coating thickness.
During this process, the aluminium surface reacts with the electrolyte to form a dense aluminium oxide (Al₂O₃) film. The coating has a porous structure, facilitating subsequent colouring.
Colouring treatment (Optional)
The surface of the aluminium workpiece after anodising has a porous structure, which facilitates the absorption of dyes. Several colouring methods can be applied according to requirements:
- In the organic dyeing method, various dye solutions are used to achieve colour penetration through adsorption via the pores.
- The electrolytic colouring method involves using metal salts to deposit metal particles in the pores, creating a colour with strong weather resistance.
- In natural colouring, they control process parameters to form specific colours (such as grey, gold, etc.) without the need for additional dyeing.
Sealing treatment
After forming the oxide film, they need to seal its pores to improve corrosion resistance and colour durability. Common sealing methods:
- Hot water sealing: The workpiece is treated in hot water at 95–100°C for about 30 minutes, allowing the pores to hydrate, expand, and seal.
- Nickel salt sealing: The workpiece is treated in a solution containing nickel salts, where a reaction occurs to form a stable substance that blocks the pores.
- Steam sealing: High-temperature, high-pressure steam is used to hydrate and expand the oxides within the pores, sealing the channels.
Drying and inspection
Finally, they must dry the workpieces in an oven or leave them to air dry. After drying, they carry out appearance inspection, coating thickness measurement (using methods such as eddy current testing), adhesion testing, and other quality control steps to ensure the product meets the required specifications.
Summary:
By properly controlling process parameters (such as electrolyte concentration, current density, temperature, and time), anodic films with varying properties and colours can be produced. The concentration of the three acids also determines the product’s quality and hardness. Therefore, Conco uses different acid concentrations for anodising depending on the product.
Common types of anodising
Soft Anodizing
- Characteristics：
- A diluted sulphuric acid electrolyte (15% to 20%) is used.
- The coating thickness is generally between 5 and 25 microns.
- The coating is semi-transparent, ranging from white to light grey, and can be dyed.
- Suitable for decorative purposes and light protective requirements.
- Applications:
- Enclosures for electronic products, aluminium alloy fittings, kitchenware, lighting fixtures, etc.
Hard Anodizing
- Characteristics：
- A more concentrated sulphuric acid solution is used, with the process carried out at low temperatures (0°C to 5°C) and high current density.
- The coating thickness can reach 25 to 150 microns.
- The coating is dense and hard, with surface hardness ranging from HV300 to 500, offering excellent wear resistance.
- The colour is generally dark grey or black, with a slight metallic sheen.
- Applications:
- Aerospace components, mechanical slides, motorcycle parts, moulds, military equipment, etc.
Colored Anodizing
- Characteristics：
- Colouring is usually done after standard anodising.
- Colours are added using methods such as organic dyes, electrolytic colouring, or photochromic techniques.
- A variety of effects can be achieved, including black, red, blue, gold, green, copper, and others.
- Applications:
- Decorative aluminium materials, signage, enclosures for consumer electronics, etc.
Chemical Anodizing
- Characteristics：
- An electroless electrolysis method is used (no external current is applied), relying on chemical oxidants for surface oxidation treatment.
- The coating is thin, with average corrosion resistance and good adhesion.
- The process is simple and suitable for complex structural components.
- Applications:
- Internal components of aircraft, military parts, and other applications where conductivity is required.
Micro-Arc Oxidation
- Characteristics：
- At high voltages (several hundred volts), micro-discharges or arcs occur, forming a ceramic-like coating on the surface.
- The coating is extremely hard, wear-resistant, corrosion-resistant, and has strong insulation properties.
- The colours are typically grey-white, black, ceramic grey, etc., with a slightly rough surface.
- It falls under the category of ‘ceramic anodising’, offering performance far superior to standard anodising.
- Applications:
- High-end automotive components, electric vehicle parts, weaponry, medical instruments, etc.
Phosphoric Acid Anodizing
- Characteristics：
- Phosphoric acid is used as the electrolyte, resulting in a thin but porous oxide film.
- Suitable for use as a surface treatment for adhesive bonding, improving adhesive adhesion.
- Applications:
- Aerospace adhesive bonded structural components (such as adhesive surfaces in aircraft riveted joints).
Chromic Acid Anodizing
- Characteristics：
- Chromic acid is used as the electrolyte, forming a very thin oxide film (a few microns).
- The coating has good flexibility and does not affect the original dimensions, making it suitable for precision components.
- Environmental concerns limit its use (hexavalent chromium is toxic).
- Applications:
- Aerospace precision structural components, spacecraft enclosures, internal components, etc.
Comparison of Advantages:
| Type | Coating Thickness | Hardness | Corrosion Resistance | Colourability | Application Features |
|---|---|---|---|---|---|
| Standard Anodising | 5–25 μm | Medium | Average | Good | Decorative, light protection |
| Hard Anodising | 25–150 μm | High | High | Average | High wear resistance, high strength applications |
| Coloured Anodising | 10–25 μm | Medium | Average | Excellent | Strongly decorative, visually prominent |
| Micro-Arc Anodising | 30–200 μm | Very High | Very High | Poor | Ceramic surface, suitable for extreme environments |
| Phosphoric Acid Anodising | <5 μm | Low | Average | Poor | Pre-treatment for adhesive bonding |
| Chromic Acid Anodising | <5 μm | Medium | High | Poor | Precision components, aerospace use |
Summary
Aluminium anodising plays an increasingly important role in the development of new materials. With the widespread demand for high-performance, lightweight, and multifunctional materials, anodising is not only continuously optimised as a traditional surface treatment technique but is also gradually evolving into a key aspect of ‘functional surface engineering’, with broad applications in advanced fields such as aerospace, new energy, electronic information, and biomedicine.
Currently, Conco has also ventured into the anodising processing of electric vehicle battery pack enclosures and outdoor battery enclosures, receiving high recognition from customers. Through our engagement with these new fields, we have continued to enhance our learning and gained valuable processing experience, laying a solid foundation for future customers.