Saturday, May 3, 2008

Comments from Tom Laundrie

I was reading the TG 211 Proposed Nace Technical Committee Report, "Report on the 100-mV Cathodic Polarization Criterion" Draft #3b. Some of the statements used in this document can be applied to the logic of leaving the -0.85 volt On Criterion in the SP0169 document. These quotes are taken out of the section on Advantages and Disadvantages for Pipeline Applications.

First I'd like to point out that it has been shown that all we really need to reduce corrosion to an acceptable level is 100 mV of polarization, and that in most cases the -0.85 Volt On Criterion is already conservative and has a "safety factor" built in to it.

Secondly, the document points out that the 100 mV criteria uses much less current and is much less likely to cause problems. This same logic can be applied to using the -.85 On Criterion as opposed to the -0.85 Volt Instant-Off Criterion.

Advantages and Disadvantages for Pipeline Applications

For pipelines, the 100-mV cathodic polarization criterion has advantages and disadvantages when compared to the -850 mVCSE criterion. As indicated in Figure 3, the current density to achieve 100 mV of cathodic polarization is typically less than to achieve the -850 mVCSE polarized potential criterion, especially in well-aerated and well-drained soils in which the native corrosion potential may be in the -200 to -400 mVCSE range. The 100-mV criterion therefore is normally more cost-effective because of the lower current requirements. The Dearing example21 demonstrated that if the -850 mVCSE criterion were to be restored on the section of pipeline under test, additional expenditures for more cathodic protection current would have had to be made, whereas to satisfy the 100-mV cathodic polarization criterion, the existing current output was sufficient and could probably have been reduced. Such a reduction in current would result in further savings in power costs and extended groundbed life. This makes the application of the 100-mV criterion to bare or poorly coated structures more appealing because the current demand on these structures is usually high. The reduction in current demand also reduces the influence of the cathodic protection system on foreign pipelines, reducing the likelihood of stray current interference.

Attempting to achieve a minimum of -850 mVCSE on a coated pipeline may result in highly negative polarized potentials that can create the risk of hydrogen embrittlement on susceptible structures, such as high-strength steels, some types of stainless steels, and prestressed concrete cylinder pipe (PCCP) as indicated in SP0169.1 Similarly, the standard cautions against the use of excessively negative polarized potentials to minimize coating damage, such as cathodic blistering and cathodic disbondment. The use of the 100-mV cathodic polarization criterion would typically minimize both these risks. On structures composed of amphoteric materials such as galvanized steel, aluminum, and lead, all of which are subject to corrosion at highly alkaline conditions, satisfying the 100-mV criterion would normally result in a lower pH than if a polarized potential criterion were utilized.

Third: They go on to point out the cost disadvantage of measuring the 100 mV Criterion and the errors that can occur and must be accounted for (sounds like "consideration").

Although there is sometimes an economic benefit to operating cathodic protection systems based on the 100-mV cathodic polarization criterion, some of this advantage is lost because the testing regimen is more complex. Because this criterion typically relies on measuring either a native corrosion potential before the system is energized or a decayed off potential after the system has operated for a period of time, this is an extra step compared to “on/off” potential surveys conducted for comparison to a polarized potential criterion. This additional step increases the survey costs and introduces the possibility of measurement errors.

Although it is not necessary to deenergize the system until all polarization has dissipated (only until a minimum of 100 mV of depolarization has been achieved), it is usual for the systems to be turned off for periods of time that may extend into weeks. Pawson22 indicated that in the majority of cases on bare pipelines, the pipeline potentials were still depolarizing after 100 days and that, for two particular bare pipelines, there was no correlation between the original native corrosion potential and the depolarized potentials.

During the depolarization time period, protection is being lost, and the risk of corrosion activity increases. Also, when the decay period is long, seasonal and weather effects can interfere with the depolarization, either accelerating or retarding the depolarization, thereby introducing error. For long periods of depolarization, potentials are typically recorded to ensure the accuracy of the data and verify that local soil conditions did not change significantly.

Fourth: The document points out the errors involved with using Coupons to represent the pipe potentials.

Coupons are often used in situations in which it is difficult to obtain accurate native corrosion potentials or depolarized potentials because of the presence of uninterruptible current sources. Nekoska35 has stated that a coupon potential “always decays to its corrosion potential.” However, the coupon depolarized potential does not necessarily represent the pipe depolarized potential nor the depolarized potential of a similar-sized holiday in similar soil conditions because depolarization at a pipe holiday is affected by the depolarization process at other pipe coating holidays. The coupon, therefore, could have different decay characteristics than a pipeline coating holiday.


Tom Laundrie
Sr. Materials Engineering Specialist
NACE Cathodic Protection Specialist