November 2010 Vol. 237 No. 11

Features

Examining The Controversy Surrounding The -850 mV CP Criteria

The 1992 version of the NACE RP0169 cathodic protection standard (CP) for pipelines[1]introduced two criteria involving a minimum potential of -850mV with respect to a saturated copper-copper sulfate reference electrode (cse).

The introduction came after about seven years of often rancorous debate between the NACE T-10-1 committee on criteria and some pipeline industry representatives.

A -850mVcse polarized potential criterion, typically measured as an instant-off potential, when the cathodic protection current is interrupted, was included along with the original -850mVcse “current-applied” criterion. This was a dramatic departure from the previous RP0169 versions, which had remained relatively unchanged dating back to the original issue of the standard in 1969.

This change rekindled a long-standing contro¬versy regarding the proper interpretation of the -850mVcse current-applied criterion, which has persisted to the present and particularly since the SP0169 standard is again being revised. The original version of the -850mVcse criterion referred to the pipe-to-soil voltage being measured with the cathodic protection “current applied” and stipulated that “consideration” be given to voltage drops in the measurement other than across the pipe-electrolyte boundary for valid interpretation of the criterion.

The -850mVcse polarized potential criterion was inserted into the standard because, un¬like the -850mVcse current-applied criterion, it was solidly anchored in the fundamental science and because the term “consideration” in the current-applied criterion was vague and non-specific. As would be expected this brought into question the efficacy of the “current-applied” criterion, especially since the only other international cathodic protection standard for land-based pipelines (ISO 15589-1)[2] specifies only a -850mVcse “instant-off” potential criterion.

The practical application of the -850mVcse polarized potential criterion is a concern primarily to gas distri¬bution companies, whose pipelines have cathodic protection systems using direct connected anodes for which the current cannot be interrupted to measure an instant-off or polarized potential. Representatives of many of these companies continue to assert that the -850mVcse “current applied” criterion provides satisfactory corrosion control, even though there is a paucity of published technical literature to support these claims.

What Is A Criterion?
According to the Oxford dictionary[3] (paraphrased) a criterion is a principle or standard by which a thing is judged. For cathodically protected pipelines the “thing” that is being judged is the corrosion rate in order to determine the level of control corrosion and to prevent pipeline failures due to external corrosion. Although the active corrosion rate can be estimated from successive in-line in-spection (ILI) runs, this technique cannot be applied to most distribution piping. Electrical resistance probes (ERP) are the only method, other than direct examination, of estimating the actual corrosion rate on gas distribution piping with direct connected galvanic anodes. Therefore, the indirect measurement of the pipe potential relative to a potential criterion is the principle method of judging whether or not there is satisfactory corrosion control. In this regard it was demonstrated by Mears and Brown[4] in, 1938, that complete cathodic protection was achieved when the structure cathodes were polarized electro-negatively to the most electronegative anode site on the structure. Accordingly, a minimum amount of cathodic polarization is required to achieve complete cathodic protection.

The -850mVcse Current-applied Criterion
This criterion, often called the on-potential criterion, originates in a 1933 paper by Robert J. Kuhn[5], who expressed the opinion that the potential criterion was “probably in the neighborhood of -0.850 volt” with respect to a saturated copper-copper sulfate reference electrode. This view was derived primarily from Kuhn’s experience in applying cathodic protection to prevent stray current corrosion on bare cast iron water mains in New Orleans in the 1920s.

The Kuhn experience is typically the only literature reference that is used to support the application of a -850mVcse current-applied criterion to steel pipe, regardless as to soil or pipe conditions. In New Orleans, it would be reasonable to conclude that voltage drop error in the potential measurements would have been small and that polarization would have readily occurred, because of the in¬herent¬ly high water table, because of the low elec¬trical resistivity of the ground water, and because of the shallow depth of the piping. Thus, a current-applied potential of -850mVcse would be close in value to the polarized potential and a significant amount of cathodic polariza¬tion would be embodied in the potential difference measurement.

