Improvement on the Repair Effect of Electrochemical Chloride ... - MDPI

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Improvement on the Repair Effect of Electrochemical Chloride Extraction Using a Modified Electrode Configuration Wei Feng, Jinxia Xu *, Linhua Jiang, Yingbin Song, Yalong Cao and Qiping Tan College of Mechanics and Materials, Hohai University, Nanjing 210098, China; [email protected] (W.F.); [email protected] (L.J.); [email protected] (Y.S.); [email protected] (Y.C.); [email protected] (Q.T.) * Correspondence: [email protected]; Tel.: +86-25-8378-6046; Fax: +86-25-8378-6046 Received: 7 January 2018; Accepted: 30 January 2018; Published: 1 February 2018

Abstract: To improve the repair effect of electrochemical chloride extraction, a modified electrode configuration is applied in this investigation. In this configuration, two auxiliary electrodes placed in the anodic and cathodic electrolytes were used as the anode and cathode, respectively. Besides this, the steel in the mortar was grounded to protect it from corrosion. By a comparative experiment, the potential evolution, various ions concentrations (Cl− , OH− , Na+ , and K+ ) in different mortar depths, the corrosion potential, and the current density of the steel were measured. The results indicate that compared to electrochemical chloride extraction with the traditional electrode configuration, this electrochemical chloride extraction method with a modified electrode configuration has a similar chloride removal ratio. Besides this, potential of steel is just about 800 mV for a saturated calomel electrode (SCE) during the treatment, which did not reach the hydrogen evolution potential. The phenomenon of the accumulation of OH− , Na+ , and K+ did not occur when the modified electrode configuration is applied. Additionally, higher corrosion potentials and lower corrosion current rates were measured after performing electrochemical chloride extraction with the modified electrode configuration. Additionally, it is a short period of time for the steel to go from activation to passivation. On this basis, the modified electrode configuration may overcome the drawbacks of electrochemical chloride extraction. Keywords: electrochemical chloride extraction; modified electrode configuration; electrochemical potential; corrosion current density; repassivation

1. Introduction The chloride-induced corrosion of steel is one of the main causes of a reduction in the durability of reinforced concrete structures exposed to a chloride-laden environment [1–3]. To prolong the service life of a chloride-contaminated concrete structure, the technique called “electrochemical chloride removal” or “electrochemical chloride extraction” has been extensively applied [4–6] due to its merits of high efficiency, relatively low cost, and having no need for maintenance. Electrochemical chloride extraction normally requires us to mount an anode at the concrete surface and have the steel reinforcement act as the cathode. A high current density (direct current (DC)) for a few weeks is applied. The applied current moves the chloride ions in the concrete away from the steel bar toward the anode, and thereby the chloride ions are removed from concrete [7]. Accordingly, an electrochemical chloride extraction treatment can remarkably reduce the chloride content near the reinforcement, leading to the reduction of the corrosion current density or the re-passivation of the steel, therefore significantly prolonging the service life of the chloride-contaminated concrete structure.

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Despite this, there are still several defects in the electrochemical chloride extraction method reported in the literature. First, hydrogen gas evolves at the reinforcing steel because of the low cathodic potential that is induced by the high current density. It provides a warning that there is a risk of hydrogen-induced brittle fracture, especially for pre-stressed concrete [8]. Second, during the electrochemical chloride extraction, the reaction at the cathode can be described as follows: 2H2 O + O2 + 4e− → 4OH−

(1)

This reaction can increase the alkalinity near the surface of the steel, which is favorable for protecting the steel from corrosion. However, an alkaline attack on the steel will happen if the pH value is raised to 14 [9]. Third, cations such as Na+ and K+ in the pore solution of the concrete move simultaneously toward the steel cathode under the applied electric current, which induces the accumulation of sodium and potassium at the interface between the steel and concrete. This has been demonstrated to increase the risk of an alkali–silica reaction in concrete containing potentially reactive aggregates [10]. Besides this, the accumulation of sodium and potassium can reduce the bond strength between the steel and the concrete [11,12]. Finally, it has been found that the corrosion current of steel is significantly increased when the power supply is disconnected at the termination of the electrochemical chloride extraction [13,14]. As a result, the above defects have limited the wide application of electrochemical chloride extraction in real concrete structures. According to the electrochemical mechanism of electrochemical chloride extraction, the above defects are mainly attributed to the direct connection of the steel with the cathode of the DC power supply. The reason is due to the fact that this connection induces a cathode reaction on the steel surface, resulting in hydrogen evolution and a significant increase of alkalinity. In addition, the electro-migration of sodium and potassium ions towards the cathode of the steel leads to the accumulation of sodium and potassium at the interface between the steel and the concrete. Based on this, it can be anticipated that if the steel reinforcement is not directly connected to the cathode of the DC power supply and used as the cathode during electrochemical chloride extraction, the drawbacks mentioned above may be defeated. To improve the repair effect of electrochemical chloride extraction, a modified electrode configuration is applied to electrochemical chloride extraction in this study. In this configuration, two auxiliary electrodes positioned in anodic and cathodic electrolytes were used as the anode and cathode, respectively. Besides this, the steel in the mortar was grounded to guard against corrosion. By a comparative experiment, the potential evolution, the content of various ions (Cl− , OH− , Na+ , and K+ ) in different mortar depths and electrolytes, the corrosion potential, and the corrosion current density of the steel were measured to evaluate the effectiveness of electrochemical chloride extraction with the modified electrode configuration. 2. Materials and Methods 2.1. Materials and Specimen Preparation The cement applied in our work was No. 42.5 ordinary Portland cement made in China. Its oxide composition is indicated in Table 1. The fine aggregate was river sand with a fineness modulus of 2.65. A mortar specimen with a water/cement ratio of 0.55 and a sand/cement ratio of 2.5 was prepared. In order to simulate chloride-contaminated concrete, chloride ions (from calcium chloride) with content in 2.0% by mass of cement were added into the mortar specimen. Besides this, the specimen had a size of 40 mm × 40 mm × 160 mm with a centrally embedded steel bar (10 mm in diameter). The surface of the steel was cleaned physically prior to the application. The detailed drawing of the mortar specimens is presented in Figure 1. Moreover, after one day of casting in plastic moulds, all of the paste samples were demolded and then cured in a 95% humidity chamber at 20 ± 2 ◦ C for 28 days.

