Definition: In general, Corrosion is a gradual degradation of materials (Metals in this context) by various Chemical reactions with its Environment.
Occurrence: When the metal is susceptible to oxidation, it reacts with water and oxygen to produce different oxides and/or salts of the original metal which ultimately produces precipitate called rust.
Mechanism: The mechanism which occurs during oxidation of a metal can be explained by taking steel as an example. Steel is an alloy of iron and carbon. Here iron is the metal and carbon is a non-metal. So, when steel comes in contact with moist air (oxygen+ water), water oxidizes iron and produces Fe2+ ions, which diffuses through the water forming a precipitate called Ferrous Hydroxide which further gets oxidized to Ferric Hydroxide which finally dehydrates to form Rust.
2Fe + 2H2O + O2 2Fe2+ + 4OH-
Fe2+ + 2OH- Fe (OH) 2
Fe (OH) 2 Fe (OH) 3 (Oxidizes)
Fe (OH) 3 Rust (Dehydrates)
To sum it up in a simple equation: Iron + Water + Oxygen = Rust.
Effects of Corrosion: In the above example, it is clearly illustrated that the iron in the solid state is getting converted into ionic state i.e., Fe2+ and finally into precipitate which ultimately results in the reduction of mechanical strength and does seriously affect other structural properties of the metal which results in the reduced service life of the metal.
The Functioning of Metals is similar to functioning of Human bodies. It is just as we Human beings get attacked by various diseases, so the Metals get attacked by various corrosions. Similarly as we take different treatments for different diseases, metals also need corrosion specific treatments. As it is important to keep our selves fit by taking good care of our body, so the metals be taken care by proper maintenance.