Cathodic protection is a technique that allows to reduce the corrosion rate of a metallic material, in an aqueous environment, by decreasing the corrosion potential of metal(cathodic polarization hence the term cathodic protection). The metal structure to be protected is then placed at a potential so that the corrosion rate becomes acceptable on all the metal surface in contact with the aqueous environment. To achieve that an electrical current flows between an auxiliary anode and the material to be protected which constitutes the cathode. The current that flows from the environment towards metal is adjusted in order to reach a potential value for which the corrosion rate of metal becomes very low. This protection applies to any metal structure in contact with an aqueous environment in particular the carbon steel structures buried or immersed, the internal surfaces of metallic storage systems with an electrolyte, as well as reinforced concrete rebars. This protection is often associated with prevention processes of corrosion such as coatings (painting, polymers…).

 

The cathodic protection current can be applied:

• with an external D.C generator connected between the structure to be protected (cathode) and an auxiliary anode (called a groundbed) using any conductive material (preferably inert): protection by impressed current
• by galvanic coupling between the structure to be protected and anodes made with a less noble metal or alloy than the metal we want to protect: protection by galvanic anodes.

 

Whatever the system, the protection efficiency depends on the density of cathodic current (and thus on the electrode potential that is achieved) on the metal surface we want to protect. It is the value of the potential that is used as a criterion to estimate the protection effectiveness.

 

 

Key to illustration text :
Source externe de courant (si courant impose) – external source of current (if impressed current)
Source interne de courant (si anode galvinque) – internal source of current (if galvanic anode)
Mileu electrolytique (sol ou eau) – electrolyte (soil or water)