Materials Performance

MAY 2017

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33 NACE INTERNATIONAL: VOL. 56, NO. 5 MATERIALS PERFORMANCE MAY 2017 ization. This value must be ≥100 mV to meet this criterion. Note that both poten- tial measurements in formation include a voltage error, but it is the same in both measurements; thus, the dif ference be- tween the instant-on and the final-on is due to polarization. The decay of polarization is determined by first measuring the polarized (instant- off ) structure-to-electrolyte potential, after which the protective current is left off, followed by a depolarized structure-to- electrolyte potential measurement. The difference between the polarized and depo- larized potentials must be ≥100 mV to meet the criterion. The depolarized potential is not expected to return to the native poten- tial crit erion , as di scussed pre viously. When the time of depolarization is limited to less than the time required for full depo- larization, it is recognized that the crite- rion achieved is conservative. The difference between the on potential and the instant-off potential is an error introduced into the on potential (often called the IR drop or voltage gradient error but called voltage error here) that must not be confused with polarization. Figure 1 illustrat es th e C P crit eria showing the polarized potential (–850 mV vs. CSE) and the two methods for establish- ing the 100-mV polarization criterion. Only one criterion needs to be met. An exception to the above criteria is a structure that is subject to stress corrosion cracking (SCC). NACE SP0169-2013 pro- vides a potential range subject to SCC d e p endin g on t emp erature, w hi l e I S O 15589-1 gives a range for SCC susceptibility of –650 to –750 mV vs. CSE. Polarized Potential (–850 mV vs. CSE) Criterion The polarized potential criterion is measured by synchronously interrupting the protective current sources and record- ing the instant-off or polarized potential. A polarized potential of –850 mV vs. CSE or more electronegative will meet the CP criterion for steel. Where a synchronous interruption is not practical, the "voltage error" must be FIGURE 1 CP criteria. removed from the on potential by sound engineering practice based on information such as historical data, other indirect and direct inspections, or coupons. This method may introduce a degree of uncertainty. Impractical to Measure a Polarized Potential There are facilities that for different reasons make it impractical to measure a true polarized potential, usually because it is not practical to interrupt all CP power sources. By extension, it is taken that the p ol ari zation crit erion cannot b e c on- firmed, or can it? Using the same premise as for the for- mation of polarization where a voltage error exists but is constant in both sets of potentials, a partially depolarized potential with a constant voltage error in both the off and partially depolarized potential could also be considered as illustrated in Figure 2 and by the examples below. Example 1 is a utility pipeline system with a combination of impressed current (rectifiers) and direct connected sacrificial anodes. W h en th e rectifiers are inter - rupted, it is recognized that the sacrificial anodes are still providing some current contributing to a voltage drop error, thus invalidating the polarized potential crite- rion (–850 mV vs. CSE). The 100-mV criterion could be estab- lished by synchronously interrupting the rectifiers and using the off potential as a base line, recognizing there is still some voltage error in the reading. When the rec- tifiers are left off, the potentials should fol- low the "partial depolarization" line in Fig- ure 2 that includes the same voltage error as the initial off potential. When the differ- ence of the off potential and the depolar- ized potential taken at the exact same loca- tion is ≥100 mV, the polarization criterion is satisfied. A polarized potential of –850 mV vs. CSE or more electronegative will meet the CP criterion for steel. Should the CP system be a mix (hybrid) of impressed current and sacrificial anodes, w hen the impressed current system is interrupted, the output of the sacrificial anodes will likely increase as the driving potential between these anodes and the structure(s) increases. This will have the effect of increasing the voltage error, thus reducing the apparent depolarization that will make the use of a 100-mV criterion more conservative. Although a cautious approach as it does not include the polar- ization from the sacrificial anodes, it is a definitive means of confirming a criterion. Example 2 is a facility that requires a large number of rectifiers to provide the necessary CP current, making it impracti-

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