Mechanism of corrosion
Metals corrode because they have a strong tendency to return to their natural, stable chemical condition. This condition is the one in which the metals occur in nature, namely in the form of ores, that means chemical compounds, mainly with non-metallic elements. The metal atoms are freed (extracted) from these compounds, for example by smelting at higher temperature. Thus every pure metal, with the exception of the noble metals that do not corrode, is in an unnatural, unstable condition. The first thing a pure metal that has just been melted usually tries to do is react with the environment in which it finds itself by covering itself with a very thin coating that seals it off from the environment and protects it from further corrosion.
Unalloyed aluminium, for example, forms a strongly adherent, pore-free layer of aluminium oxides in air by atoms from its surface chemically combining with oxygen atoms from the air. After a few days this oxide layer reaches its maximum thickness, which depending on the humidity of the air can be up to several thousandths of a millimetre. This affords the metal better corrosion resistance than most industrially used metals, namely against the elements (in air that is not strongly polluted, rain and snow), weak alkalis and acids (for example citric acid) as well as many chemicals.
Possible methods of improvement
Even refined aluminium is not resistant to very strong alkalis and acids (usually with pHs of less than five or greater than eight), and aggressive vapours and gases. Such chemicals remove the oxide layer and then dissolve the aluminium atoms either locally or over larger areas. Aluminium products can be protected against this in a controlled manner, however: by strengthening the oxide layer by anodising, by surface treatment (for example coating), by lacquering or by a combination of these processes.
One possibility for protecting aluminium against corrosion by certain substances from the start is alloying, that means adding other metals to it. For example, a few per cent of magnesium makes it resistant to seawater and suitable for use in shipbuilding. By contrast, copper and zinc can increase the susceptibility to corrosion.
In construction and in various applications one has to take care to avoid contact corrosion. At points of contact with more noble metals such as iron and copper, the ingress of moisture can lead to the formation of an electrolytic cell, which results in a small electric voltage that can lead to chemical attack of the aluminium.