All creation exists undisturbed in an equilibrium state. For any metal this is an ore or mineral state. Metals are refined by driving heat energy into the ore or mineral that contains the elemental metal. Thus any refined metal contains and has potential to release that energy to get back to its equilibum state. This is how your flashlight works or how you start your automobile in the morning. When you start your auto the battery of cells consisting of zinc and lead plates completes a circuit between the two and electrical current naturally flows between the anode Zinc and the Cathode Lead, corroding the anode which releases some of its stored energy and drives sufficient current for a short time period to turn over the engine in your car and thus start it. Thus the zinc plate has sacrificed some of its energy to start your car.
Cathodic protection is a method of preventing corrosion on any metallic structure exposed to an electrolyte. Metals corrode by releasing the heat energy stored in the elemental metal when that metal is refined. Any metal when exposed to oxygen or to an oxygen bearing electrolyte will combine with oxygen, salts, or other materials available in order to return to its equilibrium state which is generally the ore state in which it was mined.
Thus in order to prevent the metal from corroding and becoming useless or unsightly, the surface may be deprived from contact with oxygen by being painted (coated). Painting is an effective way to keep corrosion from damaging structures, proper paint and coating application is essential to maximum life of any coating system. Proper third party inspection is the only way to insure the best coating or paint application is being accomplished.
For any buried structure the coating must cover 100% of the surface area for 100% of the structure life in order to be effective. This is not practical as any structure will suffer coating damage during installation or in the course of its life. Thus a certain area of the structure will then become exposed to the corrosive environment and cause damage to the coating. Cathodic Protecton is a method of installing a DC current generating system which will sacrifice an expendable and replaceable material to keep corrosion off the structure.