All metallic structures are susceptible to corrosion if they are not in a state of equilibrium with their environment. The corrosion cell is natural and occurs to all metals with exception to the noble metals. A significant amount of energy has been put into the metal during refining process, placing it in high-energy state conditions. Corrosion will occur even if the metals are not immersed in electrolyte as the air is an aqueous (water-containing) environment, sufficient to complete the corrosion cells requirement.
Such corrosion can be suppressed when the metallic energy state is lowered to a state of electrochemical stability. This can be achieved by enveloping the material beneath a stable passive protective film (eg: Painting or Coating) or by reducing the metalermo-dynamic activity to a level at which the energy is insufficient for the corrosion reaction to proceed.
In the latter case protection can be achieved by means of galvanic action or by the influence of impressed of Direct Current. Suppression of the corrosion by such means is termed as Cathodic Protection.
Cathodic Protection (CP) is a method used to control the corrosion of a metal surface by making it as cathode of an Electrochemical cell. A simple method of protection connects protected metal to a more easily corroded "sacrificial metal" to act as the anode. The sacrificial metal then corrodes instead of the metallic structure intended to be protected. For structures such as long buried pipelines, where passive galvanic cathodic protection is not adequate, an external DC electrical power source is used to provide sufficient protective current.
Cathodic protection systems protect a wide range of metallic structures in various environments. Common applications are steel pipelines, steel piles, tank plates and various other steel structures immersed in Electrolyte such as Water, Soil & Sand. Another common application is Galvanised Steel, in which a sacrificial coating of Zinc is introduced on the surface of the metal to prevent corrosion.