Definition and basis of corrosion

Corrosion is a natural phenomenon occurring in metallic materials. Interaction with the surrounding environment leads to damage to these materials which is referred to as corrosion. Any system in the universe has a tendency to lower its internal energy to become more stable. Most metals and alloys have a higher internal energy or free energy as compared to their chemical compounds. So they try to reduce their free energy by converting into their more stable compounds and corrosion results. Metal atoms are 'oxidised' to their oxides or other compounds and become more stable.

Corrosion is an electrochemical process in the sense that when metals are converted to their compounds, electronic transfer takes place and electrical currents are produced as in a battery. Hence, corrosion may be accelerated or retarded by inducing electrical currents in appropriate directions. Galvanic corrosion and cathodic protection are examples.

Corrosion may be of various forms. Uniform corrosion, localized corrosion like pitting and gasket corrosion, dealloying, stress corrosion cracking, hydrogen embrittlement, corrosion-fatigue etc are few examples of metallic corrosion.

Corrosion leads to significant monetary losses to a nation. Corrosion damages or disfigures metallic structures or components, necessitating their repair or replacement, damage to plant components causes shut down of plants and loss of production, loss in mechanical stability due to corrosion may lead to catastrophic failures like collapse of bridges or crashing of aircrafts or explosions in boilers or storage tanks, all leading to heavy losses in money and often human lives. Because of corrosion, metals are converted to their compounds,thus leading to the wastage of all the energy and man-power spent in extracting them from their ores, as well as the loss of precious natural resources. It is estimated that the average annual loss to a nation due to corrosion amounts to about 5 % of its GNP. A proper understanding of the basic principles of corrosion enables the proper selection of materials for engineering applications which will result in much reduced corrosive loss.