Abstract: Sacrificial anodes for cathodic protection systems wherein inner portions of a tank, at least partly filled with a liquid, are exposed to a corrosive inner environment and are protected against the corrosion by the provision of a consumable sacrificial anode, the material of which gradually migrates through the liquid to and is deposited on the inner tank portions, without an external voltage being applied, and wherein the inner tank environment also promotes detrition of the sacrificial anode. The invention with the anode with an inner metal tube, provides openings in the tube wall, preferably by perforation, so that the anode material normally constitutes a barrier between the tube interior and the tank environment, and obtains an indication of progressed detrition or consumption of the anode, at which point the consumable anode should be replaced for continued effective cathodic protection.