- Email: [email protected]
Effective monitoring of corrosion in reinforcing steel in concrete constructions by a multifunctional sensor
Electrochimica Acta 56 (2011) 1881–1888
Contents lists available at ScienceDirect
Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta
Effective monitoring of corrosion in reinforcing steel in concrete constructions by a multifunctional sensor Shi-Gang Dong, Chang-Jian Lin ∗,1 , Rong-Gang Hu, Lan-Qiang Li, Rong-Gui Du State Key Laboratory of Physical Chemistry of Solid Surfaces, and Department of Chemistry, College of Chemistry and Chemical Engineering, Xiamen University, Xiamen, Fujian 361005, China
a r t i c l e
i n f o
Article history: Received 30 June 2010 Received in revised form 29 August 2010 Accepted 30 August 2010 Available online 16 September 2010 Keywords: Multifunctional sensor pH value Cl− concentration Corrosion Reinforced concrete
a b s t r a c t A novel multifunctional sensor is developed for in situ and non-destructive monitoring of the corrosion current and open circuit potential of reinforcing steel, as well as the pH and Cl− concentration of concrete. The pH and Cl− sensors show good responses to the pH and Cl− concentration of concrete pore solutions, respectively, and are able to monitor both the carbonization process of concrete and the ingress of Cl− in concrete. Combined with measurements of the corrosion potential and corrosion current density, as well as the EIS spectra of reinforcing steel in concrete, this study demonstrates that the pH and the Cl− concentration of concrete are two of the most crucial factors that determine the corrosion of reinforcing steel in concrete. The corrosion tendency and corrosion rate of reinforcing steel largely depend on the chemical environment in the concrete. The multifunctional sensor is a powerful tool for in situ monitoring corrosion of steel in concrete, and provides accurate details of the chemical condition of the concrete pore solution and the corrosion status of the reinforcing steel in concrete. These are essential for corrosion predictions and service life evaluations of concrete constructions. © 2010 Elsevier Ltd. All rights reserved.
1. Introduction Reinforced concrete is applied widely as a structural material in various infrastructures. With the development of diversified constructions and service conditions in the past decades, environment-induced corrosion and the premature degradation of steel-reinforced concrete structures have become increasingly serious and ubiquitous. The durability and safety of large-scale or key rebar reinforced constructions have become matters of great concern. Normally, reinforcing steel in concrete does not corrode because a compact and stable passive film always forms on the surface of the reinforcing steel in the highly alkaline concrete pore solutions. However, the carbonization of concrete and/or the ingress of Cl− may result in the breakdown of the passive film. Corrosion of the reinforcing steel then occurs and develops, which not only destroys the construction, but also sometimes causes accidents and safety problems. The rehabilitation and maintenance costs of a construction that has been degraded by corrosion may sometimes be several times higher than the original cost of the construction [1–4]. Thus, it is necessary to develop various techniques
∗ Corresponding author. Tel.: +86 592 2189354; fax: +86 592 2186657. E-mail address: [email protected] (C.-J. Lin). 1 ISE member. 0013-4686/$ – see front matter © 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.electacta.2010.08.089
for monitoring and studying the corrosion of reinforcing steel in concrete constructions. The essential reason for the premature degradation of reinforced concrete constructions is the corrosion of steel rebar in concrete, which is, in principle, a typical electrochemical process that happens in an occluded system. As such, electrochemical techniques have particular advantages in the detection of corrosion in concrete. The half-cell potential technique, linear polarization technique, electrochemical impedance spectroscopy, electrochemical noise technique, and electrode array technique, among others, have been widely used to measure the corrosion of reinforcing steel in concrete [5–22]. However, most of these techniques are generally used only in the laboratory and, to date, very few of them can be applied in field monitoring. Although many non-electrochemical techniques, such as ultrasonic and eddy current detections, have been developed in the past years, electrochemical sensors play the most crucial role in monitoring the corrosion of reinforcing steel because they are able to provide more and earlier in situ information of the corrosion process of steel-reinforced constructions. The measurement of the corrosion potential is essential for monitoring the corrosion process of reinforcing steel in concrete, and various embeddable solid reference sensors in concrete have been developed [23–28]. Long-term potential stable reference sensors embedded in concrete provide opportunities to follow the corro-
1882
S.-G. Dong et al. / Electrochimica Acta 56 (2011) 1881–1888
