Monday, December 13, 2010

Response to Gummow Article

November 2010
R. A. (Bob) Gummow has written a very good article “Examining The Controversy Surrounding The -850 mV CP Criteria” that is published in the November 2010 Pipeline and Gas Journal. As usual Bob does a very good job of making his points while barely mentioning some well-known “facts” that do not quite fit his theory and experience. For this reason I would like to provide comments on some of his statements that may concern some in the pipeline and CP industry.
I agree with much of what is addressed in the article. The discussion is mainly about the controversy between using an “ON” -850 mV and the “polarized” -850 mV criterions. The discussion below will be about specific statements and where I believe we “MUST” discuss each side of the debate if we are to actually learn and achieve a better understanding of the proper ways to use these criteria as separate items in the NACE International SP0169 document. Even though some say there is confusion when there are two criterions using the -850 mV as the base number, most of the end users do not have this problem. When allowed to consider their specific system and what criterion actually provides the most economical and efficient process for evaluating the cathodic protection effectiveness for their system, while at the same time providing a safe environment for the pipeline and surrounding communities is the critical issue.
Statements are made that if the CP levels are such that a polarized -850 mV (or more negative potential) or proof of at least 100 mV of polarization would result in (most cases) a corrosion rate of less than 1 mil per year (mpy). This rate of corrosion has been established in several tests using coupons and no one is questioning this fact, but it is something to keep in mind while reading this response. Just how we are going to measure this on a pipeline has not been totally addresses yet.
One of the issues with using the “ON” -850 mV criterion is that of considering IR drops in the soil. There is no doubt that we can see a difference in the “ON” and instant Off potentials. Whether this difference is truly a problem is debatable. One of the problems is that we cannot easily measure the so called “instant off” or polarized potential. That is a discussion for another time.
Assuming that we can measure the instant Off effectively (mostly we try to measure it consistently), then we can subtract the instant Off from the “ON” and that value is the IR drop that is the assumed culprit. It is interesting that this is called an “error” in the “ON” potential. This is not an “error”. It is simply part of the “ON” potential. Whether it should be considered as a problem or not is up to those responsible for interpreting these potentials using their knowledge and experiences on that particular pipeline system according to all the information available, not just CP potentials. This is sound engineering practice. Forcing all to use a polarized potential because of a few tests on coupons, is not sound engineering practice.
The companies that use the “ON” potential as one of their criterion do not try to protect an entire pipeline system at an “ON” -850 mV, but use this “ON” potential as a warning sign that the protection at that site and the surrounding area needs further testing or additional CP. I have yet to see a pipeline system that could be protected at one specific potential for its entirety. It is absurd to even bring this up when discussing the “ON” -850 mV potential criterion, yet this is one argument against using it. Bob has to admit that “likely” when using this criterion the 100 mV of polarization is met or exceeded, therefore since the “ON” potentials are typically much more negative than the -850 mV, these companies are indeed considering IR drop in these potentials. The 5th paragraph under “The -850 mV cse Current Applied Criterion” addresses this fact.
Another interesting statement is when discussing the paper by Mr. Holtsbaum. In the caption under Figure 1, he states “IR Drop versus On Potentials to Illustrate a Dangerous Use of a Constant IR Drop Factor, in this case 100 mV.” I do not think the use of “Dangerous” is called for here, but if you are trying to convince folks that do not know, that you are right then you use such words to get their attention. The statement was perfectly fine without the word “Dangerous”. Had these potentials actually been allowed to depolarize there would have likely been at least 100 mV of polarization which would have proven that these potentials were not dangerous, just not complete. Since I do have copy of Mr. Holtsbaum’s paper, I do not know if this testing was performed or published in this paper.
The last sentence of the ON criterion section says, “Because of the extreme range of the IR drop on coated pipelines, the level of CP cannot be determined by the “ON” potential alone.” This statement could be interpreted to mean that using only any “ON” criterion will result in many pipeline failures. This has not been proven to be true. Yes, there have been pipeline failures on coated pipelines which use this “ON” criterion (also the polarized and 100 mV of polarization criteria), but if we could go back and actually find the root cause of the problem, we would find that most of these failures occurred because of disbonded and CP shielding pipeline coatings, interference or other shielding issues. If this statement were true, then why would companies with FBE coated pipelines not also experience these same external corrosion failure problems? Again, there have been a few external corrosion problems on FBE coated pipelines, but nearly all can be explained as AC or DC interference or shielding from other objects placed in the ditch.
From the time I started in the pipeline industry (1980) and before companies were using an “ON” -850 mV criterion for protection of most of their pipelines. Those coated with FBE starting in 1960 did not and still do not have the same external corrosion problems as those pipes coated with other coating types. This is still true today. This has been proven out through the use of many ILI tools over the last 25 years, as well as ECDA. This points out that if using non-shielding pipeline coatings such as FBE even with an “ON” -850 mV criterion, companies will rarely have external corrosion caused from inadequate CP. This particular issue is conveniently ignored by those who propose that only a polarized -850 mV criterion is the only -850 mV criterion that can be used successfully.
For the most part everyone agrees with the fact that use of the polarized -850 mV criterion would result in more polarization and in some cases would certainly be a better choice. Of course, we must also understand that there can be problems when using this criterion just like when using the “ON” -850 mV. These facts are also conveniently left out of this article, but should be part of any discussion of this topic.
