Galvanic Corrosion requires 3 elements: 2 different types of metals in contact with each other and a solution that is corrosive such as water.
If these 3 elements are in place, the metal which is more noble will become a cathode and will corrode at a slower rate than it would if it was on its own.
On the other hand, the metal which is more active will become an anode and will corrode at a faster rate. The increased corrosion rate of the anode is called Galvanic Corrosion.