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Journal of Constructional Steel Research

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Assessment of stainless steel reinforcement for concrete structures rehabilitationJ.T. Pérez-Quiroz a,b,∗, J. Terán a, M.J. Herrera a, M. Martínez a, J. Genescá b

a Instituto Mexicano del Transporte Km 12+000, Sanfandila, Pedro Escobedo, Querétaro, CP 76700, Mexicob Depto. Ingenieria Metalurgica, Facultad Quimica, Universidad Nacional Autónoma de México, UNAM, Ciudad Universitaria, CP 04510, México, D.F., Mexico

a r t i c l e i n f o

Article history:Received 25 November 2007Accepted 15 July 2008

Keywords:Carbon steelCorrosion potentialGalvanic corrosionGalvanic couplingReinforcing barStainless steel

a b s t r a c t

The aim of this study was to revise the factors influencing the service life of galvanic coupling betweencarbon steel and stainless steel reinforcements in simulated concrete pore solution, simulating thecondition of a damaged structure repaired with stainless steel reinforcing bars. Numerous investigationshave reported that austenitic stainless steel rebar, compared to carbon steel, when embedded in concrete,offer superior corrosion resistance in aggressive environments, especially chloride contaminatedconcrete. In concrete, contact with other metals should be avoided because of the risk of galvaniccorrosion. When passive, both carbon steel and stainless steel have comparable corrosion potentials andthe coupling of the two materials is of little effect on the corrosion behavior of either material. Galvaniccurrent values measured between carbon and stainless steel are negligible.

© 2008 Elsevier Ltd. All rights reserved.

1. Introduction

Corrosion is the main cause of deterioration of traditionalcarbon-steel-reinforced concrete structures. Due to the highmaintenance costs involved in infrastructure preservation, astrategy of designing concrete structures with improved corrosionresistance in aggressive environments is highly desirable.

Theoretically, carbon steel rebar is immune to corrosion whenembedded in concrete due the high alkalinity created by theconcrete environment. Unfortunately, this is not always the casedue to surrounding environmental conditions. Typically, passivityof the embedded rebar is lost due to sufficient accumulationof chloride ions and dissolved oxygen introduced from thesurrounding environment. These conditions are encountered intropical environments due to exposure to seawater and areaswhere deicing salts are used heavily to prevent adverse drivingconditions in the winter. The critical chloride concentration thatenables corrosion initiation is known as the chloride threshold andis typically expressed as weight per volume of concrete (kg/m3) orchloride to hydroxide ion ratio (Cl−/OH−) in pore solution [1].

Corrosion of conventional carbon steel rebar in reinforcedconcrete has become a major concern for the Federal HighwayAdministration, FHWA, due to the resulting decrease in lifetime ofconcrete structures and cost associatedwith repair [2]. Repair costsassociated with the corrosion of reinforcing steel are estimatedat over $8 billion [2]. In this study, carried out between 1999

∗ Corresponding author at: Instituto Mexicano del Transporte Km 12+000,Sanfandila, Pedro Escobedo, Querétaro, CP 76700, Mexico. Tel.: +52 442 2169777.

E-mail address: jtperez@imt.mx (J.T. Pérez-Quiroz).

and 2001 and published in 2002 [2], the annual direct cost ofcorrosion for infrastructure in the category of highway bridges canbe estimated in the order of $8.3 billion, consisting of $3.8 billionto replace structurally deficient bridges over the next ten years,$2.0 billion for maintenance and cost of capital for concrete bridgedecks, $2.0 billion for maintenance and cost of capital for concretesubstructures (minus decks), and $0.5 billion for maintenancepainting of steel bridges.

Mexico is no exception to this world problem. An evaluationperformed by the Mexican Institute of Transportation [3] esti-mated that there are 10,000 bridges in Mexico. Of this total ofbridges, Fig. 1,

(a) 330 bridges require a detailed inspection for their susceptibil-ity to present corrosion damage,

(b) 65 bridges require urgent and immediate inspection,

Life-cycle analysis estimates indirect costs to the user due to trafficdelays and lost productivity at more than ten times the direct costof corrosion maintenance, repair and rehabilitation.

