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Aluminium finishes: Anodising

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Anodising finish is the electrolytic process which allows to create artificial layers of aluminium oxide on the surface of the profiles.

The aluminium oxide or alumina (Al2O3) has a very special characteristic, this is that, unlike what it happens for example with iron, its layer of aluminium oxide is uniform (0, 01 micron) and protective. Due to this property, what we pretend with anodising is to protect the aluminium with and artificial layer of alumina inherent in the material, with a much higher thickness.

Being a film of its own oxide, its union is due to chemical bond forces. This bond between the protective layer and the material makes that by this finish we can get profiles extremely resistant to corrosion, abrasion and wear.

Extrual has two advanced and modern anodising plants in which different types of finishing are made. All of them in their different shades and mechanical treatments.

Anodising process

1 Degreasing bath + rinsing

With this bath any remaining trace of dirt is eliminated after the extrusion process. As a result we get a clean and chemically processed surface.

2. Etching or chemical satin finish + rinsing

 In this step the natural oxide is eliminated, getting a silky and satin finish.

3. Neutralising + rinsing

Here the traces of the previous etching are eliminated to avoid contaminations in the next process and to have a neutral surface.

4. Anodic oxidation + rinsing.

This step is very important, and the one which define the anodising process. It is an electrolytic bath by which the anodic coat is made. By inducing an electrical current between the anode+ and cathode-, the ions of the water (H2O) tend to oxidize the aluminium, forming the desired alumina coat in the profile surface. However, this “barrier film” gets a number of pores uniformly distributed, which are weak points that must be treated.

5. Colouring bath + rinsing

This process is optional, because the material could be presented in its natural colour. In this treatment, the pores previously mentioned are filled with metallic salts or different chemical compounds which would bring the planned colour.

6. Rinsing in de-ionised water.

In this process the profiles are rinsed to leave them completely clean and free of acids.

7. Sealing of the pores

This process is very important. There are two methods to get the hermetic sealing of the pores: hydration and impregnation.

-Hydration: with the pore open, the profile is introduced in demineralized water. When the aluminium is hydrated the volume of the alumina layer increases its volume, closing the pores hermetically.

- Impregnation: In this case the water has metallic salts which complete the pores closing them hermetically.

It must be noted, that the anodising finishing of the aluminium is the only treatment that protects 100% the surface.

See the link: Vídeo "El Anodizado del aluminio" de AEA [Aquí]

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