Response to Mark and Ernie

Response to Mark and Ernie,

Mark and Ernie, thank you very much for your comments. These are the discussions that we will all learn from.

I would like to clarify a couple of points about my comments to be sure everyone is aware of what I think are some very critical points.

Mark Mateer says - “The reason FBE seems so effective is that is doesn't create any shielding problems that plague other coatings, not because it works better with any particular criterion.”

This is one of the points that I do not think everyone has understood. I am not saying FBE performance has anything to do with the criteria selected. Too the contrary, my point is that no matter which of the three criteria are used, we rarely (if ever) see external corrosion on FBE coated pipelines with the exceptions noted in the earlier presentation. The point is that most companies used an “ON” -850 mV (without considering IR drop) for the first 30+ years of FBE and some still use this criterion because it works for them. If considering IR drop is so important, why would we not have external corrosion on these pipelines? Toby Fore’s paper “First Generation of Fusion Bonded Epoxy Coatings Performance After 30 Years of Service – A Case Study” (CORROSION 2006 –Paper 06045) points this out very well. Even though the potentials at the sites studied were more negative than -850 mV “ON” versus copper/copper sulfate electrode, the point is that the criterion used was an “ON” -850 mV without IR drop consideration and there are no external corrosion problems!

There have been many cases of FBE disbondment, but because of the non-shielding property, the “ON” -850 mV (without IR drop considered) is adequate protection. If the “ON” -850 mV is adequate for the areas of non-shielding, disbonded, FBE coated pipelines it will be sufficient for any structure that has a non-shielding coating that allows the CP current to be effective or a bare structure with no shielding. Coatings that do shield CP current in disbonded areas, do not allow CP to be effective so corrosion can and many times does develop when water penetrates.

Mark Mateer says - “From a past PRCi study, we know that 850 IR considered works about 95% of the time when used correctly. In contrast, 850 polarized is about 98% effective and the 100 mV shift criteria is 100% effective.”

I have not seen a complete copy of this document, but I think there are many issues with this report. I do agree with the statement that the -850 mV criterion is more effective than -850 mV “ON” criterion, but only because you are applying more current, so you may force some current under disbonded coating. We must also include the problems that we create because of all the current we are now using. As Johnny, pointed out, the problems with interference are enough to make us stop and consider what other potential problems we are creating. Corrosion problems from interference happen much faster than those from inadequate CP. Of course, we also have to consider the potential damage from more coating disbondment problems and possible hydrogen embrittlement, etc. (Especially as we move toward higher strength steel pipelines). More energy consumption and increased cost of surveys and equipment must also be considered. Ondak and Rizzo mention this in their paper at CORROSION 2008 – “ELECTROCHEMICAL ANALYSIS OF PIPELINE CP CRITERIA” – Paper 08068.

I would not say that the 100 mV criterion is 100% effective, especially for pipelines that have disbonded and shielding pipeline coating, or in other areas of shielding, etc. Bob Gummow addresses many of the problems with this criterion in his paper “Technical Consideration on the Use of the 100 mV Cathodic Polarization Criterion” Paper 07035 from CORROSION 2007. Though I do not agree with all Bob has to say or has written, I know he has a very vast knowledge of CP criteria, etc. and I have learned many things from him.

How many times do we find that we do not have 100 mV of polarization when we have an “ON” -850 mV? Not many from my experience. When protecting bare pipe using the 100 mV of polarization do we see corrosion once the pipe is polarized? If we provide enough current to meet the 100 mV of polarization on an uncoated pipe, we rarely have external corrosion unless there is shielding or interference. Same as on coated pipelines with non-shielding coatings. How about that! But we must consider IR drop to have effective protection according to certain folks and NACE SP0169 - 2007.

Ernie Kleckha says - “A good impressed current cathodic protection system can throw some current under a disbonded coating or at least increase the pH.”

Areas where we have coatings that shield CP current in disbonded areas, we have the potential for corrosion. As Ernie mentioned in his comments, you can throw “some” current under disbonded, shielding, coatings, but this is very hard to determine and almost impossible to measure in an effective way. There is no way of knowing if adequate current will penetrate under these coatings from above ground surveys.

I have seen times when a coating that would normally shield CP, allowed enough CP current to increase the pH to a protective level under the disbonded coating. These areas did not have external corrosion, because the CP could be effective! Again, if a coating is non-shielding in that environment (for what ever reason) it will allow enough current to effectively protect these surfaces.