Located in Puerto Progreso port, in the Yucatan Peninsula,Mexico, a dock constructed between 1937 and 1941 is aninteresting case to study due to the fact that 304 stainless steelwas used as reinforcing bars. Until now no rehabilitation has beennecessary, contrasting strongly with another dock constructed in1960 in the same place, but with carbon steel reinforcing bars,which is completely destroyed, Fig. 2 [4].

One example of the costs related with maintenance, repair andrehabilitation of a Mexican bridge is presented in Table 1. Thisbridge opened in 1982. The cost associated with its maintenance,repair and rehabilitation was, at the end of 2004, of the orderof approximately $5 million. From Table 1 it is clear that after

0143-974X/$ – see front matter© 2008 Elsevier Ltd. All rights reserved.doi:10.1016/j.jcsr.2008.07.024

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Fig. 1. Mexican bridges localization. Corrosion by carbonation (left) and corrosion by chlorides (right).

Fig. 2. View of the dock constructed with stainless steel (right) and carbon steel(left) [4].

only 4 years of operation, repairs were required costing 8 millionMexican pesos (approximately $0.8 million). This bridge wasdesigned in principle for a life-cycle of 50 years. Since totalinvestment in the nation’s highway infrastructure amounts tomillions of dollars, it is extremely important that all possiblemethods applicable to controlling corrosion in existing concretebridges be developed so that these structures do not deteriorateprematurely.

Different technical practices have been developed in recentyears to solve problems that could affect both old and newstructures subjected to corrosion damage. Some of these are:

• Cathodic protection of rehabilitated structures.• Rehabilitation of reinforced carbon steel structures + unpro-

tected.• Rehabilitation of reinforced carbon steel structures+ corrosion

inhibitors.• Rehabilitation of reinforced carbon steel structures + coatings.

Some of these technical solutions involve changes in thelocal concrete environment surrounding the reinforcing steelbar. From a corrosion engineering point of view, the bestsolution is to chose the correct material for the reinforcing bar.A material with inherent corrosion resistance in the workingmedia seems to be an intelligent engineering solution. In thissense, stainless steel reinforcing bars (rebar) seems to be aninteresting alternative. Stainless rebar can provide long-termcorrosion resistancewhen concrete is exposed to aggressivemedia,such as chloride containing environments. Stainless steels havebeen used successfully as rebar to minimize the problems ofreinforcement corrosion in many structures in the last 20 years,especially in highway bridges.

Some Mexican institutions organized extensive corrosiontesting [5] to find rebar materials that could extent the lifetime of

reinforced concrete bridges to 75–100 years, when the concretewas contaminated with chlorides. Studies carried out in Mexico[6] demonstrate the influence of chloride environment on rebarcorrosion. Carpio et al. [6] found chloride concentration in therange 0.4%–2.9% byweight. At thementioned Progreso dock [7] thechloride concentration at a depth of 7.8 cmwas of the order of 1.2%while in the area surrounding the rebars was of 0.6–0.8 wt% [8].

In recent years, several research projects have been conductedto compare corrosion properties of stainless steel and stainlesssteel clad reinforcement to conventional steel. Limited investiga-tions have been published, but the results and conclusions are con-troversial. Some reports on the use of stainless steel as reinforcingbars in concrete structures are summarized below.

Treadaway et al. [9] carried out some studies with stainlesssteel rebars (304 and 316) in chloride contaminated concrete ofmediumandhigh porosity. After 10 years of exposure, no corrosionwas observed when the chloride content is as high as 1%. This Cl−content is similar to that reported by Carpio et al. [6] and Castroet al. [8] in Mexico.

Sorensen et al. [10] compared the corrosion performance oftype 304 and 316 stainless steels with that of conventional steel.The electrochemical investigation found that the Cl− contentthreshold for corrosion to occur for reinforcing bars embedded inmortar with admixed chloride was more that 10 times higher forstainless than for conventional steel. The critical chloride contentby weight of cement was less than 0.5 percent of admixed chloridefor the conventional steel, while the critical chloride content forgrade 304 stainless steel was 5%–8% and greater than 8% for type316 stainless steel.

