As a result of aluminium"s pronounced ability to react with oxygen (to "oxidise"), a compact layer of aluminium oxides forms immediately on bare surfaces on aluminium and its alloys form when they come into contact with air or water. Unlike other metal oxides, such as rust or iron oxide on iron, this natural oxide layer is very adherent. It protects the underlying aluminium against further oxidation and to a large extent makes it resistant to the effects of the weather. The natural oxide layer is corrosion resistant between pH 4.5 and 8.5. The acidic or alkaline attack outside this range depends on the strength of the acid or alkali and the exposure time. If the oxide is damaged mechanically or by pickling, a new oxide layer forms immediately. The build-up and thickness of the oxide layer depends on the humidity of the air or the composition of the water:
- In dry air and at normal temperatures, an aluminium oxide layer up to several millionths of a millimetre thick forms within minutes and during the course of the next few days grows to double or treble this thickness. Growth rate and thickness increase with increasing temperature. Above about 500 °C the oxide layer is crystalline (aluminium oxide forms as crystals), and below that it is amorphous (non-crystalline). The crystalline form is better at impeding further growth than the amorphous form.
- In moist air (for example aluminium parts used in building construction that are exposed to the weather), the oxide layer can reach a thickness of a thousandth of a millimetre, which is significantly more than in dry air. In addition, in this case it consists of two layers: An almost pore-free base or barrier layer of amorphous aluminium oxide forms on the surface of the aluminium, with a porous surface layer, which in addition to aluminium oxide also contains aluminium trihydroxide (aluminium oxide containing water) and can have a grey colour as a result of the inclusion of dirt and dust. The surface layer protects unalloyed aluminium, refined aluminium and low-copper alloys against corrosion even in aggressive industrial air. At higher humidity and where there are marked temperature changes it is possible that acidic droplets (e.g. containing sulphur dioxide) condense on the surface layer; if they stay in contact with the surface for a prolonged period, they tend to dissolve the oxide layer and after that the aluminium at a point, leading to the formation of whitish efflorescence.
- In natural or tap water, a two-part oxide layer forms as in moist air. If the water contains aggressive chemicals (for example chlorine compounds) or heavy metals, pitting is possible: for example, copper ions force their way through pores to the aluminium surface where they form a galvanic cell, which dissolves the aluminium.
The oxide layer does not offer protection against corrosion in all cases. Oxide layer and protective effect can be strengthened by the action of boiling water or hot steam. A surface layer of aluminium monohydrate (boehmite) up to one to two thousandths of a millimetre thick forms on top of the barrier layer within a matter of minutes. It is colourless to milky in appearance and provides protection, for example against fruit acid and lactic acid used in food packaging. Even more corrosion resistant oxide layers are produced artificially by anodising.