Weathering Resistance of Anodised Semi-products

The development of applications of aluminium in constructions such as metallic fittings, foldable walls, shop equipment, urban furniture stems from the excellent resistance to atmospheric corrosion of anodic layers. These layers aim at maintaining the surface appearance over a long period of time by preventing blackening and pitting corrosion. 

On anodised products, alterations of the surface appearance can occur  

In the short run, within a few months up to a few years. This leads to iridescence [16], chalking, and powdering. These forms of ageing depend more on the anodising conditions (especially sealing) than on the climatic conditions [17]. Some can be eliminated by appropriate cleaning. 

Anodising delays the occurrence of pitting, but does not prevent it once it has initiated. As with other coatings, sharp angles are more sensitive to pitting corrosion than plane or bent surfaces. That is why sharp angles should be avoided as far as possible. 

For architectural applications, the piece is anodised in a sulphuric bath, followed by sealing in boiling deionised water. Electrolytic colouring is obtained with nickel, cobalt and tin salts. 

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