The major drawback regarding employing stainless steel rebarin construction applications is the increased cost over carbonsteel. The projected increase in materials costs of stainless steelrebar is approximately 4–8 times greater than carbon steel rebardepending on stainless steel grade [11]. However, the projectedlifetime costs of using stainless steel are likely to be lower thancarbon steel rebar, due to the extended lifetime of the structurewithout the need for costly repair and rehabilitation projects [11].However, the actual degree of extension of lifetimes must beestablished and baseline properties must be established in orderto do this.

In 1998, McDonald et al. [12] reported the results of a five-yearstudy on the corrosion performance of solid 304 and 316 stainlesssteels. In that study, the lowest corrosion rates of the 304 stainlesssteel bars were obtained when the stainless bars acted as boththe anode and cathode. The test results indicated that the 304stainless steel bars were about 1500 times less corrosive than

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Table 1Mexico bridge’s rehabilitation inversion

Year Repair cost, Mexican pesos Repair cost, dollars

Operation begins 1982 N/R N/RNov. 1986–Dec. 1988 $8,515,800.00 $788,500.00May 1993–May 1994 $13,999,701.00 $1,296,268.611996 $2,043,850.97 $189,245.461999–2000 $8,700,000.00 $805,555.562002 $16,713,533.00 $1,547,549.352003–2004 $6,751,190.32 $625,110.21

Total $56,724,075.39 Total $5,252,229.20

Fig. 3. Corrosion damage in the piles of a concrete bridge.

the conventional bar specimens. However, when the stainlesssteel cathode was replaced with a conventional steel cathode,five out of ten of the 304 stainless bars exhibited moderate tohigh corrosion currents. A visual inspection showed that two ofthe stainless steel bars that had conventional steel cathodes andwere near a precracked location of the concrete had moderatered rust corrosion. No corrosion staining was observed whenstainless cathodes were used. The 316 bars had about 800 timesless corrosion than conventional steel, and were little affected bygalvanic effects when used in conjunction with conventional barsas cathodes.

Bertolini et al. [13–15] concluded from their experiments onconcrete specimens that the use of stainless steel in combinationwith carbon steel did not increase the risk of corrosion of passivecarbon steel. Galvanic coupling with stainless steel can increasethe corrosion rate of active carbon steel reinforcement in chloride-contaminated concrete, but the effect is no worse than in thecoupling with passive carbon steel. Knudsen et al. [16,17] andKlinghoffer et al. [18] suggested that the use of carbon steel withstainless steel does not increase the risk of corrosion for the carbonsteel, as long as both metals are in a passive condition. Cochrane[19] reached a similar conclusion. Hope concluded in his study [20]that high and potentially damaging corrosion rates would developin galvanically coupled carbon steel and stainless steel grades 316or 2205 if the concrete surrounding the carbon steel becomeschloride-contaminated or carbonated. These corrosion rates arelikely to be similar to, or somewhat less than, the corrosion rates ifcarbon steel alone were used.

Therefore, the galvanic coupling between passive stainless steeland active carbon steel would appear to be less troublesomethan the coupling between passive carbon steel and active carbonsteel in ordinary reinforcing applications. However, the cathodicefficiency of stainless steel was found to depend strongly on itssurface condition [21].

However,Webster [22] addressed this problemand determinedthat corrosion could take place if two different metals wereelectrically connected. He also suggested that it would benecessary to isolate the electron transfer path between the anodeand the cathode to prevent corrosion damage due to galvaniccoupling. Seibert [23] stated that coupling carbon steel with

stainless steel reinforcements is not recommended because thisgalvanic coupling would initiate corrosion on the carbon steel.

Judicious use of stainless steel rebars in both new constructionand rehabilitation of older structures seems to be a viable,cost-effective option for extending service life and reducingmaintenance costs.