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Figure 1. Detaileddrawing drawing of specimens. Figure 1. Detailed ofmortar mortar specimens. Table 1. Oxide composition of cement used (% by mass).

Table 1. Oxide composition of cement used (% by mass). Oxide Composition (wt.%)

Oxide

CaO

CaO 57.27

SiO2 24.99

Al2O3 9.32

Fe2O3 3.11

MgO 0.86

K2O 0.83

Na2O 0.28

Total Cl 0.05

drawing of mortar SiO2Figure Al21.ODetailed Fe2 O K2 Ospecimens. Na2 O 3 3 MgO

SO3 1.13

Total Cl

Composition (wt.%) 57.27Treatment 24.99 9.32 3.11 0.86 0.83 0.28 0.05 2.2. Electrochemical Table 1. Oxide composition of cement used (% by mass).

Ignition Loss 2.16 SO3 Ignition

1.13

Loss

2.16

A rectangular electrolytic tank (24 ×16 × 6 cm) from acrylic sheets was separated Oxide CaO SiO 2 Al 2O3cmFe 2O3 cm MgO K2Omade Na2O Total Cl SO 3 Ignition Loss

2.2. Electrochemical Treatment (wt.%) 57.27 24.99 by 9.32 0.86 0.83 mortar 0.28 0.05 1.13 2.16 into Composition two identical compartments placing3.11 the as-fabricated specimen in the middle of the tank. For the aim of conducting electrochemical chloride extraction with the modified electrode

A rectangular electrolytic tank (24 cm × 16 cm × 6 cm) made from acrylic sheets was separated 2.2. Electrochemical Treatment configuration (abbreviated as MEC treatment), a saturated calcium hydroxide solution was readied into two identical compartments by placing the as-fabricated mortar specimen in middle of the for use as cathodic and anodic electrolytes. Platinized titanium were appliedwas as the the anode A rectangular electrolytic tank (24 cm ×16 cm × 6 cm) mademeshes from acrylic sheets separated and cathode. In addition, the steel in the mortar specimen was grounded to be protected against tank. Forinto thetwo aim of conducting electrochemical chloride extraction with the modified identical compartments by placing the as-fabricated mortar specimen in the middle of theelectrode corrosion. Theaim detailed experimental configuration for the calcium MEC treatment indicated in Figure tank. For the of as conducting electrochemical chloride extraction withisthe modified electrode configuration (abbreviated MEC treatment), a saturated hydroxide solution was 2a. readied for In contrast, the steel in the mortar specimen was directly connected to the cathode of the DCreadied power configuration (abbreviated as MEC treatment), a saturated calcium hydroxide solution was use as cathodic and anodic electrolytes. Platinized titanium meshes were applied as the anode and supply during electrochemical chloride extraction the traditional electrode for use as cathodic and anodic electrolytes. Platinizedwith titanium meshes were applied configuration as the anode cathode. In addition, as theTEC steel in the mortar specimen was grounded to bethe protected against corrosion. (abbreviated treatment). The mortar specimen was placed into and cathode. In addition, the steel in the mortar specimen was grounded to be saturated protected calcium against The detailed experimental configuration for the MEC is indicated in Figure 2a. 2a. In hydroxide solution. The detailed experimental configuration fortreatment the TEC is treatment isin indicated in contrast, corrosion. The detailed experimental configuration fortreatment the MEC indicated Figure Figure the steel In incontrast, the2b. mortar specimen wasspecimen directly connected to the tocathode of of the the steel in the mortar was directly connected the cathode theDC DC power power supply supply during electrochemical chloride with extraction with the traditional electrode configuration during electrochemical chloride extraction the traditional electrode configuration (abbreviated (abbreviated as TEC treatment). The mortar specimen was placed into the saturated calcium as TEC treatment). The mortar specimen was placed into the saturated calcium hydroxide solution. hydroxide solution. The detailed experimental configuration for the TEC treatment is indicated in The detailed experimental configuration for the TEC treatment is indicated in Figure 2b. Figure 2b.

(a)

(b)

Figure 2. Detailed experimental configurations for the modified electrode configuration (MEC) treatment (a) and the traditional electrode configuration (TEC) treatment (b). DC: direct current.

(a) (b) were 28 days. A 100-watt The MEC and TEC treatment periods for each electrical field applied DC power supply was used. For the MEC treatment, the positive and negative poles of the power Figure 2. Detailed experimental configurations for the modified electrode configuration (MEC) Figure 2. treatment Detailed experimental forthe theanode modified electrode supply were connected with theconfigurations titanium mesh in and the cathode, configuration respectively. As(MEC) a (a) and the traditional electrode configuration (TEC) treatment (b). DC: direct current. comparison, the TEC treatment, the positive and negative poles of(b). theDC: power supply were treatment (a) andduring the traditional electrode configuration (TEC) treatment direct current. connected withand titanium mesh andperiods steel, respectively. The MEC TEC treatment for each electrical field applied were 28 days. A 100-watt DC power supply was used. For the MEC treatment, the positive and negative poles of the power

The MEC and TEC treatment periods for each electrical field applied were 28 days. A 100-watt DC supply were connected with the titanium mesh in the anode and the cathode, respectively. As a power supply was used. treatment, the positive andpoles negative the power comparison, duringFor the the TECMEC treatment, the positive and negative of the poles powerof supply were supply were connected with titanium in the anode and the cathode, respectively. As a comparison, connected withthe titanium mesh mesh and steel, respectively. during the TEC treatment, the positive and negative poles of the power supply were connected with titanium mesh and steel, respectively. Three different constant current densities (2.0, 4.0, and 8.0 A/m2 , based on the mortar specimen surface) were used. The saturated calcium hydroxide solution was replaced every two days to maintain a stable composition. All of the MEC and TEC treatments were performed at ambient temperature.

