1、Item No. 24204NACE International Publication 11100This Technical Committee Report has been preparedby NACE International Work Group T-11-4f* onReference Electrodes for Use in ConcreteUse of Reference Electrodes for AtmosphericallyExposed Reinforced Concrete Structures March 2000, NACE InternationalT
2、his 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 using products, processes, or procedures
3、 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, or as indemnifying or protecting anyone a
4、gainst 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. Unpredictablecircumstances may negate the
5、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,environmental, and regulatory documents and for
6、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 operations detailed or referredto within thi
7、s 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 existing applicable regulatory requirements p
8、rior to the use of thisreport.CAUTIONARY NOTICE: The user is cautioned to obtain the latest edition of this report. NACEInternational reports are subject to periodic review, and may be revised or withdrawn at any time without priornotice. NACE reports are automatically withdrawn if more than 10 year
9、s old. Purchasers of NACE Internationalreports may receive current information on all NACE International publications by contacting the NACEInternational Membership Services Department, P.O. Box 218340, Houston, Texas 77218-8340 (telephone+1281228-6200).ForewordThe objective of this technical commit
10、tee report is toprovide a state-of-the-art overview of the characteristics ofand the installation method for reference electrodescommonly used in atmospherically exposed reinforcedconcrete structures. It is beyond the scope of this report tofully address all factors associated with the selection and
11、use of reference electrodes in steel-reinforced concrete.Additional reference electrodes are being developed byseveral manufacturers for use in this application. Thisreport is intended as a technical resource for engineersresponsible for assessment of reinforced concretestructures or the corrosion c
12、ontrol design for suchstructures. It is also useful to owners involved in thesedecisions.This technical committee report was prepared byWork Group T-11-4f, a component of Task Group T-11-4on Cathodic Protection of Reinforcing Steel in Concrete. Itis issued by NACE International under the auspices of
13、Group Committee T-11 on Corrosion and Deterioration ofthe Infrastructure._G0D Chairman John Olson III, Electrochemical Devices Inc., Belmont, MA.NACE International2IntroductionIn the recent past the costs of damage due to corrosion ofreinforced concrete have risen dramatically. Concretestructures su
14、ch as bridges, parking garages, buildings, andmarine docks exposed to chlorides from either deicing saltsor the local environment are subject to deterioration.Chlorides that are introduced into the reinforcedconcrete structure initiate corrosion by destroying thepassive film that is naturally formed
15、 on steel in concrete.The corrosion products of steel occupy several times thevolume of the steel itself and exert tensile stresses on thesurrounding concrete. Cracking of the concrete develops,and, ultimately, spalling of the concrete takes place. Thiscan render the structure unsound for use.Defini
16、tionsAnion: A negatively charged ion that migrates through theelectrolyte toward the anode under the influence of apotential gradient.Cathodic Protection: A technique to reduce the corrosionof a metal surface by making that surface the cathode of anelectrochemical cell.Cation: A positively charged i
17、on that migrates through theelectrolyte toward the cathode under the influence of apotential gradient.Charge-Balancing Current: Current that flows as a resultof differences in potential, especially those which remainbriefly after interruption of current.Electrolyte: A chemical substance containing i
18、ons thatmigrate in an electric field.Exchange Current Density: The rate of charge transferper unit area when an electrode reaches dynamicequilibrium (at its reversible potential) in a solution; that is,the rate of anodic charge transfer (oxidation) balances therate of cathodic charge transfer (reduc
19、tion).Half-Cell: A pure metal in contact with a solution of knownconcentration of its own ion, at a specific temperature,develops a potential that is characteristic and reproducible;when coupled with another half-cell, an overall potential thatis the sum of both half-cells develops.Hysteresis: The f
