Anodising is the process by which metals can be coated with a protective layer. The metal to be treated is the anode electrode in an electrolytic cell. There are many benefits to anodising. The formation of oxide coatings can improve the performance of the treated metal. For example, treating aluminium in this way will create a layer of aluminium oxide, which makes it harder, more inert, and insulated. It can also be dyed.
The layer created by the anodising process is known as the film, and the thickness of this film usually depends on the type of material and how long the anodising process goes on. Anodised coatings adhere extremely well to the substrate as they are integral to the structure, which can be an advantage in comparison to painting or electroplating. The film will mirror the surface of the original item. Here’s a look at three main areas of anodising.
Chromic Acid Anodising
The use of chromic acid is the oldest known method of anodising. It was first used on an industrial scale in 1923, when it protected seaplane parts from corrosion. It produces a thin, opaque film that is softer and can self-heal to some extent. These surfaces can be harder to dye but are sometimes used to pretreat a metal before painting or in certain specialised applications.
Sulphuric Acid Anodising
The most widely used solution to form an anodised film is sulphuric acid. The films formed from this process are naturally light grey in colour, but they can be dyed. It can be used for a myriad of purposes, from cheap coloured items not expected to last long to high-tech specialised equipment.
This is a progression from the sulphuric acid anodising mentioned above resulting in thicker films. In this method, more process control is required, and the anodising will be conducted in refrigerated tanks close to the freezing point of water. Higher voltages are used in hard anodising than for the thinner coatings.
The benefits of hard anodising are that it enables the film to be harder, denser and thicker. This means the item in question will be more resistant to wear and corrosion. This method is ideal for a range of industrial applications, including hydraulic systems in the aerospace industry, gearbox or braking equipment and machinery required at deep sea levels as it will be more resistant to corrosive salt water.
It is also ideal for bakeware, as discussed in this article from The Independent.
There are some incredible examples of anodising being used in construction, as discussed by the Council for Aluminium in Building. The New Bodleian Library was completed in 1938 and has anodised aluminium windows. Refurbishment in 2014 only required these to be cleaned and re-glazed because the finish is still in good condition.
With the additional benefit of being more environmentally friendly than some other methods of finishing metals, anodising is a truly useful process.