Review Of CP Criteria In Five Standards

December 2011, Vol. 238 No. 12

F.M. Song, PhD, and H. Yu, PhD, Southwest Research Institute, San Antonio, TX


(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.

Recent Review
A comprehensive review and evaluation with field and laboratory data of global CP criteria were performed in a recent project for Pipeline Research Counsel International (PRCI). It was found that none of the CP criteria or even their combinations sufficiently guaranteed adequate protection of a pipeline in all field conditions, such as pitting under a shielding coating disbonded from the pipe surface.

The inability of small-scale laboratory tests to reproduce the IR drops contained in an on potential measured in the field makes lab test data inadequate to use for evaluating the effectiveness of the -850 mV on-potential criterion. The -850 mV on-potential criterion has long been used in the U.S. The effectiveness of this criterion with respect to soil resistivity may be qualitatively explained in the following.

In lower resistivity soils, the IR drop tends to be small. In high-resistivity soils, the IR drop can be large, but the corrosion rate is low and the polarization required for protection is correspondingly small. In such high-resistivity soils, CP is most useful, and can become effective, only during wet seasons when the soil becomes corrosive. This criterion may fail, however, as can other CP criteria when the soil near the pipe is wet and corrosive while the bulk soil is still dry and CP currents cannot reach the pipe surface to provide the necessary protection.

Results of the PRCI project also show that the -850 mV off-potential criterion is, in general, more stringent than the 100 mV cathodic polarization criterion. The -850 mV on-potential criterion may or may not be more stringent than the 100 mV cathodic polarization criterion, depending on specific field conditions. The PRCI project also evaluated the different off-potential criteria in ISO and EN standards for higher resistivity soils, elevated temperatures, and soils with bacteria.

With the advancement of monitoring and inspection technologies capable of assessing the CP effectiveness and adequacy for specific field conditions, the report notes that regulatory reinforcement of a given CP criterion may not be entirely appropriate; nevertheless, the CP criteria can still serve as important corrosion-control tools when a proven effective CP level is lacking or cannot be justified.

Only the results obtained from a review of all historical versions of NACE Standard RP/SP 0169 (up to the present) and from a review and comparison of five international CP standards and their CP criteria are presented here.

NACE Standard 0169 History
The field practice of CP to protect buried pipelines from corrosion began in 1928 when the first CP rectifier was installed by Robert J. Kuhn on a long-distance gas pipeline in New Orleans. The -850 mV on-potential was first reported by Kuhn at the 1928 Washington Conference for CP held by the National Bureau of Standards (NBS). Modern CP technology was founded on this potential. In his 1933 paper, Kuhn stated, “The potential to a copper-sulfate electrode, to which a pipe (potential) must be lowered in order to stop corrosion, is probably in the neighborhood of -0.850 V.”

Although the field practice of CP on buried pipelines was started in the 1920s, it was not until 1969 that the first NACE standard for CP criteria, NACE Standard RP 0169?1969, was conceived. Subsequently, it was revised in 1972, 1976, 1983, and 1992, and later reaffirmed in 1996, 2002, and 2007. In 2007, it was changed from recommended practice (RP) in the previous versions to standard practice (SP). These eight versions of the NACE Standard RP/SP 0169 were reviewed and the review results are presented.

Historically, a total of six CP criteria were cited in the NACE RP/SP 0169. They are listed in Table 1 and briefly explained next.
(a) On-potential of -850 mV with consideration of voltage drops other than across the structure-electrolyte boundary
(b) Polarized potential of at least -850 mV; measurement requires elimination of all voltage drops other than across the structure/electrolyte boundary
(c) A minimum of 100 mV of cathodic polarization between the structure surface and a stable reference electrode contacting the electrolyte; measurement requires elimination of all voltage drops other than across the structure/electrolyte boundary
(d) A negative (cathodic) voltage shift of at least 300 mV, as measured between the structure surface and a stable reference electrode, with consideration of voltage drops other than across the structure/electrolyte boundary
(e) A voltage at least as negative (or cathodic) as that originally established at the beginning of the Tafel segment of the E-logI curve; measurement requires elimination of all voltage drops other than across the structure/electrolyte boundary
(f) A net protective current from the electrolyte into the structure surface, as measured by an earth current technique applied at predetermined current discharge (anodic) points of the structure; this is measured without consideration of voltage drops

As shown in Table 1, only (e) in the list above is required to measure current; all other criteria require only the measurement of pipe-to-soil potential (a-d) or the potential difference (f).

Table 1:  Historical methods of measuring the level of Cp in NACE RP/SP 0169.

The measurement of (a) must be conducted with CP applied, while the measurements for (b) and (c) must be performed with all external current sources shut off. Measurements of both on- and off-potentials should be performed for (d?f).

The historical change of the CP criteria in the standard is summarized in Table 2. In general, the content of the CP criteria was consistent among the first four versions (1969, 1972, 1976, and 1983) and among the last four versions (1992, 1996, 2002, and 2007). The first four versions all contain the five criteria: (a) and (c-f). A significant change was made in the 1992 version.

Table 2: Comparison of historical CP criteria in NACE RP/SP 0169.

In the 1992 version, the earlier criteria (d), (e), and (f) were removed and criterion (b) was added. The term “Steel and Cast Iron Structures” in the earlier versions was replaced by “Steel and Cast Iron Piping.” The basis for removing criteria (d) and (f) was perhaps due to their inadequate theoretical and practical bases. The basis for removing criterion (e) was perhaps due to its impractical application in field conditions. The criterion (b) was included, possibly due to the availability of more laboratory and field test results and to the evolution and advancement of field instruments to measure instant off-potentials more readily. The exact reason is unclear.

