Materials Performance

MAY 2017

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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32 MAY 2017 MATERIALS PERFORMANCE NACE INTERNATIONAL: VOL. 56, NO. 5 CATHODIC PROTECTION N NACE SP0169-2013 and ISO 15589- 1:2015 provide two measured cathodic protection criteria for steel that can be summarized as a 100-mV "polarization criterion" or a voltage error-free "po- larized potential criterion" of –850 mV with respect to a copper/copper sul- fate (Cu/CuSO 4 ) reference electrode. A concern often expressed with these criteria is that it can be impractical to synchronously interrupt all influencing current sources in a specific system; thus, a true polarized potential cannot be measured, and the amount of po- larization cannot be obtained. Where polarized potentials are not practical, an approach applying the polarization criterion to partial depolarization may be useful. NACE SP0169-2013 1 lists three cathodic protection (CP) criteria for steel, of which one is the use of empirical data and the other two involve field measurements with conditions for various operating environ- ments. ISO 15589-1-2015 2 also lists similar field-measured criteria but expands to others under different conditions. The most popular criterion for steel used in the past is a polarized (instant-off ) structure-to-electrolyte potential that is equal to or more negative than –850 mV referenced to a copper/copper sulfate (Cu/ C u S O 4 ) e l e c tr o d e (C SE ) w ith c u r re n t applied but after all voltage errors have been removed or with all influencing, pro- tective sources of CP current interrupted. When Polarized Potentials Are Not Practical for a CP Criterion W. Brian Holts B aum, Corsult Associates (1980), Ltd., Calgary, Alberta, Canada The less popular criterion is a minimum of 100 mV of polarization. Conditions cover- ing these criteria must also be observed. Other discussions of the CP criteria and test methods are given in several other publications. 3-6 Typically, the polarization criterion relates the true polarized potential to a native or depolarized potential. A native potential is the structure-to-electrolyte p o t enti al j u st b efore a C P cur rent i s applied . A depolarized potential is the potential recorded after all inf luencing, protective CP sources have been turned off. It is unlikely that the depolarized potential will be the same as the native potential due to the products formed on the structure surface during polarization. Criteria Polarization (100-mV) Criterion Polarization is the change from the open-circuit potential caused by current flow across the electrode/electrolyte inter- face. NACE SP0169-2015 states: • " A minimum of 100 mV of cathodic polarization. Either the for- mation or the decay of polarization must be measured to satisfy this cri- terion." The formation of polarization can be determined by first measuring the instant- on structure-to-electrolyte potential imme- diately after the CP system is energized. After a period , the final-on potential is measured at the same exact location with the same applied current. The difference between the instant-on and the final-on potentials represents the amount of polar-

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