How Cathodic Protection Works

Consider a section of a pipe shown in the following figure. Applying a direct current upon all the cathodic areas would attain the potential of most of the anodic area and consequently corrosion would stop. A large difference in the potential between the anode and cathode is shown before the application of DC current.

After forcing the DC current, the potential of the cathode attains the potential of the anode, the difference of potential becomes insignificant and the pipe attains a more negative potential due to polarization caused by the external dc current. 

Similarly, the potential of the anode can be shifted to the potential of the cathode and all the cathodic areas can be made to attain the potential of the anodic areas and the whole structure would become anodic (+). the process of shifting the potential of any metal or alloy from its equilibrium state in either anodic or cathodic direction is called polarization. The anodic and cathodic polarization control is shown in following figures. On polarizing a structure in the negative direction, the structure will become cathode and on polarizing in the positive direction, it would become cathode.