电化学氯离子萃取处理后腐蚀钢筋的再钝化(英文)

孔祥明等：苯丙乳液改性砂浆的微观结构与性能· 115 ·第37卷第1期电化学氯离子萃取处理后腐蚀钢筋的再钝化姚武，赵正，雷芳华(同济大学先进土木工程材料教育部重点实验室，上海 200092)摘要：采用线性极化法研究了电化学氯离子萃取处理后砂浆中已被氯离子侵蚀的钢筋的再钝化。

研究表明：应用电化学氯离子萃取(electrochemical chloride extraction，ECE)后60d，钢筋经去极化和再钝化，腐蚀电位升高到–200mV以上，与基准试件低于–400mV的电位相比，腐蚀概率大为降低；腐蚀电流密度的变化规律与腐蚀电位相似，ECE时钢筋腐蚀电流密度仍较高，60d后则降到安全的范围内。

经ECE处理后，钢筋腐蚀电位和腐蚀电流密度的变化表明钢筋经过了再钝化，有效地抑制已被氯离子侵蚀的钢筋进一步腐蚀。

关键词：电化学氯离子萃取技术；腐蚀电位；腐蚀电流密度；线性极化法中图分类号：TU503 文献标志码：A 文章编号：0454–5648(2009)01–0115–05RE-PASSIV ATION OF CORRODED REBAR AFTER ELECTROCHEMICAL CHLORIDEEXTRACTION TREATMENTYAO Wu，ZHAO Zheng，LEI Fanghua(Key Laboratory of Advanced Civil Engineering Materials (Tongji University), Ministry of Education, Shanghai 200092, China)Abstract: The re-passivation of corroded rebars in mortar after electrochemical chloride extraction (ECE) was studied by the linear polarization method. The results show that after ECE for two months the corrosion potentials of corroded rebars increase to above –200mV due to depolarization and re-passivation, indicating the corrosion probability is much lower compared with those of refer-ence specimens that are below –400mV. The corrosion current densities vary in a similar way with the corrosion potentials. Right after ECE, the corrosion current densities of corroded rebars are still high, but they drop down to a safe range after two months. The variations of corrosion potential and corrosion current density after ECE are indications of the re-passivation of corroded rebars, and ECE is an effective way of protecting rebars eroded by chloride from further corrosion.Key words: electrochemical chloride extraction technology; corrosion potential; corrosion current density; linear polarization methodIn the electrochemical chloride extraction (ECE) tech-nique, an electrochemical instrument is used to extract chlorides that have penetrated or been mixed into rein-forced concrete. With this technique, an external electric field is applied between a concrete rebar and an exterior metal electrode (the cathode of the power supply is con-nected with the rebar, while the anode is connected with the exterior metal electrode), then under the force of an electric field the chloride ions are transferred through the pore solution to the exterior electrolyte. This treatment can remarkably decrease the chloride concentration near the reinforcements, keep the reinforcements from further corrosion, protect the reinforced concrete structure (RCS), and prolong its service life. For highway bridges con-taminated by de-icing salts and marine constructions eroded by seawater, the ECE is a recommended measure due to its feasibility and efficiency.The high extraction efficiency of ECE has been proven by the trials of many researchers, and the effi-ciency depends on several extraction parameters, includ-ing the current density, extraction duration, and suitable electrolyte concentration. If these parameters are properly chosen, the efficiency will reach above 50%.[1–4] How-ever, the ultimate objective of ECE is to prevent rein-forcements from further corrosion; thus together with ensuring high extraction efficiency, the variation of elec-trochemical properties of RCS (corrosion potential, cor-rosion rate) during the whole extraction process should收稿日期：2008–05–05。

