Aluminium Anodising
Aluminium is the most widely used metal in the world after steel. Engineers like aluminium for its light weight, corrosion resistance and competitive price.
But, straight aluminium has low wear resistance. If exposed to the environment, it forms a thin oxide layer that provides aluminium with its characteristic corrosion protection. But this naturally formed oxide film will erode upon reaction with some environmental elements.
The solution is to provide better protection with anodising. This procedure has other benefits as well but let’s start from the beginning.
What is Anodising?
Anodising is an electrochemical process that grows an aluminium oxide coat on the surface of the aluminium. This protects the aluminium from wear and tear and improves the aesthetics. In this process, the product to be coated acts as an anode in an electrolytic cell, and that is where it gets the name anodising.
Anodising was first done on an industrial scale in the early 1920s.
It didn’t take long for many variations of this process came into use for different materials using various electrolytic chemicals. It was around this time that Gowen and O’Brien used sulfuric acid to anodise aluminium.
This process has remained the most common for over 100 years.
Benefits of Anodised Aluminium
Anodising the aluminium increases the thickness of aluminium oxide on the surface of the product.
The aluminium oxide layer is extremely hard. On the Mohr’s scale, it has a score of 9 and is second in hardness only to diamond. It is so hard that it is commonly used as an abrasive in sandpapers. Depositing a layer of this material on the product ensures that the product will have high wear resistance.
The thickness of this layer depends on the purpose of anodising. For decorative purposes, a thin layer of 15-20microns is enough. A thicker layer (30-50microns) protects the surface as well as improving the appearance.
Having a thick layer of aluminium oxide makes the metal surface suitable for dying various colours. The pores grown on the surface are porous and desired pigments are introduced that fill the pores from the surface. This makes the pigment quite durable as it cannot be scratched away.
Anodising can also act as an excellent primer for a regular coat of paint on the surface instead of accommodating it into the actual oxide layer.
Anodising aluminium improves the insulation properties of aluminium as aluminium oxide is not a good conductor of electricity.
The size of the anodised layer or pores can be manipulated by the temperature and voltage used in the anodising process, lower temperatures and higher voltages result in smaller pores and harder surface. One disadvantage to the harder or type 3 anodising is the smaller pores are harder to colour.
Principle working
Anodising works on the principle of an electrolytic cell. In this process, the anodising tank is filled with a suitable electrolyte, usually a diluted Sulphuric acid solution. In this tank, the part is suspended to expose the surface to the electrolyte.
Then we place plates of suitable cathodes (usually lead or aluminium) in the tank. The next step is to use a suitable power source and complete the circuit between the cathode and anode.
The positive terminal of the power supply/rectifier is connected to the aluminium product (cathode), and the anode plates are connected to the negative terminal of the power supply. As the circuit is now complete, the current can pass through it.
The voltage, current and duration of the electrical current passed as well as the temperature of the electrolyte will determine final characteristics such as the thickness of the aluminium oxide layer on the anodised aluminium product.
Aluminium Anodising Process
The majority of anodising setups today still use the original sulfuric acid bath. But there are, a few new features that have been added to significantly improve the final result, improving aesthetics and functionality of the product.
The modern aluminium anodising process is very technical. Generally, aluminium anodising consists of the following steps.
Cleaning/Degreasing
The surface of the aluminium product needs to be cleaned before anodising so it is free from any trace of oil, dirt or grease from the manufacturing process. The aluminium is dipped in a cleaning solution of acidic or alkaline cleaning agents to clean grease/dirt from the surface.
Pre-treatment
This step eliminates any surface imperfections. The goal is to provide a visible finish with a clean and smooth surface.
Etching
Etching removes a layer of aluminium from the product surface to provide a matte finish. A hot solution of sodium hydroxide is used to remove surface imperfections.
Desmut
Desmut is the act of removing excess alloyed metals from the surface from aluminum after etching. Desmutting can be done in any mineral inorganic acid such as hydrochloric, sulfuric, nitric, etc. Even some old time anodizers would desmut in the anodize tank before adding current!
Anodising
After pre-treating, the product is ready for anodising. Sulfuric acid is the go-to electrolyte for aluminium anodising and this is where the electrical current is applied to the parts to produce the oxidised layer on the surface of the aluminium.
Colouring
There are several methods to add colour to anodised aluminium. At Anotech we use organic dyes by Clariant for colouring anodised aluminium.
Dyeing
Dyeing is a popular method of adding colour to an anodised aluminium product. The pores that are formed during the electrochemical process readily absorb dyes or pigments.
They fill the pores through the entire thickness of the aluminium oxide layer. Since the thickness of this layer can be up to 50 microns in some cases, this method is quite durable. Scratching or rough usage of the part doesn’t affect the colour due to the layer’s thickness. Also, the range of available colours is wide.
Sealing
Sealing is the final step in the aluminium anodising process. This prevents water leakage and improves corrosion resistance of the anodised aluminium product as well as colour fastness. At Anotech we use a chemical sealing solution at 85 degrees for different lengths of time depending on the thickness of the anodising layer.
Sealing reduces the chances of staining, scratching, colour degradation and crazing of the surface.
Types of Anodising
Based on the thickness of the aluminium hydroxide layer, there are 2 main types of anodising.
Decorative anodising
Decorative anodising, as the name implies, has its focus on providing a nice aesthetic finish first and providing protection as more of bonus.
For decorative anodising, we recommend a layer is between 15µm to 25µm.
Hard anodising
In cases where you need superior protection the aluminium (marine applications or exposure to corrosive chemicals), we offer a hard anodising option.
The thickness of the oxidation coating is usually between 25µm and 50µm for the full benefits of hard anodic oxidation coatings.
Final Finish
Anodising gives the aluminium surface a superior appearance. As we know that the surface consists of the pores with pigments as well as the uncoloured portions where the surface reacted with oxygen to prevent further oxidation. As the light strikes both these surface features at the same time, it interferes on reflection, giving the metal an attractive metallic shine.
The surface also has very few imperfections as it reacts uniformly with the electrolyte giving it a smooth finish.
Anodising vs Powder Coating
Powder coating is a type of surface treatment that is most common for coating steels but also available for aluminium. The surface of aluminium is coated with polyester powder for decoration as well as protection. Manufacturers have a choice between these two methods when they are looking for surface treatment options for aluminium.
Anodising is better than the traditional powder coating in many ways, some of which are as follows:
It is an inorganic finish and provides a superior surface finish compared to organic counterparts such as powder coating.
When it comes to appearance, anodising has a metallic sheen and is extremely well integrated with the surface compared to powder coating. An anodised surface reacts differently to both natural and artificial light.
Anodising is also better in the long run. Powder coating sometimes suffers adhesion failure and even if it doesn’t, the colour will fade over time.