Galvanic anodes are basically metal castings that do not utilize an external power supply to drive current. They rely on the natural potential differences between the two metals to drive the cathodic protection current. There are three primary types of galvanic anodes. Magnesium is the most active type of galvanic anode and is used primarily in soil applications. Comparatively, Zinc galvanic anodes are less active and commonly used in low resistivity soils and brackish water. Zinc is also the primary metal in galvanized applications. Finally, seawater applications typically use the third type of galvanic anodes, aluminum. NOTE: People often refer to galvanic anodes as sacrificial anodes because the CP reaction consumes them. This is also true for many impressed current anodes. The term sacrificial implies that a power supply does not exist, and that the anodes utilized are more active than the protected structure.