Electrochemical Theory of Rusting

Corrosion Of Iron

Corrosion is the process in which a metal is destructed as a result of its reaction with environment. Corrosion of iron is known as rusting. Rusting is the hydrated ferric oxide. Other examples for corrosion are tarnishing of silver and development of green coating on copper and bronze. In corrosion metals undergo anodic oxidation to metal oxides.

Electrochemical theory of rusting

The rusting of iron is an electrochemical process involving the following steps.

The moister containing CO2 acts as electrolyte.

H2O + CO2 ------> H2CO3
H2CO3 ------> 2H+ + CO32-

The iron is oxidized by the removal of electrons and acts as the anode.

Fe ------> Fe2+ + 2electron

The H+ ions from the electrolyte accept electrons from the adjacent areas on metal surface and function as cathode.

4H+ + 4electron ------> 2H2

The atmospheric oxygen moves hydrogen as water.

2H2 + O2 ------> 2H2O

Adding the above two equations,

4H+ +O2 + 4electron ------> 2H2O

Fe2+ formed at the anode is further oxidized to Fe3+ by atmospheric oxygen in presence of moisture.

4Fe2+ + O2 + 4H2O ------> 2Fe2O3 + 8H+

The ferric oxide gets hydrated to form rust.

Fe22O3 + xH2O ------> Fe2O3.xH2O (rust)

Rust is chemically hydrated iron oxide (Fe2O3.xH2O). It does not stick to the surface of iron. Hence fresh surface of iron is always exposed and undergo further rusting. Corrosion causes several damages to buildings, bridges, ships and other objects made of metals. Some of the important methods used to prevent corrosion of metals are barrier protection, Sacrificial protection, Electrical protection, using anti rust solution.

Prevention of Corrosion Methods

1. Barrier protection

In this method a barrier is placed between metal and atmosphere. This is actually done by coating the metals surface with paint oil or grease. The barrier avoids the direct contact of the metal surface with the environment and hence prevents corrosion.

2. Sacrificial Protection

In this method the iron object is covered with a thin coating of metal like zinc which is more reactive than iron. Here the active metal which is coated will act as anode and undergo oxidation in preference to iron. The process of coating iron objects with zinc is called galvanization.

3. Electrical Protection or Cathodic Protection

In this method, the iron object which is to be protected from corrosion is made cathode by coupling it with another more active metal. Here the active metal acts as anode and protect the iron article from destruction. The iron object will remain protected as long as the active metal in present.

4. Using anti rust Solution

Alkaline phosphate and alkaline chromates are used as anti rust solutions. When anti rust solution are applied on iron objects, the iron phosphate or iron chromate formed are act as insoluble and heat resistant coating and prevent rusting.

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