Impressed current cathodic protection is a method for ships hull protection. But, before discussing the Impressed current cathodic protection, its important to know the theory and history of cathodic protection.Cathodic Protection method is a method of controlling corrosion and is based on electrochemical process. In this method, the corrosion of the cathode is achieved by concentrating the oxidation reaction in a galvanic cell at the anode. This method of Cathodic protection was first developed and used on a small scale in 1824 by Sir Humphrey Davy for protecting the British naval ships from corrosion.During corrosion process, the direction of corrosion current within the electrolyte is from local cell anode to the local cell cathode. The current flowing through the short circuited corrosion cell, flows across a resistance Raoffered at a local anode surface. Now, if an opposing current is applied to the corroding metal, through another anode the corroding metal would receive more and more amount of electrons. The local cathodic reaction rate reduces and the corrosion current, gets suppressed, thereby suppressing corrosion. The externally applied current, thus, makes the entire metal as cathode. This technique of corrosion protection is known asCathodic Protection.The currenti1flowing through the short circuited corrosion cell, flows across a resistanceRaoffered at a local anode surface. Cathodic protection is achieved by applying external currenti2through external anode to the corroding surface. It is observed that the cathodic area offers resistance Rcto the total current i1+ i2. Therefore, using Kirchoffs laws we obtain a simple relation connecting the applied external potential and the resistances offered in terms of the current flowing in the circuitThus,

Therefore we see that In order to prevent corrosion, i1(corrosion current), should be zero. This is possible, ifEa= Ec+ i2Rc. The above equation states that the potential of the local cell cathodic site should be suppressed to the potential of local cell anodic site on the corroding metal by applying an external current through external anode to achieve cathodic protection. The cathodic protection on bare metal requires a large amount of current to be impressed externally. It can also be seen that this current demand can be appreciably reduced, if surface resistance (Ra+ Rc) is increased. The only way to increase this resistance is by application of paint film on steel surface.