1、Item No. 24213NACE International Publication 35201This Technical Committee Report has been preparedby NACE International Task Group 210* onCoupon TechnologyTechnical Report on the Application andInterpretation of Data from External Coupons Usedin the Evaluation of Cathodically Protected MetallicStru
2、ctures February 2001, NACE InternationalThis NACE International technical committee report represents a consensus of those individual memberswho have reviewed this document, its scope, and provisions. Its acceptance does not in any respect precludeanyone from manufacturing, marketing, purchasing, or
3、 using products, processes, or procedures not included inthis report. Nothing contained in this NACE International report is to be construed as granting any right, byimplication or otherwise, to manufacture, sell, or use in connection with any method, apparatus, or productcovered by Letters Patent,
4、or as indemnifying or protecting anyone against liability for infringement of LettersPatent. This report should in no way be interpreted as a restriction on the use of better procedures or materialsnot discussed herein. Neither is this report intended to apply in all cases relating to the subject. U
5、npredictablecircumstances may negate the usefulness of this report in specific instances. NACE International assumes noresponsibility for the interpretation or use of this report by other parties.Users of this NACE International report are responsible for reviewing appropriate health, safety,environ
6、mental, and regulatory documents and for determining their applicability in relation to this report prior toits use. This NACE International report may not necessarily address all potential health and safety problems orenvironmental hazards associated with the use of materials, equipment, and/or ope
7、rations detailed or referredto within this report. Users of this NACE International report are also responsible for establishing appropriatehealth, safety, and environmental protection practices, in consultation with appropriate regulatory authorities ifnecessary, to achieve compliance with any exis
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10、ForewordCoupons can be used to determine the level of corrosionprotection provided by a cathodic protection (CP) system toa buried structure, such as an operating pipeline. Structure-to-electrolyte potential measurements have long been usedas the basis for assessing cathodic protection levels andcom
11、pliance with CP criteria. It is well known that a voltage(IR) drop exists in the soil or across the coating, and thatthis IR drop can produce an error in the interpretation of thestructure-to-electrolyte potential measurement. This voltagedrop can be a function of reference electrode placement,soil
12、resistivity, burial depth of the structure, coatingcondition, and the amount of CP current applied.CP coupons have been used since the 1930s by several ofthe pioneers of the corrosion-control industry, both in NorthAmerica and in Europe. Over the last two decades, the useof CP coupons has been redis
13、covered as a practicalmethod to determine the level of polarization of a buriedstructure and to confirm the voltage drop in a potentialmeasurement. Acceptance of CP coupon technology isslowly occurring. Research sponsored by the pipelineindustry has explored the use of CP coupons and hashelped valid
14、ate the use of this technology. The purpose ofthis technical committee report is to provide an alternative tothe conventional evaluation of the effectiveness of a CPsystem.This NACE technical committee report was prepared byTask Group 210 (formerly T-10A-3a) on CouponTechnology. Task Group 210 is ad
15、ministered by SpecificTechnology Group (STG) 35 on Pipelines, Tanks, and WellCasings and is sponsored by STG 05 on Cathodic/AnodicProtection. This report is issued under the auspices of STG35 on Pipelines, Tanks, and Well Casings._*Chairman Kevin T. Parker, CC Technologies, Inc., Dublin, Ohio.NACE I
16、nternational2GeneralA CP coupon is used to simulate coating defects (holidays)on the structure being evaluated. The CP coupon isinstalled near the structure and then connected to it throughan aboveground test station. This allows the CP coupon tobe connected to the CP system on the structure, thussi
17、mulating a holiday in the coating. The CP coupon canthen be disconnected from the circuit during periodic testing,and an instant-off potential measured. The CP coupon canthen continue to be monitored and the depolarizationmeasured. These measurements approximate thepolarized and the depolarized pote
18、ntial of the structure inthe vicinity of the CP coupon and allow the operator tocalculate the IR drop. A second “free-corroding” nativecoupon has sometimes been installed at the same locationas the CP coupon to measure the native potential of thecoupon.NACE Standard RP01691states, “Voltage (IR) drop