sion potential of steel in concrete. Many galvanic sensors based on the galvanic current between anode and cathode in reinforcing steel have been applied to monitor the galvanic corrosion of reinforcing steel in concrete. It is also possible to detect the corrosion distribution in one dimension in concrete by arranging of multiple anodes in a ladder format at different depths [29–34]. Correia et al. [35] revealed a linear relationship between the limiting currents of oxygen reduction and oxygen concentration, and developed an oxygen sensor for measuring the oxygen concentration in concrete. The Ag/AgCl electrode, as a Cl− sensor, has been used for monitoring Cl− ion concentrations in concrete [36–38]. Other sensor techniques, such as resistance, fiber optic, ceramic, and microelectromechanical system sensors, have also been designed for evaluating corrosion, pH levels, cracks, deformations, temperature, and moisture in concrete [39–46]. Concrete is a typical multi-scale and non-uniform system consisting of the gas–liquid–solid phases. The corrosion process of reinforcing steel in concrete is usually much more complicated compared to the general corrosion process of steel in an open system. In generally speaking, more parameters are required to determine a complicated system because it is dependent upon many variables. A single bit of information, such as the corrosion potential or corrosion current of steel in concrete measured by an electrochemical measurement method is not enough to accurately and comprehensively describe the corrosion process of steel in concrete. Scattered data of electrochemical measurements from steel in concrete are frequently encountered due to the various variables that exist during the corrosion process. For example, when the cathodic reaction is controlled by oxygen transport in concrete, the corrosion potential of steel varies with the oxygen concentration in concrete; however, it is not directly related to the corrosion status or corrosion rate of the steel. The Cl− concentration and pH at the interface of the steel/concrete are recognized as two of the most crucial environmental factors that determine the stability and breakdown of the passive film on steel in concrete. The measurement of pH and Cl− concentration in concrete were previously performed by leaching the concrete pore solutions, and then by chemical analysis [47–50], which was usually time-consuming, not in situ and destructive to the concrete construction. Various potentiometric sensors, such as Ti and AgCl electrodes [51–55], were developed for monitoring the pH and Cl− concentration, respectively, in concrete. However, the potential of Ti in alkaline solution is not stable, and the correspondence between potential and pH is not linear [56]. The same happened with the Ag/AgCl electrode when embedded in concrete if it was not properly prepared. Climent and co-workers [38] showed that the Ag/AgCl electrodes can be stable in solution, but a large bias appeared when they were embedded in concrete with a very small amount of chloride. Also the potential evolved with time due to attack of the electrode by hydroxides. The performances of the sensors largely depended on the structure and preparation of the electrodes. It is our desire to develop a practical multifunctional sensor to be able to provide more comprehensive in situ information not only on the corrosion process of steel, but also on the chemical environment of concrete, especially for the long-term monitoring of environment-induced corrosion in concrete constructions [57]. This work attempts (1) to develop a hybrid sensor composed of Ag/AgCl, IrO2 electrodes and electrochemical three electrode system to in situ monitor the Cl− concentrations and pH of concrete, and follow the corrosion process of reinforcing steel in concrete; (2) to characterize the sensitivity, stability, and response of the prepared sensors to concrete constructions; and (3) to further understand the dependence of corrosion behavior of reinforcing steel on the Cl− concentrations and pH in concrete.
2. Experimental 2.1. pH sensor The pH sensor constructed from a Ti/IrO2 electrode was prepared from H2 IrCl6 by thermal oxidation on a Ti rod surface. The Ti rod, 6 mm in diameter and 25 mm in length, was ultrasonically cleaned in EtOH and distilled water, and then treated in a boiling 10% oxalic acid solution for 20 min. A dipping solution, composed of a mixture of chloroiridic acid and isopropyl alcohol at a specific ratio, was spread on the Ti rod’s surface by a brush, dried under an infrared lamp, and then heat-treated in a muffle furnace at 450 ◦ C for 20 min. After repeating this spreading-drying-heat treating process for 20 times, the Ti rod was heat-treated at 450 ◦ C for 120 min. A compact IrO2 film was formed on the rod’s surface and used as the pH sensor. One end of the Ti rod was welded to a conducting wire, and the welding spot and side faces were sealed with an epoxy resin. Only the top of the rod was exposed. The prepared Ti/IrO2 electrode was activated in a simulated concrete pore solution of saturated Ca(OH)2 for 1 month before it was used [58]. The XRD spectra of the Ti/IrO2 electrode were obtained by a PANalytical X’Pert PRO diffractometer. The potential responses of Ti/IrO2 to pH were measured in a series of solutions adjusted to different pH values.
2.2. Cl− sensor An Ag/AgCl electrode was used as the Cl− sensor and prepared by anodic chlorination. An Ag rod (99.99% purity), 4 mm in diameter and 10 mm in length, was welded to a conducting wire, and the welding spot and part of the rod were sealed with an epoxy resin. Only about 5 mm of the length of the rod was exposed. After burnishing with an emery cloth and ultrasonic cleaning in EtOH and distilled water, the Ag rod was anodically chloridized in an HCl solution at a given current of 0.1 mA cm−2 for 24 h. An AgCl film formed on the rod’s surface. The prepared Ag/AgCl electrode was rinsed with distilled water, and was then placed in the dark. The potential responses of the prepared Ag/AgCl electrode to various Cl− concentrations were measured in saturated Ca(OH)2 solutions with different concentrations of NaCl. The potentials (vs. a saturated calomel electrode, SCE) of the pH and Cl− sensors in solution were measured by a high-resistance digital multimeter. The electrochemical measurements were performed by an Ivium electrochemical workstation.
Fig. 1. Schematic structure of MnO2 reference electrode.
S.-G. Dong et al. / Electrochimica Acta 56 (2011) 1881–1888
1883
Fig. 2. Schematic arrangement of the multifunctional sensor.
