Corrosion is the natural tendency of refined metals, such as steel, to revert to their more chemically stable state, typically an oxide like rust. This deterioration occurs through an electrochemical reaction with the surrounding environment, like soil or water. Cathodic Protection (CP) is an engineering technique designed to control this degradation on metal structures that are submerged or buried. CP operates by transforming the entire protected structure into a cathode, the non-corroding component of an electrical circuit. This effectively halts the metal loss that would otherwise compromise the structure’s integrity and lifespan.
Understanding the Electrochemical Process
Corrosion requires four distinct components to occur, forming what is known as a corrosion cell. These components are an anode, a cathode, an electrolyte, and a metallic path connecting the anode and cathode. The anode is the location where the metal actively corrodes, losing mass as it releases electrons into the system.
The electrons released at the anode travel through the metallic structure to the cathode, where a reduction reaction consumes them. The electrolyte, such as moisture in soil or seawater, provides the conductive medium for ions to move and complete the electrical circuit. Without all four elements present, the corrosion reaction cannot proceed.
Cathodic protection works by intentionally introducing a new, more electrically active anode into the system. This sacrificial anode takes over the role of the corroding site, supplying electrons to the entire surface of the structure. By forcing a current onto the structure, its electrical potential shifts, making it the cathode. Since metal loss only occurs at the anode, the protected structure is prevented from corroding.
Galvanic vs. Impressed Current Systems
The two primary methods used are the Galvanic Anode system and the Impressed Current Cathodic Protection (ICCP) system, differing in their power source and scale of protection. Galvanic systems, also called sacrificial anode systems, operate passively by relying on the natural electrical potential difference between two metals. They connect a more reactive metal, such as an alloy of zinc, aluminum, or magnesium, to the steel structure.
The more reactive anode naturally corrodes preferentially, generating a small electrical current that flows to the protected structure. This method is simple, requires no external power source, and is used for smaller structures or those in highly conductive electrolytes, like a ship’s hull in seawater. However, the current output is fixed and low, meaning the anodes must be replaced periodically as they are consumed.
Impressed Current Cathodic Protection (ICCP) systems are active and use an external power source to drive the protective current. A transformer-rectifier unit converts alternating current (AC) into a controlled direct current (DC), which is then applied to inert anodes, such as mixed metal oxides or graphite. This external power source provides a much higher driving voltage and current output compared to galvanic systems.
The high current capacity makes ICCP suitable for protecting large, complex structures like long-distance pipelines or extensive marine structures. Since the current is supplied externally, the anodes are consumed at a much slower rate, and the current can be adjusted to compensate for changing environmental conditions. The choice between the two systems depends on the size of the structure, the required current, and the availability of external power.
Essential Uses in Infrastructure
Cathodic protection is widely applied across infrastructure where metal structures are constantly exposed to corrosive environments like soil and water. CP is indispensable for buried structures, including long-distance oil and gas pipelines and underground steel storage tanks. It is also used for well casings to maintain structural integrity and prevent leaks.
For submerged structures, CP safeguards assets in marine and freshwater environments. This includes the steel hulls of ships, offshore oil and gas platforms, and the supporting piles for jetties and bridges. Constant exposure to highly conductive seawater accelerates corrosion, making CP necessary for extending the service life of these complex installations.