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1、Item No. 24204 NACE International Publication 11100 This Technical Committee Report has been prepared by NACE International Work Group T-11-4f* on Reference Electrodes for Use in Concrete Use of Reference Electrodes for Atmospherically Exposed Reinforced Concrete Structures March 2000, NACE Internat
2、ional This NACE International technical committee report represents a consensus of those individual members who have reviewed this document, its scope, and provisions. Its acceptance does not in any respect preclude anyone from manufacturing, marketing, purchasing, or using products, processes, or p
3、rocedures not included in this report. Nothing contained in this NACE International report is to be construed as granting any right, by implication or otherwise, to manufacture, sell, or use in connection with any method, apparatus, or product covered by Letters Patent, or as indemnifying or protect
4、ing anyone against liability for infringement of Letters Patent. This report should in no way be interpreted as a restriction on the use of better procedures or materials not discussed herein. Neither is this report intended to apply in all cases relating to the subject. Unpredictable circumstances
5、may negate the usefulness of this report in specific instances. NACE International assumes no responsibility 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
6、documents and for determining their applicability in relation to this report prior to its use. This NACE International report may not necessarily address all potential health and safety problems or environmental hazards associated with the use of materials, equipment, and/or operations detailed or r
7、eferred to within this report. Users of this NACE International report are also responsible for establishing appropriate health, safety, and environmental protection practices, in consultation with appropriate regulatory authorities if necessary, to achieve compliance with any existing applicable re
8、gulatory requirements prior to the use of this report. CAUTIONARY NOTICE: The user is cautioned to obtain the latest edition of this report. NACE International reports are subject to periodic review, and may be revised or withdrawn at any time without prior notice. NACE reports are automatically wit
9、hdrawn if more than 10 years old. Purchasers of NACE International reports may receive current information on all NACE International publications by contacting the NACE International Membership Services Department, P.O. Box 218340, Houston, Texas 77218-8340 (telephone +1281228-6200). Foreword The ob
10、jective of this technical committee report is to provide a state-of-the-art overview of the characteristics of and the installation method for reference electrodes commonly used in atmospherically exposed reinforced concrete structures. It is beyond the scope of this report to fully address all fact
11、ors associated with the selection and use of reference electrodes in steel-reinforced concrete. Additional reference electrodes are being developed by several manufacturers for use in this application. This report is intended as a technical resource for engineers responsible for assessment of reinfo
12、rced concrete structures or the corrosion control design for such structures. It is also useful to owners involved in these decisions. This technical committee report was prepared by Work Group T-11-4f, a component of Task Group T-11-4 on Cathodic Protection of Reinforcing Steel in Concrete. It is i
13、ssued by NACE International under the auspices of Group Committee T-11 on Corrosion and Deterioration of the Infrastructure. _ ? Chairman John Olson III, Electrochemical Devices Inc., Belmont, MA. Copyright NACE International Provided by IHS under license with NACELicensee=IHS Employees/1111111001,
14、User=Wing, Bernie Not for Resale, 05/16/2007 01:43:06 MDTNo reproduction or networking permitted without license from IHS -,-,- NACE International 2 Introduction In the recent past the costs of damage due to corrosion of reinforced concrete have risen dramatically. Concrete structures such as bridge
15、s, parking garages, buildings, and marine docks exposed to chlorides from either deicing salts or the local environment are subject to deterioration. Chlorides that are introduced into the reinforced concrete structure initiate corrosion by destroying the passive film that is naturally formed on ste
16、el in concrete. The corrosion products of steel occupy several times the volume of the steel itself and exert tensile stresses on the surrounding concrete. Cracking of the concrete develops, and, ultimately, spalling of the concrete takes place. This can render the structure unsound for use. Definit
17、ions Anion: A negatively charged ion that migrates through the electrolyte toward the anode under the influence of a potential gradient. Cathodic Protection: A technique to reduce the corrosion of a metal surface by making that surface the cathode of an electrochemical cell. Cation: A positively cha
18、rged ion that migrates through the electrolyte toward the cathode under the influence of a potential gradient. Charge-Balancing Current: Current that flows as a result of differences in potential, especially those which remain briefly after interruption of current. Electrolyte: A chemical substance
19、containing ions that migrate in an electric field. Exchange Current Density: The rate of charge transfer per unit area when an electrode reaches dynamic equilibrium (at its reversible potential) in a solution; that is, the rate of anodic charge transfer (oxidation) balances the rate of cathodic char
20、ge transfer (reduction). Half-Cell: A pure metal in contact with a solution of known concentration 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 that is the sum of both half-cells dev
21、elops. Hysteresis: The failure of a property that has been changed by an external agent to return to its original value when the cause of the change is removed. Impedance: The opposition to current flow in an electric circuit. Ionic Mobility: The drift velocity of an ion at infinite dilution and und
22、er the force exerted by a unit electric field (1 V cm-1) on the charge of the ion (units are usually given in cm2 s-1 V-1). Liquid Junction Potential: The potential difference across the interphase region in which there is a transition from the concentration of one solution to the concentration of a
23、nother. It results from a tendency for charge separation due to differences in ionic mobilities. Luggin Probe: A small tube or capillary filled with electrolyte, terminating close to the metal surface under study, which is used to provide an ion-conducting path without diffusion between the electrod
24、e under study and a reference electrode. Passive: (1) The positive direction of electrode potential. (2) A state of a metal in which a surface reaction product causes a marked decrease in the corrosion rate relative to that in the absence of the product. pH: The negative logarithm of the hydrogen io
