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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23 MATERIALS PERFORMANCE: VOL. 57, NO. 9 SEPTEMBER 2018 (similar coils were used as anodes) and a calo mel or silver-silver sul- fide reference. The cathodic coil (ap- proximately 2.6 square inches sur- face) was designed so that changes in resistance of the exposed wire could be followed with an electrical resistance meter. Current was regu- lated with series resistors and po- tentials measured on a precision potentiometer. However, less pre- cise potential measurement would suffice. All laboratory tests were made at 86 F. In the experimental procedure a corrosion rate for the working elec- trode was established without cur- rent flow. A polarization curve then was run, and corrosion rates at vari- ou s appl ied c u r r e nt s wer e ob - served. In some solutions tested, the initial cor rosion rate was too low or the corro sion films too con- ductive to produce observable cor- rosion rates. However, the method is used in the study of polarization phenomen a, a nd it s s en sit iv it y could be increased by the use of smaller diameter wire for the work- ing electrode. Results and Discussion Cur ves i n Fig ure 2 represent types most frequently encountered in field and laboratory studies. A brief dis cussion of them will help in under standing their significance. Curve A in Figure 2, the t ype most frequently encountered, con- sists of two essentially straight line segments with a curved segment between them. Early descriptions of the method say the point of inter- cept of the exten sions of the straight line segments (Point 1 on the curve) indicates the protective current. Po- l a r i z at ion t h e or y s ug ge st s t h e upper segment should represent the region of cathodic re action only. Deviation from this line indicated iron is still dissolving; thus protec- tive current can be determined at Point 2 on the curve. Haycock 4 has presented experimental data which agree with the latter interpre tation. Curve B in Figure 2 shows more t ha n one break poi nt. Alt hough such curves have been derived, 4 it is s ome t i me s que st ioned whet her t hese are discrete segments or a gradual curve. Nevert heless, t he uppermost break is accepted as in- dicative of protective current. Feasibility Studies on Cathodic Protection of Deep Well External Casing Surfaces Figure 3—Effect of applied current on cor- rosion rate in sodium acetate-sodium sul- fide-hydrazine solution. Initial pH was 6, final pH was 8.9. Figure 4—Polarization curves for sodium acetate-sodium sulfide-hydrazine solution, pH 8.9. Rapid Time Curve is 1: less than 0.001 volt potential change in 5 minutes. Slow time cur ve is 2: constant poten- tials—38 days elapsed. Silver-silver sulfide reference electrode was used. No interpretation of Curve C can be found in the literature although such curves were obtained in field and laboratory tests. A plateau fol - lowed by an upward sweep may represent either a diffusion limited cathodic reaction, two cathodic re - actions or an anodically controlled reaction process. Data from laboratory study are summarized in Figures 3-6, and the field polarization curves are shown in Figures 7-14. The probable pro- tective current indicated by each of the field polarization curves is indi- cated by an arrow. If no arrow is shown, the curve is not considered interpretable. Discussion of labora- tory results is presented first be- c au s e ma ny of t he conc lu sion s drawn from field data depend on select ion of a protect ive current value from a curve. Laboratory Study— Interpretation of Curves To obtain laboratory polariza- tion data which are comparable to field data, some assumptions must be made about conditions behind the casing. The pH is assumed to be controlled by the calcium carbonate of natural formations or by calcium

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