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2.3. Measurements 2.3.1. Steel Potential Measurement The electrochemical potential of the steel was monitored periodically during the TEC treatment and the MEC treatment. The saturated calomel reference electrode as the reference electrode was positioned on the surface of the mortar. Then, the electrochemical potential was measured with a recorder connected to a computer. 2.3.2. Cl− , OH− , Na+ , and K+ Contents During the TEC treatment and the MEC treatment, the anodic electrolytic solutions were collected every two days for determining the cumulative amounts of chloride extraction with time. Besides this, the treated specimens were cut into slices from the mortar surface to the steel. Each slice was cut with a 5 mm thickness and powdered until it passed through a sieve of 0.16 mm. Then, 20 g powders were added into 200 mL distilled waters and 200 mL nitric acid solutions so as to determine the free Cl− content (the number of Cl− ions remaining free in the pore solution of the mortar) and the total Cl− content (including free chloride and the chloride ions bound in the cement hydrates), respectively. The mixtures were vigorously stirred for a period, and then left to stand for 24 h. All of the chloride contents were measured by potentiometric titration with 0.01 M silver nitrate (AgNO3 ). Moreover, the suspensions applied to determine the free chloride content were also employed to measure the OH− , Na+ , and K+ contents. Prior to the measurement, the suspensions were filtered. The OH− content of the leachate was measured using a pH-meter. Besides this, the contents of sodium (Na+ ) and potassium (K+ ) ions in the leachates were determined by flame atomic absorption spectrometry. 2.3.3. Corrosion Potential and Corrosion Current Density of the Steel As soon as the TEC treatment and the MEC treatment were finished, the mortar specimens were transferred to an atmospheric environment (20 ◦ C and 60% relative humidity (RH)). The corrosion potential and corrosion current density for the steel reinforcement in the mortar specimen after the TEC treatment and the MEC treatment were continuously monitored for three months by the linear polarization method. For the linear polarization measurement, the Partstat 2273 advanced potentiostat/Galvanostat/frequency response analyser system (AMETEK Corporation, Berwyn, PA, USA) was applied. Besides this, the saturated calomel electrode (SCE) and a platinum electrode had been connected to work as the reference electrode and the auxiliary electrode, respectively. The linear polarization scan was carried out between −10 mV and +10 mV with respect to the rest potential (Ecorr ). The potential was applied at a rate of 0.1 mV/s. The polarization resistance (Rp ) obtained by the linear polarization measurement was introduced into the Stern–Geary equation to calculate the corrosion current density (Icorr ). The Stern–Geary equation is: Icorr /B = Rp

(2)

where B is a constant, which is assumed to be 26 mV (its value has been determined by means of calibration against mass loss measurements) [15,16]. 3. Results 3.1. Change of Steel Potential with Time Figure 3 shows the change of steel potential with time in the mortars during the TEC treatment and the MEC treatment. Prior to the treatments, the potential of the steel was about −500 mV versus the SCE. When the TEC treatment was applied, the potentials of the steel are sharply decreased. The ultimate value of the potential is maintained at −1000 mV versus the SCE for the specimen with

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Figure 3 shows the change of steel potential with time in the mortars during the TEC treatment Figure 3 shows the change of steel potential with time in the mortars during the TEC treatment and the MEC treatment. Prior to the treatments, the potential of the steel was about −500 mV versus Materials 2018, 11, 225 5 of 12 and the MEC treatment. Prior to the treatments, the potential of the steel was about −500 mV versus the SCE. When the TEC treatment was applied, the potentials of the steel are sharply decreased. The the SCE. When the TEC treatment was applied, the potentials of the steel are sharply decreased. The ultimate value of the potential is maintained at −1000 mV versus the SCE for the specimen with a 2.0 ultimate value of the potential is maintained at −1000 mV versus the SCE for the specimen with a 2.0 2 current a 2.022 A/m density. With the increase of applied the absolute value of the A/m current density. With the increase of applied currentcurrent density,density, the absolute value of the steel’s A/m current density. With the increase of applied current density, the absolute value of the steel’s steel’s potential is obviously increased. The lowest ofsteel’s the steel’s potential TEC treatment potential is obviously increased. The lowest valuevalue of the potential for for the the TEC treatment is potential is obviously increased. The lowest value of the steel’s potential for the TEC treatment is is about −1800 versus SCE. Compared theTEC TECtreatment, treatment,the thepotential potentialof ofthe the steel steel for for the about −1800 mVmV versus thethe SCE. Compared totothe about −1800 mV versus the SCE. Compared to the TEC treatment, the potential of the steel for the 800 mV mV versus versus MEC treatment is obviously higher. higher. The The ultimate ultimate value value of of the the steel’s steel’s potential potentialisisonly only− −800 MEC treatment is obviously higher. The ultimate value of the steel’s potential is only −800 mV versus the SCE. In addition, the potential of the steel steel does does not not obviously obviously change with the increase increase of applied the SCE. In addition, the potential of the steel does not obviously change with the increase of applied current density during the MEC treatment. current density during the MEC treatment.

Figure 3. Change of steel potential with time during the TEC treatment and the MEC treatment. SCE: Figure3. 3.Change Change steel potential with during the TEC treatment andMEC the treatment. MEC treatment. Figure of of steel potential with timetime during the TEC treatment and the SCE: saturated calomel electrode. SCE: saturated calomel electrode. saturated calomel electrode.

