Aluminium is a really useful metal because it is light and strong. In many applications it is superior to steel, not least because it can be left out in the weather for years without significantly deteriorating.
Steel in similar situations would need a protective coating, such as paint or zinc. The surfaces of both aluminium and steel oxidize when exposed to the oxygen in the air. The difference is that the layer of aluminium oxide formed on aluminium is ‘waterproof’: it is impermeable to water. If there were no oxide coating, the aluminium would rapidly react with water, forming aluminium hydroxide and hydrogen gas, destroying the aluminium. The iron oxide that forms on the surface of steel is permeable — water can travel through it, forming hydrated iron III oxide, which we call ‘rust’. Eventually the steel will be destroyed as it rusts away.
The layer of aluminium oxide is only a few atoms thick, but is tough and can be polished, giving the aluminium an attractive appearance. Over time the surface of the aluminium weathers and deteriorates. It has been found that a thicker layer of oxide better protects the metal from weathering. Passing a low-voltage DC electric current through an acid electrolysis bath with the aluminium being the anode produces highly reactive oxygen atoms on the surface of the aluminium. The oxygen reacts with the aluminium to form aluminium oxide. This increases the thickness of the oxide layer, greatly extending the time that the aluminium looks shiny and new. Dyes can be added to the anodizing bath to colour the oxide layer.
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website: the-shed.nz/ home/2019/8/25/colour-anodise-thataluminium-alloy.