Theory behind Impressed current cathodic protection

Impressed current cathodic protection is a method for ship’s hull protection. But, before discussing the Impressed current cathodic protection, its important to know the theory and history of cathodic protection.

Cathodic Protection 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.

At the time of corrosion, the flow of current (also known as Corrosion-current) is always from anode to cathode in the presence of an electrolyte (just like the current flow in a galvanic cell). The current flowing through the short circuited corrosion cell, flows across a resistance R_a offered at a local anode surface. Now, if we apply a current in opposition of the existing corrosion-current, the cathodic metal will then be re-inforced again with electrons. Due to this, the rate of reaction at cathode will be boosted and the rate of reaction at anode will reduce. And so, the corrosion current, gets suppressed, thereby suppressing corrosion. The additional current, thus, makes the entire metal as cathode. This technique of corrosion protection is known as Cathodic Protection.

The current i₁ flowing through the short circuited corrosion cell, flows across a resistance R_a offered at a local anode surface. Cathodic protection is achieved by applying external current i₂ through external anode to the corroding surface. It is observed that the cathodic area offers resistance R_c to the total current i₁ + i₂. Therefore, using Kirchhoff laws we obtain a simple relation connecting the applied external potential and the resistances offered in terms of the current flowing in the circuit

 Thus,

Therefore we see that In order to prevent corrosion, i₁ (corrosion current), should be zero. This is possible, if E_a = E_c + i₂ R_c. 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 (R_a + R_c) is increased. The only way to increase this resistance is by application of paint film on steel surface.