2.3. MnO2 reference electrode As shown in Fig. 1, the reference electrode was made of MnO2 powder, graphite, and a binder, mixed at a given ratio, and mounted on a stainless steel barrel 15 mm in diameter and 45 mm in length. An alkaline slurry of Ca(OH)2 was poured into the MnO2 ring, and the top was sealed with a porous hydrated cement paste, which provided an electrical contact with the surrounding environment. The stainless steel was connected to a conductive wire and the whole assembly was encapsulated in a rubber shrink tube. The half-cell potential of the MnO2 electrode was determined by an MnO2 /Mn2 O3 equilibrium potential. The filled core was composed of an alkaline slurry with a high pH corresponding to the concrete pore solution. The porous cement paste top bonded with the concrete well, so junction potentials across the cement paste were very small [26,27].
2.4. Design and assembly of multifunctional sensors One end of HRB335 reinforcing steel with 8 mm in diameter, the same material as that used in concrete, was connected to a conductive wire. The welding spot and the side surface were sealed with an epoxy resin, and another cross section acted as the working area. The working area was mechanically polished by sand paper and ultrasonically cleaned in EtOH. Fig. 2 shows the schematic arrangement of the multifunctional sensor consisting of the Cl− sensor, the pH sensor, a steel working electrode, the MnO2 reference electrode, and a stainless steel thimble. All of the electrodes were assembled in and fixed to the stainless steel thimble with an epoxy resin at their designated positions. The stainless steel thimble, which was about 30 mm in diameter, was used as both a counter electrode for electrochemical measurements and a main holder for the multifunctional sensor. The top of the multifunctional sensor was covered with a thin layer (about 3 mm thick) of cement mortar with a water/cement ratio of 0.6.
2.5. Preparation of concrete sample The concrete sample was cast with 42.5# Portland cement at a cement:water:fine sand:coarse aggregate ratio of 1:0.4:1.57:2.36. The prepared multifunctional sensor was embedded in the concrete at a depth of about 10 mm. The concrete sample was cured at room temperature and 95% RH for 28 days.
2.6. Measurement of pH, Cl− concentration, and corrosion of reinforcing steel in concrete The prepared concrete sample was exposed to a cyclic condition of immersion in a 3.5% NaCl solution for 2 days and then drying for 2 days. At certain periods, the potentials of the Cl− and pH sensors were measured to calculate the Cl− concentration and pH of the concrete, respectively. At the same time, the corrosion potential, corrosion current density, and EIS spectra of the embedded steel electrode in sensor were obtained. The potential measurements of the pH and Cl− sensors, as well as the steel electrode, were carried out by a high-resistance digital multimeter relative to the MnO2 reference electrode. In order to be able to compare the data obtained with those in the literature, all measured potentials were converted and reported with respect to SCE. A three-electrode system was used to measure the corrosion current density and obtain the EIS spectra, which was performed by an Ivium electrochemical workstation. The reinforcing steel, MnO2 electrode, and stainless steel thimble acted as the working electrode, reference electrode, and counter electrode, respectively. The corrosion current density, icorr , was measured using the linear polarization method from a potential scan within Ecorr ± 15 mV at 0.167 mV s−1 , based on the following equation: icorr =
ˇa ˇc B = Rp 2.303 · Rp (ˇa + ˇc )
(1)
where Rp is the polarization resistance, and B is a constant related to the anodic and cathodic Tafel slopes. B was equal to 26 mV for the active state and 52 mV for the passive state [34,59,60]. In the present work, the icorr was obtained directly through a fitting procedure, carried out on an Ivium electrochemical measurement system. The EIS spectra of the reinforcing steel were obtained in the 105 –10−2 Hz range at 10 mV amplitude of the stimulant sinusoidal voltage. 2.7. Corrosion monitoring for a tunnel construction by the multifunctional sensor In order to further evaluate the viability of the prepared multifunctional sensor, the comprehensive corrosion data, including corrosion rate, polarization resistance, open circuit potential of the reinforcing steel, and pH and Cl− concentration of concrete, were monitored at a river tunnel concrete construction. Since the tunnel was an old concrete construction, several holes were drilled into the wall inside the tunnel, and the multifunctional sensors were inserted and fixed to the base of the holes using cement
1884
S.-G. Dong et al. / Electrochimica Acta 56 (2011) 1881–1888
Fig. 3. Time response of (a) pH sensor and (b) Cl− sensor.
mortar. The pH and Cl− concentration of the concrete, and the corrosion potential, polarization resistance, and corrosion rate of steel were monitored using the multifunctional sensor. The data were collected using a computer-assisted measurement system.
