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Metals
Corrosion of metals
1) What is corrosion ?
The corrosion of a matter is its degradation.
It may have mechanical causes such as frictions or chemical causes.
In the open air, metals are in contact with dioxygen, water, carbon dioxide and gaseous pollutants that can cause a chemical transformation in which the metal is one of the reactants. The latter is then consumed and some products are formed.
2) Corrosion of gold
Gold is one of the few metals to be free from corrosion. It thus preserves its shine and color despite prolonged exposure to air.
It is said that gold is unalterable and this is one of properties that makes it a precious metal.
3) Copper corrosion
Copper corrodes in contact with air: it is covered with a greenish layer called verdigris.
This layer is impermeable and isolates copper from air and reactants responsible for its corrosion.
Copper corrosion stops as soon as verdigris forms.
Copper corrosion remains superficial, it only modifies its appearance.
Example: The Statue of Liberty is covered with verdigris
4) Corrosion of iron
The iron reacts with dioxygen and water to form some reddish rust. This porous layer of rust lets air and water pass: they can reach iron and keeps reacting with it until it is completely transformed into rust.
An iron object can deeply corrode and may even totally disappear.
The iron must then be protected with a layer of paint or varnish which isolates it from the water and air.
5) Corrosion of silver
Silver oxidizes on contact with dioxygen from the air: it darkens by covering with a layer of silver oxide. This impermeable layer prevents corrosion from spreading.
6) Corrosion of aluminum and zinc
The corrosion of the zinc and aluminum is the same type as that of copper and silver: it only attacks the metal surface on which is formed an oxide layer (zinc oxide or aluminum oxide ).
However, the layers that form in both cases are greyish. It is therefore difficult to distinguish them from metal they cover.
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