Anodizing / Anodizing Process

Anodising refers to an electrochemical process in surface engineering that produces a protective, hard layer on an aluminium workpiece. The process utilises what is known as anodic oxidation. Experts also refer to anodising as ‘anodisation’ or the ‘anodic oxidation process’.

In anodising, no protective layer is applied to the aluminium; instead, the outermost layer of the workpiece is converted into a hydroxide or oxide. The layer can be between 5 and 25 micrometres thick – depending on the intended application. In contrast, the natural oxide layer that forms on aluminium when the material comes into contact with air is only a few nanometres thick. It is also less hard.

First and foremost, anodising serves to protect aluminium. The anodising process makes it more resistant to corrosion and extends its service life. It also helps the material retain its attractive appearance for longer. This is thanks to the hard, wear-resistant layer formed during the anodising process. It transforms aluminium into a durable building material.

Aluminium with a thicker anodised coating (20–25 µm) is primarily used for corrosion protection in the construction industry, for aircraft and automotive components, and for household items. Thinner coatings (8–20 µm) are mainly used for surfaces such as door fittings and MP3 players. They therefore serve a predominantly decorative purpose. In heat sinks, the coatings improve heat dissipation. The particularly hard and abrasion-resistant hard anodised coatings are used in industrial applications.

Not all aluminium alloys are suitable for anodising. Whilst, for example, the materials AIMg3 and AIMg5 are easy to anodise, this is not the case with AIMg4,5Mn.

Only aluminium materials can be anodised. Depending on the alloy used, the properties of the resulting coatings may vary.

Anodising is a process that is exclusively suitable for applying a protective coating to aluminium. The correct term for the process is the ‘Eloxal process’. The term ‘Eloxal’ is derived from the first two letters of the phrase ‘ELectrolytic OXidation of ALuminium’. It is therefore not possible to anodise stainless steel.
To anodise steel and stainless steel, a different oxidation process is used. In this case, the alloys can be treated, for example, by ‘blacking’ or ‘black oxidation’.

The following anodising colours are available for aluminium: blue, red, green, gold, violet and black. White is not an anodising colour; it can only be produced by applying dyes to a surface.