In the section on the comparison of the two -850 mV criterion, the statement is made that there have been no studies made where the current applied (ON) -850 mV potential was held at -850 mV on coated pipe in order to determine what the expected corrosion rate would be. First there is no company I know of that would attempt to do that in real life. As mention before, this NOT the way the “ON” criterion has been used or should be used. A much better test would be to see how many times 100 mV of polarization cannot be attained when the “ON” potential is a -850 mV or more negative on coated pipelines. This would really be useful information. There are a few instances where 100 mV may not be reached, but these are rare because with well coated pipe (assuming no CP shielding) CP current only has to protect the very few bare areas (holidays) and will easily find these. Polarization of these areas takes minimal current allowing current to move on to other openings. Compared to a bare structure that meets the 100 mV of polarization criterion, the well coated pipeline will require considerably less current than an uncoated pipeline. Most of the uncoated pipelines will never reach a polarized -850 mV criterion or the “ON” -850 mV criterion so the 100 mV of polarization is the only choice.
The push of this article seems to be to prepare everyone for the use of a statement in the SP0169 revision that states that if the polarized -850 mV or 100 mV of polarization criterions are used then there will be 1 mil or less corrosion rate. I think most basically agree with this concept if these criteria are used (and current can actually reach the pipe steel), but this is not a statement that should be placed in the SP0169 since there is no way to actually measure this in the field. If a statement like this is used in the SP0169, it must be well explained as to its intent, but I would still be concerned about the problems that could develop if lawyers and expert witnesses start using such statements to try to win law suits against pipeline companies. This is a discussion for another time.
Mark Mateer’s report is used to back up the theory that if everyone would use the polarized -850 mV criterion then we would have less external corrosion problems than if we use the “ON” -850 mV criterion. This may or may not be, but there are several issues that must be addressed before such assumptions can be made. I agree this is a very comprehensive report since it covers a 50 year time frame, but many other issues were not addressed in this article such as:
1. Again there is no discussion of the root cause of corrosion on these pipelines. There is no discussion of coating types, condition or if the coatings were shielding or non-shielding.
2. Even though the criterion used for the mid portion of the pipeline life was -850 mV “ON”, we are not given whether this criterion was properly applied or not.
3. What happened during the period of time when the polarized -850 mV criterion was used that may have also influenced the results beyond that of simply applying more CP?
a. As companies began to learn more about CP, coatings and reasons for external corrosion, they applied more CP and performed more detailed surveys to find areas of coating damage and repaired these areas. The same can be said from the beginning of CP on these pipeline systems until use of the “ON” -850 mV criterion.
b. The use of ILI tools also started at this time which also helped find and correct corrosion problems along with recoating programs and pipe replacements.
c. There were (probably) more and better coated pipelines laid during this 16 year period, which would sway the results. Only if the same pipelines were carried throughout the entire survey could this be reliable data.
d. More training and certification courses were offered during this time from NACE, companies and other organizations providing a much more knowledgeable and experienced work force.
e. During this time corrosion control becomes much more important in these companies and governments, which forced significant changes in the industry with large budgets and more attention paid to corrosion control efforts.
f. I would suspect that nearly every company could show similar results even if they were using an “ON” -850 mV criterion throughout the same time frame because of the reasons mentioned, not just because of using a polarized potential criterion.
Distribution companies are referenced as needing to change to the polarized potential criterion to eliminate their corrosion problems. Of course, the issue here is that most distribution companies do not have a reasonable way of performing the required testing to determine a polarized potential because most of these systems are protected with galvanic anodes that do not have access to install interrupters. Also, many systems have short segments of steel mixed with plastic pipe so electrical continuity is a problem. I have been told all they have to do is install coupons to determine what the polarized potential would be on these systems. Coupons are good tools when used properly, but are not the total answer and the cost of installing, reading and maintain these would be significant with limited improved results. Distribution companies have significantly more problems than transmission pipeline companies.
In the Summary the statement “the claim that the -850 mV current applied potential is as effective in controlling as the -850 mV polarized potential criterion is not borne out in the published literature”, is another statement that could use some discussion. Even though there have been several papers and presentations made to contrary, these are typically ignored.
The first sentence of the second paragraph under the Summary section is another bit of misinformation. I do not know a single company that only relies only on external corrosion failures to determine their external corrosion control program. It is interesting that in Mark Mateer’s article, these companies continued to have external corrosion “failures” even though they switched to a polarized potential criterion. Apparently they had corrosion rates over 1 mil per year! Are Bob and everyone else supporting this issue ready to tell us that there will be no external corrosion problems (at least less than 1 mil per year) if all switch to a polarized -850 mV criterion? I do not think so.
What we need is to continue to educate those who are responsible for corrosion control programs around the world that there are many parts to an external corrosion control program, not just CP and criteria. SP0169 is a critical document to our industry. Let’s do it right by making it flexible enough that companies can use the processes available to control the external corrosion on their systems. Those companies and their workers understand these systems better than anyone else, so all we have to do is continue to education these folks so they have a well-rounded understanding of all these processes and let them make the decision on what is best for their companies.
Another issue that needs to addressed is that of some in NACE teaching that only a polarized potential or 100 mV of polarization are acceptable criteria. If the -850 mV current applied criterion is listed it should also be taught as acceptable as long as it properly applied. Until and if the “ON” criterion is removed from the SP0169, it should be taught as a viable criterion when properly used.
As much as I have said, I want to be perfectly clear that I do respect Mr. Gummow. He is a very intelligent person and truly dedicated to what his experiences and knowledge has given him. I have had the opportunity to teach with him many times and he is a great instructor. Each time I get to teach with him, I learn and will always be grateful for the opportunity. Even though we have differences in opinions, as long as we all learn from and challenge each other we will all improve our knowledge and abilities.

Richard Norsworthy
Polyguard Products, Inc.