My fear is the committee is going to be looking for any reason to pass TG 360 without getting it right. When reviewing the comments made from members voting in favor of SPO169 in 2009 I have to ask WHY would you vote for something that you thought needed to be changed? Approximately 30 of the 41 members who voted in favor of SPO169 in 2009 left comments were requesting some kind of changes or corrections be made.
In over 25 years I have never found location that I had a -.850mV that would not also pass the 100Mv criteria. I have to ask WHAT scientific documentation did you use to come up with the -.850 mV ON or Polarize and you like to call it criteria??
I have to ask does it not look funny to have 2 different criteria we can use but BOTH with the same -.850 mV number?
I am also not in favor of the 1 MPY corrosion rate we would be required to show. I personally feel this need to be removed.
The only way I could do an interrupted survey in my tank Farms in Cushing, OK is to interrupt the local power company
Wednesday, March 10, 2010
Monday, March 8, 2010
Negative vote at this time!
Negative vote comments from Richard Norsworthy:
Forward:
This standard practice does not address the use of nonadhered polyethylene encasement.
ACTION: Remove this statement.
REASON: This document is about effective control of external corrosion on underground or submerged metallic piping systems that should include all piping systems no matter the type of coating or encasement.
Section 2: Definitions and Acronyms
Beta Curve: A plot of dynamic (fluctuating) interference current or related proportional voltage (ordinate) versus the corresponding structure-to-electrolyte potentials at a selected location on the affected structure (abscissa). For the purposes of this standard, “Beta Curve” is defined as a correlation between the pipe-to-soil potential of the affected pipeline and the open-circuit potential between the affected pipeline and the stray current source. (see Appendix A [nonmandatory]).
ACTION: Remove this definition.
REASON: Why do we need this and Appendix A? It is not a technique that is normally used. We can not put everyone’s method for solving interference. I am not saying it is not a method that can be used, but how many more such methods are there? Do we show them all? Not saying it is wrong, just that we should not include all the various methods unless they are popular and frequently used.
Coating: (1) A liquid, liquefiable, or mastic composition that, after application to a surface, is converted into a solid protective, decorative, or functional adherent film; (2) (in a more general sense) a thin layer of solid material on a surface that provides improved protective, decorative, or functional properties.
ACTION: Remove the word thin.
REASON: Unless we define “thin” we are limiting the types of coatings that are not thin. Either define it or remove it.
ADD DEFINITION: Empirical
REASON: Since this word is being used throughout the document, it needs to be defined in the context intended.
ADD DEFINITION: Non-shielding coating system
REASON: The US Department of Transportation now references “non-shielding” coatings in Part 192.112 as a requirement for pipeline operators who want to increase the MAOP to 80%. More and more companies are beginning to recognize the need for these coatings when used in conjunction with cathodic protection.
SUGGESTED DEFINITION: “Non-Shielding” in this context means if the coating system adhesion fails and water penetrates between the pipe and the coating, corrosion on the pipe is significantly reduced or eliminated because cathodic protection (CP) current is able to protect the pipeline in these disbonded areas.
ADD DEFINITION: “Shielding with no Holiday”
Shielding with no Holiday – A substantial diversion of cathodic current away from its intended target (i.e. steel substrate) due to a disbonded coating with a high dielectric characteristic and no holidays on the disbonded coating. Steel substrate is deprived of protective current and corrosion can continue undetected and unchecked, including MIC activity, until failure occurs. Traditional diagnostic techniques such as CIS and DCVG cannot identify the existence of such anomalies.
REASON: The definition is extracted from NACE document “Coating Failure Definitions in Relation to CP” and should be included as a definition in any document for external corrosion control.
Weak Acids: Acids that only partially dissociate to form hydrogen (H+) ions at moderate concentrations.
ACTION: Remove or better define
REASON: This is a “weak” definition. Surely there is a better definition. I can not find the reference (90) mentioned in the document from the CP-2 course? Which version are we referencing?
SECTION 3
3.2.2.1
ADD:
g. Coating, if present, should be evaluated for type and condition.
4.3.2 Sixth sentence - The type of support selected should not cause damage to the pipe coating or act as a shield to CP current.
ACTION: Remove the last part of the sentence.
REASON: All the casing supports shield CP. The casing itself will shield CP in most cases unless water is in the casing, then the CP current may not be sufficient to protect the pipe in the casing, etc.
SECTION 5: External Coatings
In general this is an improvement over the existing document, but all the referenced documents are confusing and not necessary. There are many NACE coating documents that are not even referenced. Please just reference the documents for the particular coating type and leave the rest of it to the particular coating document to point out all the various coating tests. Add NACE Tape coating SP0.
5.1.2.1.1 Effective electrical insulator
Change to read:
5.1.2.1.1 Effective electrical insulator when properly adhered to pipe;
REASON: There have been many studies, articles and test programs dealing with problems of disbonded coatings (including a few by DNV/CC-Technologies) that shield CP when they lose adhesion. We must address this issue, especially now that the US DOT is using the term “non-shielding coating systems”.
OR THE OPTION IS TO ADD ANOTHER CHARACTERISTIC:
5.1.2.1.? Non-shielding to CP current if disbondments occur;
Reason: Again this is a desirable characteristic for coatings used in conjunction with CP for the same reasons as changes in 5.1.2.1.1.
5.1.2.1.9 Resistance to disbonding
Change to read:
5.1.2.1.9 Resistance to disbonding from CP, soil stress, and other environmental stresses;
REASON: Disbonding can occur for many reasons we need to give some guidance.
5.1.4 Information in this section is primarily by reference to other documents (see Tables 1 through 5). It is important that the latest revision of the pertinent reference be used. Tables 1 through 5 are not intended to be all inclusive.
Change to read:
5.1.4 Information in this section is primarily by reference to other documents (see Tables 1 through 5). It is important that the latest revision of the pertinent reference be used. Tables 1 through 5 are not intended to be all inclusive. Not all tests or information listed pertains to each coating system, therefore the end user must selection the information that pertains to that particular coating type.
REASON: Some may interpret the information and tests are for every coating type and require tests that are not relative or pertinent to a particular coating type.
I am also including comments from Dale Temple that I agree with and these should also be included in my negative.
Section 5: External Coatings
Clause 5.1.2.1
These are mother hood statements and are not a characteristic or property. For
example is FBE really an effective moisture barrier or the fact that it absorbs
water a positive attribute for not shielding cathodic protection. This section
needs to be reviewed and rewritten with realistic clearly defined characteristics.
To date all coatings deteriorate with time and at different rates so we doubt that
any coating in meeting this list of “desirable attributes”.
Clause 5.1.2.2.9
Why are coating costs provided as a factor and cost is not a factor to be taken
into consideration for choosing a CP criteria? This is inconsistent logic between
sections of the document.
Clauses 5.1.3.2.1 & 5.1.3.2.4
There is inconsistency between clause 5.1.2.1 and clause 5.1.3.2. For example
when applying mechanical FBE over a anti-corrosion FBE, why wouldn’t all the
“desirable attributes” of clause 5.1.2.1 be applicable? Clause 5.1.2.2.1 and
5.1.3.2.4 are “desirable attributes” that should be included in clause 5.1.2.1.
Section 5 tables
We do not have all of the referenced standards and wonder why ISO and
Australian and New Zealand standards were not included? We cannot go
through all the standards and the following are errors that we know to exist in
the existing Tables. Based on the following NACE must go through these
standards and ensure they correctly placed in the right table (s) and table
section(s).
Table 1a
CSA Z245.20 does not provide any practices for liquid-epoxy (tests or
properties).
CSA Z245.21 does not provide any practices for polypropylene (tests or
properties) and is not included in any nature.
CSA Z245.20 does not provide any practices for polyurethane (tests or
properties) and is not included in any nature.
Table 2
Why was CSA Z245.21 not included as it is referenced in Table 1a?
Table 3
Why was CSA Z245.21 not included as it is referenced in Table 1a
Table 4a
CSA Z245.20 does not have any special requirements for application of coating
over the ditch and is limited scope to pipe coated in a mill only.
Why was CSA Z245.21 not included as it is referenced in Table 1a and is Z245.21
for mill applied polyethylene coatings applied to pipe.
Table 4b
Why was CSA Z245.21 not included as it is referenced in Table 1a and is Z245.21
for mill applied polyethylene coatings applied to pipe.
CSA Z245.20 does not have any special requirements for joint coatings and field
repairs and is limited in scope to pipe coated in a mill only.
Table 5
This table is technically incomplete and in error.