following equations, respectively [61,62]: IrO2 + H+ + e → 1/2Ir2 O3 + 1/2H2 O 0 E = EIrO
2 /Ir2 O3
0 = EIrO
2 /Ir2 O3
RT
ln[H+ ] F − 0.0592 pH(vs. SHE/V, 298 K) +
3. Results and discussion
AgCl + e → g + Cl−
3.1. Characterization of pH and Cl− sensors
0 E = EAgCl/Ag +
The typical XRD spectra of the prepared Ti/IrO2 electrode and Ag/AgCl electrode were measured. By comparison to standards, it was confirmed that a tetragonal IrO2 crystal film was coated onto the Ti rod’s surface, and a cubic AgCl crystal film on the Ag rod’s surface. To evaluate the stability and sensitivity of the prepared pH and Cl− sensors, the variations of sensor potentials with time were measured in the simulated concrete pore solution. Fig. 3a shows that the potential of the pH sensor in a solution with a pH of 9.3 changes less than 3 mV within 30 min, and then rapidly shifts and becomes stable again in a few minutes when about 2 mL of 0.1 mol L−1 NaOH solution is added in the test solution. This indicates that the prepared sensor is of good stability and high sensitivity to the pH of the test solution. As shown in Fig. 3b, the potential of the Cl− sensor is very stable in a 0.01 mol L−1 NaCl solution and shows almost no change within 40 min, quickly responding to any change in Cl− concentration in the solution. This demonstrates that the prepared Cl− sensor is stable in solutions containing Cl− and is highly sensitive to variations in Cl− concentration. The principles of the Ti/IrO2 electrode as a pH sensor and the Ag/AgCl electrode as a Cl− sensor can be described through the
RT
ln[Cl− ] F 0 = EAgCl/Ag − 0.0592 lg[Cl− ](vs SHE/V, 298 K)
(2) (3) (4) (5)
According to Eqs. (3) and (5), at a given temperature, the potential of the prepared Ti/IrO2 electrode only depends on the pH of the solution, and the potential of the Ag/AgCl electrode only relies on the Cl− concentration. By measuring the potential of the sensors, the pH and Cl− concentrations of the solution can be obtained accordingly. A three-component simulated concrete pore solution [58], 0.6 mol L−1 KOH + 0.2 mol L−1 + 0.001 mol L−1 Ca(OH)2 , was prepared to calibrate the pH sensor. The pH level of the solution was adjusted by adding 0.8 mol L−1 NaHCO3 and/or diluted H2 SO4 solutions. Fig. 4a shows a good linear relationship between the potential of the pH sensor and the pH in a wide pH range of 7–14. The linear relationship can be described by the linear regression equation: E(vs. SCE/mV) = 670.3 − 58.4 pH(R2 = 0.99925)
(6)
The response slope, 58.4 mV, is close to the theoretical value of 59.2 mV at room temperature. The pH value in a normal concrete pore solution is about 13, and may drop to 8–9 when the concrete is seriously carbonized [63–65]. The linear response region of the
Fig. 4. Calibration curves of (a) pH sensor and (b) Cl− sensor.
S.-G. Dong et al. / Electrochimica Acta 56 (2011) 1881–1888
1885
Fig. 5. Variation of pH and Cl− concentration in steel/concrete interface with time.
pH sensor is sensitive enough to follow any change in the pH level in the concrete pore solution. The potential–concentration calibration curve of the Cl− sensor was measured in saturated Ca(OH)2 solutions with different concentrations of NaCl. A perfect linear relationship, as shown in Fig. 4b, shows the potential of the Cl− sensor and the logarithm of the Cl− concentration in a range of 10−4 –2 mol L−1 , as described by the linear regression equation: E(vs SCE/mV) = 11.8 − 49.1 lg[Cl− ] (R2 = 0.99939)
(7)
The slope is smaller than the theoretical value; however, the linear relationship of the sensor is good enough for measuring the Cl− concentration in concrete. 3.2. Monitoring of pH and Cl− concentrations in steel/concrete interfaces The potentials of the pH and Cl− sensors were measured during the exposure period, and the pH value and Cl− concentrations of the concrete were calculated using Eqs. (6) and (7), respectively. As shown in Fig. 5, the Cl− concentration in concrete remained at low levels during the first 40 days of exposure, and then increased gradually afterward. The increase in the Cl− concentration of the concrete was driven by the concentration difference between the external solution and the inner concrete. The Cl− concentration of concrete reached a maximum and then becomes constant after 124 days. The accumulation of the Cl− in the concrete resulted in a high Cl− concentration; the Cl− concentration in the concrete was close to that of the test solution. The pH value decreased slowly at the beginning of the experiment due to the OH− that diffused from the inner concrete to the external solution based on the pH difference between the two zones. It then exhibited a big drop after 120 days of exposure, indicating that the depth of carbonization reached the steel/concrete interface.
Fig. 6. Variation of corrosion potential Ecorr of reinforcing steel embedded in concrete with time.
0.1 A cm−2 , thus proving that the steel in concrete had a moderate corrosion rate [66]. The time for the initial corrosion of the reinforcing steel is close to the time of the decrease in pH and the increase in Cl− concentration, indicating that the corrosion state and corrosion rate of steel in concrete are closely related to the pH and Cl− concentration of the concrete pore solution. Fig. 8 shows the EIS spectra, in Bode plots, for the reinforcing steel in concrete. The Nyquist plot presents a non-ideal semicircle at low frequencies. The EIS spectra, from the diameter of the semicircle and the phase angle, the total impedance in Bode plots, exhibited little change with time in the initial 124 days, but changed rapidly afterward. EIS plots are usually analyzed by an equivalent circuit of Rs (Rct Q). The Rs , Rct , and Q are the resistance of the concrete pore solution, the charge transfer resistance, and the constant phase element (CPE) of double layers of steel/concrete interfaces, respectively. The CPE can be defined using the following equation: ZCPE =
1 Y0
(jω)
−n
(8)
where Y0 is a constant, and n is the CPE-power ranging from 0 to 1. The CPE is a pure resistance and a pure capacitance if n = 0 and 1, respectively [19,20]. Based on the equivalent circuit of Rs (Rct Q), the fitting data for the EIS of the reinforcing steel in concrete are shown in Fig. 9. In the initial 124 days, the Rct was 1–10 M, the Y0 was 2.0–4.0 × 10−5 −1 sn , and the n was about 0.8, indicating
3.3. Corrosion behavior of reinforcing steel in concrete The corrosion potential, Ecorr , corrosion current density, icorr , and EIS spectra of reinforcing steel in concrete were measured and obtained at the same time as the measurements of the pH and Cl− concentrations of the steel/concrete interface were made. Fig. 6 shows that the Ecorr of the reinforcing steel in concrete exhibited a positive value (−250 to −350 mVSCE ) during the first 124 days of exposure, implying that the steel in the concrete maintained a passive state. It then shifted negatively after 124 days, close to about −550 mVSCE . The icorr of steel (see Fig. 7) was at low levels for the first 124 days, and then increased rapidly afterward to over
Fig. 7. Variation of corrosion current density icorr of reinforcing steel embedded in concrete with time.