20、ailure of a property that has beenchanged by an external agent to return to its original valuewhen the cause of the change is removed.Impedance: The opposition to current flow in an electriccircuit.Ionic Mobility: The drift velocity of an ion at infinitedilution and under the force exerted by a unit
21、 electric field (1V cm-1) on the charge of the ion (units are usually given incm2s-1V-1).Liquid Junction Potential: The potential difference acrossthe interphase region in which there is a transition from theconcentration of one solution to the concentration ofanother. It results from a tendency for
22、 charge separationdue to differences in ionic mobilities.Luggin Probe: A small tube or capillary filled withelectrolyte, terminating close to the metal surface understudy, which is used to provide an ion-conducting pathwithout diffusion between the electrode under study and areference electrode.Pass
23、ive: (1) The positive direction of electrode potential.(2) A state of a metal in which a surface reaction productcauses a marked decrease in the corrosion rate relative tothat in the absence of the product.pH: The negative logarithm of the hydrogen ion activitywritten as:pH = -log10 (aH+)where aH+=
24、hydrogen ion activity = the molar concentrationof hydrogen ions multiplied by the mean ion-activitycoefficient.Polarization: The change from the open-circuit potential asa result of current across the electrode/electrolyte interface.Potential: The relative voltage at a specific point comparedwith th
25、at of a known standard at the same location.Spalling: The spontaneous chipping, fragmentation, orseparation of a surface or surface coating.Standard Electrode Potential: The reversible potential foran electrode process when all products and reactions are atunit activity on a scale in which the poten
26、tial for the standardhydrogen reference electrode is zero.NACE International3Use of Reference ElectrodesMetal surrounded by an electrolyte constitutes anelectrochemical half-cell. Steel in concrete is an example ofsuch a half-cell. The potential of such a half-cell can onlybe measured relative to an
27、other half-cell. A half-cell used tomeasure this potential is known as a reference half-cell, or areference electrode. It generally conforms to certainfundamental properties described in the section of thisreport titled “Properties of an Ideal Reference Electrode.”ASTM(1)C 8761describes a test proce
28、dure formeasuring the potentials of reinforcing steel in concrete.The procedure involves the placement of a referenceelectrode on the concrete surface and the measurement ofthe potential difference between the reference electrodeand the embedded steel. This potential difference indicatesthe state of
29、 corrosion of the steel.Embedded reference electrodes are also used formeasurement of corrosion potential or the effectiveness ofcathodic protection. Portable electrodes can be used forshort-term measurements at the concrete surface whilepermanent, embedded electrodes are used for long-termmonitorin
30、g.Most of the reference electrodes suitable for use inconcrete were first developed for underwater orunderground applications. Each reference electrode type ischaracterized by its standard potential on the hydrogenscale. The potential of the standard hydrogen electrode(SHE) is, by definition, the ze
31、ro point on the hydrogen scale.Figure 1 shows the reference electrodes most commonlyused with concrete and their relative positions with respectto the SHE.Measurements can be converted from one scale toanother, provided that the scale being used is quoted.FIGURE 1Relative potentials of selected refe
32、rence electrodes and probes used in concrete._(1)American Society for Testing and Materials (ASTM), 100 Barr Harbor Dr., West Conshohocken, PA 19428-2959.Corrosionunlikely-350 mV-200 mVCorrosionlikely0.40.30.20.120.10-0.3-0.1-0.2G7FG7FG7FG7FG7FMnO2NaOH 0.39(0.5 M)G7FG7FG7FG7FG7FCu/CuSO40.32G7FG7FG7F
33、G7FG7FCalomel (Sat.) 0.24G7FG7FG7FG7FG7FAg/AgCl (Sat.) 0.20G7FG10Reported Range 0.11-0.21of GraphiteStandard HydrogenElectrodeNACE International4Properties of an Ideal Reference ElectrodeFor a reference electrode to perform satisfactorily inconcrete, it generally possesses several well-definedfundam
34、ental properties, as described below.(1) Reversible The potential of the reference electrodeis defined by a thermodynamically reversible reaction.(2) Low polarization The current transfer used tomeasure potential does not significantly affect the potentialof the reference electrode.(3) Minimum hyste
35、resis The reference electrodetypically returns to its equilibrium potential after smallcurrents are passed through it.(4) High exchange current density The referenceelectrode typically has a high exchange current density.(5) Low internal resistance The internal resistance ofthe reference electrode i
