INTRODUCTION
Cathodic potential (CP) criteria have been used as a recommended minimum requirement for controlling external corrosion of buried steel piping systems. In Part 1 of the three-part paper, discrepant and sometimes conflicting CP criteria in global CP standards were reviewed and the correlation between CP criteria and theory was discussed. In this paper, a correlation between steel pipe native/rest potentials vs. soil resistivity was developed and validated with laboratory and field data. Its relevance to CP criteria is also reported. Field and laboratory data were evaluated to understand the effectiveness of the -850 mV on- and off-potential criteria.
The CP criteria for buried piping systems are not consistent among global CP standards. This inconsistency can lead to confusion when a criterion is determined to be the best practice in the field. In addition, the global standards including ISO(1) 15589-1,1 EN(2) 129542 and AS(3) 2832.13 cited few references, if any, to provide the bases of the CP criteria. This may lead to confusion regarding how justified the CP criteria are in the global CP standards. It is also questioned whether the CP criteria can assure that pipes are effectively protected under all conditions. The CP criteria in five international CP standards were reviewed and reported in a companion paper (Part 1) including all historical versions of the NACE RP/SP01694. This paper reports the results of evaluation for the -850 mV on-and off-potential criteria. Whenever and wherever applicable during an evaluation of a CP criterion, it is defined that CP is considered effective if the overall corrosion rate (pipe or coupon or probe) is controlled below 1 mpy (0.0254 mm/y). The test setup and conditions of this Barlo 1994 study are described next to facilitate the readers’ understanding of the tests without having to refer to the original report.