3.2. Extracted Chloride, Free Chloride, and Total Chloride Contents 3.2.Extracted ExtractedChloride, Chloride,Free FreeChloride, Chloride,and andTotal Total Chloride Chloride Contents 3.2. Variations in the extracted chloride content in the anolyte versus time for the TEC treatment and Variations in in the the extracted extracted chloride chloride content content in in the theanolyte anolyteversus versustime timefor forthe theTEC TECtreatment treatmentand and Variations the MEC treatment are presented in Figure 4. For both the TEC treatment and the MEC treatment, the MEC MEC treatment treatmentare are presented presentedin in Figure Figure 4. 4. For Forboth boththe theTEC TECtreatment treatmentand andthe theMEC MECtreatment, treatment, the most of the chloride contents are extracted during the first 2 weeks of application. As the time of most of ofthe thechloride chloridecontents contentsare areextracted extractedduring duringthe thefirst first2 2weeks weeks application. time most of of application. As As thethe time of treatment is further increased, the extracted chloride content in the anolyte is slowly increased. of treatment is further increased, extracted chloride content theanolyte anolyteisisslowly slowlyincreased. increased. treatment is further increased, thethe extracted chloride content ininthe Compared to the TEC treatment, the chloride content in the anolyte for the MEC treatment is slightly Comparedto tothe theTEC TECtreatment, treatment,the thechloride chloridecontent contentin inthe theanolyte anolytefor forthe theMEC MECtreatment treatmentisisslightly slightly Compared higher when the same current density is applied. The results show that the MEC treatment did not higherwhen whenthe thesame samecurrent currentdensity densityis is applied. results show MEC treatment did higher applied. TheThe results show thatthat the the MEC treatment did not reduce, but increased the chloride removal ratio. A similar result can be also concluded from Figures not reduce, but increased the chloride removal ratio. A similar result can concluded be also concluded from reduce, but increased the chloride removal ratio. A similar result can be also from Figures 5 and 6. 5Figures and 6. 5 and 6.

Figure 4. Cumulative amounts of chloride in the anolyte during the TEC treatment and the MEC Figure amounts of chloride in the during the TEC and the MEC Figure 4.4.Cumulative Cumulative amounts of chloride in anolyte the anolyte during the treatment TEC treatment and the treatment. treatment. MEC treatment.

Figures 5 and 6 show the profiles of free chloride and total chloride in the mortar specimens after the TEC treatment and the MEC treatment, respectively. Besides this, the profiles of free

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Figures Figures 55 and and 66 show show the the profiles profiles of of free free chloride chloride and and total total chloride chloride in in the the mortar mortar specimens specimens after after the the TEC TEC treatment treatment and and the the MEC MEC treatment, treatment, respectively. respectively. Besides Besides this, this, the the profiles profiles of of free free chloride chloride and and chloride and for total chloride for the untreated mortar specimensFigures are indicated6 in Figures 5 and a6 total total chloride chloride for the the untreated untreated mortar mortar specimens specimens are are indicated indicated in in Figures 55 and and 6 in in order order to to make make a in order to make a comparison. It can bethe seen inchloride Figure 5 that thebefore free chloride contents before comparison. comparison. It It can can be be seen seen in in Figure Figure 55 that that the free free chloride contents contents before the the TEC TEC treatment treatment and and theMEC TEC treatment treatmenthave and athe MEC treatment have a uniform distribution in average the mortar specimens. the uniform distribution in the mortar specimens. The the MEC treatment have a uniform distribution in the mortar specimens. The average value value is is about about The average value is about 1.4% (by mass of cement). Besides this, a similar result can be obtained 1.4% 1.4% (by (by mass mass of of cement). cement). Besides Besides this, this, aa similar similar result result can can be be obtained obtained for for the the distribution distribution of of the the free free for the distribution of the free chloride content after the MEC treatment. However, the free chloride chloride chloride content content after after the the MEC MEC treatment. treatment. However, However, the the free free chloride chloride content content is is obviously obviously decreased, decreased, content is obviously decreased,removal indicating the obvious chloride removal capacity of thisincrease treatment. indicating indicating the the obvious obvious chloride chloride removal capacity capacity of of this this treatment. treatment. Furthermore, Furthermore, with with the the increase of of Furthermore, with the increase of applied current density, the freereduced. chloride content is further reduced. applied current density, the free chloride content is further Compared to the MEC applied current density, the free chloride content is further reduced. Compared to the MEC Compared to the MEC treatment, the free chloride contents in the specimens after theaTEC treatment treatment, treatment, the the free free chloride chloride contents contents in in the the specimens specimens after after the the TEC TEC treatment treatment display display a non-uniform non-uniform display a non-uniform distribution. The chloride content inthe theexternal depth close to the external anode is distribution. The chloride content in the depth close to anode is relatively distribution. The chloride content in the depth close to the external anode is relatively lower. lower. relatively lower. However, higher chloride content in the opposite direction of the external anode can However, However, higher higher chloride chloride content content in in the the opposite opposite direction direction of of the the external external anode anode can can be be found. found. With With be found. With the increase of density, applied the current density, the trends for the non-uniform distribution the increase of applied current trends for the non-uniform distribution of free the increase of applied current density, the trends for the non-uniform distribution of free chloride chloride of free chloride content are more remarkable. Furthermore, a higher chloride removaltreatment ratio for the content content are are more more remarkable. remarkable. Furthermore, Furthermore, aa higher higher chloride chloride removal removal ratio ratio for for the the MEC MEC treatment in in MEC treatment in the same depth is obtained compared to that for the TEC treatment. Compared to the the same same depth depth is is obtained obtained compared compared to to that that for for the the TEC TEC treatment. treatment. Compared Compared to to the the profiles profiles of of free free the profiles of free chloride, the profiles of total chloride exhibit a similar 6). regularity (seetoFigure 6). chloride, chloride, the the profiles profiles of of total total chloride chloride exhibit exhibit aa similar similar regularity regularity (see (see Figure Figure 6). Compared Compared to the the TEC TEC Comparedthe to total the TEC treatment, the total chloride contentisfor the MEC treatment is relatively lower treatment, chloride content for the MEC treatment relatively lower under the same applied treatment, the total chloride content for the MEC treatment is relatively lower under the same applied under the same indicating applied current density, indicating aremoval slightly better chloride removal capacity. current current density, density, indicating aa slightly slightly better better chloride chloride removal capacity. capacity.