Where do you run a cathodic disbondment test on an in-service pipeline?
Other methods include in-line inspections, indirect methods, and direct methods
such as bell holes or “day lighting” the pipe/coating. For example refer to clause
5.3.1.1 and 5.3.1.2 of the draft document.
Clause 5.2.3.6
Now all or a sudden the draft coating section is very specific about rock sizes.
Rock sizes depend on many other issues with coating only being one of the
concerns. Other concerns are stress created by anchoring or backfill size etc.
The rock size (clause 5.2.3.6.1) is not the limiting factor it is the impact energy,
mass of rock and drop height. What about if we have a three layered
polypropylene coating system would it not absorb more energy than a 14 mil
thick FBE coating?
Does NACE have back-up science for this 100 mm distance or is this just some
Companies internal practice?
Table 1b
ACTION: Remove from the document:
REASON: Why do we need a separate table for ductile iron?
5.3.4 Method for Evaluating an External Coating System by In-Service Field Performance Only
5.3.4.1 The purpose of this method is to qualify an external coating system when
none of the first three methods given in Paragraph 5.3 has been or will be used. It is intended that this method should be limited to minor pilot installations.
5.3.4.1.1 The use of at least one of the first two methods given in Table 5 is recommended on the basis of at least one investigation per year for five consecutive years.
ACTION: Remove this section.
REASON: This section does not offer any guidance. The first method of Table 5 is Cathodic Disbondment testing. How are we going to do this on an existing pipe? This does not make good sense the way it is written.
SECTION 6: Criteria and Other Considerations for Cathodic Protection
6.1.1 This section lists criteria for CP that indicate, when used either separately or in combination, whether adequate CP of a metallic piping system has been achieved (see also Section 1, Paragraphs 1.2 and 1.4). A paraphrasing of CP criteria contained in a number of international and country standards is contained in Appendix B (Nonmandatory). It is not intended to be all inclusive.
ACTION: Remove the last two sentences.
REASON: This is an international standard. We can reference the other documents, but why do we need to paraphrase them? If the complete document is not used, there can be significant problems from paraphrasing only the parts that refer to CP criteria. Why not paraphrase the coating sections, etc from these documents?
6.1.2
Last Sentence:
Because such methods sometimes are not practical, meeting any criterion or combination of criteria in this section is evidence that adequate CP has been achieved.
ACTION: Add a sentence:
Meeting criteria does not ensure external corrosion control when conditions such as electrical shielding exist.
REASON: We must provide guidance as to what conditions would allow external corrosion to occur even when meeting the listed criteria. There may be a better place to list this, but it needs to be included in this section.
6.2.2 The two fundamental polarization criteria in this section have been proved empirically to reduce the average corrosion rate of steel to less than 25 μm/y (1 mil/y) in soils and natural waters in the field at ambient temperatures.82,83 Situations may exist in which a single criterion for evaluating the effectiveness of CP may not be satisfactory for all conditions. A single criterion for evaluating the effectiveness of CP may not be satisfactory for all locations along a structure.
ACTION: Delete the first sentence. Then combine the last two sentences.
REASON: The T-10 committee rejected this same issue in 1990 because it made no sense then and does not today. I will use the T-10 committee’s words:
1. The responsibility for determining and selecting the level of adequate corrosion control rests with the designer and the operator, not the Task Group writing RP0-169 (now SP0-169). Specification of a rate, such as; zero, one mil per year, or some other value, denies the design engineer the opportunity to select and design the structure according to the specific design conditions and needs. Specification of one value would abrogate the selection and design process unnecessarily.
2. Specification of an acceptable corrosion rate, such as one mil per year which is most frequently referenced allowable rate of corrosion used in corrosion control research to demonstrate effective corrosion mitigation, is setting a subjective standard, “which the Task Group feels it does not have an adequate basis to establish”.
Now my own words:
When we use wording such as this it opens up an opportunity for lawyers, expert witnesses and potentially regulators to use this number to challenge operators as to whether or not their CP is adequate. The data used for determining these results were performed on coupons, not pipelines. We can not allow these types of numbers to become a burden to operators when there is no reasonable way for them to prove this number.
We have been told that coupons are the answer. Though coupons have their place, they are not the answer since they do not represent the corrosion rate on the pipe. If we calculate the corrosion rate on a pipeline to be less than 25 µm/year (1 inch/year), then we are OK? I do not think so. We could still have pitting, interference and other issues that cause localized corrosion that would allow leaks and unsafe operating conditions, but according to this statement we would be OK if the corrosion rate could be proven to be under this value.
Restate the last two sentences:
Situations may exist in which a single criterion for evaluating the effectiveness of CP may not be satisfactory for all conditions or all locations along a structure.
6.2.3.1 Adequate cathodic protection can be achieved at various levels of cathodic
polarization depending on the environmental conditions. Any criterion selected must provide a reliable indication that the corrosion control objectives of the user have been satisfied. In the absence of data listed in Paragraph 6.1.2 that demonstrate that adequate CP has been achieved, at least one of the following shall apply:
ACTION: Remove the second sentence.
REASON: If this sentence is left in the document “reliable indication” must be defined. Also the corrosion control objectives of the user can vary considerably around the world, but as long as it meets the user’s objectives, we are saying it is OK? What if their objective is to have no more than ten leaks per year?
6.2.3.1.1 A cathodic voltage of –850 mV or more negative as measured with respect to a
saturated copper/copper sulfate (CSE) reference electrode. This potential may be either a
direct measurement of the instant-off potential, or a current-applied potential. Interpretation of a current-applied measurement requires correction for voltage drops in the earth or metallic paths.
NOTE: Correction is understood to mean the application of sound engineering practice in the determination of the magnitude and significance of voltage drops by methods such as:
ACTION: Start over!
REASON: Have separate criteria for the polarized and ON criteria. Why do we have to combine these into one statement that confuses even more folks? Do not use the term “instant off” potential. Have we defined “instant off”? Even though some claim they can measure instant off with various instruments, no one can define just where instant is going to be. Even if we could reasonably measure the “instant off”, there are so many conditions that do not allow us to get a polarized potential such as multiple pipeline ROW’s. Certainly we can get “close”, but how do we know the accuracy of the meter, the skill of the operator and all the variable field conditions that will affect these potentials.
The use of certain types of equipment that give some potential at a certain point is at least consistent and will allow for some type of reading that can be used. How accurate it will be is or should be is another question.
I will provide Mr. Cherry’s article again for your consideration.
How Instant is Instant?
ISSN 1466-8858 Volume 9 Paper 6
B.W.Cherry Department of Materials Engineering, Monash University, Victoria 3800, Australia.
Abstract
The instant off technique for the determination of the value of the potential to which a metal has been polarised by the action of a cathodic protection current implicitly relies on the time scales of the various polarisation decay processes being very different. Normally measurements are taken between 0.1 and 1 second after the interruption of the current and the assumption is made that in this time the activation and diffusion polarisations have not decayed by a significant amount. This paper attempts to calculate the time constants for the decay associated with the ohmic, activation, and diffusion components of the polarisation. It suggests that there may be such an overlap between the different decay processes that differentiation between them is not possible. The calculations when applied to steel in sea-water and reinforcing bars in concrete suggest that such is the difference between these typical systems that generalised criteria are not possible.
Keywords: cathodic protection, potential, criteria, instant off, overpotential, activation polarisation, diffusion polarisation, ohmic polarisation.
This is a preprint of a paper that has been submitted for publication in the Journal of Corrosion Science and Engineering. It will be reviewed and, subject to the reviewers' comments, be published online at http://www.umist.ac.uk/corrosion/jcse in due course. Until such time as it has been fully published it should not normally be referenced in published work. � UMIST 2004.
