Cathodic protection is a technique used to prevent or minimize the corrosion of metal surfaces. It works by making the metal surface the cathode of an electrochemical cell, diverting the corrosion reactions that would normally occur. This is achieved by introducing a sacrificial anode (usually made of a more easily corroded metal, like zinc or magnesium) or by applying an impressed current to the structure.
The working principle of cathodic protection can be explained in two methods:
- Galvanic or sacrificial anode method: In this method, a more active metal (the sacrificial anode) is connected to the metal structure to be protected. The sacrificial anode subsequently corrodes, sacrificing itself to protect the structure. This occurs due to the difference in electrochemical potential between the sacrificial anode and the structure. The anode corrodes preferentially, ensuring that the protected metal remains intact.
- Impressed current method: Instead of using a sacrificial anode, an external power source is used to impose a direct current on the metal structure. This causes the metal surface to become cathodic, preventing corrosion. The power source supplies a constant flow of electrons, counteracting the natural flow of electrons that would lead to corrosion.
Both methods of cathodic protection create an electrochemical environment that shifts the structure’s potential from a corrosive state to a non-corrosive state, thereby providing protection against corrosion.