On the 100 mV cathodic polarization criterion, its original content has consistently been preserved throughout the history of this standard. The measurement method has been polarization decay, while the formation polarization method was added since the 1992 version.

Five International CP Standards
The following five global standards with CP criteria were collected and reviewed:
(1) NACE SP 0169?2007
(2) ISO 15589?1
(3) EN 12954:2001
(4) AS 2832.1?2004
(5) OCC?1?2005 (refers to the Canadian CP standard)

Table 3 shows an overall comparison of the CP criteria for all five standards. The specific CP criteria in each standard are summarized in Tables 3A-3E (which are not included here but are available from the author). In developing the tables, much effort was devoted to matching the content in the tables to the original standard as closely as possible, even though the format appears different. The consensus and difference among these standards can be clearly seen from the tables.

 Table 3: Comparison of CP criteria in five international standards.

Table 3 shows that, in general, the CP criteria in NACE SP 0169?2007 are consistent with those in OCC?1?2005, while the standards of ISO 15589?1 and EN 12954:2001 are similar in nature. Neither ISO 15589?1 nor EN 12954:2001 has the -850 mV on-potential criterion, while both contain additional detailed criteria, including the effect of soil resistivity and overprotection.

Some differences between these two standards are that ISO 15589?1 contains a CP criterion with the effect of bacteria, while EN 12954 contains CP criteria at different temperature ranges and in anaerobic soil. The criterion of overprotection is -1,200 mV with ISO and -1100 mV with EN. In addition, EN 12954 is distinguished from all other standards in that it does not have the 100 mV cathodic polarization criterion that is contained in all other standards.

AS 2832.1?2004 is unique in that its -850 mV off-potential criterion is used in conjunction with either a coupon or an ER probe, and the 100 mV cathodic polarization criterion can be used alone or in conjunction with either a coupon or an ER probe. The -850 mV on potential is recommended for use only when the IR drops are insignificant, though no specific value for the IR drop is given. The overprotection criterion in this standard is the same as that in the ISO 15589?1.

Conclusions
A review of the past eight versions of NACE Standard RP/SP 0169 shows that the CP criteria were generally consistent among the first four versions (1969, 1972, 1976, and 1983) and among the last four versions (1992, 1996, 2002, and 2007).

A significant change was made in the 1992 version, where the three earlier criteria — 300 mV polarization shift, Tafel segment, and net protective current — were removed and the -850 mV off-potential criterion was added. Putting together in parallel the -850 mV on- and off-potential criteria in one standard appears to contradict the principle on which NACE RP/SP 0169 was developed, which states that “This standard is intended to serve as a guide for establishing minimum requirements for control of external corrosion.” There is clearly a minimum between these two criteria.

The CP criteria are nearly identical between NACE SP 0169?2007 and OCC?1?2005 and similar between ISO 15589?1 and EN 12954:2001. The base CP criteria of the former two standards are the -850 mV on- and off-potential criteria, and the 100 mV cathodic polarization criterion. For the latter two standards, neither has the -850 mV on-potential criterion, while both have more detailed criteria relating to the effect of soil aeration, soil resistivity, and overprotection.

Neither provides references to cite the basis of the criteria. Their differences are that the ISO 15589?1 contains a CP criterion for soils with bacteria, while EN 12954 contains CP criteria at elevated temperatures. The overprotection criterion is -1,200 mV for ISO 15589?1 and -1,100 mV for EN 12954. EN 12954 is distinguished from all other standards in that it does not have the 100 mV cathodic polarization criterion.

AS 2832.1?2004 is unique in that its -850 mV off-potential criterion is recommended for use in conjunction with either a coupon or ER probe. The 100 mV cathodic polarization criterion can be used alone or in conjunction with either a coupon or ER probe. The -850 mV on-potential criterion is recommended for use when the IR drop is insignificant, while the limit of the IR drop is not provided. The overprotection criterion is the same as in ISO 15589?1. Contact the author: e-mail: fsong@swri.org.

Acknowledgement
This article is based on work sponsored by Pipeline Research Council International (PRCI) under Contract PR?015?0835. The advice of Bob Gummow of Correng Consulting Service Inc., program management of Mark Piazza of PRCI, and technical guidance of the PRCI Corrosion Committee, particularly David McQuilling of Panhandle Energy and Dave Aguiar of Pacific Gas & Electric Company, are appreciated. Daniela Matthews and Linda Goldberg of NACE headquarters provided all eight versions of NACE Standard RP/SP 0169.

Authors
Fengmei (Frank) Song is a senior research engineer at Southwest Research Institute in San Antonio, TX. He earned his Ph.D. (2002) from the University of Toronto. He is a leading researcher in the areas of pipeline internal corrosion, external corrosion and stress corrosion cracking, and their direct assessment methodologies. His work also involves evaluations of corrosion inhibitors and coatings and studies on microbiologically induced corrosion and corrosion fatigue. fsong@swri.org.

Hui Yu is a research engineer in the Materials Engineering Department of Southwest Research Institute. He earned his Ph.D. (2007) from Florida Atlantic University with a specialty in materials corrosion and control. He has experience conducting laboratory and field investigations for the corrosion of reinforcement in concrete and metallic corrosion in aqueous/soil environments. He has experience in cathodic protection design and evaluation for underground pipelines, tanks, vessels, and offshore structures.

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