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Extraction of Metals

Aluminium - Resistance to Corrosion.

Corrosion is the name given to the process
where a metal reacts with oxygen in the air to form a metal oxide.
Corrosion is the reverse of the process used to extract the metal.
If the metal is iron, corrosion is called rusting.
In general, the more reactive a metal is the more quickly it corrodes.

Aluminium is a reactive metal.
It is above carbon in the reactivity series but is resistant to corrosion
because aluminium reacts with oxygen in the air
and forms a thin layer of aluminium oxide (Al₂O₃).
The oxide layer covers the surface of the aluminium metal
and prevents any further reaction (corrosion) from happening.
The thickness of the oxide layer can be increased by anodising.

The oxide layer will react with an acid or alkali
to form an aluminium salt  + water.
The exposed aluminium metal will then itself react
with acid or alkali to form the aluminium salt  + hydrogen.
It is therefore best to avoid contact of aluminium cookware
with citrus fruits or vinegar (acids),
or alkali cleaners such as caustic soda (sodium hydroxide).

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