The eating up of metals by the action of air and moisture on their surface is called corrosion.
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The corrosion of iron is called rusting. While other metals are said to ‘corrode’, iron metal is said to ‘rust’.
Rusting of Iron
When an iron object is left in damp air (or water) for a considerable length of time, it gets covered with a red-brown flaky substance called rust. This is called rusting of iron.
Conditions Necessary for the Rusting of Iron
Rusting of iron (or corrosion of iron) needs both, air and water. Thus, two conditions are necessary for the rusting of iron to take place:
- Presence of air (or oxygen)
- Presence of water (or moisture)
The chemical composition of rust is hydrated iron (iii) oxide, Fe2O3. xH2O
Experiment to show that Rusting of Iron Requires Both, Air and Water
We take five test-tubes and put on clean iron nail in each.
- In the first test-tube, the iron nail is put in tap water without a cork so that it is in contact with air and water.
- In the second test-tube containing iron nail, we put some anhydrous calcium chloride and close its mouth with a tight cork. The anhydrous calcium chloride is added to absorb water (or moisture) from the damp air present in the test-tube and make it dry.
In this way, the iron nail in the first test-tubes is kept in dry air (having no water vapour in it).
- In the third test-tube containing iron nail, we put boiled water. Boiled water does not contain any dissolved air (or oxygen) in it (This is because the process of boiling removes all the dissolved air from it).
A layer of oil is put over boiled water in the test tube to prevent the outside air from mixing with boiled water. In this way, the iron nail is kept in air-free, boiled water.
The mouth of this test-tube is closed with a cork and it is kept aside for about one week.
- In the fourth test-tube containing an iron nail, oil is added so that the nail is in contact with neither air nor water.
- In the fifth test-tube, the nail is placed in sea water so that it is in contact with air, salt and water.
After one week, we observe the iron nails kept in all the three test-tubes, one by one. We find that:
- No rust is seen on the surface of iron nail kept in dry air (water-free air) in the second test tube. This tells us that rusting of iron does not take place in air alone.
- No rust is seen on the surface of iron nail kept in air-free, boiled water in the third test tube. This tells us that rusting of iron does not take place in water alone.
- No rust is seen on the surface of iron nail kept in oil in the fourth test tube, where the nail is in contact with neither air nor water
- Red-brown rust is seen on the surface of iron nail kept in the presence of both air and water together in the first test-tube.
- The nail in the fifth test-tube in contact with both air and salty water is very rusty.
- This tells us that rusting of iron takes place in the presence of both air and water together.
How to stop things rusting
Stopping air and water from reaching iron and steel can prevent rusting. Putting a thin layer on the surface of the iron or steel can stop air and water from reaching iron and steel.