1、Item No. 24214NACE International Publication 01101This Technical Committee Report has been preparedby NACE International Task Group 054 * onElectrochemical Chloride Extraction and Realkalizationof Reinforced Concrete.Electrochemical Chloride Extraction from SteelReinforced Concrete A State-of-the-Ar
2、t Report May 2001, NACE InternationalThis NACE International technical committee report represents a consensus of those individual members whohave reviewed this document, its scope, and provisions. Its acceptance does not in any respect preclude anyonefrom manufacturing, marketing, purchasing, or us
3、ing products, processes, or procedures not included in this report.Nothing contained in this NACE International report is to be construed as granting any right, by implication orotherwise, to manufacture, sell, or use in connection with any method, apparatus, or product covered by LettersPatent, or
4、as indemnifying or protecting anyone against liability for infringement of Letters Patent. This reportshould 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. Unp
5、redictable circumstances maynegate the usefulness of this report in specific instances. NACE International assumes no responsibility for theinterpretation or use of this report by other parties.Users of this NACE International report are responsible for reviewing appropriate health, safety,environme
6、ntal, and regulatory documents and for determining their applicability in relation to this report prior to itsuse. 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 opera
7、tions detailed or referred towithin 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 existi
8、ng applicable regulatory requirements prior to the use of this report.CAUTIONARY NOTICE : The user is cautioned to obtain the latest edition of this report. NACE Internationalreports are subject to periodic review, and may be revised or withdrawn at any time without prior notice. NACEreports are aut
9、omatically withdrawn if more than 10 years old. Purchasers of NACE International reports mayreceive current information on all NACE International publications by contacting the NACE InternationalMembership Services Department, 1440 South Creek Drive, Houston, Texas 77084-4906 (telephone +1281 228-62
10、00).ForewordThe purpose of this technical committee report is topresent state-of-the-art information on electrochemicalchloride extraction (ECE) from conventionally reinforcedconcrete surfaces. Included are discussions of commonindustry practices used by design engineers to controlcorrosion of reinf
11、orcing steel in portland cement concretestructures through the application of ECE.This report is intended for use by engineers attempting toprotect corroding reinforced concrete structures by use ofelectrochemical treatment techniques. The informationpresented in this report is limited to ECE for at
12、mospher-ically exposed reinforced concrete and is not applicable toprestressed or post-tensioned elements or concrete con-taining epoxy-coated reinforcing steel, galvanized, orother coated or nonferrous reinforcement.This report, focusing on electrochemical chloride extrac-tion, is Part I of a two-p
13、art series. Part II is intended tofocus on the realkalization of carbonated concrete struc-tures.This technical committee report was prepared by TaskGroup 054 on Electrochemical Chloride Extraction andRealkalization of Reinforced Concrete. It is issued byNACE International under the auspices of admi
14、nistrativeSpecific Technology Group 01 on Concrete and Rebar. _*Chairman John Broomfield, Consulting Corrosion Engineer, London, UK.NACE International2IntroductionReinforced concrete is a versatile and widely usedconstruction material. Its excellent performance and dur-ability rely on the compatibil
15、ity of the steel with theconcrete surrounding it and the ability of the concrete toprotect the steel from corrosion in most circumstances.Unfortunately, corrosion protection is not guaranteed, andcan fail if sufficient chlorides (usually in the form of seasalt, deicing salt, or chloride contaminatio
16、n of the originalmix) or atmospheric carbon dioxide (CO 2 ) penetrate theconcrete and break down the passive layer that protectsthe steel. This breakdown of the passive oxide layerleads to corrosion of the reinforcing steel if sufficientoxygen and water are available.Regardless of the cause of depas
17、sivation (chlorides orcarbonation), corrosion occurs by the movement of elec-trical charge from an anode (a positively charged area ofsteel where steel is dissolving) to the cathode (a nega -tively charged area of steel where a charge-balancingreaction occurs, turning oxygen and water into hydroxyli
18、ons). This means that the process is both electrical andchemical, i.e., electrochemical. In the case of chlorideattack, patch repairs are only a local solution to corro-sion, and repairing an anode can accelerate corrosion inadjoining areas.One solution to this problem involves applying an electro-c
