Explain the basic principle of cathodic protection and differentiate galvanic CP from impressed-current CP.

• A. CP makes the protected structure the cathode and uses either sacrificial anodes or an external power source
• B. CP makes the protected structure the anode and uses corrosion inhibitors
• C. CP neutralizes the electrolyte to stop all reactions
• D. CP increases the metal potential to accelerate corrosion

Answer: (B)

Cathodic protection works by making the metal surface act as the cathode, so oxidation (and thus corrosion) is suppressed. This is achieved by supplying electrons to the protected structure and driving its electrochemical potential to a level where reduction reactions dominate at the metal surface instead of oxidation.

In galvanic protection, a more active metal (the sacrificial anode) is connected to the protected structure. The two metals form a natural electrochemical couple, and the sacrificial anode corrodes first, delivering electrons to the structure. The protected metal becomes the cathode, staying protected without any external power source. This approach is simple and inexpensive for suitable cases, but the protection current is limited by the galvanic potential difference and the environment, and the anodes must be replaced as they are consumed.

In impressed-current protection, an external power source drives electrons onto the structure, forcing it to become the cathode. The current is regulated by a rectifier, and inert anodes (which don’t corrode easily) supply the other half of the circuit. This method provides strong, adjustable protection suitable for large or complex structures, especially in challenging soils or seawater. It requires power and ongoing monitoring but offers greater control over the protective potential and current.

Corrosion inhibitors are not the mechanism here; CP relies on shifting the metal’s electrochemical state by supplying electrons, either through a sacrificial anode or an external power source.