March 2012, Vol. 239 No. 3

Features

Steel Pipe Native Potentials In Soils Affect CP Criteria

Soil resistivity, corrosivity and steel native potentials in soils are interrelated. A chart was developed in this work to quantitatively map the rule-of-thumb relationship between steel native or rest potentials and soil resistivity.

This chart was validated with available literature data from independent sources and can be used to provide a rough estimate of the magnitude of polarization applied on buried steel pipes for a given soil resistivity and an off-potential measured. Polarization provides a more direct measure (than an off-potential) on the magnitude of corrosion rate reduction. The chart shows that meeting the -850 mV, -750 mV and -650 mV (vs. Cu/CuSO4) off-potential criteria for soil resistivity ranges of, respectively, <10K, 10K-100K and >100K ohm.cm would – in general – yield at least 100 mV cathodic polarization.

Cathodic protection (CP) has been used for many decades for the control of external corrosion of buried or submerged steel piping systems. CP criteria can be used to guide the level of CP that needs to be provided on the pipe surface. The International Standard Organization (ISO) and European (EN) CP standards (ISO 15589-11 and EN 129542 ) recommend three off-potentials -850 mV, -750 mV and -650 mV vs. Cu/CuSO4 (CSE) – for the corrosion control for soil resistivity ranges of <10K, 10K-100K and >100K ohm.cm, respectively.

Unfortunately, few references, if any, are given in these standards to justify why these criterion values (and not others) should be used with respect to different soil resistivity ranges. Nevertheless, if the steel native or rest potentials can be known, the level of cathodic polarization applied on the steel pipe surface for a given off-potential can be evaluated and provides a more direct measure (than an off-potential) on the magnitude of corrosion rate reduction. In other standards, such as the NACE Standard Practice (SP) 0169-2007 and Australian CP Standard (SAA AS 2832.1), -850 mV on-potential criterion with CP current applied is recommended to use. To clarify the different terminologies used in this article, Figure 1 is provided which results from modification of a similar chart reported elsewhere.(5)

Figure 1 shows schematically the two methods used in the field to measure the polarization on buried piping. The meanings of on- and off-potentials, polarization (decay or growth) and ohmic voltage drop (IR), native and rest potentials are labeled and shown clearly. Figure 1(a) shows the method of polarization growth, and Figure 1(b) shows the method of polarization decay. The potentials shown in each of the figures include the native potential (Ecorr), the on- and off-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.

The difference between on- and off-potentials measured under the same conditions may be generally considered as the IR voltage drop, with the on-potential being generally more negative than the off-potential. Open circuit potential (OCP) is measured with no external current applied to a metal surface. It is generally referred to as a steel native potential or a free corrosion potential, although it can also be an instant off-potential, a decayed off-potential, or a rest potential. The values of these potentials can be measured correctly only when there is absence of interference by stray currents or long-line currents.

In field practice, an off-potential is usually measured by interrupting all possible external current sources within a sufficiently small time interval. In this work, it may be regarded as a polarized potential, although these two potentials differ from each other. A polarized potential can only be measured local at the exposed structure-electrolyte interface. By contrast, the off-potential is usually measured on-ground and represents an average covering a section of the pipe to be measured.

The goal of this article is to provide an understanding and a rule-of-thumb relationship between steel native/rest potentials and soil resistivity from which the relationship between off-potential criteria in different soil resistivity ranges and the magnitude of cathodic polarization can be better understood.

Chart Development
Let us discuss the development of the steel native/rest potential vs. soil resistivity chart.

Table 1 shows a soil resistivity classification.(6,7) The low or medium soil resistivity
is considered to be below 10K ohm.cm; high or very high soil resistivity between 10K
and 100K ohm-cm; ultra high or super high soil resistivity above 100k ohm-cm. The terminology used for the resistivity classification in Table 1 is different from that given in the ISO CP standard.(1)

In addition, the ranges of soil resistivity given in Table 1 are more detailed and data are unavailable to support developing a correlation between soil resistivity and corrosivity. Table 2 shows a relationship between soil resistivity and corrosivity.(8) In general, the higher the soil resistivity, the less corrosive the soil is to steel (CP effect not considered). When the soil resistivity is below 10K ohm.cm, the soil corrosivity to steel is classified as varying from very corrosive (0-500 ohm.cm), to corrosive (500-1K ohm.cm), to moderately corrosive (1K-2K ohm.cm), and to mildly corrosive (2K-10K ohm.cm). When soil resistivity is greater than 10K ohm.cm, the soil corrosivity to steel is classified as being progressively less corrosive.