1886
S.-G. Dong et al. / Electrochimica Acta 56 (2011) 1881–1888
Fig. 8. EIS spectra for reinforcing steel in concrete: (a) total impedance and (b) phase.
that the passive film helped prevent corrosion and that no serious corrosion occurred. This is because the pH value and Cl− concentration of the steel/concrete interface did not reach the critical values for corrosion to begin. After 124 days, large decreases in Rct (dropping to ∼10 k) and n (<0.5), and an increase in Y0 were observed, implying the appearance of corrosion on the reinforcing steel. 3.4. Dependence of the corrosion of reinforcing steel on the pH and Cl− concentration in concrete The above results show that a decrease in the pH and an increase in the Cl− concentration of the steel/concrete interface induce the initiation and acceleration of the corrosion process of reinforcing steel in concrete. When the steel/concrete interface maintains a high pH value (∼13.4) and a low Cl− concentration (∼1 mmol L−1 ), the reinforcing steel maintains a positive Ecorr and a lower icorr , indicating that the passive film on the reinforcing steel is stable in this condition. However, when the pH level dropped and the Cl− concentration increased significantly on the steel/concrete interface after 124 days, the Ecorr shifted negatively to −0.5 V and the icorr increased to 0.2 A cm−2 , implying that the reinforcing steel began to corrode. The EIS spectra also reflect the change in the surface condition of the steel in concrete, charactering as a decrease in Rct and in the diameter of the semicircle in the Nyquist plot. The exper-
Fig. 9. Fitting results for EIS spectra of reinforcing steel in concrete; (a) Rct , (b) Y0 , and (c) n.
iment was conducted in accelerated conditions, including a thinner concrete layer, and also dry and wet cycles, so that the initial time for corrosion to occur was shorter than usual. In the same way, the corrosion behavior of steel is affected by changes in the chemical environment of the concrete. The multifunctional sensor is helpful in deducing the environmental corrosion conditions and the corrosion process of steel in concrete. It is promising that the sensor becomes a powerful tool in evaluating the service life of concrete constructions.
S.-G. Dong et al. / Electrochimica Acta 56 (2011) 1881–1888
1887
Fig. 10. Variations of pH and Cl− concentration in a tunnel concrete construction.
Fig. 12. Variations of polarization resistance (Rp ) and corrosion rate of reinforcing steel in a tunnel concrete construction.
3.5. Corrosion monitoring for a real concrete construction in the field
term monitor pH and Cl− concentration, the most pivotal corrosion species in concrete. The multifunctional sensor is promising to become a powerful tool for monitoring of chemical environment and corrosion of steel in concrete.
Fig. 10 shows a typical result of variations of pH and Cl− concentrations in concrete for a real tunnel construction. The pH in the concrete pore maintained at levels higher than 11.5. The Cl− concentration was always lower than 0.006 mol L−1 , implying that the Cl− concentration in the concrete was very low. As shown in Fig. 11, the Ecorr of the reinforcing steel stayed positive, at about −0.25 VSCE , indicating that the steel in the concrete was in a passive state. Fig. 12 shows that the polarization resistance of steel was large, at about 35 k cm2 , and the corrosion rate was very small, less than 0.007 mm a−1 . In other words, the results demonstrate that the passive film on the reinforced steel remains stable and its corrosion is negligible due to the high pH and low Cl− ion concentration in the concrete, implying that the tunnel construction is safe. Comparing to the previously developed sensors, the multifunctional sensor in this work, integrated with the pH/Cl− sensors, working electrode, reference electrode and counter electrode, is able to perform a more accurate and comprehensive measurement of chemical environment and corrosion behavior of steel in concrete. It is demonstrated in both laboratory measurement and monitoring for a real concrete construction, that the prepared Ti/IrO2 and Ag/AgCl electrodes in this work are suitable to long-
4. Conclusion (1) A multifunctional sensor was developed in this work not only for in situ detection of the pH and Cl− concentration in concrete, but also for determining the corrosion behavior of reinforcing steel in concrete. (2) It was demonstrated that the prepared Ti/IrO2 pH sensor and Ag/AgCl Cl− sensor are of very good potential responses, sensitivity, and stability in a wide pH and Cl− concentration range in solution. They are suitable for in situ and long term monitoring the pH and Cl− concentrations in concrete. (3) The multifunctional sensor may become a powerful tool for in situ and non-destructive monitoring both the corrosion of reinforcing steel in concrete and the chemical environment of the concrete. The corrosion potential, corrosion current density, and EIS behavior of the reinforced steel are closely related to the pH and Cl− concentration in concrete. (4) Corrosion of reinforcing steel occurs after 124 days of exposure, corresponding to a distinct increase in Cl− concentration and decrease in pH, indicating a critical condition of chemical environment to corrosion of reinforcing steel exists in the concrete. (5) The availability of the prepared multifunctional sensor has been demonstrated by the field corrosion monitoring of a real tunnel construction. It is promising to become a powerful tool for providing comprehensive and accurate information on the environment-induced corrosion of reinforcing steel in concrete constructions. Acknowledgements This work has been supported by the National Natural Science Foundation of China (50731004) and Technology Support Programs of China (2007BAB27B04). References
Fig. 11. Variation of corrosion potential of reinforcing steel in a tunnel concrete construction.