修改稿收到日期：2008–08–18。

基金项目：大连理工大学海岸和近海工程国家重点实验室开放基金(LP0707)；教育部博士点基金(20070247063)资助项目。

第一作者：姚武(1966—)，男，博士，教授。

Received date:2008–05–05. Approved date: 2008–08–18. First author: YAO Wu (1966–), male, Ph.D., professor.E-mail: yaowuk@第37卷第1期2009年1月硅酸盐学报JOURNAL OF THE CHINESE CERAMIC SOCIETYVol. 37，No. 1January，2009硅 酸 盐 学 报· 116 ·2009年be considered, they are very useful to estimate the effects of ECE on preventing corrosion.[5] In this paper, the cor-rosion states of the rebars before and after ECE treatment were studied by means of the linear polarization method, and the relationships and mechanisms of the changes of corrosion potentials and corrosion current densities with ECE and re-passivation were analyzed and discussed. The linear polarization method may underestimate the pitting danger caused by chloride ions, but it is still feasi-ble to compare the differences of the rebars’ passivation states between treated specimens and untreated speci-mens, and thereby validate the efficiency of the ECE technique.1 Experimental procedure1.1 Mix proportion and casting of mortar specimens The materials used included P·O Cement 42.5, me-dium river sand, naphthalene-based super plasticizer, plain steel rebars (nominal diameter 8 mm), saturated solution of calcium hydroxide, sodium chloride (analyti-cal reagent), and ordinary tap water. The mix proportion of water, cement, sand was 0.5:1:3, and the dosage of superplasticizer was 0.5% of the cement mass. The so-dium chloride was added into the tap water during mix-ing in order to simulate an environment in which rebars were eroded. Demoulding was done 24 h after casting, and then the specimens were cured in a standard curing room for 28 d.The size of the column mortar specimens was φ 70 mm × 150 mm. A single 200 mm-long rebar was embedded vertically into the very center of the specmen’s end face 130 mm inside the mortar. The specimens were divided into two groups denoted by A and B which represents that the initial chloride contents were 2% and 4% by cement mass respectively. Each group contained four specimens, three of which were treated by ECE for 5 weeks with the current densities of 1, 2 A/m 2 and 3 A/m 2 of the rebar surface, and another one as reference specimen was left untreated (shown in Table 1). The specimens were encir-cled with titanium mesh as external anode, and the elec-trolyte was a saturated solution of calcium hydroxide.Table 1 Experimental schemeSample Impressed current density/(A ·m –2) Initial chloride added/%A0 0 2A1 1 2A2 2 2A3 3 2 B0 0 4B1 1 4B2 2 4B3 3 4 1.2 Determination of corrosion potentials andcorrosion current densitiesThe linear polarization test was conducted according to the direct current (DC) potentiostatic method. After immersion in saturated solution of Ca(OH)2 for 24 h, the specimens were tested with a 273A Potentiostatic In-strument produced by EG&GPARC. Each specimen was placed in a 0.1 mol/L NaOH solution, using the rebar as working electrode, a saturated calomel electrode (SCE) as reference electrode, and a stainless steel rod as auxil-iary electrode. The instrument was operated using M352 system software. An initial potential of 20 mV below the corrosion potential (E corr ) was ramped to a final potential of 20 mV above E corr at a rate of 0.1 mV/s. The testing was carried out before ECE, right after ECE and 60 d after ECE, respectively.2 Results and discussions2.1 Corrosion potentials of rebarFrom the point of view of electrochemistry, the re-bar-concrete system is regarded as an electrode having an equilibrium potential. This electrode has a potential dif-ference compared with the reference electrode, which is called self-corrosion potential of reinforced concrete. The corrosion potential differs depending on the electro-chemical states of rebar. If the rebar is under passivation, the corrosion potential will increase, while if the passive state is shifted to the active state, the corrosion potential of the rebar will decrease, and thereby the corrosion state of the rebar can be qualitatively estimated according to the change of self-corrosion potential, as shown in Table 2.Table 2 Standard of estimating rebar corrosion probabilityfrom ASTM–C876Corrosion potential/mVCorrosion probability/%–(276–426)≥95–(126–276) 50*≥–126 5*Probable pitting corrosion.As chlorides had been added into mortar specimens before casting, after cured for 28 d the passivating film ofeach rebar must have been eroded by chlorides. Table 3shows that before ECE treatment all the corrosion poten-tials of the rebars were below –500 mV . Referring to Ta-ble 2, the rebar corrosion probabilities were all above95%. Although the corrosion potentials of the treatedspecimens and reference ones (A0 and B0) all rose afterECE, the reasons are essentially different. The potential values of the reference specimens basically represent the corrosion degree as a result without ECE treatment.There are two primary reasons for the increase in thecorrosion potential of the reference specimens. First, the姚 武 等：电化学氯离子萃取处理后腐蚀钢筋的再钝化· 117 ·第37卷第1期Table 3 Corrosion potential values of rebars mV Sample No. Before ECE Right after ECE After ECE for 60 d A0 –644 –492 –467A1 –617 –360 –135A2 –625 –470 –128 A3 –579 –550 –175 B0 –532 –478 –405B1 –532 –262 –122B2 –548 –293 –161B3 –547 –775 –152cathodal electrode reaction consumed oxygen, which re-duced the initial oxygen concentration around the rebar 22(aq)2H O(l)O (g)4e 4OH −−++→ (1) and then the corrosion reaction was controlled by oxygendiffusion.