Figure Figure5. 5. Free Freechloride chlorideconcentrations concentrationsversus versuslocations locationsfrom fromthe thesteel steelto tothe theanolyte anolyteor orthe thecatholyte catholyte Figure 5. Free chloride concentrations versus locations from the steel to the anolyte or the catholyte (untreated, after the TEC treatment and the MEC treatment). (untreated, after the TEC treatment and the MEC treatment). (untreated, after the TEC treatment and the MEC treatment).

Figure 6. Total chloride concentrations versus locations from the steel to the anolyte or the catholyte Figure chloride concentrations versus locations from Figure 6. 6. Total Total locations fromthe the steel steel to to the the anolyte anolyte or or the the catholyte catholyte (untreated, afterchloride the TECconcentrations treatment and versus the MEC treatment). (untreated, after the TEC treatment and the MEC treatment). (untreated, after the TEC treatment and the MEC treatment). − + + 3.3. 3.3. OH OH−,, Na Na+,, and and K K+ Contents Contents

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− Figure 7 presents thethe locations forfor the the TECTEC treatment and the Figure presents the theOH OH− contents contentsininrelation relationtoto locations treatment andMEC the Figure 7 presents the OH− contents in relation to the locations for the TEC treatment−and the treatment. From the figure, an increase in applied current density leads to an increase in OH content MEC treatment. From the figure, an increase in applied current density leads to an increase in OH−− MEC treatment. From the figure, an increase in applied current density leads to an increase in OH − ions at the same position for the TEC treatment. The content OH− ions near the near steel surface increased content at the same position for the TEC treatment. Theofcontent of OH the steelissurface is content at the same position for the TEC treatment. The content of OH− ions near the steel surface is 2 2 is applied. by 30% (by of cement) a current 2.0 A/m is A/m applied. However, this value is increased bymass 30% (by mass of when cement) when adensity currentof density of 2.0 However, this increased by 30% (by mass of cement) when a current density of 2.0 A/m2 is 2applied. However, this 2 increased up to 80% by mass of cement when a current density of 8.0 A/m is applied. Compared value is increased up to 80% by mass of cement when a current density of 8.0 A/m 2 is applied. value is increased up to 80% by mass of cement when a current density of 8.0 A/m is applied. to the TECto treatment, the contentthe of content OH− near the−steel remarkably reduced for the MEC Compared the TEC treatment, of OH near surface the steelissurface is remarkably reduced for Compared to the TEC treatment, the content of OH− near the steel surface is remarkably reduced for treatment, only having value the of 1.2~1.3% mass of this, with the of the MEC treatment, onlythe having value of (by 1.2~1.3% (bycement). mass ofBesides cement). Besides this,increase with the the MEC treatment, only having the value of 1.2~1.3% (by mass of cement). Besides this, with the − is slightly − near − the current density, the content of OH increased. Furthermore, the content of OH increase of the current density, the content of OH − is slightly increased. Furthermore, the content of increase of the current density, the content of OH is slightly increased. Furthermore, the content of the−steel the mortar after the MEC is only slightly higher than that in the that mortar OH nearsurface the steelinsurface in the mortar after treatment the MEC treatment is only slightly higher than in OH− near the steel surface in the mortar after the MEC treatment is only slightly higher than that in without the treatment. the mortar without the treatment. the mortar without the treatment. Figures88and and 99 show show the the profiles profiles of of Na++ and andKK++contents, contents,respectively. respectively.ItItcan canbe beseen seenthat thatboth both Figures Figures+ 8 and 9 show the profiles of Na+ and K+ contents, respectively. It can be seen that both + + contents Na++ and andKK contents exhibit a similar regularity of change with location after the TEC treatment. Na exhibit a similar regularity of change with location after the TEC treatment. An Na and K+ contents exhibit a similar regularity of change with location after the TEC treatment. An + and + contents + + An obvious increase of Na K near the steel surface can be obtained, which is similar to obvious increase of Na + and K + contents near the steel surface can be obtained, which is similar to the obvious increase of Na and K contents near the steel surface can be obtained, which is +similar to the + + + the previous literature [8,12]. With increase applied currentdensity, density,the theNa Na and andKK contents contentsare are previous literature [8,12]. With thethe increase of of applied current previous literature [8,12]. With the increase of applied current density, the Na+ and K+ contents are furtherincreased. increased. In Incontrast, contrast, the the Na Na+++ and andKK+++contents contentsnear nearthe thesteel steelsurface surfaceare arenot notincreased, increased,but butare are further further increased. In contrast, the Na and K contents near the steel surface are not increased, but are softlyreduced reducedafter afterthe theMEC MECtreatment. treatment.Therefore, Therefore,ititcan canbe beconcluded concludedthat thatthe theMEC MECtreatment treatmentcannot cannot softly softly reduced after the MEC treatment. Therefore, it can be concluded that the MEC treatment cannot inducethe theaccumulation accumulationof ofsodium sodiumand andpotassium potassium near near the the steel steel surface. surface. induce induce the accumulation of sodium and potassium near the steel surface.

− Figure a function location Figure7. 7.Concentrations Concentrationsof ofOH OH−−ions ionsas functionof location(untreated, (untreated,after afterthe theTEC TECtreatment treatmentand and Figure 7. Concentrations of OH ions asasaafunction ofoflocation (untreated, after the TEC treatment and the MEC treatment). the MEC treatment). the MEC treatment).