[1] A. Bentur, S. Diamond, N.S. Berke, Steel Corrosion in Concrete: Fundamentals and Civil Engineering Practice, E & FN Spon, London, 1997. [2] J.P. Broomfield, Corrosion of Steel in Concrete, E & FN Spon, London, 1997. [3] E. Meck, V. Sirivivatnanon, Cement Concrete Res. 33 (2003) 1113. [4] J.A. Gonzalez, S. Feliu, P. Rodriguez, Corrosion 53 (1997) 65.
1888
S.-G. Dong et al. / Electrochimica Acta 56 (2011) 1881–1888
[5] ASTM C 876-91, Standard Test Method for Half-Cell Potentials of Uncoated Reinforcing Steel in Concrete. America Society of Test and Materials, 1991. [6] B. Elsener, Constr. Build. Mater. 15 (2001) 133. [7] G. Baronio, M. Berra, L. Bertolini, T. Pastore, Cement Concrete Res. 26 (1996) 683. [8] T. Parthiban, R. Ravi, G.T. Parthiban, Adv. Eng. Softw. 37 (2006) 375. [9] C. Andrade, J.A. Gonzalez, Mater. Corros. 29 (1978) 515. [10] S. Feliu, J.A. González, M.C. Andrade, V. Feliu, Corros. Sci. 29 (1989) 105. [11] K.R. Gowers, S.G. Millard, J.S. Gill, R.P. Gill, Br. Corros. J. 29 (1994) 25. [12] B. Elsener, Corros. Sci. 47 (2005) 3019. [13] Y.S. Choi, J.G. Kim, K.M. Lee, Corros. Sci. 48 (2006) 1733. [14] M.F. Montemor, A.M.P. Simoes, M.M. Salta, Cement Concrete Compos. 22 (2000) 175. [15] C. Andrade, M. Keddam, X.R. Nóvoa, M.C. Pérez, C.M. Rangel, H. Takenouti, Electrochim. Acta 46 (2001) 3905. [16] A. Carnot, I. Frateur, S. Zanna, B. Tribollet, I. Dubois-Brugger, P. Marcus, Corros. Sci. 45 (2003) 2513. [17] I. Mohamed, O. Masayasu, Constr. Build. 20 (2006) 458. [18] J. Zhang, P.J.M. Monteiro, Cement Concrete Res. 31 (2001) 603. [19] B. Zhao, J.H. Li, R.G. Hu, R.G. Du, C.J. Lin, Electrochim. Acta 52 (2007) 3976. [20] G.F. Qiao, J.P. Ou, Elctrochim. Acta 52 (2007) 8008. [21] A. Nagiub, F. Mansfeld, Corros. Sci. 43 (2001) 2147. [22] A. Legat, Electrochim. Acta 52 (2007) 7590. [23] S. Muralidharan, V. Saraswathy, A. Madhavamayandi, K. Thangavel, N. Palaniswamy, Electrochim. Acta 53 (2008) 7248. [24] S. Muralidharan, T.H. Ha, J.H. Bae, Y.C. Ha, H.G. Lee, D.K. Kim, Measurement 40 (2007) 600. [25] S. Muralidharan, T.H. Ha, J.H. Bae, Y.C. Ha, H.G. Lee, K.W. Park, D.K. Kim, Sens. Actuators B 113 (2006) 187. [26] S. Muralidharan, T.H. ha, J.H. Bae, Y.C. Ha, H.G. Lee, K.W. Park, D.K. Kim, Mater. Lett. 60 (2006) 651. [27] S. Muralidharan, V. Saraswathy, K. Thangavel, N. Palaniswamy, Sens. Actuators B 130 (2008) 864. [28] G.S. Duffo, S.B. Farina, C.M. Giordano, Electrochim. Acta 54 (2009) 1010. [29] J.H. Yoo, Z.T. Park, J.G. Kim, L. Chuang, Cement Concrete Res. 33 (2003) 2057. [30] Z.T. Park, Y.S. Choi, J.G. Kim, L. Chung, Cement Concrete Res. 35 (2005) 1814. [31] J. Gulikers, Constr. Build. 11 (1997) 143. [32] M. Raupach, P. Schiel, NDT&E Int. 34 (2001) 435. [33] B. Elsener, Cement Concrete Compos. 24 (2002) 65. [34] W.J. McCarter, Ø. Vennesland, Constr. Build. 18 (2004) 351. [35] M.J. Correia, E.V. Pereira, M.M. Salta, I.T.E. Fonseca, Cement Concrete Compos. 28 (2006) 226.
