The National Electrical Code (NEC) mentions cathodic protection only once; however, dealing with cathodic action and protection can be difficult to diagnose and time-consuming to fix. The 1999 NEC first introduced cathodic protection in 250.6, generally dealing with objectionable current, and specifically in 250.6(E) as isolation of objectionable direct current (DC) ground currents. The text stated, “Where isolation of objectionable DC ground currents from cathode protection systems is required, a listed AC coupling/DC isolating device shall be permitted in the equipment grounding conductor path to provide an effective return path for AC ground-fault current while blocking DC current.”


The NEC does not explain the cathodic action phenomena, nor does the NEC provide any design applications for cathodic action or protection. There are ways to recognize cathodic action and deal effectively with the results of the potential destructive action.


Also commonly called “galvanic action,” cathodic action is an electrochemical process where one dissimilar metal with a different electrical potential reacts to another dissimilar metal with another potential. One of the metals becomes an anode, the other becomes a cathode, and there is an electrolyte in between them. The anode metal dissolves its molecules (ions) into the electrolyte, the electrolyte provides the medium for the ion migration, and the cathode metal (or earth) collects the molecules or ions from the anode metal. The anode metal corrodes more rapidly than normal due to the electrically conductive path from the anode metal through the electrolyte to the cathode metal. The effects of cathodic action are evident in the way the anode metal quickly develops rust and deteriorates.


An example of this cathodic action can be readily visible in swimming pools with stainless steel ladders, flashings and other similar decorative parts. With swimming pool water circulation systems, high acid levels or an imbalance in the water alkalinity or PH will also cause galvanic action or corrosion. 


Another example of cathodic action often occurs with evaporative coolers where water is stored in a tank or pan. The water is pumped to the top of the evaporative cooler and dripped through pads. An air blower forces air through the water-laden pads, providing cooler air into the building. Within one or two years, the water pan often deteriorates from cathodic action. A common remedy is to attach a metal strip to the metal of the tank that will act as a sacrificial anode. This sacrificial anode must be replaced periodically and is called “passive cathodic protection.”


For the cathodic protection to function properly, the anode or sacrificial material must have a lower (more negative) electrical potential than the cathode metal (the steel tank). The normal electrical potential for clean (not rusted) steel is minus 0.5 to minus 0.8 volts (V), whereas aluminum alloy containing approximately 5 percent zinc would be about minus 1.05V. Zinc would be about minus 1.1V, and pure magnesium is about minus 1.75V.


A passive system using a sacrificial element may be acceptable for small tanks (e.g., evaporative coolers); however, larger structures, such as large metal tanks in the ground or swimming pools, may require an active cathodic protection system. These active systems often use a DC source, such as photovoltaic modules or wind power, or an AC source supplying a transformer rectifier that develops DC. The negative output terminal (the cathode) of the active system is attached to the structure to be protected, and insulated conductors connect the positive output terminal (the anode) to solid cast iron, graphite or metal oxide rods or tubes installed in the earth that act as sacrificial elements.


With a thorough understanding of the cathodic action concept and proper monitoring of the protection system, the underground metal structure or gas piping system can be protected from corrosion.