Aluminium's ability to respond to anodising makes it an important metal. The fact that aluminium can be made in this attractive, durable and tough finish makes it adaptable to a wide range of applications from partitions to vehicle components. Its strength and lightness makes it important in the building and construction industries.

Anodising is an electro-chemical process in which the natural oxide film on aluminium is thickened by passing an electric current through certain acid electrolytes with the aluminium part as the anode. It is a conversion of the metal. The anodised surface is a protective inert film of aluminium oxide. This film will not crack, peel, or chip unless subject to severe stress or extreme thermal changes.

anodising process

The basic anodising process consists of a chemical pre-cleaning dip, followed by etching in a caustic soda base solution, anodising electrolytically in a sulphuric acid or other solution, and finally sealing to reduce porosity. Varying the electrolyte composition and modulating the process parameters allows the production of coatings bearing specific properties. For example, very hard anodic films can be developed to provide abrasion resistant surfaces on bearings, pistons, gears, and other components.

Anodising also provides a means of colouring the oxidised metal. Anodic films are coloured by a variety of methods. Sulphuric acid films are microscopically porous, so pigments and organic or inorganic dyes can be combined and sealed in the film. The more durable coloured films necessary in exposed environments can be produced as part of the anodic layer and are quite permanent.

The thickness of the aluminium oxide coating can be varied by processing time. The following figures are minimum standards.

  • 10 Micron
    is suitable for internal applications, and outdoor applications where cleaning is very frequent, for example, caravan trim.

    15 Micron
    is recommended for the majority of ordinary architectural requirements.

  • 25 Micron
    is recommended for heavy duty external permanent architectural applications where little deterioration can be tolerated.

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