[6–7]Second, during the corrosion process a compact mixed layer of concrete and rust formed, whichblocked the diffusion of oxygen, hedged the transfers of the corrosion product, and slowed down the harmful an-odic reaction. Although these two factors led to the drop of the corrosion rate, it was just temporary and the corro-sion was not always be slowed down at that rate.[8] The treated specimens were polarized because the rebar was connected with the cathode of the external power source, and as a result the potential values became more negative. Table 3 shows that the higher impressed current led to more severe polarization and resulted in lower potential values. Thus, if oxygen depletion is also taken into con-sideration, the determination of the corrosion potentials right after ECE could not fully reflect the corrosion state. After ECE treatment for two months, due to a sharp re-duction of chlorides, the corrosion activity was gradually eased and the treated rebars inside the mortar becamerepassivated in a restored alkaline condition. The corro-sion potentials of the treated specimens were all above–200 mV , which indicated low corrosion probability. Compared with the values of lower than –400 mV of thereference specimens, the corrosion probability of the treated specimen was much lower, which somehow provedthe effects of ECE in protecting eroded rebars from furthercorrosion. During the process of ECE treatment, cathodalreactions produced OH – continuously, and in the meantimecations like Na + and K + transferred toward the vicinity ofthe rebar under the driving force of the electric field, whichincreased the alkalinity of the pore solution and was help-ful for the repassivation of rebars. This is another advan-tage of ECE, which is called realkalization.Marcotte et al .[9] found out that after ECE treatment the corrosion potentials dropped to below –1 000mV , and after the rebars were pulled out from concrete, about 50%–100% of their surfaces were found to be covered bydark brown substances. They attributed this to the occur-rence of alkali corrosion because the strong cathodal po-larization caused very low potential and the pH value rose to above 14 due to the ECE treatment, and the cor-rosion reaction produced HFeO 2– and FeO 22–(refer toFig.1). However, after depolarization and redistributionof OH –, black Fe 3O 4 became primarily resultant. No suchphenomena were found in our trials. After pulling out the rebars, red Fe 2O 3 was found to be primarily resultant.This may be attributed to the difference of the impressed current density. The current densities we chose were be-tween 1 A/m 2 and 3 A/m 2of the rebar surface embedded in the mortar, while Marcotte et al . adopted 1 A/m 2 of thetreated concrete surface, which would be 8.37 A/m 2 ifconverted to the rebar surface scale. Under these circum-stances, cathodal polarization would be extreme and more OH – would be created by electrode reactions; thiswould result in alkali corrosion. Thus it can be concludedthat when ECE is applied, the impressed current density should be strictly controlled.Fig.1 Potential–pH diagram of Fe–H 2O system at 25 ℃ and 101.3 kPa [9]2.2 Corrosion rates of rebar The corrosin potential is only determined to qualita-tively estimate the corrosion probability of rebar, but itcan not be used to figure out the corrosion rate of a cer-tain moment or to quantify the corrosion state. However,the corrosion rate is in direct proportion with the corro-sion current density, so it can be calculated if the corro-sion current density is determined. Andrade et al .[10] pro-posed a division mode of corrosion rate rank (see Table 4) based on abundant trials. This rank division was applied in the present work to forecast the residual service life of the reinforced concrete structure. Table 4 Ranking of rebar corrosion rate Corrosion rate/(μA·cm –2) Rebar corrosion ranking≤0.1 Can be ignored0.1–0.5 Low 0.5–1 Intermediate≥1 High硅 酸 盐 学 报· 118 ·2009年Figures 2 and 3 respectively show the corrosion cur-rent density values of specimens with 2% and 4% initial chloride content. Before ECE treatment all the corrosion current densities of the specimens were above 1 μA/cm 2. According to Table 3, all the rebars were severely cor-roded, and the more chlorides were initially added, the more badly the rebars were corroded. After treatment, the corrosion current of most specimens notably declined, mostly because the chloride contents around the rebars dropped sharply (see Fig.4) and the active corrosion caused by chlorides was well restrained. However, the current densities here were not at a safe level and there were several possible reasons for this: (1) the corrosion products on the surface of the rebar could not be