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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nized criteria may not be adequate in analyzing the degree of protection being afforded a storage tank resting on the ground. Experience has shown the following: 1. That structure-to-earth potential measurements obtained at the perimeter of the tank do not indicate actual CP lev- els at the center, particularly on larger-diameter tanks; 2. That potential measurements at the center of the tank and at other areas under the tank are necessary if tank bottom CP surveys are to be complete; 3. That the liquid level in a tank is a significant factor in determining the CP status of the tank bottom at the center; 4. That certain types of CP designs may not be effective in providing adequate CP to the tank bottom in certain areas; 5. That an improved design for distributed groundbed, using slant or angle drilling techniques (Figure 4) to locate anodes under the tank bottom, can improve the current distribution to the center by reducing the ten- dency for most of the current from conventional distrib- uted beds to flow to the tank at the perimeter. The standard method of determining the effectiveness of CP on any structure is the structure-to-electrolyte potential measurement. These measurements are performed using a high-impedance volt meter and a stable, reproducible refer- ence electrode contacting the electrolyte. The natural potential of a carbon steel structure in contact with an electrolyte is usually on the order of –0.600 V when measured with respect to a copper-copper sulfate reference electrode (Cu -CuSO 4 ). More positive values are typically indic- ative of steel structures that have undergone corrosion, while more negative values are indicative of new, well-coated struc- tures or structures that are under the Influence of CP. A structure is considered to be effectively protected when a potential measurement of –0.850 V (Cu-CuSO 4 ) is obtained 3 (one of several accepted criteria). For a true representation of electrical potential measurements, the reference electrode is to be placed as close as practicable to the structure under investigation (either directly over or adjacent to). The problem associated with monitoring CP systems on tank bottoms is the inability to place a portable reference elec- Cathodic protection of external tank bottoms FIGURE 2 — Anodes installed under a tank bottom. FIGURE 3 — Commonly installed deep well impressed current anodes. FIGURE 4 — New system approach: angled, impressed current anodes. MATERIALS PERFORMANCE: VOL. 57, NO. 9 SEPTEMBER 2018 A41

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