[36] M.A. Climent-Llorca, E. Viqueira-Pérez, M.M. López-Atalaya, Cement Concrete Res. 26 (1996) 1157. [37] M.F. Montemor, J.H. Alves, A.M. Simões, J.C.S. Fernandes, Z. Lourenc¸o, A.J.S. Costa, A.J. Appleton, M.G.S. Ferreira, Cement Concrete Compos. 28 (2006) 233. [38] G. de Vera, M.A. Climent, C. Antón, A. Hidalgo, C. Andrade, J. Electroanal. Chem. 639 (2010) 43. [39] H.S. Lee, S.W. Shin, J.M. Ahn, Y.C. Kim, Y.T. Kho, Mater. Corros. 54 (2003) 229. [40] S.Y. Li, Y.G. Kim, S. Jung, H.S. Song, S.M. Lee, Sens. Actuators B 120 (2007) 368. [41] M. Blumentritt, K. Melhorn, J. Flachsbarth, M. Kroener, W. Kowalsky, H.H. Johannes, Sens. Actuators B 131 (2008) 504. [42] M. Ghandehari, C.S. Vimer, NDT&E Int. 37 (2004) 611. [43] C.K.Y. Leung, NDT&E Int. 34 (2001) 85. [44] Z. Zhang, F. Ansari, Sens. Actuators A 126 (2006) 107. ´ N. Simon, Cement Concrete Compos. 22 (2000) 115. [45] B. Gliˇsic, [46] A. Norris, M. Saafi, P. Romine, Constr. Build. 22 (2008) 111. [47] L.F. Li, A.A. Sagüés, N. Poor, Cement Concrete Res. 29 (1999) 315. [48] L. Cáseres, A.A. Sagüés, S.C. Kranc, R.E. Weyers, Cement Concrete Res. 36 (2006) 492. [49] L.F. Li, J. Nam, W.H. Hartt, Cement Concrete Res. 35 (2005) 277. [50] M. Castellote, C. Alonso, C. Andrade, P. Castro, M. Echeverria, Cement Concrete Res. 31 (2001) 233. [51] Y.H. Liao, J.C. Chou, Mater. Chem. Phys. (2009) 542. [52] S.A.M. Marzouk, Anal. Chem. 75 (2003) 1258. [53] M. Castellote, C. Andrade, C. Alonso, Corros. Sci. 44 (2002) 2409. [54] B. Elsener, L. Zimmermann, H. Böhni, Mater. Corros. 54 (2003) 440. [55] U. Angst, B. Elsener, C.K. Larsen, Ø. Vennesland, Cement Concrete Res. 39 (2009) 1122. [56] P. Castro, A.A. Sagüés, E.I. Moreno, L. Maldonado, J. Genesca, Corrosion 52 (1996) 609. [57] R.G. Du, R.G. Hu, R.S. Huang, C.J. Lin, Anal. Chem. 78 (2006) 3179. [58] C.J. Kitowski, H.G. Wheat, Corrosion 53 (1997) 216. [59] J.P. Broomfield, K. Davies, K. Hladky, Cement Concrete Compos. 24 (2002) 27. [60] M.F. Montemor, A.M.P. Simões, M.G.S. Ferreira, Cement Concrete Compos. 25 (2003) 491. [61] K. Kinoshita, M.J. Madou, J. Electrochem. Soc. 131 (1984) 1089. [62] D. Ammann, Ion-selective Microelectrodes: Principles, Design and Application, first ed., Springer, New York, 1986. [63] V. Kumar, Corros. Rev. 16 (1988) 317. [64] M. Moreno, W. Morris, M.G. Alvarez, G.S. Duffó, Corros. Sci. 46 (2004) 2681. [65] O.A. Kayyali, M.N. Haque, Mag. Concrete Res. 47 (1995) 235. [66] S¸. Erdo˘gdu, T.W. Bremner, I.L. Kondratova, Cement Concrete Res. 31 (2001) 861.
Contents lists available at ScienceDirect
Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta
Effective monitoring of corrosion in reinforcing steel in concrete constructions by a multifunctional sensor Shi-Gang Dong, Chang-Jian Lin ∗,1 , Rong-Gang Hu, Lan-Qiang Li, Rong-Gui Du State Key Laboratory of Physical Chemistry of Solid Surfaces, and Department of Chemistry, College of Chemistry and Chemical Engineering, Xiamen University, Xiamen, Fujian 361005, China
a r t i c l e
i n f o
Article history: Received 30 June 2010 Received in revised form 29 August 2010 Accepted 30 August 2010 Available online 16 September 2010 Keywords: Multifunctional sensor pH value Cl− concentration Corrosion Reinforced concrete
a b s t r a c t A novel multifunctional sensor is developed for in situ and non-destructive monitoring of the corrosion current and open circuit potential of reinforcing steel, as well as the pH and Cl− concentration of concrete. The pH and Cl− sensors show good responses to the pH and Cl− concentration of concrete pore solutions, respectively, and are able to monitor both the carbonization process of concrete and the ingress of Cl− in concrete. Combined with measurements of the corrosion potential and corrosion current density, as well as the EIS spectra of reinforcing steel in concrete, this study demonstrates that the pH and the Cl− concentration of concrete are two of the most crucial factors that determine the corrosion of reinforcing steel in concrete. The corrosion tendency and corrosion rate of reinforcing steel largely depend on the chemical environment in the concrete. The multifunctional sensor is a powerful tool for in situ monitoring corrosion of steel in concrete, and provides accurate details of the chemical condition of the concrete pore solution and the corrosion status of the reinforcing steel in concrete. These are essential for corrosion predictions and service life evaluations of concrete constructions. © 2010 Elsevier Ltd. All rights reserved.
