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A method of protecting metal structures, such as steel tanks and piping, from corrosion.
In most locations, an unprotected buried steel tank will corrode. Tiny flakes of metal will pull away and, as the corrosion process worsens, holes will begin to appear in the tank walls. Product contained in the tank will leak out into the ground, or groundwater will enter.
This corrosion process is a result of complex electrolytic action. In a moist-soil environment, the tank behaves somewhat like part of an electric battery. An area of the tank acts as an anode–a positive pole. Ions go out from this positive pole, seeking a nearby negative pole (a cathode). The cathode may be located on a separate structure or at another location on the tank itself. When the ions move away from the anode on the tank, they carry with them tiny bits of baggage–microscopic pieces of the tank itself. As the process continues and accelerates, more and more bits of the tank are carried away. The corrosion becomes increasingly severe.
One way of combating this process is to cover the entire surface of the tank with a protective coating. Traditional coatings, however, do not provide sure-fire protection. If there is a pin-hole size opening somewhere in the coating, it is at this point that the ions, seeking to flee the positive pole, will concentrate their escape activities. It is here that corrosion will set in.
Corrosion engineers discovered years ago that another way of combating this process was to convert the buried tank from an anode to a cathode. That is, if the tank could be made to act as a negative pole (a cathode), ions in the soil would not seek to move away from the tank, carrying along tiny bits of material as they departed. Instead, the ions would seek to move toward the tank and no metal would be lost.
To create this condition, corrosion engineers buried a bar, usually made of magnesium, in the soil near the tank and connected it by a wire to the tank. The idea was that in this closed environment the magnesium bar would act as the anode. The tank’s role, in turn, would be reversed: It would become the cathode. Ions would flow from the magnesium to the tank. The magnesium bar, the “sacrificial anode,” would sacrifice itself to protect the steel tank, now the cathode.
From these concepts, cathodic protection has emerged. Today, buried steel tanks are routinely protected from corrosion through the use of sacrificial anodes, typically made of zinc although magnesium is also used. Many steel underground tanks, manufactured by members of the Steel Tank Institute, are shipped with anodes attached. Steel core bars running through the zinc anodes are welded to the tanks.
See also Impressed current.