When a metal is attacked by substances around it such as moisture, acids, etc. is said to corrode and this process is called corrosion.
Corrosion is a natural process, it converts a metal to a more chemically stable form such as its oxide, hydroxide, or sulfide Gradual destruction of materials by chemical reactions with the environment is known as corrosion.
It is the process where pure metals react with water or air and they transform into useless substances because their useful and desirable properties such as strength, appearance, etc get degraded by the process of corrosion.
Fun Fact: Did you know that corrosion is an irreversible process?
Which metals can get corroded?
Metals like zinc, iron etc get easily corroded because they are placed higher in the reactivity series and thus have greater tendencies to get oxidised whereas the metals lower in the reactivity series do not corrode example, gold and platinum.
Silver articles become black after some time when exposed to air because they react with sulphur in the air to form a coating of silver sulphide on the surface of silver articles.
Iron when exposed to moist air for a long time acquires a coating of a brown flaky substance. This brown flaky substance is known as rust.
Copper reacts with moist carbon dioxide in the air and slowly loses its shiny brown surface and acquires a green coat. This green substance is called rust.
Under which conditions Iron can rust?
Now we will learn about the conditions under which iron rusts through an experiment.
- Take 3 test tubes
- Place clean iron nails in each of them
- Label them A, B, C
- Pour some water in the test tube A and cork it
- Pour boiled distilled water in test tube B. Add about 1 ml of oil and cork it. The oil will float on the surface of water and prevent the air from dissolving in water.
- Put some calcium chloride in test tube C and cork it. Calcium chloride will absorb the moisture, if any from the air in the test tube.
- Leave the test tube for few days
What do you observe?
1. You will observe that iron nails rust in test tube A but not in test tube B and C
2. In test tube A, the nails are exposed to both air and water
3. In test tube B, nails are exposed to only water, but not air
4. In test tube C, nails are exposed to dry air but not moisture
This shows that conditions under which iron rusts are:
- Availability of water
- Availability of oxygen
Prevention of Corrosion
Let us learn how we can prevent the corrosion of metals in detail.
Rusting of iron can be prevented by- Painting, oiling, greasing, galvanizing, chrome plating, anodizing or making alloy.
- Reduce exposure to rain/water in the environment.
- Control the amount of sulfur, chlorine, or oxygen in the surrounding environment.
- Coating of Powder: Powders or composition of these powders such as Acrylic, polyester, epoxy, nylon, and urethane can be applied on the clean surface of the metal and heated to fuse the powder on the metal as an unbroken film which can protect the metal from corrosion.
- Coating of Paints: One of the most affordable ways of preventing corrosion is to coat the painting on the metal which will act as a barrier between the corrosive solution and the metal.
- Galvanizing: The process of coating zinc over steel or iron to prevent the metal from rusting is called galvanization. Galvanizing the metal makes the metal rust-proof!