• Generally metals that are most commonly used are strong, ductile and malleable.
  • It get exposed to the environment when it is used due to which many of them will lose their shine, and get covered with coloured layer on them.
  • Some even lose their strength and become weak and brittle.
  • This occurs due to chemical interaction between the metals and their environment.
  • The spontaneous destruction of metals as they interact with environment is called corrosion.
  • Corrosion takes place in the open or exposed surface.
  • When the upper layer of the metal gets corroded, then the inner layer of the metal gets exposed, and the corrosion then continues up-to certain depth.
  • Corrosion is said to have taken place when for example, silver gets tarnished, i.e., it loses its shine, iron gets coated with a brittle brown-coloured layer, copper and brass get a green-coloured deposit, aluminium surface becomes dull, and loses its shine, and so on.
  • General corrosion affects the entire surface of the metal, while the local corrosion is confined to certain localized spots on the surface.
  • Corrosion causes enormous damage to the buildings, bridges, ships and many other things made of iron.

A) Classification of Corrosion Process

Depending up on the nature of corrosion, and the factors affecting it, the corrosion may be classified as follows:

i) Chemical corrosion

  • Such corrosion generally takes place when,

a) Reactive gases come in contact with metals at high temperatures e.g., corrosion in chemical industry

b) Slow dissolution of metal takes place when it is kept in contact with non-conducting media containing organic acids.


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ii) Bio-chemical corrosion or Bio-corrosion

  • This is caused by the action of microorganisms.
  • Soils of definite composition, stagnant water and certain organic products greatly favour the bio-corrosion.

iii) Electrochemical corrosion

  • It occurs in a gaseous atmosphere in the presence of moisture, in soils and in solutions.
  • For example, the following corrosions are electrochemical in nature:

a) Corrosion of insoluble anodes.

b) Corrosion of pipelines with current carrying liquids flowing through them.

c) Corrosion of underground metal structures.


B) Factors Affecting the Rate of Corrosion

Rate of corrosion is affected by various factors. Some of those factors are as follows:

i) Electropositive nature of metal

  • More electropositive metals are more reactive. Therefore, more electropositive metals get corroded fast.
  • For example, iron gets corroded faster than silver, because iron is more electropositive than silver.

ii) Purity of metal

  • Pure metals do not corrode. The presence of other metals (as impurity) tends to increase the rate of corrosion.
  • This happens because different metals set up local galvanic cells.
  • These local cells produce small currents, and oxidize the more reactive metal.

iii) Presence of reactive gases in the air

  • Gases such as oxides of sulphur (SO2, SO3), oxides of nitrogen (NOx) and carbon dioxide in the presence of moisture produce acids, and accelerate the rate of corrosion.

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iv) Presence of electrolytes in water

  • The presence of electrolytes such as sodium chloride in water increases the rate of corrosion.
  • For example: iron corrodes faster in sea water than in distilled water (sea water contains many dissolved electrolytes).

v) Temperature

  • A rise in temperature (within a reasonable limit) increases the rate of corrosion.


C) Corrosion Protection

  • Corrosion of metals can be prevented in many ways. Some of the methods commonly used are given below:

a) By surface coating

Corrosion of metals can be prevented by coating their surfaces with any of the following:

  • By applying oil, grease, paint or varnish on the surface.
  • By coating/depositing a thin layer of any other metal which does not corrode. For example, iron surface can be protected from corrosion by depositing a thin layer of zinc, nickel or chromium on it. Copper/ Brass can be protected by coating it with a thin layer of tin. Tinning of brass utensils is a very common practice.

b) By connecting metal to a more electropositive metal

  • A metal can be protected from corrosion by connecting it to amore electropositive metal.
  • As long as the more electropositive metal is there, the given metal does not get corroded.
  • For example, iron can be protected from corrosion by connecting it to a block/plate of zinc or magnesium.
  • This method of corrosion protection is called cathodic protection.

c) By forming insoluble phosphate or chromate coating

  • Metal surfaces are treated with phosphoric acid to form an insoluble phosphate coating on the surface.
  • This phosphate coating protects the metal from corrosion.
  • Formation of a thin chromate layer also prevents the corrosion of metals.