Aluminium is a widely used material in the manufacturing industry due to its strength, lightness, and versatility. However, aluminium is prone to corrosion, which can affect its appearance and structural integrity. To overcome this issue, anodising aluminium has become a popular process in the industry. In this article, we will discuss everything you need to know about anodizing aluminium, including the process, types, interesting facts, and common mistakes.
What is Anodising?
Anodizing is an electrochemical process that enhances the natural oxide layer on the surface of the metal. The process involves immersing the aluminium in an electrolyte solution and passing an electric current through it. This causes the formation of a thicker and more durable oxide layer on the surface of the aluminium, which provides increased corrosion resistance, improves durability, and enhances the appearance of the metal.
What is the Aluminium Anodization Process?
The process of anodizing aluminium involves several steps, including pre-treatment, anodizing, colouring, and sealing. Here’s a detailed overview of each step:
Pre-treatment
Before the anodizing process, the aluminium surface needs to be cleaned thoroughly to remove any dirt, grease, or other contaminants. The pre-treatment process includes the following steps:
Degreasing: The aluminium parts are immersed in a hot alkaline solution to remove any oil or grease on the surface.
Etching: The aluminium surface is treated with an acidic solution to create a microscopically rough texture that helps the anodizing solution to penetrate the metal.
Desmutting: The aluminium parts are rinsed in a solution to remove any remaining smut or etching residue.
Anodizing
The anodizing process involves immersing the pre-treated aluminium parts in an electrolytic solution and applying a direct current. The current causes the aluminium oxide layer to grow on the surface of the metal. The anodizing process can be performed using one of the following types of electrolytes:
Sulfuric acid: This is the most commonly used electrolyte in the anodizing process. It produces a hard, dense, and corrosion-resistant anodized layer.
Chromic acid: This electrolyte produces a thin and soft anodized layer and is mainly used for decorative purposes.
Phosphoric acid: This electrolyte produces a porous and thick anodized layer, mainly used for adhesive bonding.
Colouring
After the anodizing process, the aluminium parts can be coloured using various techniques, such as dyeing or electrolytic colouring. The colour of the anodized layer depends on the type of dye or pigment used in the colouring process. Some common colours used in the anodizing process are black, red, blue, green, and gold.
Sealing
The final step in the anodizing process is sealing, which involves closing the pores of the anodized layer to improve its corrosion resistance and durability. The sealing process can be performed using hot water, steam, or chemical sealing agents.
3 Main types of Anodising
Type I: Chromic Acid Anodising
Chromic Acid Anodising, also known as type I anodising, is a process that produces the thinnest anodic coat compared to the other two types. The thickness typically ranges up to 5µm per surface. Despite being thin, Chromic Anodising provides the same level of corrosion protection as the thicker sulfuric and hard coat anodizing when properly sealed.
Compared to other types of anodising, Chromic Anodizing appears greyer in colour and can only be dyed black, making it less practical for decorative purposes. However, it is suitable for non-reflective, protective coatings on housings for optical components. Chromic Anodising has good bonding capabilities, is non-conductive, and is ideal for precision machined components, aerospace components, welded parts and assemblies, and as a paint/primer base. It is also beneficial for tight tolerance parts, as it will not change their dimensions.
Type II: Sulfuric Acid Anodising
Sulfuric Acid Anodising, also known as MIL-A-8625 Type II, is the most commonly used method for anodizing. It produces a coating ranging from 5 to 15µm for clear (natural) and 10 to 25µm for dyed (coloured). This type of anodising is particularly suited for applications where hardness and resistance to abrasion are required.
Compared to other types of anodising, Sulfuric Acid Anodising is less expensive with respect to the chemicals used, heating, power consumption, and length of time to obtain the required thickness. It is also harder than Chromic Anodising and has a clearer finish, allowing for dying with a greater variety of colours. Waste treatment is easier than Chromic Anodising, making it more cost-effective. This type of anodization is used for a variety of applications, including optical components, hydraulic valve bodies, military weapons, computer and electronic enclosures, and mechanical hardware.
Type III: Hard Coat Anodising
Hard coat Anodising, also known as Hard Anodising, is an anodic coating of aluminium oxide that is denser, harder, thicker, and more abrasion resistant than other types of anodization. It is applied by converting a cleaned and deoxidized aluminium alloy component into an oxide film using a suitable electrolyte, typically sulfuric acid, at very low temperatures and high voltage with an applied current density of 24-36 amps per square foot.
Compared to Type I and Type II anodising, Hard coat Anodising has increased abrasion, wear, and corrosion resistance, and improved dielectric properties. It can be sterilized, is non-contaminating, and has improved lubrication when PTFE sealed. The thickness of the coating can range from 20 to 70µm for hard anodising, with thicknesses up to 100µm achievable on some alloys.
