The process of anodising produces thick, oxide coatings, often to be used on aluminium and aluminium alloys. The thickness of the layer of oxide varies, but it generally measures between 5 and 30µm. The layer increases the resistance of the coated material and helps prevent corrosion.

Alternatively, or even additionally, it is used to give a decorative finish to items, as explained by the Aluminium Anodizers Council.

How does anodising work?

Firstly, the items that are to be covered in the oxide coating are treated to prepare them for the process. The surface of the component to be treated is oxidised, which causes an oxide film to form. This coherent oxide film is vital to the whole process as it is strongly adhered to the metal substrate that lies beneath it. Most anodising is carried out on aluminium and aluminium alloys, but it’s also done on titanium and magnesium.

As part of the preparation for anodising, the surface of the aluminium must undergo a pre-treatment. This process will impact on the finished appearance of the anodised coating, as well as a number of its other properties. There are several ways to pre-treat the aluminium alloy, including abrasive polishing, electrolytic polishing and chemical brightening. If the component needs drilling, machining or welding, this should all be done before the anodising takes place.

There are three different types of electrolyte solution used in the process of anodising. They are as follows:

– a 10-15% sulphuric acid solution, at a temperature of 25°C. When this solution is used, it will give a coating formation rate of approximately 25µm/hr.
– a mixture of oxalic and sulphuric acid, at a temperature of 30°C. This solution offers a rate of formation of around 30µm/hr.
– a 10% chromic acid solution, at a temperature of 38-42°C. This gives a lower formation rate of approximately 15µm/hr.

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What is hard anodising?

This is when thicker coatings are used; these coatings offer a higher hardness and are generally used to increase the wear resistance of the aluminium alloy underneath. In order to achieve these increased levels of thickness and hardness, oxalic acid and sulphuric acid are mixed at higher concentrations.