19、hemical treatment that suppresses corrosion. Figure 1shows the basic components of an electrochemical treat-ment system. The components are a direct-current (DC)power source and an anode (temporary or permanent)usually distributed across the surface of the concrete.Electrochemical methods work by ap
20、plying an externalanode and passing current from it to the reinforcing steelso that all of the steel becomes a cathode.Figure 1 : Schematic Diagram of Electrochemical Treatment SystemThree electrochemical techniques are used to countercorrosion of steel in concrete. The first of these tech-niques is
21、 cathodic protection. A newer alternative forchloride-contaminated structures is electrochemical chlor-ide extraction (ECE), also known as electrochemicalchloride removal (ECR), or desalination, as the process iscalled in Europe. A method for treating carbonated con-crete has been developed and is g
22、aining rapid accept-ance as a rehabilitation method for carbonation in build-ings and other structures. This is known as realkaliza-tion.Chloride removal was the subject of two major studiesconducted under Federal Highway Administration ( 1)(FHWA) contracts in the 1970s. 1,2 Both of these studies,as
23、 well as follow-up reports, concluded that chlorideremoved by electrochemical migration is a promisingtechnique for use on salt-contaminated concrete._(!) Federal Highway Administration (FHWA), 400 7 th St. SW, Washington, DC 20590.NACE International3Further research was undertaken in Norway by a pr
24、ivatecompany, and under the Strategic Highway ResearchProgram ( 2) (SHRP). As a result of that research anumber of patents were published. A list of some of theprincipal U.S. patents directly relating to ECE is given inthe bibliography. The list is not comprehensive and doesnot include patents from
25、other countries. The chloride ion acts as though it is a catalyst to corro-sion, and is not consumed in the corrosion reaction.Chlorides enable corrosion to develop and expand oncethey are present beyond a threshold level at the steelsurface. Because chlorides are negatively charged, theelectrochemi
26、cal process can be used to repel the chlorideion from the steel surface and move it toward an externalanode. The ECE process uses an external anode that isinstalled for the duration of the treatment process. Ahigher electrical current density is applied than that usedfor cathodic protection (see NAC
27、E Standard RP0290 3 oncathodic protection and NACE Standard TM0294 4 fortesting embeddable anodes for cathodic protection ofatmospherically exposed steel-reinforced concrete). Nor-mally, the ECE system runs for a limited time (typicallyfour to eight weeks), and is then dismantled and removedfrom the
28、 structure. No permanent system is installed.TheoryWhen direct current is passed through an electrolyte,anions migrate toward the anode and cations migratetoward the cathode. This is the mechanism by whichdirect current is carried through concrete. All mobile ionspresent migrate under the influence
29、of the electric fieldand therefore carry a portion of the current passed. Inchloride-contaminated concrete, these ions include chlo-ride ions and other ions typically found in concrete, inclu-ding hydroxyl, sodium, potassium, and calcium ions. Theamount of current carried by each ion depends on seve
30、ralfactors, including the total current passed, the mobility ofthe ion, the concentration of the ions present, andpossibly temperature.In the case of ECE, a principal objective is the extractionand removal of chloride ions from the concrete. This hasbeen accomplished by placing an anode and electrol
31、ytetemporarily on the concrete surface. The passage ofdirect current between the reinforced steel and the exter-nal anode has resulted in the transfer of chloride ionstoward the anode and into the electrolyte. At the con-clusion of the treatment process the electrolyte, whichnow contains a portion o
32、f chloride ions, can be removed.The efficiency of chloride extraction can be expressed interms of the chloride transference number, which can becalculated using Equation (1).current totalCl by carried current of amount= t -Cl (1)Operating conditions typically maximize this parameter.Chloride transfe
33、rence numbers in chloride-contaminatedconcrete range from 0.05 to 0.40. 5, 6The passage of direct current through concrete is alsocharacterized by electrochemical reactions that occur atthe anode and at the embedded steel (cathode). Catho-dic reactions, which result in an increase in alkalinity atth
34、e surface of the steel, are considered carefully. Theseinclude the reduction of oxygen and water as shown inReactions (2) and (3).)(OH4 4e + O2H O -22 + (2)(OH2 + H 2e + O2H -2-2 (3)The first of these reactions takes place very slowlybecause the availability of oxygen in concrete is limited.Most of
