A cathodic protection (CP) system controls the natural process of metal degradation known as corrosion. This technique is applied to metallic structures immersed in an electrically conductive environment, such as soil, seawater, or fresh water. The purpose of a CP system is to prevent the structure from losing material by halting the electrochemical reaction responsible for rust and decay. Controlling this reaction significantly extends the lifespan and maintains the structural integrity of infrastructure assets.
The Science Behind Protecting Metal
Corrosion is an electrochemical reaction requiring four components: an anode, a cathode, an electrolyte, and a metallic path. The anode is the site where the metal dissolves, releasing ions and electrons. The cathode is the site where a reduction reaction occurs, consuming these electrons. The electrolyte, such as moist soil or water, is the conductive medium that completes the electrical circuit.
Cathodic protection disrupts this natural corrosion cell by introducing an external electrical current. This current shifts the entire potential of the metallic structure to a more electrically negative state. By making the entire surface of the protected structure act as the cathode, the CP system eliminates the anodic sites where metal loss occurs.
This shift in electrical potential effectively suppresses the oxidation reaction, which is the dissolution of metal atoms into ions. The protective current ensures that metal atoms remain bonded within the structure, preventing the formation of corrosion products like rust. This method is highly effective because it controls the flow of electrons, treating the cause of degradation rather than simply coating the surface.
Industry standards define a specific negative potential that must be maintained for a steel structure in soil or water to be considered protected. This potential is typically around -850 millivolts relative to a copper-sulfate reference electrode. Maintaining this minimum potential confirms the CP system is successfully diverting corrosion activity away from the structure, preventing metal deterioration.
Distinguishing Between System Types
The two primary methods used to implement cathodic protection are the Sacrificial Anode Cathodic Protection (SACP) system and the Impressed Current Cathodic Protection (ICCP) system. Both systems transform the protected structure into a cathode but use different mechanisms to supply the necessary protective current. The choice depends on factors like the structure’s size, the required current output, and the accessibility of external power.
Sacrificial Anode Systems
Sacrificial Anode Systems, also called Galvanic CP, are passive and require no external power source. They utilize a more electrochemically active metal, such as zinc, aluminum, or magnesium, which is electrically connected to the structure needing protection. Since these materials are naturally less noble than the protected steel, they willingly become the anode in the electrochemical cell.
The active metal sacrifices itself by corroding and continuously releasing electrons, which flow to the protected structure. This natural current flow is driven by the potential difference between the two metals, functioning like a simple battery. Sacrificial anodes are typically used for smaller structures, such as ship hulls, water heaters, and short pipelines, but they must be replaced once consumed.
Impressed Current Cathodic Protection (ICCP)
Impressed Current Cathodic Protection (ICCP) systems are active and utilize an external source of direct current. These systems employ a rectifier to convert standard alternating current (AC) power into the necessary low-voltage, high-current direct current (DC). The DC current is fed through a circuit to anodes made of durable, inert materials, such as high silicon cast iron or mixed metal oxides.
The ICCP system forces current to flow from the inert anodes through the electrolyte and onto the protected structure, which connects to the rectifier’s negative terminal. This method allows for a higher and adjustable current output, making it suitable for large-scale applications like long-distance pipelines and complex offshore platforms. Engineers can adjust the protection level to compensate for changing environmental or structural conditions over time.
Real-World Uses of Cathodic Protection
Cathodic protection is widely deployed across multiple industries where metal assets are exposed to corrosive environments. The method defends structures that are buried or submerged, where water and soil act as a powerful electrolyte. Without CP, the longevity of these structures would be compromised, leading to costly failures.
A major application is the protection of underground structures, notably oil and gas transmission pipelines and large storage tanks. These buried assets are exposed to varying soil conditions, which can be highly corrosive. CP systems ensure the external steel surface remains protected, preventing leaks resulting from corrosion damage.
Marine environments are an extensive use case for CP, as the high salinity of seawater creates an aggressive electrolyte. CP is applied to the submerged portions of ship hulls, offshore wind turbine foundations, and fixed platforms. Structures like steel pier pilings and jetties also rely on CP to resist the corrosive action of tides and currents.
CP is also used to protect steel reinforcement bars, or rebar, embedded within concrete structures like bridges, parking garages, and tunnels. Chlorides from road salt or seawater can penetrate the concrete and initiate corrosion. CP systems supply current to the rebar, stopping the corrosion process and preventing the internal expansion that causes concrete to crack.