The goal of this work is to evaluate the possible use of 304stainless steel bars for rehabilitation of damaged Mexican bridges,taking into account the possibility of increased corrosion due togalvanic effects.

This paper presents an investigation of the galvanic couplingbehavior between carbon steel and 304 type stainless steel alloys.Testswere performed in electrochemical cells containing saturatedcalcium hydroxide Ca(OH)2 simulating concrete pore solution.

Fig. 3 shows the typical damage produced by corrosion in abridge. Immediately after construction the concrete of some pilingexhibited cracks up to 4m in length above the sea level (tidal level)and more than 1 cm in width in some cases, which necessitatedmajor repairs. The repair method consisted of filling the crackswith a resin and encasing the piles within a steel mesh reinforcedcement jacket in moulds of fiberglass. Then, by capillarity waterrising, the repaired part of the columns produced corrosion as aresult of the oxygen concentration cells formed in a very humidchloride environment. Probably this repairing method was not themost adequate.

The repair proposal, Fig. 4, according to technical guidelines [24]proposed by International Concrete Repair Institute, ICRI, involvedthe removal of the damaged old concrete as well as the reinforcedbars and replacement with new stainless steel reinforcing barsembedded in new concrete. To simulate this repair procedure,experiments were carried out in a saturated calcium hydroxidesolution used to simulate the concrete pore solution environment.As a first step the chloride permeation is not considered.

2. Experimental

Carbon steel and stainless steel rod samples of 0.5 × 0.5 ×

1 cm dimensions were embedded in epoxy resin. Each metallicspecimen was welded to a copper wire for potential measurement

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Fig. 4. Schematic of the repairing proposal with stainless steel rebar of a concretestructure subjected to corrosion.

purposes. Experiments were conducted by triplicate in a plasticcell, Fig. 5, filled with an aqueous solution of saturated Ca(OH)2to simulate the concrete environment (simulated concrete poresolution). This solution simulates the liquid substance found in thesaturated pores and capillaries of concrete. Synthetic pore solutiontests have been used elsewhere for simulating the concreteenvironment. The pH of the solution is 12.5 approximately.

Metallic surfaces were polished with 180, 220, 320, 600 and1000 grades of emery paper. After polishing, samples were driedin hot air and placed in a dessicator until use. The stainless steelused in this work was AISI 304 type (see Table 2 for nominalcomposition)

Open circuit potential (corrosion potential) measurementswere performed with a Fluke 867B multimeter using a saturatedcalomel electrode (SCE) as reference electrode. To measurecorrosion potential, the positive lead of the voltmeter is connectedto a carbon or stainless steel bar. The negative lead of the voltmeteris connected to the SCE. The voltage reading on the voltmeter isthe corrosion potential of the steel with respect to the SCE. pHmeasurement was carried out periodically to detect any changesof this parameter in the saturated Ca(OH)2 solution.

Those that have studied and researched the effects of macrocellcorrosion have been led to believe that the ratio of the cathodearea (stainless steel) to the anode area (carbon steel) greatly affectsthe rate of corrosion. The greater the ratio of the cathode area to

the anode area, the greater the contribution of the macrocell tothat of corrosion. To take into account this possible effect, anotherexperimental arrangement consisted of threaded carbon steel andstainless steel rods immersed in a saturated Ca(OH)2 solution.Rectangular cells were used, Fig. 6, and different cathode/anodesurface areas between stainless steel and carbon steel weretested to be able to determine the effect of cathode-to-anodebar area ratio on corrosion potential and galvanic current. Thesearrangements were proposed only for experimental reasons and inorder to take into account the effect of area ratio and avoid someeffect due to corrosion crevice during the testing; of course, it is notadequate for field application. The joining method normally usedin the field consists of welding or joining carbon steel and stainlesssteel reinforcing bars with stainless steel connectors.

3. Experimental results and discussion

3.1. Potential test procedure

Electrochemical potential measurements of steel with respectto a standard reference electrode indicate the corrosion ‘‘state’’ ofthe steel. The absolute potential, or energy, of a metal cannot bemeasured directly. However, the potential difference between twochemical reactions can be measured directly. Reference electrodescontain materials that undergo known chemical reactions.