19、s otherthan those across the structure-to-electrolyte boundarymust be considered. . .” in order to assess the effectivenessof a CP system using fixed potential measurements.Several methods are used to evaluate the IR drop valueand thus the effectiveness of CP systems: Minimizing the distance between
20、 the referenceelectrode and the surface of the structure; Measuring the potential when the current flow isinterrupted (the “instant-off” potential); or Installing CP coupons in the vicinity of thestructure to replicate a coating holiday.All of these methods have their advantages anddisadvantages. NA
21、CE Standard TM04972includes some ofthe methods used to satisfy NACE Standard RP0169.CP coupons have several advantages. IR-drop error-reduced potentials can be obtained without interruptingmultiple CP sources. CP coupons can also be used onburied structures with direct-connected galvanic anodes,whic
22、h cannot be interrupted. Using CP coupons,depolarization testing can be performed without de-energizing the CP system. An additional advantage is theability to record interference-reduced structure potentials onstructures affected by stray currents.A disadvantage of using CP coupons is the possibili
23、ty ofdiscrepancy between the polarized potential of the structureand that of the CP coupon. Because the CP couponrepresents a holiday of similar size on the structure, thepolarized potential of the CP coupon does not alwaysduplicate the polarized potential of the structure at the samelocation. The h
24、oliday location (as observed by the polarizedpotential of the structure measured at grade) is notnecessarily at the CP coupon location, and can be at alocation some distance away.An additional complication in measuring a structurepotential is the effect of IR drops from uninterruptablecurrent source
25、s. By design, CP coupons can bedisconnected from the structure and CP system, therebyeliminating the IR drop attributable to these current sources.Even when all current sources have been interrupted, long-line currents can still affect the structure-to-electrolytepotential readings measured at grade
26、 on a pipeline.Because the effective reference point of a CP coupon isvery close to the CP coupon surface, IR drops caused bylong-line currents are minimized.DefinitionsAutomated Coupon Reader: A portable electronicinstrument capable of making several types ofmeasurements at multiple coupon test sta
27、tions and storingthese values for later uploading to a computer.Buried Stationary Reference Electrode: A referenceelectrode, usually copper sulfate (CuSO4), that is designedto last many years in a permanently buried position.Corrosion Potential (Ecorr): The potential of a corrodingsurface in an elec
28、trolyte relative to a reference electrodeunder open-circuit conditions (also known as nativepotential, rest potential, open-circuit potential, or freelycorroding potential).CP Coupon: The coupon connected to the structure beingprotected by CP.Depolarized Potential: The steady-state potential that th
29、eCP coupon reaches some time after disconnecting from thestructure. In most cases this time ranges from one to 48hours.Depolarizing Wave Form: A recorded plot of potentialversus time from just prior to disconnecting the CP couponfrom the structure, to some time thereafter, typically 48hours or more.
30、 This wave-print can be used to determinethe instant-off potential and the depolarized potential.Dip Tube: See Soil Access Tube.Electrical Isolation: The condition of being electricallyseparated from other metallic structures or the environment.Electrometer: A high impedance, low-current digital met
31、er.The input resistances are typically many tetraohms. Theseinstruments can be used to measure low currents, voltagesfrom high resistance sources, charges, or high resistances.Freely Corroding Coupon: See Native Coupon.NACE International3Free Corrosion Potential: See Corrosion Potential.Foreign Stru
32、cture: Any metallic structure that is notintended as a part of a system under cathodic protection.Galvanic Anode: A metal that provides sacrificialprotection to another metal that is more noble whenelectrically coupled in an electrolyte. This type of anode isthe electron source in one type of cathod
33、ic protection.Holiday: A discontinuity in a protective coating thatexposes unprotected surface to the environment.Impressed Current: An electric current supplied by adevice employing a power source that is external to theelectrode system. (An example is direct current for cathodicprotection.)Instant
34、-Off Delay Time: The time at which the instant-offpotential is recorded after the coupon is disconnected fromthe structure. This value is measured in milliseconds.Instant-Off Potential: A measurement of a structure-to-electrolyte potential made without perceptible delayfollowing the interruption of
