Cathodic potential (CP) criteria have been used as a recommended minimum requirement for controlling external corrosion of buried steel piping systems. In this last part of a three-part paper, field and laboratory data were evaluated to understand the effectiveness of the -100 mV polarization criterion, which was compared with the effectiveness of the -850 mV on-potential criterion and various off-potential criteria in different soil resistivity ranges. The effectiveness of various CP criteria associated with elevated temperatures and for soils with bacteria was also evaluated and reported.
The CP criteria for buried piping systems are not consistent among global CP standards (Part 1 of this paper). This inconsistency can lead to confusions when a criterion is determined for best use in the field. The global standards including ISO(1) 15589-1,1 EN(2) 129542 and AS(3) 2832.13 cited few, if any, references for justifying the bases of their CP criteria. This may lead people to wonder how well justified the CP criteria are in these standards. It is also questioned whether the CP criteria can assure that pipes are effectively protected under all conditions. There is a need for an evaluation of the CP criteria, and understanding fundamental bases of the CP criteria. In two companion papers (Part 1 and Part 2), the CP criteria in five international CP standards were reviewed, a chart showing the correlation between the steel pipe native/rest potentials vs. soil resistivity and its relevance to CP criteria were revealed, and the results for an understanding of the effectiveness of the -850 mV on- and off-potential criteria were discussed. This paper is focused on presenting the results for an understanding of the -100 mV polarization criterion and several CP offpotential criteria associated with higher resistivity soils, elevated temperatures and soils containing bacteria.