The chemical reaction that has been chosen to have zeropotential with respect to all other chemical reactions is 2H+

+

2e−↔ H2, which occurs in the standard hydrogen electrode

(SHE). The copper/copper sulfate electrode (CSE) is widely used tomeasure the potential of steel in bridge decks. ASTM C 876 [25] isthe standard test method for determining the corrosion potentialof uncoated bars in concrete using the CSE [25].

The corrosion state of steel can be classified as passive, active, orindeterminate, depending on the difference in potential betweenthe steel and reference electrode. The potential difference isdetermined by setting up an electrochemical cell between the steeland a reference electrode, andmeasuring the voltage drop betweenthe electrodes using a voltmeter.

The potential test measures the corrosion potential (open circuitpotential) of the specimen exposed to an aqueous simulatedconcrete pore solution. Potential readings are recorded on a dailybasis for 90 days. The measurements are taken with respect to asaturated calomel electrode (SCE).

The ranges of potential values that signify a high or lowcorrosion risk for steel in concrete have been determined inprevious studies [26–28]. A potential range between +100 and

Table 2Nominal chemical composition of tested carbon steel and stainless steel alloys (wt%)

Type/composition % C % Cr % Ni % Mn % S % Si % Fe

AISI 304 Austenitic stainless steel 0.05–0.052 18.40–19.2 8.31–8.5 0.78–1.8 0.016–0.03 0.21–0.90 BalanceAISI 1018 carbon steel 0.15–0.20 – – 0.6–0.9 0.050 – Balance

Fig. 5. Cell design, stainless and carbon steel probes.

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Fig. 6. Experimental arrangement of electrochemical cells with different area.

−200 mV (versus SCE) indicates that the steel is in a non-corroding, passive state. Corrosion potentials between −200 and−500 mV (versus SCE) imply localized corrosion is occurring, andpotentials between −450 and −600 mV (versus SCE) indicate agood probability that general corrosion is underway.

ASTMC 876 [25] states that a corrosion potential more negativethan −350 mV versus CSE (−273 mV versus SCE) has a greaterthan 90% probability that reinforcing steel corrosion is occurringin that area at the time of measurement. If the potential is morepositive than −200 mV versus CSE (−123 mV versus SCE), there isa greater than 90% probability that no reinforcing steel corrosionis occurring in that area at the time of measurement. For valuesbetween−200 and−350mV, the corrosion activity is uncertain. Ingeneral, reinforcing steel is considered passive at potentials morepositive than −200 mV (versus SCE).

Sistonen et al. [29] stated that austenitic stainless steelrebar performed very well in chloride-contaminated concrete.No corrosion was detected when observed visually. The halfcell and polarization resistance measurements supported theseobservations. Outdoor concrete structures under severe conditionsreinforced with austenitic stainless steel rebar will have aconsiderable longer service life than structures reinforced withany other steel rebar. Approximately half-cell potential limits fordifferent rebar materials measured with copper/copper sulfatehalf-cell (reference electrode), CSE, were proposed on the basis oftheir corrosion tests (experiments), Table 3.

A variety of electrochemical techniques are now being widelyused for monitoring the corrosion damage of structures. However,the advantages and disadvantages of each respective method havenot led to an unambiguous test method. For example, open-circuit potential measurements provide no information on thekinetics of the corrosion process, but this method provides a fastmeasurement to identify if corrosion is present.

The corrosion potential variation with time in a givenenvironment is the first concern when evaluating the galvaniceffect. Fig. 7 shows the variation of the open-circuit potentialof carbon steel, CS and stainless steel, SS, as a function of time.The potential data reveal what was expected with respect to theexistence of galvanic couples. The potential of SS stabilized atvalues comprised between −100 and −250 mV (SCE), CS andgalvanic couple CS/SS at −250 mV (SCE) approximately and insome cases at a more negative values.

