The process of chemical decay of metals due to the action of the surrounding medium is called corrosion.


When metals come in contact with atmospheric gases, the surface of metals is coated with oxides, sulfides, carbonates, etc. sometimes these compounds form a compact layer on the surface. Thus metal is protected from further attack.

However, if water is present then layers of oxides, sulfides, and carbonates are dissolved in the water. Thus corrosion penetrates the metal. Further water promotes an electrochemical process which is the main cause of corrosion.


Two critical theories of corrosion are

                       Acid Theory

                       Electrochemical theory  


100% pure metal does not corrode.

Impurities present in metal promote corrosion.

Consider an example

Let Al is present in contact with Cu.

Moisture and CO₂ are present on the surface of the metal.

Water ionizes to H+ and OH ions.

CO2 is dissolved in water to produce H₂CO3 which ionizes into H+ and HCO3  ions.

                    H₂O            H+   +   OH  

                    CO2     +    H2O              H₂CO3

                    H₂CO3             H+    +      HCO3

Others words metals are immersed in a solution of H+, OH-, and HCO3-  ions.

Thus a Galvanic call is set up. In this cell, Al releases electrons and changes into Al³+ ion. It acts as a negative electrode while Cu acts as a positive electrode it is because Al is more reactive than Cu and is present above Cu in the electrochemical series.

Al³+ ions combine with OH–  ­ions to form Al (OH), ions. Thus Al starts dissolving.  

                Al         à      Al³+     +   3e­­­-

                      3H+       3e­­­-             à     3/2H₂

Thus Al corrodes rapidly when in contact with Cu. The Cu receives electrons and releases H₂ gas from its active surface.


When an active metal (higher in electrochemical series) is in contact with less active metal (lower in chemical series). The active metal corrodes rapidly while the other remains intact.


Prevention of corrosion is very important. In the case of iron, its corrosion is about 1/4th of its annual production. Thus corrosion must be prevented.

The following methods are generally used.

Coating of Metals

It is the simplest method. In this metal surface is coated with oil, paint, varnish, or enamel.


Corrosion can be prevented by alloying metal with other metals. e.g decay of Fe is prevented by alloying it with Ni, Cr, etc.

Metallic Coating

A protective layer of another metal on the surface of the metal can also prevent corrosion.


In this process, a clean sheet of iron is dipped in the molten tin. It is then passed through hot pairs of rollers The surface of iron is coated with a thin layer of tin. The tin itself is very stable. Thus it prevents the rusting of metal.

It is used in the manufacture of tin cans, oil containers, and other similar articles. If the protective layer is damaged, iron comes in direct contact with moisture. A galvanic cell is set up in which tin acts as a cathode while iron acts as an anode. Electrons flow from iron to tin. Iron is oxidized to Fe3+ which is from Fe (OH)3, in combining with OH-  ions. On the surface of tin, H+  ions are discharged to H₂ gas. The dissolves more rapidly.

Hence, it can be concluded that tin-plated iron is rusted more rapidly than non-plated iron if the tin coating is damaged.


It is done by dipping clear iron sheets in a ZnCl₂ bath and heating. Iron sheets are removed and rolled bath and air cooled.

If the protective coating of Zn is destroyed then a galvanic cell is set up. In this cell, Zn acts as an anode, and iron act as a cathode. Electrons flow from Zn to iron.

Thus Zn decays while Fe remains intact. This is called sacrificial corrosion.

          Fe²+     +      Znà     Zn²+     + Fe  

This type of galvanizing is used in water pipes.

Benefits of Metal Coating


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