Anodising (Topic 11050)

In the anodising process, the natural oxide layer on aluminium is replaced by a thicker oxide which is produced by an electrochemical process. The corrosion protection achieved from the anodising, depends on the thickness and quality of the oxide film, for example with regards to sealing. In corrosive environments the thickness of the oxide layer should be about 25 µm , while thinner layers may be sufficient in milder conditions. The anodic layer is quite porous and in order to get good corrosion protection a sealing of the layer is normally done in hot water or steam. The parameters of this process needs to be carefully controlled to get maximum corrosion protection. In most conditions anodising will give an excellent corrosion protection against pitting and general corrosion. The oxide layer is an electrical insulator and as long as it is free from faults it will be a very good protection against galvanic corrosion.

Whenever appearance and weathering resistance is important, anodising is an attractive alternative. Before sealing different dyes can be added to the oxide layer for colouring. The surface will also become hard and fairly resistant to wear, abrasion and scratching. However, anodising will not give a long term protection against strong acids or bases. This is due to the decreased stability of the oxide layer at high and low pH.

Hard anodising is a special variant of the anodising process which provides a hard and wear resistant surface with a thick oxide layer. This is normally not sealed.

It is not possible to do anodising on site, but on the other hand, the work done in a plant will normally be of a high and even quality. Structures that have been welded will have the be anodised after the welding. Since the process is done in baths there will be a limitation on the size of structures that are possible to treat.

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