Corrosion potential of carbon steel, stainless steel and galvaniccouple as a function of immersion time was shown in Fig. 7. Thetest period was of 90 days, as a first step, but the study continues(March 2008). In all the cases, corrosion potential shows a trendto steady-state values after 60 days of immersion. Stainless steelshows the more positive potential values, while carbon steel andgalvanic couple show a similar behavior. More negative potentialvalues were obtained for a carbon steel sample as well as CS/SSgalvanic couple.

Fig. 7(a) presents the results of the corrosion potential testsusing bare conventional carbon steel specimens (CS1, CS2 andCS3). Potentials are measured with respect to a saturated calomelreference electrode (SCE). Within the first 10 days of testing, the

Fig. 7. Corrosion potential, Ecorr , values of stainless steel, carbon steel and galvanicpair in Ca(OH)2 solution at room temperature.

potentials of all three specimens drop below −0.5 V, indicating apotential for rapid corrosion. After 30 days, the potentials becomemore positive, ranging between −0.25 and −0.45 V (SCE) beforereturning below −0.45 to −0.5 V (SCE).

The corrosion potential test results for stainless steel barspecimens (SS1, SS2 and SS3) are shown in Fig. 7(b). One of thethree specimens exhibits potentials below−0.2 V (between−0.25and −0.30 V (SCE)), signifying that the bars have a tendency tocorrode. The two specimens that maintained a potential above−0.2 V did not show any signs of corrosion. Stainless steel barspecimens (SS1 and SS2) have potentials more positive than−0.2 V, indicating that the stainless steel is passive (Fig. 7(b)).

When two different metals are in contact within concrete, cor-rosion can occur because each metal has a unique electrochemical

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Table 3Half-cell potential limits when using ‘‘black steel’’ and austenitic stainless steel rebar vs copper/copper sulfate half-cell, CSE, mV (After Sistonen et al. [29])

Corrosion condition ‘‘Black steel’’ rebara Austenitic stainless steel rebarmV vs CSE mV vs CSE

Chloride-contaminated concrete Chloride-contaminated concreteLow risk of corrosion, about 10% >−200 >−100Intermediate corrosion risk −200 to −350 −100 to −250High risk of corrosion, <90% −350 to −500 −250 to −400Severe corrosion risk <−500 <−400

a ASTM C876.

Fig. 8. Galvanic current of the couple stainless steel/carbon steel in Ca(OH)2 solution at room temperature.

potential. Galvanic probe corrosion monitors are simple, ruggedinstruments constructed of two dissimilar metals that are sub-merged in the electrolyte to be studied. The direct current gener-ated by the dissimilar metals is a function of the corrosiveness ofthe electrolyte in which the probe is submerged.

To define the initiation of corrosion on the different bars, themacrocell (galvanic) current flowing between bars for the threedifferent area ratio studiedwasmeasured daily from the beginningof this ongoing investigation. This measurement was made using azero voltage ammeter, ZRA, which was connected between eachcarbon steel bar and stainless steel bar and then the positiveterminal of ZRA was connected to the carbon steel bar while thenegative terminal was connected to the stainless steel bars. Inthis manner, a negative galvanic current was determined, i.e., thecarbon steel bars would be anodic in comparison to the stainlesssteel bars.

Fig. 8 shows the macrocell (galvanic coupling) currents forthe three experimental sets of carbon steel/stainless steel barscontaining 304 bars, which are known for their excellent corrosionresistance. For those with the higher cathode bar (SS) to anodebar (CS) area ratio, SS/CS = 1/7, their daily macrocell currents,even after 90 days of simulated concrete pore solution exposure,were still practically indiscernible or insignificant, averaging only0.0 µA at a standard deviation of ±0.2 µA during that period,which indicates that the SS bars were still passive.

