For the electrons that are formed at the anode to move to the cathode there must be a path that they can follow. This usually means a physical contact between the anode and the cathode.
There must also be a way for the ions produced to come together so they can react to form the corrosion products. This usually is provided by a liquid (water, electrolyte) or moist conductor.
If both the connection and electrolyte are present then the circuit is completed.
The essential components mentioned are:
All of these components need to be present for corrosion to occur. If you remove even one of them then there will be no corrosion. This is the theory behind corrosion prevention.
An example of an electrolytic cell is a copper fitting on a steel pipe carrying water. The steel pipe acts as the anode, the copper fitting is the cathode . The pipe is in direct contact with the fitting for the connection. The water acts as the electrolyte, allowing for migration of the ions, and completion of the circuit.