Corrosion is a natural electrochemical process that causes metals to revert to their more stable, oxidized state, such as iron turning into rust or iron oxide. This deterioration is a major concern for metallic structures submerged in water or buried in soil, leading to significant structural weakening. Cathodic Protection (CP) is an effective electrical method used to control this degradation by forcing the metal structure to become the cathode in an electrochemical cell. Sacrificial Anode Cathodic Protection (SACP) is a passive technique that achieves protection by electrically connecting the asset to a more reactive metal. This highly active metal sacrifices itself by corroding preferentially, ensuring the primary, valuable structure remains intact.
Understanding the Electrochemical Process
Corrosion requires four components: an anode, a cathode, a metallic path, and an electrolyte, which is the conductive medium like seawater or damp soil. The primary metal structure naturally develops anodic (corroding) and cathodic (protected) sites across its surface when exposed to this electrolyte. The SACP system works by establishing a controlled “galvanic cell” that exploits the natural electrical potential difference between two dissimilar metals.
A metal significantly more “active” or “less noble” than the protected structure is selected to become the sacrificial anode. This selection is based on the Galvanic Series, a ranking of metals by their electrochemical potential. Once the anode is electrically connected to the structure and placed in the electrolyte, the more active anode metal spontaneously oxidizes.
This oxidation reaction releases electrons, which flow through the electrical connection to the structure being protected, effectively turning the entire structure into a cathode. By continuously supplying this protective current of electrons, the corrosive reaction on the structure’s surface is suppressed. The protective current is driven solely by the inherent voltage difference between the two metals, eliminating the need for an external power source. The sacrificial anode dissolves slowly over time, diverting all corrosive action away from the valuable asset.
Materials Used for Sacrificial Anodes
The selection of the sacrificial metal depends upon the specific environment, with the three most common being Magnesium, Zinc, and Aluminum. The most active of the group is Magnesium, which has the most negative electropotential on the Galvanic Series.
Magnesium is the preferred choice for high-resistivity environments, such as freshwater applications or buried steel pipelines in soil. Its high driving voltage is necessary to overcome the poor conductivity of these less saline or less moist environments. Conversely, magnesium anodes are unsuitable for highly conductive saltwater, where their rapid corrosion rate would lead to excessively fast consumption.
Zinc remains a reliable choice for stable saltwater environments. However, zinc anodes can become inactive in freshwater, where they often develop a dense, insulating oxide coating that prevents the electron flow. Modern Aluminum alloys are increasingly favored in marine applications due to their high ampere-hour capacity and lighter weight. These alloys are highly efficient in both saltwater and brackish water conditions, offering a better lifespan in seawater compared to zinc, and are often the material of choice for large offshore structures.
Where Sacrificial Anodes Are Used
SACP systems are utilized across various industries to protect metallic assets exposed to corrosive electrolytes. Marine environments represent a major area of application, where anodes protect the hulls, propellers, and ballast tanks of ships. They are also installed on offshore platforms, port pilings, and submerged infrastructure constantly exposed to seawater or brackish water.
Another significant application is the protection of buried structures, including oil and gas pipelines and underground storage tanks. In these contexts, magnesium anodes are often utilized to counteract the corrosive effects of the surrounding soil, which acts as the electrolyte.
Sacrificial anodes are also commonly found in domestic settings, specifically inside residential water heaters, where a magnesium or aluminum rod protects the steel lining of the tank. For smaller, well-coated structures requiring a low current, SACP is often preferred over Impressed Current Cathodic Protection (ICCP), which uses an external power supply.
Monitoring Anode Performance and Lifespan
Because the sacrificial anode is designed to be consumed, ongoing monitoring is necessary to confirm that the protected structure maintains its safe, non-corroding status. The primary method for assessing performance involves measuring the electrical potential of the structure relative to the electrolyte. This measurement is taken using a specialized tool called a reference electrode, or half-cell, which provides a stable baseline voltage.
A potential reading that is sufficiently negative confirms that the protective current is flowing correctly and that the structure is fully polarized. For example, steel structures often require a potential more negative than -720 millivolts to confirm protection. Technicians also perform regular visual inspections to check the anode’s consumption rate and look for signs of surface contamination, known as passivation.
The lifespan of an anode depends on its original mass, the total surface area of the structure, and the overall current demand of the system. Environmental factors, such as higher water temperature or increased water flow, can increase the current demand and accelerate the anode’s consumption rate. Regular monitoring allows maintenance teams to predict the remaining life and schedule a timely replacement before the anode is completely consumed.