Introduction
The "instantaneous off" technique for the determination of the potential to which a cathodically protected structure has been polarised has been used for at least fifty years and is the basis for national standards in a number of countries. Typically, Australian Standard AS 2832.5 records that protection is obtained on the Absolute Potential Criterion when, "An instant off potential (measured between 0.1 s and I s after switching the d.c. circuit open) more negative than -720 mV with respect to Ag/AgCI/0.5M KCI." is obtained. It seems to be generally assumed that the ohmic component of the polarisation dissipates in times that are very much shorter than the activation and diffusion components of the polarisation, and although it is the combination of the activation and diffusion components of the overpotential that moves the potential of the metal being protected to a value corresponding to a lower corrosion activity, the relative magnitudes of these components and therefore the means to modify them remains unclear. Neglecting any uncertainties in the ability of the switching system to provide a step discontinuity in the current there are in general four separate decay processes in the polarisation decay process. These are, the activation, concentration and resistance polarisations and the capacitive effects of the electrolyte in which the system is immersed. It has sometimes been assumed that the decay constants for these are well separated in magnitude. Polak [1] suggested that the ohmic component of the polarisation potential disappears in picosec whereas the electrochemical component (activation and concentration overpotential) disappears in the interval from 10 millisec up to several seconds. Schwenk [2] calculated that the time constant for the activation component of the polarisation was between 10-5 and 101 sec. but did not attempt the calculation of the time constant for the concentration polarisation. Wallen and Linder [3] suggested that because the time constant for the decay of the ohmic component of the polarisation overpotential could be of the order of milliseconds then it was not possible with the instrumentation that they had 2 available to them at that time to separate the IR drop from from the other kinds of polarisation. Cherry [4] suggested that the capacitive effects of the soil around a buried pipeline could be of the order of seconds. The object of this paper is therefore to examine the possible time constants for the decay of activation and concentration polarisation and to suggest that the sharp differentiation between the time constants for the various potential decay processes that is assumed by a number of monitoring standards may not always obtain in practice. In particular it is concerned with the extent to which activation and diffusion polarisation play their different roles in determining the so-called absolute potential measured using instant off techniques. In order to illustrate the relative roles played by the different components of the total polarisation two situations will be considered; bare steel in sea water and reinforcing bars in concrete. Bare steel in Sea-water The calculations to determine the relative magnitudes of the components of the total polarisation for bare steel in sea-water will be based upon the assumption that a cathodic current of 100 mAm-2 results in a diffusion overpotential of 10 mV and an activation overpotential of 100 mV. Although it is a very greatly simplifying assumption, the time constant for the activation polarisation of a bare electrode in a conductive electrolyte solution can be calculated by assuming a simple Randles circuit that incorporates the polarisation resistance of the solid/electrolyte interface and the double layer capacity. Putting the polarisation resistance as 5 x 10 -2 ohm m-2 and the double layer capacity as 1 F m-2 [5] yields a time constant of about 5 x 10-2 sec. The activation overpotential component of the polarisation falls to about 1% of its original value in about 0.13 sec and consequently is not captured by an instantaneous off potential measurement one second or even 0.1 second after the current supply to a bare metal surface in a conductive electrolyte is switched off. 3 The time constant for the concentration polarisation that may obtain for a similar system may also be calculated if it is assumed that the cathodic process is O2 + 2H2O + 4e- 4OHand the rate determining step in this process is the transport of oxygen to the rebar surface. The diffusion overpotential is given by RT c c ln s D zF o c where cs is the concentration of oxygen at the surface of the metal and co is the concentration of oxygen in the external solution. cs is determined by the balance between the rate at which oxygen is consumed at the cathode and the rate at which it can diffuse to the cathode through the solution. Hence in order to carry out the calculation for a diffusion overpotential component of the total polarisation of 10 mV, it will be assumed that the bulk solution is in equilibrium with the atmosphere and has an oxygen content of 10 mg/l. The concentration of oxygen at the surface of the metal can then be calculated to be 0.020 mg/l or 0.6 mole.m-3. The rate of diffusion of oxygen to the cathodic surface may be assumed to follow Ficks second law of diffusion. If it is assumed that in a steady state, the rate at which oxygen is consumed at the surface corresponds to the protection current that is being applied then there will be a concentration profile in the solution. The form of this concentration has been calculated by Sands [6] and is reported as x2 iz 1 c x c0 . exp .dt 4 Dt nF t D0 t (1) where x is the distance from the electrode, t the time for which the current has been flowing, i is the current density flowing on to the surface, z is the number of the reactive molecules taking part in the reaction, n is the number of electrons involved in the reaction, F the Faraday and D the diffusion coefficient of the reactant in the solution. 4 Equation (1) can be integrated for increasing values of time to examine the development of the concentration profile. However for an "instant off" potential measurement the impressed current is reduced to zero, theoretically instantaneously, but in practice over a period that is determined by the efficiency of the switching gear. It will be assumed that at the moment that the current falls to zero then for a short period at least the concentration gradient of the reactants in the solution will remain and that these will cause a flow of the reactants to the metal surface with a consequent reduction in the diffusion overpotential. These concentration gradients supported the flow of reactants to the surface at a rate that corresponded to the impressed current. Hence it is suggested that initially the flow of reactant to the surface will continue at the same rate as before the current interruption but that since the reactants are no longer being consumed by the electrode reaction they will be increasing the concentration at the interface and decreasing the diffusion overpotential. The variation of the concentration at the surface of the reactant with time can therefore be calculated following Vetter [7] by putting x = 0 in equation (1) and integrating to yield c s co 2 iz t nF D The initial rate of change of the diffusion potential will then be given by d RT iz dc s dt nF c s dt RT 1 iz 1/ 2 t nF cs nF 1 /2 D (2) Putting cs = 0.6 mole m-3 as derived previously, assuming that the polarising current was 10-1 Am-2 and D = 1.54 x 10 -1 m2sec. [8]. d yields an initial value for of 9 x 10-5 t -1/2 V/sec. Equation (2) is dt only valid for the instantaneous decay rate of the diffusion overpotential component of the polarisation, but the time taken for this component to fall by 1% of its original value may be approximated by the integration of equation (2) to suggest that this may take some 5 25 sec. It seems therefore that an instantaneous off potential measurement carried out at between 0.1 and 1 sec is likely to capture most of the diffusion overpotential component of the total polarisation. However since the time constant for the decay of the activation polarisation has been calculated as of the order of 50msec, the magnitude of the activation overpotential component of the polarisation at one second or even 0.1 seconds after switching off the current is vanishingly small and will not be captured by an instant off potential measurement. It may therefore be suggested that the polarisation that may be determined by an instantaneous off measurement for an electrode that is being cathodically protected in (say) sea-water is effectively entirely determined by the concentration overpotential and not at all by the activation polarisation. Reinforcing bars in Concrete For reinforcing bars in concrete the relative magnitudes of the components of the total polarisation will be based upon the assumption that a cathodic current of 5 mAm -2 yields a diffusion overpotential of 100 mV and an activation overpotential of 50 mV. The approximate time constant for the decay of the activation polarisation of a reinforcing bar electrode in a concrete can be calculated as was done previously by assuming a simple Randles circuit. For a reinforcing bar in the passive state, putting the polarisation resistance as 210 ohm m2 [9] and the double layer capacity as 31 x 10-2 F m -2 yields a time constant of 65 sec. If the activation overpotential component of the polarisation under cathodic protection had been 50 mV then this time constant corresponds to a retention of 98% of the activation polarisation and a depolarisation rate of 0.7 mV/sec at 1 second after the current has been switched off. An instantaneous off potential measurement carried out between 0.1 and 1 sec is therefore likely to capture most of the activation overpotential component of the total polarisation. In a similar fashion, the rate of depolarisation of the diffusion overpotential for a reinforcing bar in concrete may be calculated. If it 6 is assumed that the original diffusion overpotential is 100 mV then by the same method as was used previously it is assumed that the concentration of the oxygen in the pore water adjacent to the electrode surface is O.3 mole m -3 . Then if the diffusion coefficient for oxygen in concrete is put as 2 x 10-8 m2sec [9] and assuming that a protection current of 5 x 10-3 Am-2 is being applied, then equation [2] d yields a value for of 2.5x10-6 t - � V/sec. Hence calculating as dt before the time taken for this component to fall by 1% of its original value, this may be approximated as 200 sec.. It seems therefore that an instantaneous off potential measurement carried out at between 0.1 and 1 sec is likely to capture most of the diffusion overpotential component of the total polarisation. It is therefore suggested that in contrast to the depolarisation of an unprotected electrode in solution, the polarisation that is measured one second after the interruption of a cathodic protection current contains both the activation and diffusion overpotential components. The depolarisation that may be measured in the following four hours may contain all the activation polarisation but only a fraction of the diffusion overpotential component.
Conclusions The "instantaneous" shift in potential that occurs when a cathodic protection current is interrupted is very rarely solely the iR drop that results from the passage of the current from the anode through the surrounding electrolyte. It will normally also contain one or more of the other components of the polarisation that is being applied to the electrode. This means that the potential that is reported as the result of such a measurement is rarely the actual polarised potential, but a potential that includes unknown portions of the activation and diffusion overpotentials. The extent to which the subsequent drift of the potential to more positive values may be the result of the decay of activation or diffusion overpotential components of the total polarisation will depend upon the system which is being protected and it would not be wise to 7 assume, as has sometimes been the case in the past, that diffusion effects control this phase of the depolarisation process. It therefore seems that the cathodic protection criteria that have been adopted, the absolute potential, the depolarisation criterion, the absolute potential after depolarisation, should all be regarded as purely empirically based and may be very dependent upon the nature of the system that is being protected.