Figure 8. Concentrations of Na++ ions as a function of location (untreated, after the TEC treatment and Figure Figure 8. 8. Concentrations Concentrations of of Na Na+ions ionsas asaafunction functionof oflocation location(untreated, (untreated,after afterthe theTEC TECtreatment treatment and and the MEC treatment). the the MEC MEC treatment). treatment).

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Figure 9. Concentrations of K+ ions as a function of location (untreated, after the TEC treatment and

Figure 9. Concentrations of K+ ions as a function of location (untreated, after the TEC treatment and the MEC treatment). the MEC treatment). + Figure 9. Concentrations of K ions as a function of location (untreated, after the TEC treatment and

3.4. Corrosion and Corrosion Current Density of the Steel the MEC Potential treatment).

3.4. Corrosion Potential and Corrosion Current Density of the Steel

Figure 10 presents the changes of measured corrosion potentials with time after the TEC 3.4. Corrosion Potential Corrosion Current Density of the Steel Figure 10and presents theand changes ofThe measured with time afterthe the TEC treatment treatment the MEC treatment. corrosioncorrosion potential potentials for the specimen without treatment is Figure 10 presents of measured corrosion potentials after the TECabout onlyMEC about −500 mV versus thechanges SCE. However, the potentials for with allthe of time the TEC treatments and the treatment. Thethe corrosion potential forcorrosion the specimen without treatment is only treatment and the MEC The potential for specimen without the treatment is lower initial valuetreatment. of about −1100 mV versus the SCE. It the is for sharply increased with the increase −500have mVaversus the SCE. However, thecorrosion corrosion potentials all of the TEC treatments have a only about −500 mVabout versus SCE. However, the corrosion for all ofattain thewith TEC treatments of time. After a time of 60 days, themV corrosion TEC treatments a stable value. lower initial value of −the 1100 versuspotentials the SCE.forItthe ispotentials sharply increased the increase of have a lower initial value of about −1100 mV versus thetreatments SCE. It isTEC sharply increased with contrast, the corrosion potentials for all ofpotentials the MEC havetreatments the value ofattain −600the mV versusvalue. time.In After a time of 60 days, the corrosion for the a increase stable of time. time of 60 lower days, the corrosion for the TEC treatments attainFurthermore, a stable value. the SCE,After whicha is slightly than those forpotentials the specimen without the treatment. it In contrast, the corrosion potentials for all of the MEC treatments have the value of −600 mV versus the In contrast, corrosion fortreatments all of the MEC treatments of −600 mV versus only takes 40the days for all potentials of the MEC to attain a stablehave valuethe ofvalue corrosion potential. We SCE, which is slightly lower than those for the specimen without the treatment. Furthermore, it only the also SCE,discover which isthat slightly thanof those for the specimen without are the all treatment. Furthermore, can the Elower corr values the TEC treatment specimens lower than those of theit takesMEC 40 days for of for the MEC treatments to stability. attain stable value value of corrosion potential. We We can also only takes 40 all days all of the MEC treatments to aattain a stable of corrosion potential. treatment when the value has achieved discover that the E values of the TEC treatment specimens are all lower than those of the corr can also that the Ecorr values of the TEC treatment specimens arethe all TEC lowertreatment than those of the theMEC The discover changes of measured corrosion current density with time after and treatment when thewhen value has achieved stability. MEC treatment the value achieved stability. MEC are indicated inhas Figure 11. As can be seen, the corrosion current density of the 2. after The changes of corrosion current density with time afterthe theTEC TECinitial treatment and the MEC specimen without themeasured treatment has a current value ofdensity about 0.9 µA/cm However, the values of the the The changes of measured corrosion with time treatment and 2 MEC are treatment are indicated in11. Figure 11. As seen, can bethe seen, the the corrosion current density the corrosion current density are increased about 10 µA/cm after TEC treatments. Besides this, treatment indicated in Figure As to can be corrosion current density of theofspecimen 2. corrosion 2 . µA/cm specimen withoutofthe treatment hasabout a value about 0.9 However, the values initialthe values the with thetreatment increase time, is obviously decreased. Similar to the potential, without the has a it value of 0.9ofµA/cm However, the initial ofcorrosion theofcorrosion 2 corrosion current density are increased to about 10 µA/cm after the TEC treatments. Besides this, densities for all of the TEC treatments attain a stable value after the waiting time of 60 days. 2 current density are increased to about 10 µA/cm after the TEC treatments. Besides this, with the with the increase of time, it is obviously decreased. the corrosion the Compared with this, the initial values of the corrosionSimilar currenttodensity are onlypotential, more than 1.3corrosion µA/cm2 increase of time, it is obviously decreased. Similar to the corrosion potential, the corrosion densities densities for alltreatments, of the TEC treatments a stable after for the the waiting time without of 60 days. after the MEC which are onlyattain slightly more value than those specimen the for all of the TEC treatments attain a stable value after the waiting time of 60 days. Compared2 with ComparedFurthermore, with this, the itinitial ofdays the corrosion are only moreathan 1.3value µA/cm treatment. only values takes 40 for all of current the MECdensity treatments to attain stable of 2 after the MEC this, corrosion the values of the corrosion density are than only those morefor than µA/cmwithout afterinitial the MEC treatments, which arecurrent only slightly more the1.3 specimen the current density. treatments, which are only slightly more 40 than those forofthe withouttothe treatment. treatment. Furthermore, it only takes days for all thespecimen MEC treatments attain a stable Furthermore, value of it only takes 40current days for all of the MEC treatments to attain a stable value of corrosion current density. corrosion density.

Figure 10. Changes of steel corrosion potential with time (untreated, after the TEC treatment and the MEC treatment).