35、the current entering the steel at current densitiestypical of those used for chloride extraction results in theproduction of hydrogen at the steel surface.The electrochemical reactions that typically take place atan inert anode include the oxidation of water and chlorideions as shown in Reactions (4
36、) and (5).-+22 4e + 4H + O O2H (4)-2e + Cl -Cl2 2 (5)If the electrolyte is allowed to become very acidic (pH 7) and the evolution of oxygen becomes the favoredanodic reaction. The small amount of chlorine that canbe evolved under this condition is rapidly hydrolyzed tohypochlorous acid and hypochlor
37、ite ion shown byReactions (6) and (7).+-22 H+ Cl + HClO OH + Cl (6)+- H + ClO HClO (7)_(2) Strategic Highway Research Program (SHRP), National Research Council, National Academy of Sciences, Box 289, Washington, DC 20055.NACE International4All of the reactions that occur at an inert anode produce aq
38、uantitative amount of acid (H +) according to FaradaysLaw. If an anode (such as steel) that corrodes during theprocess is used, a significant amount of the current canbe consumed by anodic dissolution of the metal, a lesseramount of acid (if any) is produced, and chlorine gas isnot generated.Both th
39、e extraction of ions and the electrochemical reac-tions that occur cause concentration gradients to developbecause ionic diffusion, which tends to restore a uniformconcentration, is slow in concrete. These concentrationgradients, which can persist for years, have a significanteffect on the long-term
40、 effectiveness of the treatment.Chloride ions can still be present in the bulk of theconcrete, but these ions are typically greatly depletednear the surface of the steel. This depletion of chlorideions, together with the increase in alkalinity from cathodicreactions, creates passive conditions that
41、effectively miti-gate corrosion. Such effects are generally consideredmore important than the simple removal of chloride fromthe concrete, because the real objective of this treatmentis to prevent or mitigate the corrosion of embedded steel.PracticeA. Tests and Evaluations Prior to TreatmentTesting
42、of Cores from Candidate StructuresThe application of ECE is unique for each concrete struc-ture. The total charge used (and therefore the duration ofthe process) depends on the amount and distribution ofchloride present in the concrete. The current densityused depends on the resistivity of the concr
43、ete and depth-of-cover over the rebars. The ECE process can aggra-vate the reaction of alkali-sensitive aggregate if inapprop-riate electrolytes are used. For these reasons, laboratorytreatment of cores removed from the candidate structureis often completed before site work. These laboratorytests ar
44、e typically considered optional, but they help topredict in advance the current density, total charge, treat-ment time, and efficiency for chloride extraction. The lab-oratory process has been conducted with and withoutlithium ion if alkali-sensitive aggregate has been known tobe present.Typical Cri
45、teria for Cores Submitted for ECE Testing1. Duplicate cores are typically submitted for eachstructure or for each 10 ,000 ft 2 (900 m 2 ) of treatmentarea if variations within the structure are anticipated.2. Cores typically include reinforcing steel.3. Cores are typically at least 4 in. (100 mm) in
46、diameter.4. Cores are typically representative of the struc-ture in chloride content and depth-of-cover to thereinforcing steel.Laboratory ProceduresThe sides of each core are typically sealed and an elec-trolyte reservoir created on the top surface of the core. Choice of anode and electrolyte is ge
47、nerally consistentwith that intended for use on the candidate structure.Monitoring of current, voltage, and total charge allowsestimation of treatment time and efficiency of extraction.Samples for chloride analyses are normally taken frompreselected levels of each core before and after the test.If a
48、lkali-silica-reactive (ASR) aggregate is present, theextent and nature of the alkali-silica reaction in the testspecimen are often analyzed before and after ECE usingstandard petrographic procedures found in ASTM ( 3) C856. 7 A gel fluorescence procedure found in SHRP-C-315 8 is also often used. Bas
49、ed on these results the needfor ASR mitigation can then be assessed.Results of ECE testing include: expected current density, recommended total charge, expected time of treatment, chloride removal efficiency, effect on alkali-sensitive aggregate, and effectiveness of lithium ion in mitigating ASR (iflithium is used in the electrolyte).B. System Selection and InstallationIn order to achieve the full potential of an ECE project,consideration and planning go into the design and selec-tion of