But, it does not follow that this criterion would be as effective in other soil conditions. Indeed Kuhn, who subse¬quently spent many years applying cathodic protection to steel oil and gas piping, concluded in 1950[6 ] that a more appropriate current-applied criterion for coated steel pipelines was -1000mVcse, which is a substantial increase from the -850mVcse on-potential criterion and, an indication that the -850mVcse current-applied criterion was insufficient.

It is generally recognized that achieving a -850mVcse current-applied potential is not equal to a -850mVcse polarized potential, since there will always be some voltage drop error in the mea-surement. Nevertheless, there are probably some soil conditions where the current-applied potential of -850mVcse includes at least 100mV of cathodic polariza¬tion, which should result in a corrosion rate of less than 1 mpy and, which is comparable to what would be expected when achieving a -850mVcse polarized potential criterion. Also, in high resistivity soils, where the free corrosion rate is already low, a small amount of polarization is all that is needed to obtain a corrosion rate of less than 1mpy.

Many companies, who claim that they are utilizing the -850mVcse current-applied criterion, are actually using more negative values, such as -900mVcse, -950mVcse, or -1000mVcse, thereby providing an allowance for some IR drop in the measurement. But even this pro¬ce¬dure may not provide a level of protection equivalent to the -850mVcse polarized potential criterion as revealed in a study conducted by Brian Holtsbaum[7] as illustrated in Figure 1. (Note number of readings below protection and above 950 mV.)

On-potentials are compared to the polarized potentials using data collected on pipelines in mid-western Canada where a -950mVcse on-potential criterion was being used. It is apparent that in many instances an on-potential of -950mVcse or more electronegative contained IR drops that, when subtracted from the on-potential value, resulted in polarized potentials that were less negative than a polarized potential of -850mVcse. In these instances the corrosion rate is unknown and the -950mVcse on-potential criterion does not satisfy the -850mVcse current-applied criterion as intended when considering IR drops other than across the structure-electrolyte boundary.

The original intent of the -850mVcse current-applied criterion was reiterated by L.F. Heverly, chairman of the T-10-e work group on Criteria for Cathodic Protection for RP0169 in 1985[8]. He stated that the reason for the on-potential criterion was “to include the voltage (IR) drop across the structure-electrolyte boundary, but not the voltage (IR) drop through the soil for a valid interpretation of the voltage measurement”. Therefore, for valid interpretation of this criterion a minimum potential of -850mVcse must be obtained after removal of IR drops except across the structure-electrolyte boundary.

It is likely that when the current-applied criterion is interpreted in this manner, the resulting corrosion rate would be comparable to the 100mV of cathodic polarization criterion and the -850mVcse criterion (e.g. <1mpy). Unfortunately, users of the -850mVcse current-applied criterion have rarely interpreted the criterion as intended, which has led to persistent rhetoric to have this interpretation loosened without a clear expectation of the consequences. Some cathodic protection practitioners have even claimed that IR drop in the -850mVcse current-applied criterion can be ignored. The large IR drop component in the Holtsbaum data is not unusual. A study by Thompson and Beaver[9] involved the measurement of IR drop free pipe-to-soil potentials at 115 test sites in the U.S. They found that 1) the on-potentials at 64% of the test locations contained IR drop greater than 30% of the on-potential value and 2) the IR drop was less than 10% of the on-potential value at only 19% of the test locations. Because of the extreme range of the IR drop on coated pipelines, the level of cathodic protection cannot be determined by the on-potential alone. The -850mVcse Polarized Potential Criterion
The -850mVcse polarized potential criterion, sometimes referred to as the instant-off potential, arose from a seminal research investigation by Schwerdtfeger and McDorman at the National Bureau of Standards in the early 1950s[10]. They reasoned that, in an air-free soil, the cathode reaction would be the reduction of hydrogen ions, whose potential is thermodynamically related to the soil pH. They then measured the corrosion potential of steel coupons in various soils, ranging in pH from 2.9 to 9.6, and found that the corrosion potential vs. pH line intersected the hydrogen line at a potential of -0.77Vsce (~ -850mVcse).

