Materials Performance

SEP 2018

Materials Performance is the world's most widely circulated magazine dedicated to corrosion prevention and control. MP provides information about the latest corrosion control technologies and practical applications for every industry and environment.

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T A N K B O T T O M P R O T E C T I O N trode in close proximity to the underside. Most of the testing in the past has relied on readings that have been taken at the perimeter of the tank. Placement of the reference electrode at the perimeter of the tank may yield erroneous results because of potential gradients created in the soil as a result of the cur rent discharge from the anode. This is particularly true when using distributed anodes installed along the periphery of the tank. In certain cases, it has been determined that potential mea surements at the perimeter satisfy one of several criteria, but through special testing, as the reference cell is advanced toward the center of the tank, the potentials fall off to the point at which little if any potential shift is measured, indicating a lack of effective protection at the center. Table 1 indicates actual measurements obtained through this testing technique. A comparison of the potentials observed at the perimeter of the tank and at the center of the tank with a certain applied current are, in this case, indicative of effective protection. However, it should be noted that the potential differ ence between the perimeter and the center is on the order of 300 mV. It is concluded that an adequate survey to determine satisfac tory CP levels on tank bottoms must include at least one structure to earth potential measurement taken at the center of the tank. For new tank construction, the problem of potential testing at the center can be solved by installing either (1) permanent reference electrodes and a lead wire underneath the tank pad to the perimeter of the tank where it can be terminated in a test station for future use in testing (Figure 5) or (2) a perforated polyvinyl chloride (PVC) or fiber reinforced plastic (FRP) pipe (Figure 6) for use in profiling the potential from the perimeter to the center. For existing tanks, the problem is somewhat more difficult but not at all impossible. Through a water jetting procedure, a hole can be bored from the perimeter of the tank to the center of the tank and a perforated PVC or FRP pipe installed under neath the existing tank (Figure 6). Once this is accomplished, a reference electrode can be passed from the perimeter to the center of the tank while profiling the potentials in between. Under these conditions, a representative depiction of the pro tective levels at the center of the tank can be observed readily. Cathodic protection system typical case studies The following typical case studies give an indication of some of the problems that can be encountered with standard designs and testing techniques. Typical Case No. 1 In this situation (Figure 7), the structure is a 36.5 m diameter by 14.6 m high, carbon steel, ground storage tank that has been cathodically protected by a deep anode system installed in close proximity to the tank. Potential measurements observed around the periphery of the tank indicate values of –0.715 to –0.885 V (Cu CuSO 4 ). Potential values obtained at the center of FIGURE 5 — Permanently installed reference cell and test station. FIGURE 6 — Perforated pipe installed for reference cell access to tank bottom. SEPTEMBER 2018 W W W.MATERIALSPERFORMANCE.COM A42

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