35、CP.Interrupted Wave-Form: A recorded plot of potentialversus time from just prior to disconnecting the CP couponfrom the structure, to some time thereafter typically a fewseconds. This wave-print is used to record or determine theinstant-off potential.IR Drop: The difference between the potential wi
36、th thecathodic protection system energized and the polarizedpotential.Native Coupon: The coupon not connected to thestructure for use in measuring native potentials. Alsoknown as “freely corroding” coupon.Native Potential: See Corrosion Potential.Noble Metal: (1) A metal that occurs commonly in natu
37、re inthe free state. (2) A metal or alloy whose corrosionproducts are formed with a small negative or a positive free-energy change.Polarization: The change from the open-circuit potential asa result of current across the electrode/electrolyte interface.Polarized Potential: The potential across thes
38、tructure/electrolyte interface that is the sum of thecorrosion potential and the cathodic polarization.Reference Electrode: An electrode whose open-circuitpotential is constant under similar conditions ofmeasurement, which is used for measuring the relativepotentials of other electrodes.Reference Tu
39、be: See Soil Access Tube.Soil Access Tube (Reference Tube or Dip Tube):Nonconductive and impermeable (polyvinyl chloride PVC,polyethylene, polycarbonate) tube used to mount couponand can be filled with electrolyte.Structure-to-Soil Potential: See Structure-to-ElectrolytePotential.Structure-to-Electr
40、olyte Potential: The potentialdifference between the metallic surface of a structure andthe electrolyte that is measured with reference to anelectrode in contact with the electrolyte. This measurementis commonly called structure-to-soil potential.Telluric Current: The current in the earth resulting
41、fromgeomagnetic fluctuations.ApplicationsCP coupons are most commonly used for buried orsubmerged pipelines. They are also used for otherstructures, such as underground storage tanks,aboveground storage tank bottoms, reinforcing bars inconcrete, and various structures in marine and freshwaterenviron
42、ments. Pipelines that use CP coupons includetransmission, distribution, gathering, utility, and in-plantpiping. CP coupons are sometimes used for cable-carryingpiping or conduit that are buried or submerged and areprotected from external corrosion with CP.CP coupons can be used to evaluate the level
43、 of CP onthose structures in areas where more conventionalmeasurement methods, such as those described in NACEStandard TM0497, cannot be applied. CP coupons havealso been used as a complementary tool to investigate andsupport the results of more conventional measurements.3Structure-to-electrolyte po
44、tential measurements have beena basis for evaluating levels of CP for many years. The-850-mV current applied and -850-mV polarized structure-to-electrolyte potential criterion measured with respect to asaturated copper/copper sulfate (Cu/CuSO4) electrode(CSE) are often used to evaluate CP levels, an
45、d there isgrowing interest in the use of the 100-mV polarizationcriterion.4These criteria have assumed that considerationhas been made for the IR drop in the structure-to-electrolytepotential measurement when CP current is flowing fromimpressed and galvanic CP systems.3-6Current interruption has com
46、monly been used whenconsidering this IR drop error, but in some situations, theCP or stray current cannot be interrupted. Stray currentsflowing in the soil and across the structure coating canproduce ohmic or IR drops. Some situations in which CPcoupons can be, or have been, applied to resolve suchp
47、roblems include the following:NACE International4Multiple Influencing RectifiersIn areas where multiple impressed current sourcesinfluence the structure-to-electrolyte potential,interruption of all current sources is not alwayspractical. CP coupons can be disconnected from thestructure and the insta
48、nt-off potential of the CP couponmeasured to evaluate the protection level with respectto the -850-mV (CSE) polarized potential criterion.Additionally, the CP coupon can be allowed todepolarize, permitting evaluation of the CP couponprotection level with respect to the 100-mV polarizationcriterion.D
49、ynamic or Static Stray Direct Current (DC)CP coupons have been used to assess the level ofprotection on pipelines affected by stray currents.Stray current sources include DC traction systems,foreign rectifiers, telluric earth currents, and high-voltage direct current (HVDC) electrodes. CP couponshave been used extensively in parts of Europe, Japan,and Australia to monitor and evaluate the level ofprotection on pipelines affected by stray DC tractionsystems, and are being used in Alaska and Canada,where telluric earth currents can corrupt accuratestructure-to
