Monday, March 8, 2010

Whitt Trimble negative vote

TG 360 has done a very extensive overhaul of the SP0169 document which was necessary at this time in the useful life of the document.

Given the contentious “criteria” issues, however, it is my recommendation that the balloting task be split into two efforts. The first ballot could address the entire document with the exception of Section 6: Criteria and the second ballot could then address Section 6: Criteria by itself.

TECHNICAL
GENERAL
1) All CP criteria are subject to inadequacies when/where localized stray currents and shielding are not detected. There should be some note or caution, in the criteria section, that the use of an adequate number of potential measurements is required to provide representative cathodic protection information and to minimize instances of external corrosion (and inadequacy of cathodic protection) due to localized shielding and stray currents.
2) Pipe-to-soil potentials should be used as CP criteria while also considering pH and Marcel Pourbaix’s work and diagram. (Polarized potential criteria especially so, as they are typically more conservative and result in more negative pipe-to-soil potentials on the cathode surface.) “More negative” cathodic protection potentials have been reported to increase corrosion rates by shifting the carbon steel from a state of passivation to a state of corrosion. (And this invalidates the “The two fundamental polarization criteria in this section have been proved empirically to reduce the average corrosion rate of steel to less than 25 um/y (1 mil/y) in soils and natural waters in the field at ambient temperature.” statement in subsection 6.2.2. Please see the work of Marcel Pourbaix.) As a result, either a maximum negative polarized potential should be specified (based upon Pourbaix’s work and which has nothing to do with coating disbondment) or pH should be considered for both polarized and current applied criteria just as consideration of “voltage drops other than those across the structure-to-electrolyte boundary” is required when using current applied potential criteria.


SPECIFIC
1) Subsection 6.1.4 should be revised to “A number of measurement techniques relating to ….” As it stands, the current wording implies that ALL of the CP criteria measurement techniques are covered in TM0497 and I don’t believe that that position should be taken by SP0169. (Does TM0497 include measurement techniques for ALL other current applied criteria such as net current flow, 300 mV shift, E log I, etc. ?)
2) Subsection 6.2.1 should be revised to “This section contains a number of criteria; however ….” As it stands, the current wording implies that ALL of the CP criteria are contained herein while then stating that “other approaches have been successful in the past”.
3) “Empirical” and “empirically” need to be defined in the document as quite a bit of reliance is placed upon these terms in the criteria section and both operators and regulators will need to know what is intended by their use.
4) Subsection 6.2.2 - One of the last 2 sentences should be retained while the other is deleted as they state the same position. There is also an inherent problem with the fact that the paragraph starts off defending the “two fundamental polarization” criteria and concludes that more than one criterion may be necessary. So does the acceptance of the use of more than one criterion include only the “two fundamental polarization” criteria or is the use of a combination of “fundamental” and non-fundamental polarization (and/or multiple non-fundamental) criteria also allowed?
5) “Cathodic voltage” (Subsection 6.2.3.1.1) needs to be defined in the document if it is going to be used. Both operators and regulators will benefit.
6) Subsection 6.2.3.1.1 – “Correction” needs to be revised to “Consideration” in 2 places. Not all “current-applied measurement” requires correction for voltage drops. The net current flow measurement technique only requires current direction, not a voltage drop correction.
7) Subsection 6.2.3.2.6 should be revised to “The criteria for the most active metal in the system may need to be used.” which then fits “may be invalid” in the previous sentence and which is correct. The -100 mV cathodic polarization criterion is valid when sufficient measurements are taken to establish that all of a mixed-metal piping system is cathodically protected. (This is, after all, how “hot spot” protection and some distributed anode ICCP systems function.)
8) Subsection 6.2.3.2.8 should be revised in accordance with GENERAL COMMENT #2. This will encourage the proper selection and use of a cathodic protection potential criterion based upon a specific environment rather than an arbitrary number that may or may not be adequate. (See Pourbaix’s diagram. A “weak acid” environment may need an instant-off potential of -930 mV, -950 mV, -960 mV, -980 mV, etc.)

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