This intersecting potential meant that there could be no potential difference between a cathode site and an anode site on the coupon, and hence no corrosion. When they then applied this potential, measured as an instant-off potential by interrupting the current using the Hickling method, to various steel coupons, they found that corrosion was “negligible”. Moreover, they concluded that the -850mVcse potential was “ in agreement with the practice for cathodic protection used by many corrosion engineers, in those cases where the measurements are free of IR drop external to the electrical boundary of the corrosion circuit”.

The interruption of the applied current, in order to eliminate the IR drop, was known to be necessary to obtain a polarized potential, as explained in 1944 by Pearson[11] who stated, “It is clear that any measurement of the polarization of a buried structure must be made to differentiate between the effects of purely IR drop and the electrochemical results of the current flow. Only the latter is of any use in controlling the rate of corrosion”.

In the early 1980s, cathodic protection tests were carried out by Barlo and Berry[12] on steel coupons in both air-free and aerated soils using experimental cells that were constructed to be identical to those used by Schwerdtfeger and McDorman. Their results were predicated on achieving a general corrosion rate, but not a pitting corrosion rate, of less than 1mpy. Among their conclusions was the verification of the effectiveness of the -850mVcse polarized potential in all soils tested but also that the polarized potential to prevent corrosion becomes less negative than -850mVcse as the free corrosion potential becomes less negative.

The Barlo and Berry laboratory studies were succeeded by an American Gas Association sponsored 5 year criteria field testing program involving 14 sites spread over three different countries[13]. This study verified the effectiveness of the -850mVcse polarized potential criterion for controlling corrosion on steel coupons, installed adjacent to a 10-foot length of bare 24-inch diameter pipe, based on a general corrosion rate of less than 1mpy, as shown in Figure 2.

Figure 2: General Corrosion Rate of the Coupons at All of the Sites as Influenced by the Criteria and Related Parameters.

Comparison Of Criteria
How do the two -850mVcse criteria compare in corrosion control performance? Unlike the -850mVcse polarized potential, there have been no studies where the current-applied potential was held at -850mVcse on coated pipe in order to determine what would be an expected corrosion rate. Clearly, a -850mVcse current-applied criterion could in practice be effective in controlling the corrosion rate to less than 1mpy in some specific circumstances. For instance, if the amount of polarization contained in the current-applied potential was equal to or greater than 100mV or where the natural corrosion rate is very small.

But without knowing how much polarization has been obtained, the resulting corrosion rate is unknown, which makes it difficult to determine the corrosion control effectiveness. Therefore, when using the current-applied criterion, one must employ other measures, such as ILI, corrosion rate probes and cathodic protection coupons to evaluate the corrosion control efficacy. These additional measures increase the cost of operating the cathodic protection system in terms of maintenance, materials, and monitoring, and hence are seldom utilized, especially on distribution piping. This leaves the external corrosion failure rate as the only indicator of the degree of corrosion control.

Mark Mateer[14] has probably produced the most comprehensive comparative study involving the two -850mVcse criteria, based on cumulative corrosion failures over a 50-year time period on a very large gas transmission system with thousands of miles of piping. The results, shown in Figure 3, indicate a significant reduction in failures when the -850mVcse instant-off criterion was adopted, after about 16 years of operation using the -850mVcse on-potential criterion.

Figure 3: External Corrosion Failure Probability Plot for “Test Structure”.

His analysis of the data concluded that “The benefit of the ’off’ or polarized potential, criterion was a five-fold decrease in the number of failures”. This result would likely be typical if the -850mVcse polarized potential criterion was applied to gas distribution piping, which typically experiences more corrosion failures than transmission piping and, where the -850mVcse current-applied criterion has been used for many years. It is interesting to note that the last several years of data for the -850mVcse polarized potential criterion resulted in a negligible increase in corrosion failures which was attributed to the adoption of an ILI program.

