Corrosion: Description and Mechanism

Have you ever witnessed an old car or a train body or a sunken ship which has been dumped in a junkyard for years? If so, you might have noticed a reddish- or yellowish-brown flaking coating formed on their surface. This is because the metal has undergone corrosion.

corrosion

It is a gradual deterioration of metals on interaction with their environment as a result of chemical reactions between them. Almost every metal is susceptible to degradation. Rusting of iron and tarnishing of silver are common examples of corrosion.

Corrosion makes the metals flaky and friable and decreases their intended usage period. The strength, appearance, and permeability are also degraded. A considerable amount of damage is done by corrosion every year, which includes damaging of cars, grills, buses, railings, sinking of ships, falling off bridges, deterioration of statues, etc.

It occurs due to the formation of metal oxides when the metal is oxidized by losing its electrons. It is an electrochemical phenomenon. Let us see how.

We take the example of rusting of iron. Rust is formed in the presence of oxygen and moisture. The site of iron which undergoes oxidation can be considered as anode. Thus, iron loses electrons at anode which rush to another site and reduce oxygen in the presence H+ ions available from acids dissolved in water. This is the site of the cathode. The reactions are:

At anode:  2 Fe à2 Fe2+ + 4 e

At cathode: O2- + 4 H+ + 4 e à 2 H2O

The Fe2+ reacts with oxygen in the air and is oxidized to form rust, which is a chemically hydrated ferric oxide, that is, Fe2O3.xH2O.

A Huge sum of money is spent for the replacement and repair of damages done by corrosion. Thus, it is important to take steps to prevent it. As we have seen earlier, its prime cause is the exposure of metal to the atmosphere. So, if we don’t let the surface of the metal to come in direct contact with air, we can prevent corrosion. Coating and painting, application of grease and galvanization are some of the effective methods used for prevention.

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Practise This Question

Metal can be prevented from rusting by