What is the function of a 'sacrificial anode' in a cathodic protection system and what materials are commonly used for this purpose?

The function of a 'sacrificial anode' in a cathodic protection system is to corrode in place of the metal structure being protected, thus preventing or minimizing corrosion of that structure. Cathodic protection is a technique used to control the corrosion of a metal surface by making it the cathode of an electrochemical cell. In a corrosion cell, metal corrodes at the anode, where oxidation occurs. A sacrificial anode, also known as a galvanic anode, is a metal that is more easily oxidized than the metal being protected. It is electrically connected to the structure to be protected, creating a galvanic cell. Because the sacrificial anode is more reactive, it corrodes preferentially, while the protected structure acts as the cathode and experiences a reduction reaction instead of oxidation. In essence, the sacrificial anode provides electrons to the protected structure, shifting its electrochemical potential to a level where corrosion is thermodynamically unfavorable. Common materials used for sacrificial anodes include zinc, magnesium, and aluminum alloys. These materials are chosen because they have a more negative electrochemical potential than steel, which is commonly used in water infrastructure. The choice of anode material depends on several factors, including the environment (soil or water resistivity), the operating temperature, and the desired service life. For example, magnesium anodes are often used in high-resistivity soils because they provide a higher driving voltage, while aluminum anodes are often used in seawater applications due to their higher current capacity and resistance to passivation.