INTRODUCTION
Cathodic potential (CP) criteria have been used as a recommended minimum requirement for controlling external corrosion of buried or submerged steel piping systems. A critical review of CP criteria in five international CP standards and historical publications was undertaken. Discrepant and sometimes conflicting CP criteria were found among the CP standards. This may confuse practitioners and lead to misdirected decisions. The impact on the operating cost can also be significant when an inappropriate criterion is used, such as the -850 mV on- vs. off-potential criterion in areas with current interferences. The discrepancy associated with diverse and sometimes conflicting CP criteria may be attributed to both the lack of sufficient field data for validating the criteria and the lack of clarity with the fundamentals behind the criteria. In the first part of a three-part paper, the historical versions of the CP criteria in NACE RP/SP0169 were reviewed, the criteria in five international CP standards were compared, and an understanding of the correlation between CP criteria and theory was provided.
The difference between on- and off-potentials measured under the same conditions may generally be considered as the IR voltage drop. With the on-potential being more negative than the offpotential, the -850 mV off-potential criterion is a more stringent criterion than the -850 mV on-potential criterion. The standard states that “This standard is intended to serve as a guide for establishing minimum requirements for control of external corrosion.” The on-potential criterion is clearly a minimum requirement among these two CP criteria. The potentials shown in each of the figures include the native potential (Ecorr), the on- and off- (or polarized) potentials, the “decayed-off” potential (potential measured during depolarization), and the “rest potential” (potential when depolarization becomes steady), or the polarization growth or decay and the IR drop.
