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34 MAY 2017 MATERIALS PERFORMANCE NACE INTERNATIONAL: VOL. 56, NO. 5 rectifiers only, thus as many rectifiers as practical should be interrupted. If the dif- ference between the partially depolarized and off potentials is ≥100 mV, this CP crite- rion is satisfied. This approach is expected to be conser- vative as polarization from the remote rec- tifiers is not measured. Thus, as many recti- fiers as possible should be interrupted. In the absence of another method to confirm a criterion, this approach will document that a CP criterion has been established. Special considerations for mixed metal systems is required, as discussed below. Misapplication of the 100-mV Criterion The 100-mV criterion is intended to represent an increase in polarization from the fully depolarized potential of steel. It can be misapplied, especially if the steel is tied to another more noble metal such as would be found in pipeline stations, power- generating stations, petrochemical plants, or refineries. In such cases, copper may be used as a material for electrical grounding, and there can be a mixture of other metals, such as steel in concrete or stainless steels, that have unique characteristic potentials. When interconnected, the potential mea- sured is a "mixed metal" potential of all metals rather than a potential of steel. In this case, applying 100 mV to a mixed metal potential is a misapplication, as shown in Figure 3 and the example below. Given a steel and copper couple: • Potential of isolated steel— –600 mV vs. CSE • Potential of isolated copper— –200 mV vs. CSE • Mixed potential of steel and copper—–400 mV vs. CSE • Apply 100 mV to mixed poten- tial—–400 + –100 = –500 mV vs. CSE • Apply 100 mV to steel native poten- tial—–600 + –100 = –700 mV vs. CSE These values are, of course, approxima- tions. The actual mixed potential depends on the surface of each metal that is in con- tact with the environment. Should the mixed metal potential be considered and the 100-mV criterion be applied to it, the incorrect target potential for CP is –500 mV vs. CSE; however, this is not even back to the potential of isolated steel (–600 mV vs. FIGURE 2 Confirming 100-mV criterion with partial interruption of direct current sources. FIGURE 3 Example of 100-mV criterion applied to mixed metals. cal to install portable synchronized inter- rupters. The preferred solution in this case is to equip them with remote monitoring units that have synchronizable interrup- tion features to allow a polarized potential measurement. In the absence of these unit s, th e fo l low in g appro a ch can b e considered. By prior testing, select the rectifiers that most influence a given portion of the system and install portable synchronized interrupters. Measure the off potentials in the area of inf luence, recognizing that it includes a voltage error from the more remote rectifiers. Turn off only those recti- fiers that were interrupted and allow time for depolarization (Figure 2) while keeping the other rectifiers at the same output. Care should be taken when selecting the rectifiers, as when the structure-to-electro- lyte potential becomes less electronegative during rectifier interruption, the output of the rectifiers still in service can increase and influence the area under test. Leave the selected influencing rectifiers off and measure the partially depolarized potentials at the same exact locations. The depolarization is that from the interrupted CATHODIC PROTECTION