Galvanisation

The process in which depositing a layer of zinc on iron occurs is known as galvanisation. The iron pipes we use in our homes to carry water are galvanised to prevent rusting. You know that ships are made of iron and a part of them remains under water. On the part above water also water drops keep clinging to the ship’s outer surface. Moreover, the water of the sea contains many salts. The salt water makes the process of rust formation faster. Therefore, ships suffer a lot of damage from rusting in spite of beingpainted. So much so, that a fraction of ship’s iron has to be replaced every year.

Corrosion of Aluminium:

Due to the formation of a dull layer of aluminium oxide when exposed to moist air, the aluminium metal loses its shine very soon after use. This aluminium oxide layer is very tough and prevents the metal underneath from further corrosion (because moist air is not able to pass through this aluminium oxide layer). This means sometimes corrosion is useful.

Corrosion of Copper:

When a copper object remains in damp air for a considerable time, then copper reacts slowly with carbon dioxide and water of air to form a green coating of basic copper carbonate [CuCO₃.Cu(OH)₂] on the surface of the object. Since copper metal is low in the reactivity series, the corrosion of copper metal is very, very slow.

Corrosion of Silver:

Silver is a highly unreactive metal, so it does not reacts with oxygen of air easily. But, air usually contains a little of sulphur compounds such as hydrogen sulphide gas (H₂S), which reacts slowly

with silver to form a black coating of silver sulphide (Ag₂S). Silver ornaments gradually turn black due to the formation of a thin silver sulphide layer on their surface and silver is said to be tarnished.