Review Of CP Criteria In Five Standards

(Editor’s Note: Tables 3A through 3E and a list of literature references are not included here due to space constraints. They are available from the author. E-mail: fsong@swri.org.)

Cathodic potential (CP) criteria are used as a recommended minimum requirement for controlling external corrosion of buried or submerged steel piping systems. Different and sometimes conflicting CP criteria are given in different global CP standards. This can lead to confusion regarding what criterion is best used for a given condition.

This article is aimed at reviewing all historical versions of recommended practices/standard practices (RP/SP) of the CP criteria in NACE Standard RP/SP 0169 and comparing CP criteria with five international CP standards. The use of an inappropriate CP criterion can lead to either excessive cost or failures.

The current NACE Standard SP 01692007 Section 6 mainly emphasizes three CP criteria:
1) -850 mV vs. saturated copper/copper sulfate reference electrode (CSE) with CP current applied, or -850 mV on-potential considering voltage drops (IR);
2) -850 mV off-potential or polarized potential;
3) 100 mV cathodic polarization.

A polarized potential cannot be measured directly in the field; practically, it is represented by an off-potential. A polarized potential is a potential measured locally at an exposed pipe surface while an off-potential, like an on-potential, represents an average potential over a pipe segment. 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 off-potential, the -850 mV off-potential criterion is a more stringent criterion than the -850 mV on potential criterion. All conditions meeting the off-potential criterion meet the on-potential criterion. Putting together these two criteria in one standard appears to contradict the principle on which NACE RP/SP 0169 was developed. The standard states, “This standard is intended to serve as a guide for establishing minimum requirements for control of external corrosion.” There is clearly a minimum requirement among these two CP criteria.

The CP criteria among global CP standards are not consistent. For instance, the standard of the International Standard Organization (ISO 15589-1) and the European standard (EN 12954) offer more specific CP criteria with respect to environmental conditions, such as soil resistivity, aeration, presence of bacteria, pipe temperature, and overprotection, while they exclude the -850 mV on-potential criterion contained in NACE Standard SP 01692007. The Australian National Standard SAA AS 2832.1 recommends the use of coupons or an electrical resistance (ER) probe in conjunction with the -850 mV off-potential or the 100 mV cathodic polarization criterion, not included in the previously noted standards.

The inconsistency of CP criteria can lead to confusion when determining the best standard or CP criterion for a given field condition. Because the global standards of ISO 155891, EN 12954, and AS 2832.1 cited few, if any, references for justifying the bases of the CP criteria, questions may arise as to whether or how well-justified these criteria are, or whether alternative CP levels can be used to provide adequate protection.