Table 3 shows soil corrosiveness vs. steel native potential.(8) The soil resistivity (the right most column of Table 3) can be derived by a comparison of soil corrosivity in Tables 2 and 3. The higher the soil resistivity, the more aerated the soil or the more likely passivated the steel by the soil, and thus, the less negative the steel native potential.

Although it is likely that the criteria used to classify soil corrosiveness in Tables 2-3 are different, the criteria may be similar. A conservative soil corrosivity ranking would assume that the “very corrosive and corrosive” categories in Table 2 (soil resistivity less than 1K ohm.cm) correspond with “severe” in Table 3 (native potential more negative than -600 mV), “moderately and mildly corrosive” in Table 2 (soil resistivity of 1K-10K ohm.cm) correspond with “moderate” in Table 3 (native potential between -500 and -600 mV), “progressively less corrosive” in Table 2 (soil resistivity greater than 10K ohm.cm) correspond with “slight” (native potential between -400 and -500 mV) and “noncorrosive” (native potential less negative than -400 mV) in Table 3.

Figure 2 was created with a 50-100 mV potential range expansion to cover resistivity ranges not overlapped by Tables 2 and 3. When the soil resistivity is less than 1K ohm.cm, it is rare that the native potential can be measured to be more negative than -800 mV in the field. When that happens, it may be related to high alkaline solution (due to CP) following the Pourbaix potential vs. pH diagram for iron.(9) At such a high pH, it is likely that the soil is not corrosive.

Figure 2 also shows a rule-of-thumb relationship between soil resistivity and steel native potentials in soils. It is likely that some native potentials, (often estimated by OCPs), measured in soils fall out of the mapped zones. For instance, a decayed off-potential or a rest potential with insufficient depolarization time may still be more negative than the native potential, or fall below the potential range shown in Figure 2 due to prior cathodic polarization. A true rest potential is often more positive than the native potential due to formation of oxides after a long exposure of the steel in soil.

In Figure 2, the potentials of -850 mV, -750 mV and -650 mV, relevant to CP criteria in different soil resistivity ranges, are labeled by the dashed horizontal lines across the respective soil resistivity ranges given in the standards of ISO 15589-11 and EN 12954:20012 . It is clear that meeting the -850 mV off-potential would generally achieve a polarization of 100 mV for the entire soil resistivity range shown in the figure. For the soil resistivity range of 10K-100K ohm.cm and the range of 100K ohm.cm or greater, meeting the off-potential criteria of -750 mV and -650 mV would respectively yield at least 200 mV cathodic polarization. This result suggests that the off-potential criteria with different ranges of soil resistivity are generally more stringent than the 100 mV cathodic polarization criterion.

Validation Of Chart
The general steel native potential vs. soil resistivity chart shown in Figure 2 can be supported by data from a number of independent sources. Figure 3(a) shows the average native potentials (averaged for the entire test duration between five and seven years) of the unpolarized bare pipes in 14 test sites vs. their respective soil resistivity (data superimposed on Figure 2). (10-11) The solid blue circles are the actual test data and the blue line is the best fit line to the data. Only three of the 14 data points fall outside the mapped zones, and the best fit line passes well through each mapped zone.

In the soil resistivity range of 10K-100K ohm.cm or greater, the free corrosion potentials listed in EN 12954 were plotted and shown as the two gray bands in Figure 3(a). These two bands fall well within the respective mapped potential vs. soil resistivity zones.

Figure 3(b) shows data of the test coupons in the 14 test sites corresponding to the pipe native potentials mentioned earlier.(10-11) The error bars were determined from a calculation of the standard deviation of the 15 coupons accompanying each pipe segment. Similar to Figure 3(a), most data points fall in the mapped zones and the best fit line passes well through the center of each zone, suggesting that the native potential vs. soil resistivity chart reasonably represents the pipe or coupon native potentials vs. soil resistivity for the 14 field sites with soil resistivity ranging from less than 1K to 1.47M ohm.cm.

