Galvanic and CP

Nature has endowed each metallic substance with a certain natural energy level or potential. When two metals having different energy levels or potentials are coupled together, current will flow. The direction of positive current flow will be from the metal with the more negative potential through the soil to that which is more positive. Corrosion will occur at the point where positive current leaves the metal surface. A dry cell battery is one example of a corrosion cell.


DC railways and other machinery often generate direct current. When this current flows through the soil indiscriminately, it is referred to as "stray" DC. The current may contact and follow a buried metallic structure such as a pipeline, but wherever it leaves that structure to return to it's origin, corrosion will occur.

Cathodic protection is an electrical method of preventing corrosion on metallic structures which are in electrolytes such as soil or water. It has had widespread application on underground pipelines, and ever increasing use as the most effective corrosion control method for numerous other underground and underwater structures such as lead cables, water storage tanks, lock gates and dams, steel pilings, underground storage tanks, well casings, ship hulls and interiors, water treatment equipment, trash racks and screens. It is a scientific method which combats corrosion by use of the same laws which cause the corrosion process.

Source: www.bushman.cc/pdf/corrosion_theory.pdf