Physio-chemical interaction between a metal and its environment which results in changes in the properties of the metal and which may often lead to impairment of the function of the metal, the environment, or the technical system of which these form a part is called corrosion (ISO 8044-1986).
Or in general we can say, loss of electrons of metal reacting with water and oxygen. According to U.S. FHA study on Corrosion Costs and Preventive Strategies in 2002 (on direct costs associated with metallic corrosion in every U.S. industry sector), for 1998 the estimated total annual direct cost of corrosion was approximately $276 billion (which was around 3.1% of US GDP). Economic damage caused by electrochemical corrosion itself is between $8 billion and $128 billion per year in the US alone, degrading structures, machines and containers. For example, rust is one of the most common causes of bridge accidents. Rust has a much higher volume than the originating mass of iron, its build-up causes failure by forcing apart adjacent parts. Thus there is a need to control corrosion to prevent this damage both on economic and safety fronts.
Corrosion is caused by the flow of electric current from areas of a metal surface through a conducting solution (or environment). Eating away of metal due to this electric current occurs at areas which are called anodes, where the electricity leaves the metal, entering the environment exposed to it.
Bonds between metal ions contain a lot of free potential energy, which has a tendency to be released through the process of corrosion (i.e. oxidation) converting the metal to its natural state. This difference in the binding energies between the metal atoms is the driving potential towards the oxidation of metal atoms, resulting in the loss of one or more electrons and the production of the ionic form of the metal.
Corrosion occurs in the presence of moisture. For example when iron is exposed to moist air, it reacts with oxygen to form rust, the steps are:
It is possible that the rust forms at some distance away from the actual pitting or erosion of iron shown in the figure. This occurs as the electrons produced due to the initial oxidation of iron can be conducted through the metal and the iron ions can diffuse through the water layer to another point on the metal surface where oxygen is available. Electrochemical cell results in the process in which iron serves as anode, oxygen as cathode and aqueous solution of ions serving as a 'salt bridge' as shown below.
For corrosion to occur, an anode, a cathode and electrolyte are essential as shown in the corrosion triangle. If by any means linkage within this triangle is broken, corrosion can be controlled and prevented.
Basic methods of controlling corrosion and protection are:
 Corrosion and corrosion protection, Philip A. Schweitzer, 1989 second edition.
 Principles and prevention of corrosion, Denny A. Jones, 1996 second edition.
 Corrosion: Understanding the basics, J. R. Davis, 2000
 Corrosion Mechanisms, Florian Mansfeld, 1986
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