Fig. 8 shows that after approximately 20 days, the specimenswith a combination of 304 stainless steel bars and carbon steelbars with an area ratio SS/CS = 1/1 began to exhibit negativemacrocell currents. This indicated that carbon steel bars becamemore anodic than the 304 bars and were, therefore, corroding. Asindicated in Fig. 8, the macrocell currents for these two subsets ofSS/CS arrangements reached as much as −3.0 to −4.0 µA. Thesecurrent values are similar to found by Qian et al. [30]. Firstly itis important to note the lack of any significant macrocell currentin the SS/CS set containing 304 SS bars, even after 90 days of thesimulated concrete pore exposure regime. This implies that allthe bars had, so far, exhibited the same good resistance to attack

by simulated concrete pore solution, (SCPS) as the solid 304 barshad. Secondly, the appearances of undesirable negative macrocellcurrents (after 20 to 30 days) in the SCPSwherein carbon steel barswere used.

Such negative macrocell currents could mean that the carbonsteel bars were already corroding before the apparently morecorrosion-resistant bars of SSwere. But it is interesting to note thatthese bars repassivate after 20–30 days, with macrocell currentsof approximately 0.2 µA. This level of macrocell currents can beconsidered insignificant, since the accuracy of the measurementwas estimated to be ±0.2 µA. When passive, both stainless steeland carbon steel have comparable corrosion potentials and thecoupling of the two materials is of little effect on the corrosionbehavior of either material.

4. Galvanic coupling test

Galvanic corrosion occurs when two (or more) dissimilarmetals are electrically connected and exposed to an electrolyte(electrically conductive solution). The potentials of the twometals,after connection, are forced to shift to a common potential, thegalvanic corrosion potential, Egc . The metal with an initially morenegative potential (corroding carbon steel) is subjected to anoxidation (anodic process) process, since it is polarized towardthe positive direction. The more noble metal (stainless steel) withan initially more positive potential is polarized to the negativedirection and subjected to a reduction (cathodic) process. Theelectrons transfer from the active metal (anode) to the noble one(cathode).

The galvanic behavior of carbon steel coupled with passivestainless steel in aqueous concrete pore solution in the absenceof chlorides was investigated. The specimens were kept in anenvironmental chamber with a temperature around 25 ◦C. Fig. 9show the galvanic coupling potential in the absence of chlorideions. Both, carbon steel and stainless steel specimens wereimmersed in the same solution. Before coupling the two rebars, theopen circuit potential of the carbon steel was more negative than

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Fig. 9. Effect of cathode/anode area ratio on galvanic potential.

that of the stainless steel as can be shown in Fig. 7(a) and (b). It canbe noted that for coupled galvanic CS/SS, Fig. 9, the open-circuitpotentialwas less negative (more positive) than that of the CS3 (seeFig. 7(a)) and CS/SS3 case, Fig. 7(c). Since the carbon steel wasmoreactive in the concrete pore solution, it was very likely in an activecorrosion condition due. After the two rebars were connected, thecoupling potential was about −0.20 to −0.35 V (SCE) over 30 days(see Fig. 9). This trend can be associated to the anodic behavior ofcarbon steel.

The effect of SS/CS area ratio is not clear, but it seems that thecoupling potential shifted to more negative values (about −0.25 Vto −0.35 V) for SS/CS = 1/7.

5. Conclusions

The following observations and conclusions are based on thetest results presented in this work.

The results of the corrosion potential and macrocell (galvanic)corrosion tests indicate that 304 stainless steel bars havesignificant corrosion resistance in simulated concrete pore solutionmaking the reinforcement a good candidate for use in concretestructures.

Based on the above investigation, it can be concluded thatuse of stainless and carbon steel reinforcing bars in the sameconcrete pore solution will not increase the corrosion risk oncarbon steel, evenwhen these bars are in direct (electrical) contact.In fact, the increase in the corrosion rate of carbon steel dueto galvanic coupling of stainless steel with corroding carbonsteel was less than that of the combination of non-corrodedcarbon steel with corroding carbon steel. Therefore, the judicioususe of stainless steel with carbon steel in the high-corrosion-risk areas of a concrete structure can be a cost-effective optionfor reducing corrosion and greatly extending the service life ofconcrete structures.

Acknowledgement

One of the authors, J.T.P.Q. gratefully acknowledge CONACYT(Mexico) for a PhD grant.

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