References
1. Polak, J. "Conference on the internal and external protection of pipes", Brit. Hydr. Res. Assn., Durham, 1975 Paper F4
2. Schwenk , W. "Determination of the potential of current-carrying electrodes with special reference to their application in cathodic corrosion protection". Werkstoffe und Korrosion (1962), 13 21218.
3. Wallen, B. and Linder, B. "Ohmic Potential Drop in Cathodic Protection of Buried Structures: Evaluation of some Measuring Methods", British Corrosion Journal, 8, (1973), 7 - 14
4. Cherry, B.W. "An analysis of the Potential Decay when Cathodic Protection is Interrupted", 4th Asian-Pacific Corrosion Control Conference, Tokyo, 1985, 1243
5. von Baeckmann. W., Schwenk, W. and Prinz, W. "Cathodic Corrosion Protection", Gulf Publishing, 1989, 44
6. Sand, H. J. S. "On the Concentration at the Electrodes in a Solution", Philosophical Magazine 1, (1900), 45 - 79
7. Vetter, K.J. "Electrochemical Kinetics" trans Scripta Technica Inc. Academic Press, New York (1967), 207
8. Ferrell, R.T. and Himmelblau, D.M."Diffusion Coefficients of Nitrogen and Oxygen in Water" J of Chem. and Eng. Data, 12, (1967), 111 - 115 8
9. Birbilis, N., Nairn, K.M. and Forsyth, M., "Transient Response Analysis of Steel in Concrete" Corrosion Science, 45, (2003), 1895
10. Bertolini L., Elsener, B , Pedeferri, P. and Polder, R. "Corrosion of Steel in Concrete", Wiley VCH, KGaA, Weinheim, 2004, 42 9
Last Sentence and note:
“Interpretation of a current-applied measurement requires correction for voltage drops in the earth or metallic paths.
NOTE: Correction is understood to mean the application of sound engineering practice in the determination of the magnitude and significance of voltage drops by methods such as:
ACTION: Rewrite entire section:
Suggestion:
6.2.3.1.1 A cathodic voltage of –850 mV or more negative as measured with respect to a
saturated copper/copper sulfate (CSE) reference electrode with current applied. This potential may include voltage drops that should be considered. Potentials shall be interpreted by those properly trained using sound engineering practices to determine when and where it is necessary to consider voltage drops.
NOTE: When sound engineering practices indicate it is necessary to consider voltage drops, consideration is understood to mean the determination of the magnitude and significance of voltage drops by methods such as:
List methods from 2007 version.
Also list the areas where voltage drop may be a “potential” problem. Surely not everywhere!
6.2.3.1.2 A minimum of 100 mV of cathodic polarization. Either the formation or the decay of polarization must be measured to satisfy this criterion.
6.2.3.1.3 A polarized potential of –850 mV or more negative as measured with respect to a saturated copper/copper sulfate (CSE) reference electrode.
Make changes to numbers as needed.
6.2.3.1.3(4) Criteria that have been shown to successfully control corrosion through empirical evidence on specific piping systems may continue to be used on those piping systems or others with the same characteristics. These criteria include –850 mV on with consideration for voltage drops other than those across the structure-to-electrolyte boundary, other current applied criteria, net current flow, 300 mV shift, or E log I.
ACTION: Remove the last sentence.
REASON: If we list some, we have to list all possible criteria that may work!
6.2.3.1.4 Other criteria or methods that are demonstrated to achieve the corrosion control
objectives of the user; for example,
ACTION: Restate
6.2.3.1.4 Other criteria or methods that are demonstrated to achieve the corrosion control
objectives of the user; for example,
REASON: Objectives of the user should not be a reason for criteria.
6.2.3.2.2 When active MIC has been identified or is probable, (e.g., caused by acid-producing or sulfate-reducing bacteria), an instant-off potential of –950 mV CSE or more
negative or as much as –300 mV of cathodic polarization may be required.
ACTION: Remove or restate
REASON: MIC can be and usually is present in nearly all environments in which pipelines are laid, therefore MIC are probable. Again the use of “instant off” is not defined etc, as above. Usually, the external corrosion caused by MIC on pipelines is found under disbonded coating that shields the CP, so a different criterion will likely not result in adequate protection.
6.2.3.2.3 At elevated temperatures (> 40 °C [104 °F]), an instant-off potential more
electronegative than –850 mV CSE, or more than 100 mV of cathodic polarization may be required. At temperatures greater than 60 °C (140 °F), the instant-off potential should be –950 mV CSE or more negative.
ACTION: Remove or restate
REASON: There many pipeline systems around the world that operate at these elevated temperatures without seeing external corrosion when using one of the 3 criterion mentioned above. Again the use of “instant off” is not defined etc, as above.
6.2.3.2.8 In weak acid environments, an instant-off potential of –950 mV or more negative with respect to a copper/copper sulfate reference electrode is required. Weak acids are those acids that only partially dissociate to form H+ ions at moderate concentrations.
ACTION: Restate or remove
REASON: This is a “weak” definition. Surely there is a better definition. I can not find the reference (90) mentioned in the document from the CP-2 course? Which version are we referencing?
6.2.3.2.9 When operating pressure and conditions are conducive to stress corrosion cracking, the use of polarized potentials in the cracking range relative to the temperature indicated in Figure 1 is not advised. (See references on stress corrosion cracking at the end of this section.)
ACTION: Either add more information or simply make cautionary statement.
REASON: The way this is shown now leads one to believe this is a common problem therefore, if operating in these ranges there is going to be stress corrosion cracking. This is not true. Yes there are a few instances, but is this true for all systems? How much of a problem is it? Make this statement only to caution that there may be stress corrosion cracking, but there are many more factors that must be put into the equation. I will let the “experts” make the statement.
6.2.3.2.10 Cathodic polarization levels that result in excessive generation of atomic hydrogen should be avoided on all metals susceptible to hydrogen embrittlement.
ACTION: Define “excessive hydrogen”. Also, is there so list of metals that are susceptible?
REASON: Again we either must give more guidance or make it more of a cautionary statement.
Section 7: Design of Cathodic Protection Systems
7.1 Introduction
7.1.1 This section provides procedures for designing CP systems that will provide effective external corrosion control by satisfying one or more of the criteria listed in Section 6 and exhibiting acceptable reliability over the intended operating life of the systems.
ACTION: Restate
REASON: Satisfying criteria does not necessarily provide effective external corrosion control.
SUGGESTION: This section provides procedures for designing CP systems that will should provide effective external corrosion control by while satisfying one or more of the criteria listed in Section 6 and exhibiting acceptable reliability over the intended operating life of the systems.
I may add more later!
Forward:
This standard practice does not address the use of nonadhered polyethylene encasement.
ACTION: Remove this statement.
REASON: This document is about effective control of external corrosion on underground or submerged metallic piping systems that should include all piping systems no matter the type of coating or encasement.
Section 2: Definitions and Acronyms
Beta Curve: A plot of dynamic (fluctuating) interference current or related proportional voltage (ordinate) versus the corresponding structure-to-electrolyte potentials at a selected location on the affected structure (abscissa). For the purposes of this standard, “Beta Curve” is defined as a correlation between the pipe-to-soil potential of the affected pipeline and the open-circuit potential between the affected pipeline and the stray current source. (see Appendix A [nonmandatory]).
ACTION: Remove this definition.
REASON: Why do we need this and Appendix A? It is not a technique that is normally used. We can not put everyone’s method for solving interference. I am not saying it is not a method that can be used, but how many more such methods are there? Do we show them all? Not saying it is wrong, just that we should not include all the various methods unless they are popular and frequently used.
Coating: (1) A liquid, liquefiable, or mastic composition that, after application to a surface, is converted into a solid protective, decorative, or functional adherent film; (2) (in a more general sense) a thin layer of solid material on a surface that provides improved protective, decorative, or functional properties.
ACTION: Remove the word thin.
REASON: Unless we define “thin” we are limiting the types of coatings that are not thin. Either define it or remove it.
ADD DEFINITION: Empirical
REASON: Since this word is being used throughout the document, it needs to be defined in the context intended.