Interesting Facts about Anodising Aluminium
Anodising Thickness Can Vary
One interesting fact about anodising is that the thickness of the oxide layer can vary depending on the process. The thickness of an anodised coating can range from less than 5µm (for chromic anodising) up to 100µm (for hard anodising on certain alloys),depending on the desired outcome. For example, a thicker layer may be desired for increased wear resistance, while a thinner layer may be desired for a more uniform appearance.
Anodising Aluminium Can Be Conductive
While it may seem counterintuitive, anodised surfaces can actually be conductive. This is because the aluminium oxide layer is not a perfect insulator, allowing for some electrical conductivity. In fact, anodised aluminium is often used in electronic devices due to its electrical properties.
Anodising Aluminium Can Create Unique Colours
Anodising is a process that can create a range of unique colours on metal surfaces by using dyes that are absorbed into the porous oxide layer. The resulting colours can be subtle pastels or bold, vibrant hues. The pore size of the oxide layer determines the range of colours that can be achieved.
Common anodising processes create a porous surface that can absorb dye, offering a wide range of colour options. Custom colours are also available, in addition to standard colours like yellow, green, blue, black, and red. Interestingly, the oxide layer’s pore size is too small for white dye molecules to be absorbed, so white is not a colour option.
Anodising Aluminium Is Environmentally Friendly
Compared to other surface treatments, anodising is considered to be an environmentally friendly option. This is because the process does not involve the use of heavy metals or other harmful chemicals. Additionally, the aluminium oxide layer is non-toxic and can be recycled.
Anodising Aluminium Can Improve Adhesion
Finally, anodising can improve the adhesion of paints and other coatings to metal surfaces. This is because the rough surface created by the oxide layer provides a better surface for the coating to adhere to. This can lead to increased durability and a longer lifespan for the coated surface.
Common mistake in Anodising Aluminium
Poor surface preparation
The first mistake is poor surface preparation. Before anodising, it is essential to thoroughly clean the surface of the aluminium to remove any dirt, oil, or other contaminants that can interfere with the anodising process. If the surface is not properly prepared, the oxide layer will not form uniformly, resulting in a patchy or uneven finish. To avoid this, the surface should be cleaned with a degreaser and then rinsed with clean water before anodising.
Inconsistent anodising parameters
The second mistake is using inconsistent anodising parameters. Anodising parameters such as voltage, current density, and electrolyte concentration need to be carefully controlled to ensure a uniform oxide layer. If these parameters are not consistent, the oxide layer will not form evenly, resulting in a patchy or blotchy finish. To avoid this, it is essential to monitor and control these parameters carefully throughout the anodising process.
Inadequate sealing
The third mistake is inadequate sealing of the anodised surface. After anodising, the surface of the aluminium is porous and can absorb water, dirt, and other contaminants. To prevent this, the surface must be sealed by immersing it in a hot water or chemical solution that closes the pores in the oxide layer. If the sealing process is inadequate or skipped entirely, the anodised surface will be vulnerable to corrosion and wear.
Over-anodising
The fourth mistake is over-anodising the aluminium. Over-anodising occurs when the aluminium is left in the electrolyte solution for too long, resulting in a thicker than desired oxide layer. A thicker oxide layer can result in a darker finish than intended, making it difficult to achieve the desired colour or appearance. To avoid over-anodizing, it is essential to carefully monitor the anodising time and remove the aluminium from the solution when the desired thickness is reached.
Inadequate rinsing The fifth mistake is inadequate rinsing after anodizing. After anodising, the aluminium should be rinsed thoroughly with clean water to remove any residual electrolyte solution. If the aluminium is not adequately rinsed, the residual solution can react with the sealed oxide layer, resulting in a patchy or discoloured finish. To avoid this, it is essential to rinse the aluminium thoroughly with clean water and then dry it carefully.
Conclusion
Anodising aluminium is a process that enhances the natural oxide layer on the surface of the metal, providing increased corrosion resistance, improved durability, and an enhanced appearance. There are several types of aluminium anodization, each with its own benefits and drawbacks. While anodising aluminium may be more expensive than applying other coatings, it provides a thicker and more durable layer, and it’s more environmentally friendly.
Anodising Aluminium FAQs
1. How long does anodized aluminium last?
Anodized aluminium can last for many years, depending on the application and environmental conditions.
2. Can anodized aluminium be repaired?
Minor scratches and blemishes can be repaired using touch-up paint, while larger damage may require re-anodizing.
3. Is anodized aluminium more expensive than other coatings?
Anodized aluminium can be more expensive compared to other coatings, but it provides a thicker and more durable layer.
4. Can anodized aluminium be recycled?
Yes, anodized aluminium can be recycled and reused for various applications.
