A potential difference usually exists between two dissimilar metals when they are immersed in a corrosive or conductive solution (seawater). If these metals are placed in contact (or otherwise electrically connected), this potential difference produces electron flow between them causing corrosion. The injurious corrosive attacks occur on those areas of steel where the electric current dissipates into the seawater. Such areas are called "anodic" and the corresponding areas of the steel where the current flows in are called "cathodic". Corrosion of the less corrosion resistance metal is usually increased and attack on the more corrosion resistant metal is decreased, as compared with the behavior of these metals when they are not in contact.

The less resistant metal becomes anodic and more resistance metal cathodic. Usually the cathode or cathodic metal corrodes very little or not at all in this type of couple. Because of the electric currents and dissimilar metals involved, this form of corrosion is called galvanic, or two-metal, corrosion. It is electrochemical corrosion. The driving force for current and corrosion is potential developed between two metals.

The principle of Cathodic protection when first applied, blocks of the lesser noble metals such as zinc or is being utilized to conserve the copper sheathing of the ships built of wood. The galvanic current, which flows from the less noble metal to the copper, keeps the latter protected. As the less noble metal "sacrificed" itself to protect the more noble metal, this type of protection is called "Sacrificial Anodes" Cathodic protection. The life span of sacrificial anodes may be between 2 & 4 years or more, depending upon their application and dimensions.

The principle has been widely applied, being used on land for protection underground pipelines and under water fittings and sea structures etc. As a source of current both sacrificial anodes and ICCP systems may be utilized based on the commercial and technical requirements.

At the cathodic areas on metal dissolution takes place. The principle of cathodic protection means to make the whole under water hull a cathode, and let the anodic reaction take place at the sacrificial or impressed current anodes. Instead of iron dissolution one gets dissolution of sacrificial anodes or production of chlorine gas at permanent anodes in the case ICCP system.

Cathodic protection effected in accordance with the principle outlined above, has become an extremely effective method for protection of hull, rudder and propeller. In many cases a sufficient supply of electric current may be obtained from sacrificial anodes welded onto the hull, but with large modern hulls of today, it is both economically and technically more attractive to get the current from the vessel's own power supply. To obtain maximum benefits from the method it is essential that the strength of protection current is carefully balanced in accordance with the conditions on the underwater hull. The current strength may be quite simply adjusted by measurements of the hull's potential.