A low voltage, high current D.C. charge is passed through the material, which is used as the anode. The charge forms a hard non-corroding film on the surface. A porous, less dense layer is formed on top, this layer provides the means for further oxidation, which in turn builds up the thickness.

When the actual anodising operation is completed, the surface film is porous and in a condition to accept a colouring agent if required. If no colouring were required then the material would then be placed in a tank of boiling water. The chemical reaction caused by the immersion seals the pores against further moisture penetration, thus giving a hard, weather resistant surface.

If colouring is required then there are two choices, an organic die (as used in textile industry) will give primary colours, or, metallic salts which will give grey, dark brown and black.

Factors that must be considered for a finished aluminium extrusion are durability, colour stability, aesthetics, cost and Health & Safety. Anodising is a highly effective and desirable means of achieving these six considerations for a high quality finish.

Anodising is a safe process that is not harmful to human health. A finished anodised surface is non-toxic, and it is chemically stable, meaning it will not decompose. The profile is also heat-resistant to the melting point of aluminium.

The anodising process is a reinforcement of a naturally occurring oxide process. Therefore, it is non-hazardous and produces no harmful or dangerous by-products. Chemical baths used in the anodising process often are reclaimed, recycled, and reused.

These benefits of anodised profiles make them advantageous in any number of applications, from appliances to commercial and residential building products.

BS 1615 covers all general anodised coatings for aluminium and BS 3987 covers external architectural applications. European standards are covered by the Qualanod quality scheme.

The average thickness of film is designated by an AA value, the figure part of this value being the film thickness in microns.

In component anodising, the heat-affected zone of welded or brazed joints will show a slightly different colour to the rest of the section; this may be in the form of a slightly darker area or even black.

There can also be slight colour variations between batches so an upper and lower colour limit should be agreed prior to the anodising-taking place.

Electrical contact is vital between flight bars and the aluminium section during anodising. This is obtained by jigging up with non-metallic clamps. The contact areas do not anodise or colour and will therefore leave an easily recognisable patch on the material. Non visible surfaces should be clearly marked on the customer's drawings so that the clamps can be placed in such a position as not to compromise the finish. If all surfaces are visible then an extra 50mm per side should be allowed on the bar ends for clamping, which can then be cut off after anodising.

This is a low temperature process, which used a considerably higher voltage than traditional anodising. The final finished surface produced is rough and very hard, and is available up to 125 microns thick. This film is normally left unsealed but it can be treated with a wax or oil. In both cases the resistance to abrasion is high, very similar to that of tool steel. Hard anodised films have excellent electrical insulation and corrosion resistance; their resilience makes them ideal for use in very aggressive environments.