What is the purpose of using cathodic protection in a geothermal power plant's piping system, and what materials are commonly used?

The purpose of using cathodic protection (CP) in a geothermal power plant's piping system is to prevent or significantly reduce corrosion of the metal pipes. Corrosion is a natural process in which a metal is gradually destroyed by chemical reactions with its environment. In geothermal plants, the geothermal fluids are often highly corrosive due to the presence of dissolved salts, acids, and gases like hydrogen sulfide (H2S). Cathodic protection works by making the metal piping the cathode in an electrochemical cell. This is achieved by either connecting the piping to a more easily corroded metal (a sacrificial anode) or by applying an external electrical current (impressed current). In the sacrificial anode method, the sacrificial anode corrodes instead of the piping, thus protecting the piping. Common materials used for sacrificial anodes include magnesium, zinc, and aluminum alloys. These materials are chosen because they have a more negative electrochemical potential than the steel or other metal used for the piping. In the impressed current method, an external power supply provides a direct current that flows from an anode, typically made of a durable material like high-silicon cast iron or platinized titanium, through the electrolyte (the soil or water surrounding the pipe) to the piping. The current makes the piping more negative, preventing corrosion. The choice of CP method depends on factors such as the soil resistivity, the size and configuration of the piping system, and the availability of power. CP is a cost-effective way to extend the lifespan of the piping system and prevent costly leaks and repairs. Regular monitoring is required to ensure that the CP system is functioning properly and providing adequate protection.