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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Page 86 of 128

28 SEPTEMBER 2018 W W W.MATERIALSPERFORMANCE.COM C L A S S I C Time Interval Is Significant Importance of the time interval be tween potential readings and the magnitude of each current incre- ment has been evident in the pre- ceding discussion. Effect of the cur- rent in crement-time interval factor was studied on each of the seven wells, and the results are summa- rized in Table 5. If the current is in- creased too rapidly or in too large increments, the protective current is not well de fined or appears at too high a value as shown in Figures 11 and 12. As the time interval is ex- t e n d e d , s h a p e o f t h e c u r v e s changes. There is more curvature near the break point, and the slope of t he st raight li ne seg ment de- creases. For West Texas wells, a double break similar to those ob - tained in the laboratory study (Fig - ure 6) is observed with the longer time intervals. In each case, how- ever, there is no appreciable change in the indicated current require- ment if some minimum time inter- val was used. For these wells the optimum current in crement-time interval appears to be 0.4 ampere per f ive m i nutes. However, t he proper time interval should be es- tablished in each area. It is important to maintain a con - stant time interval over the entire run. Because potentials tend to shift to more negative values with time, true equilibrium probably is not es- tablished in the time intervals of these curves; the indicated break in the curves would be masked by in - consistent time intervals. This ef- fect has been obser ved for bot h labora tory and field curves. Effect of Continued Application on Current Requirements The gradual shift in potential ob - ser ved for ma ny wel ls leads to specu lation that current require- ments are reduced after long appli- cation of cur rent. In the final phase of this st udy, permanent ground beds were installed on test wells, and current values above and below the indicated pro tective currents were applied for five to six months. After these intervals, current re- quirements were redeter mined. Results are shown in Figures 13 and 14. For each case in which the ap plied current was less than the pre viously indicated protective cur- rent, t he curves show a defin ite break at almost the same current value as pre vious curves. For Wells C a nd D, applied c urrents were Figure 14—Effect of extended current appli- cation on current requirements. Curves 1, 2, and 3 represent Wells D, E and F respec- tively. Cathodic current applied for about 6 months was 6.6 amperes for Well D, 3.5 amperes for Well E and 5 amperes for Well F. Curve 1 began at 6.6 amps. After comple- tion of "run," current was set at 5.5 amps and the potential read after 24 hours. Cur- rent then was lowered to 3.9 amps and the potential again read after 24 hours. Figure 13—Effect of extended current appli- cation on current requirements. Curves 1, 2, and 3 are for Wells A, B and C respec- tively. On Well A, 1 ampere of cathodic current was applied for about 6 months. On Well B, 2.2 amperes of cathodic current was applied for about 6 months and 4.55 amperes for the same time on Well C.

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