36、s low enough that it does notsignificantly affect the accuracy of the measurementinstruments.(6) Low contact resistance The contact resistanceresulting from the electrical and mechanical coupling of thereference electrode to the concrete is low enough that itdoes not significantly affect the accurac
37、y of measurementinstruments.(7) Minimum response to impurities The potential of thereference electrode is constant regardless of the concrete inwhich it is used or the impurities present in the concrete.(8) Contamination The reference electrode is notcontaminated by and does not contaminate theenvir
38、onment.(9) Known temperature response A reference electrodeintended for permanent embedment in outdoor concretestructures typically has a known response to temperaturethat remains predictable through a temperature range ofabout 0 to 50oC (32 to 120oF).(10) Freeze-thaw cycling An embedded referenceel
39、ectrode is able to accept repeated freeze-thaw cycling dueto temperature extremes.(11) Rugged The reference electrode is constructed fromdurable components in order to withstand the expectedenvironmental conditions for its design life as well aspossible mistreatment during installation.(12) Reproduc
40、ible Reference electrodes are consistentlymanufactured to provide reproducible results.(13) Available Reference electrodes are readily available.(14) Low cost Reference electrodes are relativelyinexpensive.Reference Electrodes in Use Today(1) True reference electrodes are those electrodes thatmost c
41、losely meet the criteria defined above. They includethe following: silver/silver chloride/potassium chloride(Ag/AgCl/KCl), saturated calomel (SCE), manganesedioxide (MnO2), and saturated copper/copper sulfate(Cu/CuSO4).(2) Pseudo reference electrodes do not have all of theproperties of a true refere
42、nce electrode. Determination of aconsistent reference potential, therefore, is either difficult orimpossible. They include the following: graphite,molybdenum/molybdenum oxide (Mo/MoO), platinum,mixed metal oxide, lead, and zinc.Portable Reference Electrodes(1) Saturated copper/copper sulfate (Cu/CuS
43、O4 or CSE)This reference electrode is the one that is most widelyused for potential measurements on land-based structures.It is robust; it can be easily dismantled, cleaned, recharged,and reassembled in the field. Because of this, the CSE hasbeen used with concrete. However, these electrodes aresens
44、itive to contamination from alkaline or chloride-containing solutions that can be present on the concretesurface.(2) Silver/silver chloride/potassium chloride (Ag/AgCl/KCl)This reference electrode is available as a small, fragile,glass-bodied or plastic-bodied electrode for laboratory useand as a du
45、rable electrode for field work. It is a commonlyused portable electrode for steel in concrete. The potentialreadings of the Ag/AgCl/KCl electrode depend on thecomposition of the internal electrolyte, or gel, as shown inTable 1. For laboratory use, the electrolyte is a 4.0-M KClsolution saturated wit
46、h AgCl. This composition is slightlyless than full saturation for KCl. Many field-gradeelectrodes use saturated KCl (4.17 M) as the electrolyte.The potential difference between the two is about 5 mV.For marine applications, seawater is allowed to contact theAg/AgCl element directly. This constructio
47、n, known as a dryelectrode, is not applicable for concrete. When potentialsmeasured against an Ag/AgCl/KCl electrode are reported,the concentration of the KCl solution is also recorded.(3) Saturated calomel (SCE)This reference electrode is available as a small, fragile,glass-bodied electrode for lab
48、oratory use and as a durableNACE International5electrode for field work. This electrode is used as areference for the calibration of other electrodes. Its field useis restricted in some circumstances, however, because itsinternal element contains mercury.Table 1Ag/AgCl/KCl Potentials vs. Standard Hy
49、drogen Electrode (SHE) at 25CAg/AgCl/X KCl Potential vs. SHE (mV) SourceX = 4.17 M (sat.) 199 Bates(A)X = 3.5 M 205 Treseder(B)X = 1.0 M 235 Hampel(C)X G7C 0.6 M (seawater) 250 Treseder(B)X = 0.1 M 288 Hampel(C)(A) R.G. Bates, Determination of pH (New York, NY: Wiley-Interscience, 1973), pp. 334-335.(B) C.A. Hampel, ed., The Encyclopedia of Electrochemistry (New York, NY: Reinhold Publishing Co., 1964), p. 433.(C) R.S. Treseder, ed., NACE Corrosion Engineers Reference Book (Houston, TX: NACE Internatio
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