Figure 4 shows the native or rest potentials of operating pipelines or installed coupons vs. soil resistivity superimposed on Figure 2. (12) The potentials and soil resistivity were measured very near the pipe-soil interface. The straight line was drawn manually based on visual observation of the data. The mapped zones cover a majority of the data points. It is interesting to note that some potentials are very negative in the high soil resistivity range (>10K ohm.cm), perhaps due to insufficient time of depolarization before the measurement was taken.

Native Potential Vs. Time
Data analysis published elsewhere (10-11) shows that of 14 unpolarized bare pipe specimens tested in 14 field sites with soil resistivity varying from less than 1K to 1.47M ohm.cm, only one site shows the native potential shifting in the more negative direction. For the other 13 pipes, their native potentials all shift in the more positive direction, accounting for a significant majority (93%). The potential shift in the more positive direction is commonly observed in the field because the pipe surface tends to form an oxide film over time and the surface is becoming passivated.

The rest potentials of three accompanying pipe specimens at each test site polarized at different off-potentials (potentials were measured annually for five to seven years and for each measurement five days was given for depolarization) all were shown shifting in the more positive direction except two of the total of 14 test sites. (10-11)

This shift of native/rest potentials in the more positive direction implies that – at a controlled off-potential – the cathodic polarization increases over time. It also implies that if the soil resistivity is stable over time on a yearly basis, maintaining a given on-potential means increasing polarization over time.

Conclusions
A chart validated with available literature data was provided to show the rule-of-thumb relationship between steel native/rest potentials and soil resistivity. This chart shows that meeting the off-potential criteria for different soil resistivity ranges given in the ISO standards would in general yield at least 100 mV cathodic polarization, suggesting that the off-potential criteria are more stringent than the 100 mV cathodic polarization criterion.

The tendency of steel native/rest potentials shifting in the more positive direction over time implies that at a controlled off-potential, the polarization increases over time. If the soil resistivity is stable over time on a yearly basis, maintaining a given on-potential means increasing polarization over time.

Acknowledgment
This work was 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 David McQuilling of Panhandle Energy and Dave Aguiar of Pacific Gas & Electric Company, are appreciated.

REFERENCES
1. ISO Standard 15589-1 (Latest Version), “Petroleum and Natural Gas Industries–otection of Pipeline Transportation Systems–Part 1: On Land Pipelines” (Geneva, Switzerland: ISO).
2. EN 12954, “Cathodic Protection of Buried or Immersed Metallic Structures. General Principles and Application for Pipelines,” 2001.
3. NACE, “Control of External Corrosion on Underground or Submerged Metallic Piping Systems,” NACE Standard SP 0169?2007.
4. AS 2832.1 (latest revision), “Cathodic Protection of Metals–Pipes and Cables,” (Sydney, Australia: Standards, Australia).
5. NACE TG 211, “Report on the 100-mV Cathodic Polarization Criterion,” (Houston, TX: NACE) March 2008.
6. H. C. Van Nouhuys, “Cathodic protection and high resistivity soil”, Corrosion, Vol.9,
Dec, 1953, 448-459.
7. H. C. Van Nouhuys, “Cathodic protection and high resistivity soil- A sequel”, Corrosion, Vol.14, No.11, 1958, pp.583-587.
8. “State-of-the-art survey on corrosion of steel piping in soils”, NACE Technical Committee (Task group 018) Report, Item No. 24216, Houston, TX, Dec, 2001.
9. D. A. Jones, “Principles and Prevention of Corrosion”, 2nd Edition, Prentice-Hall, Inc. NJ (1996) 50-54.
10. L. A. “Roy” Bash, “Robert J. Kuhn’s -0.85V, CSE, cathodic protection criterion for buried coated steel pipelines is scientifically sound”, CORROSION/2006, paper No. 06086.
11. T. J. Barlo, “Field testing the criteria for cathodic protection of buried pipelines”, PRCI final report, PR-208-163, February, 1994.
12. F. King, G. V. Boven, K. Lawson, et al., “Cathodic protection of pipelines in high resistivity soils and the effect of seasonal changes”, Corrosion/2006, paper No. 06163.
13. Song, F. M. and H. Yu, “Variable CP Criteria”, PRCI Contract PR-015-083500, Final Report, October 2010.

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.