Attaching a piece of silver to an iron storage tank will not provide cathodic protection to the tank. Cathodic protection is a method of preventing corrosion on a metal surface by making it the cathode of an electrochemical cell. In order to provide cathodic protection, a more active metal must be connected to the metal being protected, and this active metal will act as the anode in the electrochemical cell.
In the case of an iron storage tank, attaching a piece of silver will not provide cathodic protection because silver is less active than iron. This means that the silver will not corrode to protect the iron, as the anode in the electrochemical cell needs to be more reactive than the cathode to provide cathodic protection. Instead, the silver will act as a cathode, and the iron tank will continue to be the anode, resulting in continued corrosion of the iron.
To provide cathodic protection to an iron storage tank, a more reactive metal, such as zinc or magnesium, would need to be attached to the tank as the anode. This anode would then corrode preferentially to protect the iron tank, effectively preventing further corrosion.