6 - Galvanic (Sacrificial) Anodes

Abstract

In contrast to impressed current anodes, the possibility of using galvanic anodes is limited by their electrochemical properties. The rest potential of the anode material must be sufficiently more negative than the protection potential of the object to be protected so that an adequate driving voltage can be maintained. Galvanic anodes should exhibit as low polarizability as possible. The extent of their polarization is important for their current output. The cathodic protection of plain carbon and low-alloy steels can be achieved with galvanic anodes of zinc, aluminum, or magnesium. For materials with relatively more positive protection potentials, galvanic anodes of iron or of activated lead can be used. Currently, iron anodes are used for objects with a relatively positive protection potential, especially if only a small reduction in potential is desired. There is a relationship between the grounding resistance of anodes and the specific soil resistivity, which is subjected to annual variations. To avoid these variations and to reduce the grounding resistance, anodes in soil are surrounded by bedding materials, so-called backfill. In addition, all cast galvanic anodes have specially shaped feeder appendages as anode supports by which the anodes are fixed by screws, brazing, or welding. This guarantees a very low resistance for the flow of current from the anode to the object to be protected.