ADD DEFINITION: Non-shielding coating system
REASON: The US Department of Transportation now references “non-shielding” coatings in Part 192.112 as a requirement for pipeline operators who want to increase the MAOP to 80%. More and more companies are beginning to recognize the need for these coatings when used in conjunction with cathodic protection.
SUGGESTED DEFINITION: “Non-Shielding” in this context means if the coating system adhesion fails and water penetrates between the pipe and the coating, corrosion on the pipe is significantly reduced or eliminated because cathodic protection (CP) current is able to protect the pipeline in these disbonded areas.
ADD DEFINITION: “Shielding with no Holiday”
Shielding with no Holiday – A substantial diversion of cathodic current away from its intended target (i.e. steel substrate) due to a disbonded coating with a high dielectric characteristic and no holidays on the disbonded coating. Steel substrate is deprived of protective current and corrosion can continue undetected and unchecked, including MIC activity, until failure occurs. Traditional diagnostic techniques such as CIS and DCVG cannot identify the existence of such anomalies.
REASON: The definition is extracted from NACE document “Coating Failure Definitions in Relation to CP” and should be included as a definition in any document for external corrosion control.
Weak Acids: Acids that only partially dissociate to form hydrogen (H+) ions at moderate concentrations.
ACTION: Remove or better define
REASON: This is a “weak” definition. Surely there is a better definition. I can not find the reference (90) mentioned in the document from the CP-2 course? Which version are we referencing?
SECTION 3
3.2.2.1
ADD:
g. Coating, if present, should be evaluated for type and condition.
4.3.2 Sixth sentence - The type of support selected should not cause damage to the pipe coating or act as a shield to CP current.
ACTION: Remove the last part of the sentence.
REASON: All the casing supports shield CP. The casing itself will shield CP in most cases unless water is in the casing, then the CP current may not be sufficient to protect the pipe in the casing, etc.
SECTION 5: External Coatings
In general this is an improvement over the existing document, but all the referenced documents are confusing and not necessary. There are many NACE coating documents that are not even referenced. Please just reference the documents for the particular coating type and leave the rest of it to the particular coating document to point out all the various coating tests. Add NACE Tape coating SP0.
5.1.2.1.1 Effective electrical insulator
Change to read:
5.1.2.1.1 Effective electrical insulator when properly adhered to pipe;
REASON: There have been many studies, articles and test programs dealing with problems of disbonded coatings (including a few by DNV/CC-Technologies) that shield CP when they lose adhesion. We must address this issue, especially now that the US DOT is using the term “non-shielding coating systems”.
OR THE OPTION IS TO ADD ANOTHER CHARACTERISTIC:
5.1.2.1.? Non-shielding to CP current if disbondments occur;
Reason: Again this is a desirable characteristic for coatings used in conjunction with CP for the same reasons as changes in 5.1.2.1.1.
5.1.2.1.9 Resistance to disbonding
Change to read:
5.1.2.1.9 Resistance to disbonding from CP, soil stress, and other environmental stresses;
REASON: Disbonding can occur for many reasons we need to give some guidance.
5.1.4 Information in this section is primarily by reference to other documents (see Tables 1 through 5). It is important that the latest revision of the pertinent reference be used. Tables 1 through 5 are not intended to be all inclusive.
Change to read:
5.1.4 Information in this section is primarily by reference to other documents (see Tables 1 through 5). It is important that the latest revision of the pertinent reference be used. Tables 1 through 5 are not intended to be all inclusive. Not all tests or information listed pertains to each coating system, therefore the end user must selection the information that pertains to that particular coating type.
REASON: Some may interpret the information and tests are for every coating type and require tests that are not relative or pertinent to a particular coating type.
I am also including comments from Dale Temple that I agree with and these should also be included in my negative.
Section 5: External Coatings
Clause 5.1.2.1
These are mother hood statements and are not a characteristic or property. For
example is FBE really an effective moisture barrier or the fact that it absorbs
water a positive attribute for not shielding cathodic protection. This section
needs to be reviewed and rewritten with realistic clearly defined characteristics.
To date all coatings deteriorate with time and at different rates so we doubt that
any coating in meeting this list of “desirable attributes”.
Clause 5.1.2.2.9
Why are coating costs provided as a factor and cost is not a factor to be taken
into consideration for choosing a CP criteria? This is inconsistent logic between
sections of the document.
Clauses 5.1.3.2.1 & 5.1.3.2.4
There is inconsistency between clause 5.1.2.1 and clause 5.1.3.2. For example
when applying mechanical FBE over a anti-corrosion FBE, why wouldn’t all the
“desirable attributes” of clause 5.1.2.1 be applicable? Clause 5.1.2.2.1 and
5.1.3.2.4 are “desirable attributes” that should be included in clause 5.1.2.1.
Section 5 tables
We do not have all of the referenced standards and wonder why ISO and
Australian and New Zealand standards were not included? We cannot go
through all the standards and the following are errors that we know to exist in
the existing Tables. Based on the following NACE must go through these
standards and ensure they correctly placed in the right table (s) and table
section(s).
Table 1a
CSA Z245.20 does not provide any practices for liquid-epoxy (tests or
properties).
CSA Z245.21 does not provide any practices for polypropylene (tests or
properties) and is not included in any nature.
CSA Z245.20 does not provide any practices for polyurethane (tests or
properties) and is not included in any nature.
Table 2
Why was CSA Z245.21 not included as it is referenced in Table 1a?
Table 3
Why was CSA Z245.21 not included as it is referenced in Table 1a
Table 4a
CSA Z245.20 does not have any special requirements for application of coating
over the ditch and is limited scope to pipe coated in a mill only.
Why was CSA Z245.21 not included as it is referenced in Table 1a and is Z245.21
for mill applied polyethylene coatings applied to pipe.
Table 4b
Why was CSA Z245.21 not included as it is referenced in Table 1a and is Z245.21
for mill applied polyethylene coatings applied to pipe.
CSA Z245.20 does not have any special requirements for joint coatings and field
repairs and is limited in scope to pipe coated in a mill only.
Table 5
This table is technically incomplete and in error.
Where do you run a cathodic disbondment test on an in-service pipeline?
Other methods include in-line inspections, indirect methods, and direct methods
such as bell holes or “day lighting” the pipe/coating. For example refer to clause
5.3.1.1 and 5.3.1.2 of the draft document.
Clause 5.2.3.6
Now all or a sudden the draft coating section is very specific about rock sizes.
Rock sizes depend on many other issues with coating only being one of the
concerns. Other concerns are stress created by anchoring or backfill size etc.
The rock size (clause 5.2.3.6.1) is not the limiting factor it is the impact energy,
mass of rock and drop height. What about if we have a three layered
polypropylene coating system would it not absorb more energy than a 14 mil
thick FBE coating?
Does NACE have back-up science for this 100 mm distance or is this just some
Companies internal practice?
Table 1b
ACTION: Remove from the document:
REASON: Why do we need a separate table for ductile iron?
5.3.4 Method for Evaluating an External Coating System by In-Service Field Performance Only
5.3.4.1 The purpose of this method is to qualify an external coating system when
none of the first three methods given in Paragraph 5.3 has been or will be used. It is intended that this method should be limited to minor pilot installations.
5.3.4.1.1 The use of at least one of the first two methods given in Table 5 is recommended on the basis of at least one investigation per year for five consecutive years.
ACTION: Remove this section.
REASON: This section does not offer any guidance. The first method of Table 5 is Cathodic Disbondment testing. How are we going to do this on an existing pipe? This does not make good sense the way it is written.
SECTION 6: Criteria and Other Considerations for Cathodic Protection
6.1.1 This section lists criteria for CP that indicate, when used either separately or in combination, whether adequate CP of a metallic piping system has been achieved (see also Section 1, Paragraphs 1.2 and 1.4). A paraphrasing of CP criteria contained in a number of international and country standards is contained in Appendix B (Nonmandatory). It is not intended to be all inclusive.
ACTION: Remove the last two sentences.
REASON: This is an international standard. We can reference the other documents, but why do we need to paraphrase them? If the complete document is not used, there can be significant problems from paraphrasing only the parts that refer to CP criteria. Why not paraphrase the coating sections, etc from these documents?
6.1.2
Last Sentence:
Because such methods sometimes are not practical, meeting any criterion or combination of criteria in this section is evidence that adequate CP has been achieved.
ACTION: Add a sentence:
Meeting criteria does not ensure external corrosion control when conditions such as electrical shielding exist.
