CORROSION

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           CORROSION

The process of slowly eating up of metals due to their conversion into oxides, carbonates, sulphides, sulphates, etc. by the action of atmospheric gases & moisture is called Corrosion.

1. The surface of silver metal gets tarnished when exposed to air. This because it reacts with Sulphur in the air to form a coating of black silver sulphide.

2. Copper objects lose their lustre or shiny brown surface after sometime. This is beacuse when the surface after sometime. This is because when the surface of copper remain exposed to air, it's Surface is attacked by moist carbon dioxide present in the air forming a green coating of basic copper carbonate.

 • Rusting of Iron

When iron is exposed to moist air for a long time, it's surface acquires a coating of brown, flaky substance called rust. This is due to attack of oxygen & water vapour present in the air on the surface of iron. It is called rusting of Iron.

• Prevention of rusting:

Rusting of iron can be prevented if damp air, that is, air & moisture are not allowed to come in contact with them.
The various methods commonly used for preventing the Rusting of iron are: 
• painting
• oiling & greasing
• Galvanisation
• Coating with tin, chromium & nickel
• Alloying with nickel & chromium

1. Painting:   The most common method of preventing rusting of iron objects is to coat their surface with a paint. By painting the surface, air & moisture are not allowed to come in contact with its surface & thus rusting does not occur. 

2. Oiling & greasing:   When a thin film of oil or grease is applied on the surface of an iron object, the air & moisture cannot come in contact with it & thus rusting is prevented.

3. Galvanisation:   The method of protecting steel & iron from rusting by coating them with a thin layer of zinc is called Galvanisation.
Galvanisation is generally done by immersing the iron object in molten zinc. When zinc is coated on the iron object, iron cannot come in contact with air & moisture & carbon dioxide of the atmosphere to form an invisible thin layer of basic zinc carbonate.

4. Coating with tin, chromium & nickel:   The Metals such as tin, chromium & nickel are resistant to Corrosion. Hence, when a thin layer of these metals is applied in the iron & steel objects by electroplating, they are protected from rusting.

5. Alloying with nickel & chromium:   When iron is alloyed with chromium, nickel & some carbon, stainless steel is obtained. Stainless steel resists Corrosion & therefore, does not rust at all.  For ex, cooking utensils, surgical instruments, etc. are made of stainless steel & thus do not rust at all.

Corrosion of Aluminium

When aluminium remains exposed to moist air, its surface is covered of with a thin impervious layer of aluminium oxide. The aluminium oxide layer does not allow moist air to come in contact with the metal & thus prevents the metal underneath from further damage. 
Thus, a common metal which is highly resistant to Corrosion is aluminium.


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