INTRODUCTION TO CATHODIC PROTECTION

Corrosion of Buried and Submerged Structures
Corrosion is an electrochemical process by which steel and other metals attempt to return to their natural ore condition.  In this process, the metal is corroded by discharges of electrical metallic ions to earth. There are many different causes of corrosion, among which the most common are dissimilar metals that are electrically tied together, dissimilar soils, differential aeration, anaerobic bacteria and outside sources of D.C. current.
In order to determine the degree of possibility of corrosion, the resistively of the soil and/or water in which the structure is buried and/or submerged is measured.  The ground soil classifications relating the possible degree of corrosivity to its resistivity are 0 to 2,000 ohm/cm, extremely corrosive; 2,000 to 10,000 ohm/cm, moderately corrosive; 10,000 to 30,000 ohm/cm, mildly corrosive and 30,000 and over, progressively less corrosive. Water on the other hand can be generally classified as 25 to 30 ohm/cm, seawater; 150 to 300 ohm/cm, brackish water, and 3,000 to 5,000 ohm/cm, potable water. The lower the resistivity, the more corrosive the environment. These classifications are general.  Under certain conditions, severe corrosion can occur in the higher resistivity soil/water.
Uncoated Versus Coated Underground Structures
Contrary to the belief of many people unfamiliar with the process of corrosion, a coated structure not protected with cathodic protection could develop leaks in a corrosive environment much sooner than an uncoated structure. The reason for this is that an uncoated structure, corrosion tends to spread itself uniformly over the area of the structure, therefore delaying the complete penetration of the structure wall.  On the other hand, on a coated structure, corrosion concentrates at small areas (that is, at faults in the coating), thus accelerating the possibility of total penetration of the structure.
Cathodic Protection
In order to stop corrosion, the flow of current to earth from the metallic structure has to be stopped.  This can be accomplished easily by the use of sacrificial anodes, which supply current to earth, which is picked up by the structure, thus reversing the flow of current and stopping the corrosion process; this is called cathodic protection.
There are two main types of cathodic protection - Galvanic anode system and Impressed current system. The type of cathodic protection utilizing galvanic anodes makes use of the voltage difference between the structure metal and the anode material.  The anode is always more negative than the structure; therefore, the flow of current through the wire connection is from structure to anode, and the flow of current through the electrolyte (that is the earth or water) is from anode to structure. There are galvanic anodes made of difference materials such as magnesium, zinc and aluminum. The impressed current type of cathodic protection makes use of a rectifier or other source of D.C. current.  The current output from the D.C. source is discharged to earth through sacrificial anodes. The current, in turn, is picked up by the structure in contact with the earth or water (providing cathodic protection in the process) and returns through cable connections to the negative terminal of the rectifier.  Sacrificial anodes are made of diversified types of material such as scrap metal, graphite, lead-silver alloys, platinum, and a high silicon alloy cast iron.