Cathodic Protection is the controlled prevention of corrosion to underground Pipes, such as water, sanitary, gas or steam lines, storage tanks to hold propane, gasoline, or underground metal structures.
Corrosion is an electrochemical process, which causes metals to deteriorate in the presence of moisture. Electric current flows in two ways. Primarily, it flows within a metal when the electrons of the metal move in response to an electrical pressure or voltage. Metal atoms exchange their electrons readily and these move freely without affecting the atoms themselves. This type of flow does not damage the metal structure. This is how electricity works.
The other type of current flow that causes the corrosion process is where electric current leaves its metal path and flows into a liquid. Instead of electron movement you have ion movement. Corrosion occurs, as these ions break free from the metal surface.
To get some idea of how widespread this problem of corrosion is, it is estimated that 100 billion dollars is spent each year in the United States for Corrosion Control.