REASON: We must provide guidance as to what conditions would allow external corrosion to occur even when meeting the listed criteria. There may be a better place to list this, but it needs to be included in this section.
6.2.2 The two fundamental polarization criteria in this section have been proved empirically to reduce the average corrosion rate of steel to less than 25 μm/y (1 mil/y) in soils and natural waters in the field at ambient temperatures.82,83 Situations may exist in which a single criterion for evaluating the effectiveness of CP may not be satisfactory for all conditions. A single criterion for evaluating the effectiveness of CP may not be satisfactory for all locations along a structure.
ACTION: Delete the first sentence. Then combine the last two sentences.
REASON: The T-10 committee rejected this same issue in 1990 because it made no sense then and does not today. I will use the T-10 committee’s words:
1. The responsibility for determining and selecting the level of adequate corrosion control rests with the designer and the operator, not the Task Group writing RP0-169 (now SP0-169). Specification of a rate, such as; zero, one mil per year, or some other value, denies the design engineer the opportunity to select and design the structure according to the specific design conditions and needs. Specification of one value would abrogate the selection and design process unnecessarily.
2. Specification of an acceptable corrosion rate, such as one mil per year which is most frequently referenced allowable rate of corrosion used in corrosion control research to demonstrate effective corrosion mitigation, is setting a subjective standard, “which the Task Group feels it does not have an adequate basis to establish”.
Now my own words:
When we use wording such as this it opens up an opportunity for lawyers, expert witnesses and potentially regulators to use this number to challenge operators as to whether or not their CP is adequate. The data used for determining these results were performed on coupons, not pipelines. We can not allow these types of numbers to become a burden to operators when there is no reasonable way for them to prove this number.
We have been told that coupons are the answer. Though coupons have their place, they are not the answer since they do not represent the corrosion rate on the pipe. If we calculate the corrosion rate on a pipeline to be less than 25 µm/year (1 inch/year), then we are OK? I do not think so. We could still have pitting, interference and other issues that cause localized corrosion that would allow leaks and unsafe operating conditions, but according to this statement we would be OK if the corrosion rate could be proven to be under this value.
Restate the last two sentences:
Situations may exist in which a single criterion for evaluating the effectiveness of CP may not be satisfactory for all conditions or all locations along a structure.
6.2.3.1 Adequate cathodic protection can be achieved at various levels of cathodic
polarization depending on the environmental conditions. Any criterion selected must provide a reliable indication that the corrosion control objectives of the user have been satisfied. In the absence of data listed in Paragraph 6.1.2 that demonstrate that adequate CP has been achieved, at least one of the following shall apply:
ACTION: Remove the second sentence.
REASON: If this sentence is left in the document “reliable indication” must be defined. Also the corrosion control objectives of the user can vary considerably around the world, but as long as it meets the user’s objectives, we are saying it is OK? What if their objective is to have no more than ten leaks per year?
6.2.3.1.1 A cathodic voltage of –850 mV or more negative as measured with respect to a
saturated copper/copper sulfate (CSE) reference electrode. This potential may be either a
direct measurement of the instant-off potential, or a current-applied potential. Interpretation of a current-applied measurement requires correction for voltage drops in the earth or metallic paths.
NOTE: Correction is understood to mean the application of sound engineering practice in the determination of the magnitude and significance of voltage drops by methods such as:
ACTION: Start over!
REASON: Have separate criteria for the polarized and ON criteria. Why do we have to combine these into one statement that confuses even more folks? Do not use the term “instant off” potential. Have we defined “instant off”? Even though some claim they can measure instant off with various instruments, no one can define just where instant is going to be. Even if we could reasonably measure the “instant off”, there are so many conditions that do not allow us to get a polarized potential such as multiple pipeline ROW’s. Certainly we can get “close”, but how do we know the accuracy of the meter, the skill of the operator and all the variable field conditions that will affect these potentials.
The use of certain types of equipment that give some potential at a certain point is at least consistent and will allow for some type of reading that can be used. How accurate it will be is or should be is another question.
I will provide Mr. Cherry’s article again for your consideration.
How Instant is Instant?
ISSN 1466-8858 Volume 9 Paper 6
B.W.Cherry Department of Materials Engineering, Monash University, Victoria 3800, Australia.
Abstract
The instant off technique for the determination of the value of the potential to which a metal has been polarised by the action of a cathodic protection current implicitly relies on the time scales of the various polarisation decay processes being very different. Normally measurements are taken between 0.1 and 1 second after the interruption of the current and the assumption is made that in this time the activation and diffusion polarisations have not decayed by a significant amount. This paper attempts to calculate the time constants for the decay associated with the ohmic, activation, and diffusion components of the polarisation. It suggests that there may be such an overlap between the different decay processes that differentiation between them is not possible. The calculations when applied to steel in sea-water and reinforcing bars in concrete suggest that such is the difference between these typical systems that generalised criteria are not possible.
Keywords: cathodic protection, potential, criteria, instant off, overpotential, activation polarisation, diffusion polarisation, ohmic polarisation.
This is a preprint of a paper that has been submitted for publication in the Journal of Corrosion Science and Engineering. It will be reviewed and, subject to the reviewers' comments, be published online at http://www.umist.ac.uk/corrosion/jcse in due course. Until such time as it has been fully published it should not normally be referenced in published work. � UMIST 2004.
Introduction
The "instantaneous off" technique for the determination of the potential to which a cathodically protected structure has been polarised has been used for at least fifty years and is the basis for national standards in a number of countries. Typically, Australian Standard AS 2832.5 records that protection is obtained on the Absolute Potential Criterion when, "An instant off potential (measured between 0.1 s and I s after switching the d.c. circuit open) more negative than -720 mV with respect to Ag/AgCI/0.5M KCI." is obtained. It seems to be generally assumed that the ohmic component of the polarisation dissipates in times that are very much shorter than the activation and diffusion components of the polarisation, and although it is the combination of the activation and diffusion components of the overpotential that moves the potential of the metal being protected to a value corresponding to a lower corrosion activity, the relative magnitudes of these components and therefore the means to modify them remains unclear. Neglecting any uncertainties in the ability of the switching system to provide a step discontinuity in the current there are in general four separate decay processes in the polarisation decay process. These are, the activation, concentration and resistance polarisations and the capacitive effects of the electrolyte in which the system is immersed. It has sometimes been assumed that the decay constants for these are well separated in magnitude. Polak [1] suggested that the ohmic component of the polarisation potential disappears in picosec whereas the electrochemical component (activation and concentration overpotential) disappears in the interval from 10 millisec up to several seconds. Schwenk [2] calculated that the time constant for the activation component of the polarisation was between 10-5 and 101 sec. but did not attempt the calculation of the time constant for the concentration polarisation. Wallen and Linder [3] suggested that because the time constant for the decay of the ohmic component of the polarisation overpotential could be of the order of milliseconds then it was not possible with the instrumentation that they had 2 available to them at that time to separate the IR drop from from the other kinds of polarisation. Cherry [4] suggested that the capacitive effects of the soil around a buried pipeline could be of the order of seconds. The object of this paper is therefore to examine the possible time constants for the decay of activation and concentration polarisation and to suggest that the sharp differentiation between the time constants for the various potential decay processes that is assumed by a number of monitoring standards may not always obtain in practice. In particular it is concerned with the extent to which activation and diffusion polarisation play their different roles in determining the so-called absolute potential measured using instant off techniques. In order to illustrate the relative roles played by the different components of the total polarisation two situations will be considered; bare steel in sea water and reinforcing bars in concrete. Bare steel in Sea-water The calculations to determine the relative magnitudes of the components of the total polarisation for bare steel in sea-water will be based upon the assumption that a cathodic current of 100 mAm-2 results in a diffusion overpotential of 10 mV and an activation overpotential of 100 mV. Although it is a very greatly simplifying assumption, the time constant for the activation polarisation of a bare electrode in a conductive electrolyte solution can be calculated by assuming a simple Randles circuit that incorporates the polarisation resistance of the solid/electrolyte interface and the double layer capacity. Putting the polarisation resistance as 5 x 10 -2 ohm m-2 and the double layer capacity as 1 F m-2 [5] yields a time constant of about 5 x 10-2 sec. The activation