cleaned out and the passivating film was still broken, which might have left a chance for some residual chlorides to cause new active corrosion. (2) OH – generated by the cathodal reaction concentrated in the vicinity of rebar, and after treatment they would diffused to other regions of the mortar as a result of concentration disparity, which might have made the determined corrosion current devi-ate from the actual values. (3) after ECE treatment, the non-uniform distribution of all kinds of cations and anionsFig.2 Diagram of corrosion current density variety of specimenswith 2% chloride addedFig.3 Diagram of corrosion current density variety of specimenswith 4% chloride addedFig.4 Chloride extraction efficiency near rebarmight have been caused, and it took time to restore the balance.Green et al .[11] found that the corrosion current densi-ties of treated specimens rose from an original 8–35 μA/cm 2 to 320 μA/cm 2. This could also be caused by al-kali corrosion due to high impressed current densities, but in our researches no such phenomenon was observed. It should be pointed out that although oxygen depletion is beneficial to deter corrosion, the corrosion current densi-ties of the reference specimens were always high (>0.8 μA/cm 2). Most of the chloride ions were extracted, all kinds of ions were redistributed and the rebars were re-passivated in alkaline condition; thus, the corrosion cur-rent densities of the treated specimens dropped to within safe levels (0.09–0.45 μA/cm 2).3 Conclusions(1) Right after ECE treatment the corrosion potentials of the specimens all rose to a certain extent, but they could not be used to estimate the corrosion probabilities because of cathodal polarization of the rebars. Two months later, the corrosion potentials of all the treated specimens rose to above –200 mV which indicated a low corrosion probability, while the potentials of the un-treated specimens were still below –400 mV . ECE treat-ment reduces the corrosion probability of rebar compara-tively, and provides beneficial repassivating and realka-lizing effects on chloride-corroded rebar.(2) Before ECE treatment, the corrosion current densi-ties of all specimens reached more than 1 μA/cm 2, which indicated a high corrosion rate. Right after treatment the corrosion current densities dropped to a certain extent, but they still maintained a higher lever because repas-sivation was blocked by the former corrosion products covering and surrounding the rebar surfaces and because all kinds of ions were transferred inside the mortar to rebuild the balance. The repassivating films were recov-ered gradually in alkaline conditions, and ionic move-ment restored the balance, at which time the corrosion current densities dropped to a safe level.姚武等：电化学氯离子萃取处理后腐蚀钢筋的再钝化· 119 ·第37卷第1期(3) Low impressed current densities (1–3A/m2) will not lead to alkali corrosion. Under the proper current density, ECE not only extracts chlorides from mortar efficiently, but also effectively recovers passivation of reinforcement and protects corroded rebar from further corrosion.References:[1] GARCES P, SANCHEZ M J, CLIMENT M A. Effect of the reinforce-ment bar arrangement on the efficiency of electrochemical chloride removal technique applied to reinforced concrete structures[J]. Corros Sci, 2006, 48(3): 531–545.[2] SWAMY R N, MCHUGH S. Effectiveness and structural implicationsof electrochemical chloride extraction from reinforced concrete beams [J]. Cem Concr Compos, 2006, 28(8): 722–733.[3] FAJARDO G, ESCADEILLAS G, ARLIGUIE G. Electrochemicalchloride extraction (ECE) from steel reinforced concrete specimens contaminated by “artificial” sea-water [J]. Corros Sci, 2006, 48(1): 110–125.[4] MIRANDA J M, COBO A, OTERO E, et al. Limitations and advan-tages of electrochemical chloride removal in corroded reinforced con-crete structures [J]. Cem Concr Res, 2007, 37(4): 596–603.[5] MIRANDA J M, GONZALEZ J A, COBO A, et al. Several questionsabout electrochemical rehabilitation methods for reinforced concrete structures [J]. Corros Sci, 2006, 48(8): 2172–2188.[6] IHEKWABA N M, HOPE B B, HANSSON C M. Structural shapeeffect on rehabilitation of vertical concrete structures by ECE tech-nique [J]. Cem Concr Res, 1996, 26(1): 165–175.[7] YAN P, YOU Y, CUI L. Controlling factor of macro-cell corrosion rateof reinforcement in concrete with high proportion of chloride [J]. Ind Constr (in Chinese), 2000, 30(5): 6–9.[8] JI Y, YUAN Y. Change characteristic and mechanism of rebar corrosionrate with time in concrete under constant climate [J]. J Chin Univ Min Technol (in Chinese), 2007, 36(2): 153–158.[9] MARCOTTE T D, HANSSON C M, HOPE B B. The effect of theelectrochemical chloride extraction treatment on steel-reinforced mor-tar Part I—electrochemical measurements [J]. Cem Concr Res, 1999, 29(10): 1555–1560.[10] ANDRADE C. Corrosion rate monitoring in the laboratory and on-site[J]. Constr Build Mater, 1996, 10(5): 315–328.[11] GREEN W K, LYON S B, SCANTLEBURY J D. Electrochemicalchanges in chloride-contaminated reinforced concrete following ca-thodic polarization [J]. Corros Sci, 1993, 35(5–8): 1627–1631.。