The effectiveness of cathodic protection on the exterior bottoms of aboveground storage tanks is best measured with permanent zinc or copper-copper sulfate reference electrodes placed under the tank bottom. The use of these electrodes is preferable to measurements taken at the periphery of the tank since peripheral measurements do not reflect the potentials deep under the tank. The asphalt storage tanks were maintained at a temperature of about 350F (175C) to keep the asphalt in a liquid state. The temperature of the soil at electrode depth was about 300F (150C), too hot for the use of conventional electrodes. Corrosion measurement probes with high temperature cables were substituted. Zero corrosion rate as measured on the probe indicates effective cathodic protection. The probes could also be used as coupons for measuring polarized potentials.
To determine the effectiveness of cathodic protection on the exterior bottoms of above grade storage tanks, reference electrodes are best installed underneath the tank bottom. Several electrodes may be used, depending on the diameter of the tank. One is usually placed under the center of the tank and the others at various locations between the center and the edge of the tank. The leads from the electrodes are led to a junction box placed usually on the wall of the tank or at a nearby rectifier location. The use of under-tank electrodes is preferable to taking measurements solely around the periphe~ of the tank.
With a portable electrode located at the periphery of the tank, one cannot measure the effectiveness of cathodic protection at any appreciable distance from the tank wall. Adequate measurement of the conditions at the center and other areas well under the tank requires the use of electrodes placed at strategic locations under the bottom.
In the case of tanks operating at ambient temperatures, zinc or copper-copper sulfate electrodes are commonly used. The soil at electrode depth under the asphalt tanks was about 300F (150C). At this temperature, copper-copper sulfate electrodes would be destroyed and zinc electrodes would likely passivate very quickly.
The problem developed recently at two locations. It was decided to use corrosion monitoring probes in lieu of the usual electrodes. These probes, which measure corrosion rates by changes in their electrical resistance, are often used in chemical or other vessels. They are also used to measure the effectiveness of cathodic protection on pipelines and other structures.
In the case of the asphalt tanks, the probes, which were each provided with a special high temperature resistant insulated cable, were placed under the tank bottoms in the same manner as zinc or copper- copper sulfate electrodes would have been. The probes can be used initially to determine polarization potential changes under the bottom. After being connected to the tank bottom, they can be used to monitor cathodic protection in two ways; first by showing a zero corrosion rate and second as coupons to measure polarized potentials.