THE POINT IS THAT IF WE USE COATINGS THAT ALLOW CP CURRENT TO BE EFFECTIVE, WE DO NOT HAVE EXTERNAL CORROSION PROBLEMS WHEN WE USE AN “ON” -850 mV WITHOUT IR DROP CONSIDERATION CRITERION! [Exceptions as mentioned earlier] WHY DO WE CONTINUE TO USE COATINGS THAT SHIELD CP IF DISBONDMENTS OCCUR?

Ernie Kleckha says - “I think we should use the IR drop and not just “consider the IR drop.” We can use IR drop to find pickup points, discharge points, AC and DC interference, and many other potentials damaging conditions.”

I agree with Ernie! There are times when considering IR drop is useful. As I and others have said many times, TRAINING cathodic protection personnel when it is important to “consider” and use IR drop is where we should be spending our time, not forcing everyone to consider IR drop for all readings!

There is certainly much confusion in the industry about what to do with “IR” drop. Roy Bash covers many topics in his CORROSION 2008 paper “Pipe-to-Soil Potential Measurements, The Basic Science”. Again, I may not understand or agree with all Roy has to say, but I certainly learn from his discussions and do agree with much of it.

Conclusion

There have been many papers written about this subject, but few actually discuss the real reasons for corrosion that is found on the pipelines or test structures. I think we are missing what is right in front of us. If FBE coated pipelines do not have external corrosion problems when using the “ON” -850 mV criterion without consideration for IR drop (with the exceptions listed before), even though there are many cases of disbonded FBE, then why are we having such any issue with using this criterion? We have over forty years of proof!

The issue is not the criterion being used. It is the use of coatings that shield CP when there is a disbondment, interference from all the CP in the ground, high powered AC interference and such related issues. The GTI Report # GRI-00-0231 (in which Kevin Garrity was an author) even states “Disbonded coating does not affect the cathodic protection currents but does not (?) significantly affect the electrical currents outside of the disbonded region.” What this is saying is that under disbonded coating that shields you cannot effectively control the corrosion with CP. Just because you have corrosion, does not mean your CP is inadequate. Just because you meet or exceed an certain SP0169-2007 criterion does not mean you will not have corrosion.

We encourage more debate and comments on these topics.

Thanks for visiting SP0169.com!

Richard Norsworthy

Mark Mateer Comments

In response to Richrad's request for information about FBE and CP, I believe the correct explanation for the success of FBE with the 850 IR considered criteria does not relate to criteria at all. From a past PRCi study, we know that 850 IR considered works about 95% of the time when used correctly. In contrast, 850 polarized is about 98% effective and the 100 mV shift criteria is 100% effective. The reason FBE seems so effective is that is doesn't create any shielding problems that plague other coatings., not because it works better wtih any particular criteria.

I don't believe FBE wotks any better with 850 IR considered than any other criteria, it is just a good coating that works well under almost all conditions. 850 IR considered will work well if done properly. It does have more room for error, which is the point of contention.

Thanks

Mark Mateer

Ernie Klechka Comment

Richard,

I really liked you NACE presentation. Are the photographs available for inclusion in the CCCP class?

I do disagree with your comments that IR drop is not important. As you are aware we can use IR drop to find stray current on pipelines.

You also seem to imply that IR drop is a constant. You and I both know that IR drop will change in current pickup and discharge areas, near anode ground beds, and at areas with high current demand.

I agree that much of the problem centers around shielding. Coating that shield the pipeline cause erroneous conclusions concerning cathodic protection. Probably the area of shielding that causes the most concern is casings. However, shielding is not the only problem on pipelines.

Your slides show several poorly coated field welds. To me this points to poor field coating repair practices. Sure shrink sleeves can be poorly applied or subject to soil stresses that cause disbondment and shielding, but the cathodic protection system should not be allowed to be compromised because of a poor coating. A good impressed current cathodic protection system can through some current under a disbonded coating or at least increase the pH.

I think we should use the IR drop and not just “consider the IR drop.” We can use IR drop to find pickup points, discharge points, AC and DC interference, and many other potentials damaging conditions.



ERNEST W. KLECHKA P.E. ( ALASKA AND OHIO)