A device for cathodic corrosion protection such as for hot water tanks having a passive anticorrosive layer on the inner wall of the tank. The cathodic corrosion protection employs the use of an external current anode. The device includes a holder which bears the external current anode and a reference electrode. The holder is adapted to be inserted into a single opening in the tank wall. A potentiostat employs the external current anode and reference electrode so as to establish a predetermined potential between reference electrode and the tank by regulating the current flowing through the external current anode to the tank. This provides the desired corrosion protection. Two embodiments of the device are described.