Corrosion is defined as destruction of a metal or alloy by chemical or electrochemical reaction with its environment. In most instances, the reaction is electrochemical in nature: a flow of electricity between certain areas of a metal surface through a solution capable of conducting an electric current. This electrochemical action causes destructive alteration (eating away) of a metal at areas.
Though corrosion is a complex function of many factors, the three most important are-
– a voltage differential between pure and impure areas
– physical conditions of temperature and humidity
– and oxygen in the air.
Higher humidities may lead to higher condensation of water on the metal surfaces. The concentration of molecules of water vapour increases with increasing RH. This molecular thickness of the layers of water eventually permits ionic conduction which accelerates the rate of corrosion.
With iron or steel, the ferrous ion may react with hydroxyl ion in water to form ferrous hydroxide and with oxygen to produce ferric hydroxide (rust).
The rate of corrosion is faster where surfaces are exposed to polluted air in combination with high relative humidity. Industrial pollutants like sulphur dioxide enhance the corrosion rate.
Corrosion or rusting of stored material can be seen in every Industry-Defence, Marine, Precision parts etc.