Summary
The claim that the -850mVcse current-applied criterion is as effective in controlling corrosion as the -850mVcse polarized potential criterion is not borne out in the published literature.

The effectiveness of the -850mVcse current-applied criterion ultimately relies on the frequency of external corrosion failures, since the corrosion rate is generally unknown. Perhaps there is pertinent data in the corrosion control records of pipeline companies to support the tendentious claims for the continued use of the -850mVcse current-applied criterion but, to this point, such data have not been presented. Thus, increasing the volume of the rhetoric around the merits of this criterion is the only defense to its continued use, despite the fact that there are techniques available to assess the level of protection in terms of corrosion rate or polarization.

It is likely that the controversy and confusion between the merits of the two -850mVcse criteria will continue as long as the current-applied criterion is included in the NACE SP0169 standard and is not included in the ISO standard.

To persist in using a criterion when 1) the expected corrosion rate is unknown, 2) when the original intent to remove the IR drop in the measurement except across the structure-electrolyte boundary is ignored, and 3) to wait for external corrosion failures to assess the corrosion control performance, exposes the public to an unnecessary risk and is therefore, not sound engineering practice.
(end)

Author
R.A. (Bob) Gummow is a corrosion engineer and a NACE accredited Corrosion Specialist, having over 40 years of experience in the application of cathodic protection to a wide variety of structures in many industries. His recent activities have included pipeline projects pertaining to induced AC corrosion and telluric currents. He is presently offering corrosion consulting services through Correng Consulting Service Inc, Markham, Ontario. Ph: 416-630-2600, ext. 245, e-mail: bgummow@corrosionservice.com.

References
1 Control of External Corrosion on Underground or Submerged Metallic Piping Systems, National Association of Corrosion Engineers, RP0169-92, April 1992.
2 Petroleum and natural gas industries – Pipeline transportation systems – Cathodic protection – Part 1 : Onshore pipelines, ISO 15589-1, 2003.
3 The Oxford Encyclopedic English Dictionary, Thumb Index Edition, Oxford University Press, 1991.
4 R.B. Mears and R.H. Brown, A Theory of Cathodic Protection, Trans Electrochemical Society, 74, pp519-531, 1938.
5 R.J. Kuhn, Cathodic Protection of Underground Pipe Lines from Soil Corrosion, API Proceedings, Nov. 1933, Vol. 14, p157.
6 R.J. Kuhn, Cathodic Protection on Texas Gas System, AGA, April 1950.
7 W. Brian Holtsbaum, Use of Historical IR drops for Interpretation of “ON” Potential Criterion, NACE International, Northern Area Western Conference, Saskatoon, Saskatchewan, Feb 2000.
8 L.F.Heverly, Discussion on “An Assessment of the Current Criteria for cathodic Protection of Buried Steel Pipelines” MP, May 1985, p55.
9 N.G. Thompson, and J.A. Beaver, Measurement of IR-Drop Free Pipe-to-Soil Potentials on Buried Pipelines, ASTM STP 1056, 1990, pp168-179.
10 W.J. Schwerdtfeger and O.N. McDorman, Potential and Current Requirements for the Cathodic Protection of steel in Soils, NBS, vol.47, No.2, Research Paper 2233, August 1951, pp104-112.
11 J.M. Pearson, Concepts and Methods of Cathodic Protection, the Petroleum Engineer, March 1944, p218.
12 T.J. Barlo and W.E. Berry, An Assessment of the Current Criteria for Cathodic Protection of Buried Steel Pipelines, MP, Sept 1984, pp 9-16.
13 Field Testing the Criteria for Cathodic Protection of Buried Pipelines, AGA, Pipeline Research Committee, PR-208-163, Feb 1994, p179.
14 M. Mateer, Using Failure Probability Plots to Evaluate the effectiveness of “Off” vs. “On” Potential CP Criteria, MP, Sept 2004, pp22-24.