overpotential component of the polarisation falls to about 1% of its original value in about 0.13 sec and consequently is not captured by an instantaneous off potential measurement one second or even 0.1 second after the current supply to a bare metal surface in a conductive electrolyte is switched off. 3 The time constant for the concentration polarisation that may obtain for a similar system may also be calculated if it is assumed that the cathodic process is O2 + 2H2O + 4e- 4OHand the rate determining step in this process is the transport of oxygen to the rebar surface. The diffusion overpotential is given by RT c c ln s D zF o c where cs is the concentration of oxygen at the surface of the metal and co is the concentration of oxygen in the external solution. cs is determined by the balance between the rate at which oxygen is consumed at the cathode and the rate at which it can diffuse to the cathode through the solution. Hence in order to carry out the calculation for a diffusion overpotential component of the total polarisation of 10 mV, it will be assumed that the bulk solution is in equilibrium with the atmosphere and has an oxygen content of 10 mg/l. The concentration of oxygen at the surface of the metal can then be calculated to be 0.020 mg/l or 0.6 mole.m-3. The rate of diffusion of oxygen to the cathodic surface may be assumed to follow Ficks second law of diffusion. If it is assumed that in a steady state, the rate at which oxygen is consumed at the surface corresponds to the protection current that is being applied then there will be a concentration profile in the solution. The form of this concentration has been calculated by Sands [6] and is reported as x2 iz 1 c x c0 . exp .dt 4 Dt nF t D0 t (1) where x is the distance from the electrode, t the time for which the current has been flowing, i is the current density flowing on to the surface, z is the number of the reactive molecules taking part in the reaction, n is the number of electrons involved in the reaction, F the Faraday and D the diffusion coefficient of the reactant in the solution. 4 Equation (1) can be integrated for increasing values of time to examine the development of the concentration profile. However for an "instant off" potential measurement the impressed current is reduced to zero, theoretically instantaneously, but in practice over a period that is determined by the efficiency of the switching gear. It will be assumed that at the moment that the current falls to zero then for a short period at least the concentration gradient of the reactants in the solution will remain and that these will cause a flow of the reactants to the metal surface with a consequent reduction in the diffusion overpotential. These concentration gradients supported the flow of reactants to the surface at a rate that corresponded to the impressed current. Hence it is suggested that initially the flow of reactant to the surface will continue at the same rate as before the current interruption but that since the reactants are no longer being consumed by the electrode reaction they will be increasing the concentration at the interface and decreasing the diffusion overpotential. The variation of the concentration at the surface of the reactant with time can therefore be calculated following Vetter [7] by putting x = 0 in equation (1) and integrating to yield c s co 2 iz t nF D The initial rate of change of the diffusion potential will then be given by d RT iz dc s dt nF c s dt RT 1 iz 1/ 2 t nF cs nF 1 /2 D (2) Putting cs = 0.6 mole m-3 as derived previously, assuming that the polarising current was 10-1 Am-2 and D = 1.54 x 10 -1 m2sec. [8]. d yields an initial value for of 9 x 10-5 t -1/2 V/sec. Equation (2) is dt only valid for the instantaneous decay rate of the diffusion overpotential component of the polarisation, but the time taken for this component to fall by 1% of its original value may be approximated by the integration of equation (2) to suggest that this may take some 5 25 sec. It seems therefore that an instantaneous off potential measurement carried out at between 0.1 and 1 sec is likely to capture most of the diffusion overpotential component of the total polarisation. However since the time constant for the decay of the activation polarisation has been calculated as of the order of 50msec, the magnitude of the activation overpotential component of the polarisation at one second or even 0.1 seconds after switching off the current is vanishingly small and will not be captured by an instant off potential measurement. It may therefore be suggested that the polarisation that may be determined by an instantaneous off measurement for an electrode that is being cathodically protected in (say) sea-water is effectively entirely determined by the concentration overpotential and not at all by the activation polarisation. Reinforcing bars in Concrete For reinforcing bars in concrete the relative magnitudes of the components of the total polarisation will be based upon the assumption that a cathodic current of 5 mAm -2 yields a diffusion overpotential of 100 mV and an activation overpotential of 50 mV. The approximate time constant for the decay of the activation polarisation of a reinforcing bar electrode in a concrete can be calculated as was done previously by assuming a simple Randles circuit. For a reinforcing bar in the passive state, putting the polarisation resistance as 210 ohm m2 [9] and the double layer capacity as 31 x 10-2 F m -2 yields a time constant of 65 sec. If the activation overpotential component of the polarisation under cathodic protection had been 50 mV then this time constant corresponds to a retention of 98% of the activation polarisation and a depolarisation rate of 0.7 mV/sec at 1 second after the current has been switched off. An instantaneous off potential measurement carried out between 0.1 and 1 sec is therefore likely to capture most of the activation overpotential component of the total polarisation. In a similar fashion, the rate of depolarisation of the diffusion overpotential for a reinforcing bar in concrete may be calculated. If it 6 is assumed that the original diffusion overpotential is 100 mV then by the same method as was used previously it is assumed that the concentration of the oxygen in the pore water adjacent to the electrode surface is O.3 mole m -3 . Then if the diffusion coefficient for oxygen in concrete is put as 2 x 10-8 m2sec [9] and assuming that a protection current of 5 x 10-3 Am-2 is being applied, then equation [2] d yields a value for of 2.5x10-6 t - � V/sec. Hence calculating as dt before the time taken for this component to fall by 1% of its original value, this may be approximated as 200 sec.. It seems therefore that an instantaneous off potential measurement carried out at between 0.1 and 1 sec is likely to capture most of the diffusion overpotential component of the total polarisation. It is therefore suggested that in contrast to the depolarisation of an unprotected electrode in solution, the polarisation that is measured one second after the interruption of a cathodic protection current contains both the activation and diffusion overpotential components. The depolarisation that may be measured in the following four hours may contain all the activation polarisation but only a fraction of the diffusion overpotential component.
Conclusions The "instantaneous" shift in potential that occurs when a cathodic protection current is interrupted is very rarely solely the iR drop that results from the passage of the current from the anode through the surrounding electrolyte. It will normally also contain one or more of the other components of the polarisation that is being applied to the electrode. This means that the potential that is reported as the result of such a measurement is rarely the actual polarised potential, but a potential that includes unknown portions of the activation and diffusion overpotentials. The extent to which the subsequent drift of the potential to more positive values may be the result of the decay of activation or diffusion overpotential components of the total polarisation will depend upon the system which is being protected and it would not be wise to 7 assume, as has sometimes been the case in the past, that diffusion effects control this phase of the depolarisation process. It therefore seems that the cathodic protection criteria that have been adopted, the absolute potential, the depolarisation criterion, the absolute potential after depolarisation, should all be regarded as purely empirically based and may be very dependent upon the nature of the system that is being protected.
References
1. Polak, J. "Conference on the internal and external protection of pipes", Brit. Hydr. Res. Assn., Durham, 1975 Paper F4
2. Schwenk , W. "Determination of the potential of current-carrying electrodes with special reference to their application in cathodic corrosion protection". Werkstoffe und Korrosion (1962), 13 21218.
3. Wallen, B. and Linder, B. "Ohmic Potential Drop in Cathodic Protection of Buried Structures: Evaluation of some Measuring Methods", British Corrosion Journal, 8, (1973), 7 - 14
4. Cherry, B.W. "An analysis of the Potential Decay when Cathodic Protection is Interrupted", 4th Asian-Pacific Corrosion Control Conference, Tokyo, 1985, 1243
5. von Baeckmann. W., Schwenk, W. and Prinz, W. "Cathodic Corrosion Protection", Gulf Publishing, 1989, 44
6. Sand, H. J. S. "On the Concentration at the Electrodes in a Solution", Philosophical Magazine 1, (1900), 45 - 79
7. Vetter, K.J. "Electrochemical Kinetics" trans Scripta Technica Inc. Academic Press, New York (1967), 207
8. Ferrell, R.T. and Himmelblau, D.M."Diffusion Coefficients of Nitrogen and Oxygen in Water" J of Chem. and Eng. Data, 12, (1967), 111 - 115 8
9. Birbilis, N., Nairn, K.M. and Forsyth, M., "Transient Response Analysis of Steel in Concrete" Corrosion Science, 45, (2003), 1895
10. Bertolini L., Elsener, B , Pedeferri, P. and Polder, R. "Corrosion of Steel in Concrete", Wiley VCH, KGaA, Weinheim, 2004, 42 9
Last Sentence and note:
“Interpretation of a current-applied measurement requires correction for voltage drops in the earth or metallic paths.
NOTE: Correction is understood to mean the application of sound engineering practice in the determination of the magnitude and significance of voltage drops by methods such as:
ACTION: Rewrite entire section:
Suggestion:
6.2.3.1.1 A cathodic voltage of –850 mV or more negative as measured with respect to a
saturated copper/copper sulfate (CSE) reference electrode with current applied. This potential may include voltage drops that should be considered. Potentials shall be interpreted by those properly trained using sound engineering practices to determine when and where it is necessary to consider voltage drops.