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Figure of steel corrosion potential with time (untreated, after the TEC treatment and the9 of 12 Materials 2018,10. 11,Changes 225 MEC treatment).

Figure 11. Changes of the steel corrosion current density with time (untreated, after the TEC treatment Figure 11. Changes of the steel corrosion current density with time (untreated, after the TEC treatment and treatment). and the the MEC MEC treatment).

4. Discussion 4. Discussion In the TEC treatment, the steel reinforcement is directly connected to the cathode of the power In the TEC treatment, the steel reinforcement is directly connected to the cathode of the power supply. Under this circumstance, a high negative potential of the steel can be attained due to the supply. Under this circumstance, a high negative potential of the steel can be attained due to the application of a high current density. Also, the cathode reaction (1) normally happens, producing a application of a high current density. Also, the cathode reaction (1) normally happens, producing a large large amount − of OH− so that the alkalinity of the pore solution near the steel surface is remarkably amount of OH so that the alkalinity of the pore solution near the steel surface is remarkably increased. increased. Besides this, the cations+ such as Na+ and K+ in the pore solution of the concrete move Besides this, the cations such as Na and K+ in the pore solution of the concrete move simultaneously simultaneously toward the cathode of the steel, which induces the accumulation of sodium and toward the cathode of the steel, which induces the accumulation of sodium and potassium at the potassium at the interface between the steel and concrete. In contrast, the MEC treatment has an interface between the steel and concrete. In contrast, the MEC treatment has an obvious difference obvious difference in the experimental setup with the TEC treatment. Two auxiliary electrodes are in the experimental setup with the TEC treatment. Two auxiliary electrodes are installed on both installed on both sides of the mortar. As a result, the potential of the steel is expected to be increased, sides of the mortar. As a result, the potential of the steel is expected to be increased, and the chemical and the chemical reaction (1) does not take place on the steel surface. Besides this, the accumulation reaction (1) does not take place on the steel surface. Besides this, the accumulation of sodium and of sodium and potassium does not happen at the interface between the steel and the concrete. potassium does not happen at the interface between the steel and the concrete. Furthermore, the steel Furthermore, the steel reinforcement is grounded in the MEC treatment. Such an arrangement is reinforcement is grounded in the MEC treatment. Such an arrangement is helpful for protecting steel helpful for protecting steel from corrosion by the leakage of current, and increases obviously the from corrosion by the leakage of current, and increases obviously the potential of the steel. potential of the steel. From the obtained results, the negative potentials of the steel in the mortars ranges from −1000 mV From the obtained results, the negative potentials of the steel in the mortars ranges from −1000 to approximately 1800 mV versus the SCE during the TEC treatment, which is dependent on the applied mV to approximately 1800 mV versus the SCE during the TEC treatment, which is dependent on the current density. According to the literature [17,18], such extremely low potentials can lead to hydrogen applied current density. According to the literature [17,18], such extremely low potentials can lead to evolution near the steel surface, which can increase the expansion stress around the steel. Once the hydrogen evolution near the steel surface, which can increase the expansion stress around the steel. expansion stress attains a critical value, a micro-crack in the mortar around the steel will happen. Once the expansion stress attains a critical value, a micro-crack in the mortar around the steel will Besides this, the hydrogen evolution may cause the loss of steel elasticity so that a brittle fracture happen. Besides this, the hydrogen evolution may cause the loss of steel elasticity so that a brittle of the steel will happen [19]. Accordingly, it is generally assumed that the TEC treatment cannot be fracture of the steel will happen [19]. Accordingly, it is generally assumed that the TEC treatment applicable to pre-stressed concrete structures [20,21]. cannot be applicable to pre-stressed concrete structures [20,21]. Compared to the TEC treatment, the potentials of the steel for the MEC treatment are obviously Compared to the TEC treatment, the potentials of the steel for the MEC treatment are obviously increased. The values for all of the MEC treatments with different current densities are only maintained increased. The values for all of the MEC treatments with different current densities are only at around −800 mV versus the SCE. According to the literature [17], the electrochemical reaction that maintained at around −800 mV versus the SCE. According to the literature [17], the electrochemical takes place on the steel surface is dependent on the potential of the steel. The lower the potential of reaction that takes place on the steel surface is dependent on the potential of the steel. The lower the the steel is, the more easily hydrogen evolution happens. Moreover, the hydrogen evolution can be potential of the steel is, the more easily hydrogen evolution happens. Moreover, the hydrogen affected by some other factors, such as the alkalinity of the pore solution and the saturation of the evolution can be affected by some other factors, such as the alkalinity of the pore solution and the concrete on the steel surface. As a rule, a potential of steel less than −1200 mV versus the SCE will saturation of the concrete on the steel surface. As a rule, a potential of steel less than −1200 mV versus induce hydrogen evolution. Contrary to this, a potential of steel more than −1100 mV versus the the SCE will induce hydrogen evolution. Contrary to this, a potential of steel more than −1100 mV SCE will not lead to hydrogen evolution, but a reaction of dissolved oxygen [17,18]. This reaction versus the SCE will not lead to hydrogen evolution, but a reaction of dissolved oxygen [17,18]. This cannot produce hydrogen evolution so that hydrogen embrittlement of the steel can be avoided by the MEC treatment.

