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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Cathodic protection of external tank bottoms the tank are considerably more positive than those obtained at the perimeter of the tank and indicate no protection at the cen- ter of the tank with an applied current of 2.1 A from a deep anode groundbed. Typical Case No. 2 In this situation (Figure 8), the structure is a 33.5-m diameter by 14.6-m high, steel, ground storage tank using a deep anode CP system delivering –2 A of current. Potential measurements obtained around the perimeter of the tank are indicative of complete protection; however, the potential value at the center of the tank [–0.640 V (Cu-CuSO 4 )] does not satisfy the minimum accepted criterion of –0.850 V (Cu-CuSO 4 ). These examples indicate the critical nature of reference elec trode placement in assessing the degree of protection being afforded a ground storage tank. The case histories also demonstrate that there is a general tendency for protective cur- rent to flow from the anode onto the tank bottom at the perim- eter of the tank. This condition is prevalent as a result of the fact that the tank bottom at the perimeter usually has the lowest resistance to earth, particularly when the tank is low on product level or empty. Under conditions of maximum product level, it has been observed that the weight of the product in the tank creates inti- mate contact between the tank bottom and the electrolyte. This lowers the contact resistance of the center area of the tank, thereby necessi tating a larger current drain to the tank as a result of the increased surface area of the cathode. Testing performed on an empty tank may indicate adequate CP even at the center as a result of the limited amount of sur- face area that is being protected by a CP system. Once the tank has been filled, the surface area in contact with the electrolyte increases and the total current density to the metal decreases. Under these conditions, testing has shown that the higher the liquid level in a tank, the higher the current drain to the tank and the lower the potential of the tank at the center. Although this testing is not entirely conclusive, the increase and decrease in product level in the tank may result in depolarization as a result of physical abrasion when the tank bottom undergoes movement. These conditions must be considered when design- ing and testing CP systems for tank bottoms. In certain cases, deep anode groundbeds have been installed and operated effectively in providing adequate CP to the entire underside surface. The success of a deep anode groundbed system is usually predicated on relatively low uni- form soil resistivities from the ground surface to the total depth of the deep groundbed. FIGURE 7 — Typical Case Study No. 1: deep anode groundbed installed in close promixity to the tank (current = 2.10 A). FIGURE 8 — Typical Case Study No. 2: deep anode groundbed installed in close proximity to the tank (current = 2.00 A). In areas of nonuniform soil resistivity, particularly where high resistivity strata may lie between the tank bottom and the active anode area, it becomes difficult to direct the groundbed current to the tank bottom, and more of the current is distrib- uted elsewhere to structures that are electrically continuous with the tank. Distributed anode groundbeds that are installed around the periphery of the tank are sometimes intended to provide a more uniform distribution of current to the tank. In either case, the electrical shielding effect of the tank perimeter sometimes precludes adequate distribution of cur- rent to the center of the tank, as evidenced by the data pre- sented in Case Studies 1 and 2. New system approach Recent advances in CP system installations using distrib- uted anodes that are augered at a 35- to 45-degree angle at the perimeter of the tank to a depth of 7.6 to 10.7 m (Figure 4) indi- cate a more uniform distribution of current and higher protec- tive levels at the center of the tank. In one case, a tank using a conventional distributed groundbed yielded potential measure- ments of –0.800 V at the perimeter and –0.600 V at the center of the tank (Cu-CuSO 4 ). A new installation of distributed anodes, using anodes angle drilled, operating at the same current output, yielded potentials of –1.00 V at the perimeter and –0.860 V at the center of the tank after one day of operation. Test data obtained on two other simi- lar installations show that the results of the angle-drilled dis- tributed groundbeds are far more favorable than those of con- v e n t i o n a l d i s t r i b u t e d g r o u n d b e d s i n s t a l l e d a r o u n d t h e periphery. In both cases, the angle-drilled installation signifi- cantly outperformed the conven tional with respect to the over- all level of protection, particularly at the center of the tank. Installations of this nature can be conducted easily through the use of prepackaged or canistered anodes which permit the anode to be centered properly in a uniform column of coke breeze backfill and facilitate the installation of the entire anode assembly in the slanted hole. The annular space can be filled afterward by pumping a coke breeze slurry, while the remainder of the hole should be backfilled with sand. Summary The application of CP is an effective means of controlling corrosion on a tank bottom groundside. The success of the CP system is dependent on proper design and selection of the type of installation and effective monitoring of the level of protec- tion being afforded the entire tank bottom (groundside). The use of angle-drilled anodes around the periphery of the storage MATERIALS PERFORMANCE: VOL. 57, NO. 9 SEPTEMBER 2018 A43

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