Any metal, that has been extracted from a naturally combined state, has a tendency to revert to that state under the action of oxygen and water. This action is corrosion, the commonest example being the rusting of steel.

Corrosion is a process that occurs naturally to metals and alloys. The rate at which it occurs, depends on the metal and the surrounding environment.

When a metal is immersed or buried in an electrolyte, such as sea water or damp soil, an electrical potential is setup at the metal / electrolyte interface. The potential will be characteristic of the particular combination of metal and electolyte involved.

Corrosion is an electro-chemical process that involves the passage of electric currents on a micro or macro scale. The change from a metallic to the combine form occurs by an "anodic" reaction.

This reaction produces a free electron which passes through the metal to another side on the surface, the cathode, where it is consumed by the cathodic reaction.

The anode and cathode in a corrosion process, may be of two different metals connected together, forming a bimetallic couple, or they may be close together on a common surface. This corrosion process may also be the result of variations in the state of the metal at different points, or a result of local difference in the environment, such as variations in the supply of oxygen at the surface.

The principle of Cathodic Protection consists of connecting an external anode to the metal to be protected and the passing of an electric current so that all areas of the metal become cathodic and, therefore, do not corrode. In electro-chemical terms, the electrical potential between the metal and the electrolyte solution, which is in contact, is lowered to a value at which corroding anodic reactions are silent and only cathodic reactions can take place.