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The capacity for chloride removal of the MEC treatment is slightly better than that of the TEC treatment. Beyond this, the experimental results show the uneven distribution of free and total (Figure 6) chloride in the mortar specimens after the TEC treatment. A higher content of free chloride at a location far from the anolyte can diffuse towards the steel, which may cause corrosion again [18]. Based on physical considerations, this behavior can be attributed to the fact that the electrical field does not pass through the part of mortar far away from the anolyte. In contrast, the free and total chloride contents in the mortar specimens after the MEC treatment are uniformly distributed. It can be explained by the current with a uniform distribution throughout the entire mortar specimen during the MEC treatment. Consequently, a chloride ion in any part of the mortar can be removed effectively and uniformly. As a result, the MEC treatment may reduce the risk of secondary corrosion due to the redistribution of surviving chloride ions in the mortar. The large amounts of OH− produced at the steel surface after the TEC treatment are mainly due to the cathode reaction. Although the increase of OH− content near the steel surface is normally assumed to be helpful for protecting steel from corrosion, too high a pH value will accelerate corrosion. According to the literature [9], a pH value that is too high will put the steel in the alkaline corrosion region of the E/pH diagram (the equilibrium phases of iron in aqueous solutions as a function of pH and electrochemical potential vs. SHE (Standard Hydrogen Electrode)). In addition, as described previously in the introduction, the high alkalinity on the steel surface after the TEC treatment increases the risk of an alkali–silica reaction. Therefore, it is necessary to maintain a moderate alkalinity to protect the steel from corrosion and guard against the alkali–silica reaction. In contrast, the MEC treatment only can increase slightly the alkalinity near the steel surface due to the electro-migration of the hydroxyl ions in the electrolytic solution to the steel surface in the mortar. Such a small increase of the alkalinity not only is helpful to protect the steel from corrosion, but will not lead to the alkaline corrosion of the steel. Also, it cannot increase obviously the risk of an alkali–silica reaction. In the TEC treatment, the Na+ and K+ concentrated at the interface will react with OH− ions produced by the cathode reaction so as to form hydroxide. Then, the further reaction of alkaline hydroxide with calcium silicate (C-S-H) can lead to the softening of the concrete. This result has been assumed to be the main cause for a strength decrease of the bonding between the steel reinforcement and the concrete [19,22]. In contrast, the MEC treatment does not induce a strength decrease of the bonding due to not forming accumulations of sodium and potassium at the interface between the steel and the concrete. Some previous works [9,14,23] have indicated that the extraction treatment has positive effects on the corrosion potential (Ecorr ) of the steel bars. In this study, the corrosion potential of the steel in the mortar specimens is also moved to the positive direction after the TEC treatment and the MEC treatment, indicating that both the TEC treatment and the MEC treatment reduce the possibility of corrosion. However, the change trends of the potential are obviously different between the TEC treatment and the MEC treatment when the power supply is disconnected. The corrosion potential of the steel after the TEC treatment is maintained at an extremely low level for a long duration due to the slow restoration of the steel’s polarization. In contrast to this, the corrosion potential of steel after the MEC treatment only has a moderate value, and it is easy to attain a stable value because the steel reinforcement is grounded to be depolarized. Similar to the potential of steel, similar change trends for the corrosion density can be obtained after the TEC treatment and the MEC treatment. Furthermore, it is well-known that the corrosion current density of 0.1 µA/cm2 has been extensively accepted as the critical value for the determination of the reinforcement state from active to passive [24,25]. By means of this standard, the corrosion state of the steel reinforcement after the TEC treatment and the MEC treatment can be determined. Compared to the TEC treatment, the re-passivation of the steel after the MEC treatment can happen more quickly. Besides this, the corrosion current density at the equilibrium is relatively lower. From the obtained results, the MEC treatment has a slightly higher chloride removal ratio than the TEC treatment. Besides this, the MEC treatment may defeat the defects of the traditional

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electrochemical chloride extraction technology, such as: hydrogen embrittlement, alkaline corrosion, and alkali–silica reaction. Also, the steel reinforcement after the MEC treatment may be easier to obtain re-passivation for than after the TEC treatment. Therefore, the MEC treatment shows promise for application in real concrete structures on a wider range, even for pre-stressed reinforced concrete. 5. Conclusions To improve the repair effect of electrochemical chloride extraction, a modified electrode configuration is presented in this study. By a comparative experiment, the potential evolution, the content of various ions (Cl− , OH− , Na+ , and K+ ) in different mortar depths and electrolytes, the corrosion potential, and the corrosion current density of steel were measured. Some conclusions have been obtained: (1)

(2)

(3)

(4)

The potential of the steel during the MEC treatment has a value of −800 mV versus the SCE, which is far lower than that during the TEC treatment. Besides, the applied current density does not change obviously the potential of the steel during the MEC treatment. This potential cannot induce hydrogen evolution so that hydrogen embrittlement of the steel can be avoided by using the MEC treatment. For both the TEC treatment and MEC treatment, most of the chloride contents are extracted during the first 2 weeks of application. Compared to the TEC treatment, the chloride content in the anolyte and the free chloride content in the mortar are obviously decreased, but the total chloride content in the mortar for the MEC treatment is increased when the same current density is applied. Therefore, the MEC treatment has a better chloride removal ratio than the TEC treatment. An obvious increase of OH− , Na+ , and K+ contents near the steel surface can be obtained for the TEC treatment. In contrast, the OH− , Na+ , and K+ contents near the steel surface are softly reduced after the MEC treatment. Therefore, the MEC treatment cannot induce the accumulation of OH− , Na+ , and K+ near the steel surface. The corrosion potentials for the specimens after all of the TEC treatments have an extremely lower initial value than the corrosion potential of the specimen without the treatment. Contrary to this, all of the potentials of the steel have a higher value when the equilibrium state of the steel is attained. However, the corrosion potentials for the specimens after all of the MEC treatments are slightly lower than the corrosion potential for the specimen without the treatment. The initial values of the corrosion current density after the MEC treatments are lower than those after the TEC treatments. In addition, the re-passivation of the steel is more quickly attained after the MEC treatment.

Acknowledgments: This work was supported by the National Natural Science Foundation of China (No. 51478164). Author Contributions: W.F. and J.X. conceived and designed the experiments; W.F. performed the experiments; W.F. and J.X. analyzed the data; W.F., J.X. and L.J. contributed to write paper; Y.S., Y.C. and Q.T. collected and analyzed the data. Conflicts of Interest: The authors declare no conflict of interest.

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