CORROSION

 

CORROSION

Corrosion is one of the most common phenomena that we observe in our daily lives.  You must have noticed that some objects made of iron are covered with an orange or reddish-brown coloured layer at some point in time. The formation of this layer is the result of a chemical process known as rusting, which is a form of corrosion.

Cossorion, in general, is a process through which refined metals are converted into more stable compounds such as metal oxides, metal sulfides, or metal hydroxides.

rusting of iron involves the formation of iron oxides via the action of atmospheric moisture and oxygen. 

an electrochemical process since it usually involves redox reactions between the metal and certain atmospheric agents such as water, oxygen, sulphur dioxide, etc.

Examples

1. When copper metal is exposed to the environment, we observe copper turning bluish-green in colour

2. Silver reacts with sulphur and sulphur compounds in the air, giving silver sulphide (Ag2S), which is black in colour.

 

3. Corrosion of Iron (Rusting)

Rusting of iron refers to the formation of rust, a mixture of iron oxides, on the surface of iron objects or structures. This rust is formed from a redox reaction between oxygen and iron in an environment containing water.

When Iron is exposed to air, the oxygen atoms bond with iron atoms, resulting in the formation of iron oxides. This weakens the bonds between the iron atoms in the object/structure.

The reaction of the rusting of iron involves an increase in the oxidation state of iron, accompanied by a loss of electrons. 

Oxygen is a very good oxidizing agent whereas iron is a reducing agent. Therefore, the iron atom readily gives up electrons when exposed to oxygen. The chemical reaction is given by:

Fe → Fe2+ + 2e–

The oxidation state of iron is further increased by the oxygen atom when water is present.

4Fe2+ + O2 → 4Fe3+ + 2O2-

Now, the following acid-base reactions occur between the iron cations and the water molecules to produce hydroxides of iron.

Fe2+ + 2H2O ⇌ Fe(OH)2 + 2H+

Fe3+ + 3H2O ⇌ Fe(OH)3 + 3H+

The resulting hydroxides of iron now undergo dehydration to yield the iron oxides that constitute rust. 

How can Rusting be Prevented?

1. Galvanization is the process of applying a protective layer of zinc on a metal. This can be done by dipping the metal to be protected in hot, molten zinc or by the process of electroplating.

2. Cathodic Protection - This can be done by making the iron as a cathode by attaching a sacrificial anode to it. This anode must have an electrode potential that is more negative than that of iron.

Commonly used as sacrificial anodes are magnesium, zinc, and aluminium. 

3. Coatings - Many types of coatings can be applied to the surface of the exposed metal in order to prevent corrosion. Common examples of coatings that prevent corrosion include paints, wax tapes, and varnish.