Galvanic corrosion: When two dissimilar electrodes such as Zn & Cu are used for the study of electrochemical corrosion then it is essential that of electrolytic solutions such as $CuSO_4$ & $ZnSO_4$ must be same.

It is also requirement for the formation of cell that 2 electrolytic solutions must be separated by either porous partition or salt bridge for the flow of electrons from anodic area to cathodic area.

For electrical contact to electrodes must be connected to each other either by battery or dry cell.

As zinc occupies lower position than cu it behaves like anode & undergoes corrosion due to oxidation reaction by dissolution.

Process & electrons are transferred from anodic area to cathodic area. $Cu_7$ behaves like cathode due to its lower position in electrochemical series.

Cu accepts electrons donated by zinc & double layer is developed over the surface of cathode & it undergoes reduction & protected from corrosion.