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Equalization Currents and Metallic IR Drop: Impediments to True Potential Measurements

Published: 20180417 by NACE Corrsion
Author(s): Daniel Fingas
Co-Author(s): Derek Leach - TransGas Limited; Jean-Patrick Boudreault

Recent close-interval potential surveys have raised concerns about the accuracy of potential measurements on some older pipelines. Two significant sources of error were identified: metallic IR-drops and equalization currents. These resulted in measured on and off potentials which differed by hundreds of millivolts from the true potentials, even though the measurements were conducted in accordance with industry best practice.

Examples of these phenomena will be discussed and the electrical theory will be explained. Both interrupted test post surveys and close-interval potential surveys are susceptible and these errors can be difficult to detect. This can have a significant impact on interpretation of survey data, External Corrosion Direct Assessments, and remedial program decisions.

Guidance for identifying when these issues may be relevant and methods for detecting and compensating for these issues will be provided.