NOTE: When sound engineering practices indicate it is necessary to consider voltage drops, consideration is understood to mean the determination of the magnitude and significance of voltage drops by methods such as:
List methods from 2007 version.
Also list the areas where voltage drop may be a “potential” problem. Surely not everywhere!
6.2.3.1.2 A minimum of 100 mV of cathodic polarization. Either the formation or the decay of polarization must be measured to satisfy this criterion.
6.2.3.1.3 A polarized potential of –850 mV or more negative as measured with respect to a saturated copper/copper sulfate (CSE) reference electrode.
Make changes to numbers as needed.
6.2.3.1.3(4) Criteria that have been shown to successfully control corrosion through empirical evidence on specific piping systems may continue to be used on those piping systems or others with the same characteristics. These criteria include –850 mV on with consideration for voltage drops other than those across the structure-to-electrolyte boundary, other current applied criteria, net current flow, 300 mV shift, or E log I.
ACTION: Remove the last sentence.
REASON: If we list some, we have to list all possible criteria that may work!
6.2.3.1.4 Other criteria or methods that are demonstrated to achieve the corrosion control
objectives of the user; for example,
ACTION: Restate
6.2.3.1.4 Other criteria or methods that are demonstrated to achieve the corrosion control
objectives of the user; for example,
REASON: Objectives of the user should not be a reason for criteria.
6.2.3.2.2 When active MIC has been identified or is probable, (e.g., caused by acid-producing or sulfate-reducing bacteria), an instant-off potential of –950 mV CSE or more
negative or as much as –300 mV of cathodic polarization may be required.
ACTION: Remove or restate
REASON: MIC can be and usually is present in nearly all environments in which pipelines are laid, therefore MIC are probable. Again the use of “instant off” is not defined etc, as above. Usually, the external corrosion caused by MIC on pipelines is found under disbonded coating that shields the CP, so a different criterion will likely not result in adequate protection.
6.2.3.2.3 At elevated temperatures (> 40 °C [104 °F]), an instant-off potential more
electronegative than –850 mV CSE, or more than 100 mV of cathodic polarization may be required. At temperatures greater than 60 °C (140 °F), the instant-off potential should be –950 mV CSE or more negative.
ACTION: Remove or restate
REASON: There many pipeline systems around the world that operate at these elevated temperatures without seeing external corrosion when using one of the 3 criterion mentioned above. Again the use of “instant off” is not defined etc, as above.
6.2.3.2.8 In weak acid environments, an instant-off potential of –950 mV or more negative with respect to a copper/copper sulfate reference electrode is required. Weak acids are those acids that only partially dissociate to form H+ ions at moderate concentrations.
ACTION: Restate or remove
REASON: This is a “weak” definition. Surely there is a better definition. I can not find the reference (90) mentioned in the document from the CP-2 course? Which version are we referencing?
6.2.3.2.9 When operating pressure and conditions are conducive to stress corrosion cracking, the use of polarized potentials in the cracking range relative to the temperature indicated in Figure 1 is not advised. (See references on stress corrosion cracking at the end of this section.)
ACTION: Either add more information or simply make cautionary statement.
REASON: The way this is shown now leads one to believe this is a common problem therefore, if operating in these ranges there is going to be stress corrosion cracking. This is not true. Yes there are a few instances, but is this true for all systems? How much of a problem is it? Make this statement only to caution that there may be stress corrosion cracking, but there are many more factors that must be put into the equation. I will let the “experts” make the statement.
6.2.3.2.10 Cathodic polarization levels that result in excessive generation of atomic hydrogen should be avoided on all metals susceptible to hydrogen embrittlement.
ACTION: Define “excessive hydrogen”. Also, is there so list of metals that are susceptible?
REASON: Again we either must give more guidance or make it more of a cautionary statement.
Section 7: Design of Cathodic Protection Systems
7.1 Introduction
7.1.1 This section provides procedures for designing CP systems that will provide effective external corrosion control by satisfying one or more of the criteria listed in Section 6 and exhibiting acceptable reliability over the intended operating life of the systems.
ACTION: Restate
REASON: Satisfying criteria does not necessarily provide effective external corrosion control.
SUGGESTION: This section provides procedures for designing CP systems that will should provide effective external corrosion control by while satisfying one or more of the criteria listed in Section 6 and exhibiting acceptable reliability over the intended operating life of the systems.
I may add more later!
Further comments from Roy Bash
Richard:
There is no definition in section 2 of the committee’s draft for “cathodic voltage” or “instant-off potential”.
Actually there is no such thing as a cathodic or anodic voltage because there is no such thing as a positive or negative voltage. When there is a voltage between two objects, one of them will be positive and the other one will be negative, but the voltage has no polarity. The polarity of the pipeline is designated with the P/S potential measurement because it is the anode (negative electrode) in the P/S potential measurement, but that does not mean it is a negative voltage.
In the natural corrosion process the anode is always the negative electrode of the corrosion cell (battery cell).
Magnesium is negative to steel and that is the reason it can be used as an anode to cathodically protect steel.
I realize that you already understood this, but the committee does not understand it. Please read the committee’s second definition of “cathodic polarization” in Section 2 of their draft. They obviously believe that forcing the pipeline (cathode) more negative renders it more active (anodic) rather than more passive (cathodic).
Oh well, I guess it could worse, maybe.
L.A. (Roy) Bash
Global Cathodic Protection, Inc.
There is no definition in section 2 of the committee’s draft for “cathodic voltage” or “instant-off potential”.
Actually there is no such thing as a cathodic or anodic voltage because there is no such thing as a positive or negative voltage. When there is a voltage between two objects, one of them will be positive and the other one will be negative, but the voltage has no polarity. The polarity of the pipeline is designated with the P/S potential measurement because it is the anode (negative electrode) in the P/S potential measurement, but that does not mean it is a negative voltage.
In the natural corrosion process the anode is always the negative electrode of the corrosion cell (battery cell).
Magnesium is negative to steel and that is the reason it can be used as an anode to cathodically protect steel.
I realize that you already understood this, but the committee does not understand it. Please read the committee’s second definition of “cathodic polarization” in Section 2 of their draft. They obviously believe that forcing the pipeline (cathode) more negative renders it more active (anodic) rather than more passive (cathodic).
Oh well, I guess it could worse, maybe.
L.A. (Roy) Bash
Global Cathodic Protection, Inc.
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.)
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.)
Sunday, March 7, 2010
Roy Bash Comments
Richard:
The 1 mpy corrosion rate in the committee’s draft is something that has to go. The CP process is based on science that can produce a zero corrosion rate, and I do not want to be involved in any part of publishing a Standard proving that as CP practitioners we are not striving to achieve a zero corrosion rate on steel pipelines carrying hazardous materials. It may not be possible but I am sure going to keep trying. The negative 850 mV, CSE instant-off CP criterion for steel pipelines is grossly overly stringent in too many cases, and it is clear from a scientific standpoint that it can achieve a zero corrosion rate, but that is not the issue, the issue is how much over protection damage is it going to inflict on steel pipelines? Also how much wasted money is it going to cost the public?
Why can’t we accept the science and available historical proof that the negative 850 mV, CSE on CP criterion can and has achieved a zero corrosion rate on many buried steel pipelines for many years?
What a drag this has turned out to be.
L.A. (Roy) Bash
Global Cathodic Protection, Inc.
The 1 mpy corrosion rate in the committee’s draft is something that has to go. The CP process is based on science that can produce a zero corrosion rate, and I do not want to be involved in any part of publishing a Standard proving that as CP practitioners we are not striving to achieve a zero corrosion rate on steel pipelines carrying hazardous materials. It may not be possible but I am sure going to keep trying. The negative 850 mV, CSE instant-off CP criterion for steel pipelines is grossly overly stringent in too many cases, and it is clear from a scientific standpoint that it can achieve a zero corrosion rate, but that is not the issue, the issue is how much over protection damage is it going to inflict on steel pipelines? Also how much wasted money is it going to cost the public?
Why can’t we accept the science and available historical proof that the negative 850 mV, CSE on CP criterion can and has achieved a zero corrosion rate on many buried steel pipelines for many years?
What a drag this has turned out to be.
L.A. (Roy) Bash
Global Cathodic Protection, Inc.
Subscribe to:
Posts (Atom)