25、n activity written as: pH = -log10 (aH+) where aH+ = hydrogen ion activity = the molar concentration of hydrogen ions multiplied by the mean ion-activity coefficient. Polarization: The change from the open-circuit potential as a result of current across the electrode/electrolyte interface. Potential
26、: The relative voltage at a specific point compared with that of a known standard at the same location. Spalling: The spontaneous chipping, fragmentation, or separation of a surface or surface coating. Standard Electrode Potential: The reversible potential for an electrode process when all products
27、and reactions are at unit activity on a scale in which the potential for the standard hydrogen reference electrode is zero. Copyright NACE International Provided by IHS under license with NACELicensee=IHS Employees/1111111001, User=Wing, Bernie Not for Resale, 05/16/2007 01:43:06 MDTNo reproduction
28、or networking permitted without license from IHS -,-,- NACE International 3 Use of Reference Electrodes Metal surrounded by an electrolyte constitutes an electrochemical half-cell. Steel in concrete is an example of such a half-cell. The potential of such a half-cell can only be measured relative to
29、 another half-cell. A half-cell used to measure this potential is known as a reference half-cell, or a reference electrode. It generally conforms to certain fundamental properties described in the section of this report titled “Properties of an Ideal Reference Electrode.” ASTM(1) C 8761 describes a
30、test procedure for measuring the potentials of reinforcing steel in concrete. The procedure involves the placement of a reference electrode on the concrete surface and the measurement of the potential difference between the reference electrode and the embedded steel. This potential difference indica
31、tes the state of corrosion of the steel. Embedded reference electrodes are also used for measurement of corrosion potential or the effectiveness of cathodic protection. Portable electrodes can be used for short-term measurements at the concrete surface while permanent, embedded electrodes are used f
32、or long-term monitoring. Most of the reference electrodes suitable for use in concrete were first developed for underwater or underground applications. Each reference electrode type is characterized by its standard potential on the hydrogen scale. The potential of the standard hydrogen electrode (SH
33、E) is, by definition, the zero point on the hydrogen scale. Figure 1 shows the reference electrodes most commonly used with concrete and their relative positions with respect to the SHE. Measurements can be converted from one scale to another, provided that the scale being used is quoted. FIGURE 1 R
34、elative potentials of selected reference electrodes and probes used in concrete. _ (1) American Society for Testing and Materials (ASTM), 100 Barr Harbor Dr., West Conshohocken, PA 19428-2959. Corrosion unlikely -350 mV -200 mV Corrosion likely 0.4 0.3 0.2 0.12 0.1 0 -0.3 -0.1 -0.2 ?MnO2 NaOH0.39 (0
35、.5 M) ?Cu/CuSO40.32 ?Calomel (Sat.)0.24 ?Ag/AgCl (Sat.)0.20 ?Reported Range 0.11-0.21 of Graphite Standard Hydrogen Electrode Copyright NACE International Provided by IHS under license with NACELicensee=IHS Employees/1111111001, User=Wing, Bernie Not for Resale, 05/16/2007 01:43:06 MDTNo reproductio
36、n or networking permitted without license from IHS -,-,- NACE International 4 Properties of an Ideal Reference Electrode For a reference electrode to perform satisfactorily in concrete, it generally possesses several well-defined fundamental properties, as described below. (1) Reversible The potenti
37、al of the reference electrode is defined by a thermodynamically reversible reaction. (2) Low polarization The current transfer used to measure potential does not significantly affect the potential of the reference electrode. (3) Minimum hysteresis The reference electrode typically returns to its equ
38、ilibrium potential after small currents are passed through it. (4) High exchange current density The reference electrode typically has a high exchange current density. (5) Low internal resistance The internal resistance of the reference electrode is low enough that it does not significantly affect t
39、he accuracy of the measurement instruments. (6) Low contact resistance The contact resistance resulting from the electrical and mechanical coupling of the reference electrode to the concrete is low enough that it does not significantly affect the accuracy of measurement instruments. (7) Minimum resp
40、onse to impurities The potential of the reference electrode is constant regardless of the concrete in which it is used or the impurities present in the concrete. (8) Contamination The reference electrode is not contaminated by and does not contaminate the environment. (9) Known temperature response
41、A reference electrode intended for permanent embedment in outdoor concrete structures typically has a known response to temperature that remains predictable through a temperature range of about 0 to 50oC (32 to 120oF). (10) Freeze-thaw cycling An embedded reference electrode is able to accept repeat
42、ed freeze-thaw cycling due to temperature extremes. (11) Rugged The reference electrode is constructed from durable components in order to withstand the expected environmental conditions for its design life as well as possible mistreatment during installation. (12) Reproducible Reference electrodes
43、are consistently manufactured to provide reproducible results. (13) Available Reference electrodes are readily available. (14) Low cost Reference electrodes are relatively inexpensive. Reference Electrodes in Use Today (1)True reference electrodes are those electrodes that most closely meet the crit
44、eria defined above. They include the following: silver/silver chloride/potassium chloride (Ag/AgCl/KCl), saturated calomel (SCE), manganese dioxide (MnO2), and saturated copper/copper sulfate (Cu/CuSO4). (2)Pseudo reference electrodes do not have all of the properties of a true reference electrode.
45、Determination of a consistent reference potential, therefore, is either difficult or impossible. 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/CuSO4 or CSE
46、) This reference electrode is the one that is most widely used 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 has been used with concrete. However, these electrodes are sensiti
47、ve to contamination from alkaline or chloride- containing solutions that can be present on the concrete surface. (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 use and as a
48、 durable electrode for field work. It is a commonly used portable electrode for steel in concrete. The potential readings of the Ag/AgCl/KCl electrode depend on the composition of the internal electrolyte, or gel, as shown in Table 1. For laboratory use, the electrolyte is a 4.0-M KCl solution satur
49、ated with AgCl. This composition is slightly less than full saturation for KCl. Many field-grade electrodes 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 the Ag/AgCl element directly. This construction, known as a dry electrode, is not applicable fo