1. Introduction Reinforced concrete is applied widely as a structural material in various infrastructures. With the development of diversified constructions and service conditions in the past decades, environment-induced corrosion and the premature degradation of steel-reinforced concrete structures have become increasingly serious and ubiquitous. The durability and safety of large-scale or key rebar reinforced constructions have become matters of great concern. Normally, reinforcing steel in concrete does not corrode because a compact and stable passive film always forms on the surface of the reinforcing steel in the highly alkaline concrete pore solutions. However, the carbonization of concrete and/or the ingress of Cl− may result in the breakdown of the passive film. Corrosion of the reinforcing steel then occurs and develops, which not only destroys the construction, but also sometimes causes accidents and safety problems. The rehabilitation and maintenance costs of a construction that has been degraded by corrosion may sometimes be several times higher than the original cost of the construction [1–4]. Thus, it is necessary to develop various techniques
∗ Corresponding author. Tel.: +86 592 2189354; fax: +86 592 2186657. E-mail address: [email protected] (C.-J. Lin). 1 ISE member. 0013-4686/$ – see front matter © 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.electacta.2010.08.089
for monitoring and studying the corrosion of reinforcing steel in concrete constructions. The essential reason for the premature degradation of reinforced concrete constructions is the corrosion of steel rebar in concrete, which is, in principle, a typical electrochemical process that happens in an occluded system. As such, electrochemical techniques have particular advantages in the detection of corrosion in concrete. The half-cell potential technique, linear polarization technique, electrochemical impedance spectroscopy, electrochemical noise technique, and electrode array technique, among others, have been widely used to measure the corrosion of reinforcing steel in concrete [5–22]. However, most of these techniques are generally used only in the laboratory and, to date, very few of them can be applied in field monitoring. Although many non-electrochemical techniques, such as ultrasonic and eddy current detections, have been developed in the past years, electrochemical sensors play the most crucial role in monitoring the corrosion of reinforcing steel because they are able to provide more and earlier in situ information of the corrosion process of steel-reinforced constructions. The measurement of the corrosion potential is essential for monitoring the corrosion process of reinforcing steel in concrete, and various embeddable solid reference sensors in concrete have been developed [23–28]. Long-term potential stable reference sensors embedded in concrete provide opportunities to follow the corro-
1882
S.-G. Dong et al. / Electrochimica Acta 56 (2011) 1881–1888
sion potential of steel in concrete. Many galvanic sensors based on the galvanic current between anode and cathode in reinforcing steel have been applied to monitor the galvanic corrosion of reinforcing steel in concrete. It is also possible to detect the corrosion distribution in one dimension in concrete by arranging of multiple anodes in a ladder format at different depths [29–34]. Correia et al. [35] revealed a linear relationship between the limiting currents of oxygen reduction and oxygen concentration, and developed an oxygen sensor for measuring the oxygen concentration in concrete. The Ag/AgCl electrode, as a Cl− sensor, has been used for monitoring Cl− ion concentrations in concrete [36–38]. Other sensor techniques, such as resistance, fiber optic, ceramic, and microelectromechanical system sensors, have also been designed for evaluating corrosion, pH levels, cracks, deformations, temperature, and moisture in concrete [39–46]. Concrete is a typical multi-scale and non-uniform system consisting of the gas–liquid–solid phases. The corrosion process of reinforcing steel in concrete is usually much more complicated compared to the general corrosion process of steel in an open system. In generally speaking, more parameters are required to determine a complicated system because it is dependent upon many variables. A single bit of information, such as the corrosion potential or corrosion current of steel in concrete measured by an electrochemical measurement method is not enough to accurately and comprehensively describe the corrosion process of steel in concrete. Scattered data of electrochemical measurements from steel in concrete are frequently encountered due to the various variables that exist during the corrosion process. For example, when the cathodic reaction is controlled by oxygen transport in concrete, the corrosion potential of steel varies with the oxygen concentration in concrete; however, it is not directly related to the corrosion status or corrosion rate of the steel. The Cl− concentration and pH at the interface of the steel/concrete are recognized as two of the most crucial environmental factors that determine the stability and breakdown of the passive film on steel in concrete. The measurement of pH and Cl− concentration in concrete were previously performed by leaching the concrete pore solutions, and then by chemical analysis [47–50], which was usually time-consuming, not in situ and destructive to the concrete construction. Various potentiometric sensors, such as Ti and AgCl electrodes [51–55], were developed for monitoring the pH and Cl− concentration, respectively, in concrete. However, the potential of Ti in alkaline solution is not stable, and the correspondence between potential and pH is not linear [56]. The same happened with the Ag/AgCl electrode when embedded in concrete if it was not properly prepared. Climent and co-workers [38] showed that the Ag/AgCl electrodes can be stable in solution, but a large bias appeared when they were embedded in concrete with a very small amount of chloride. Also the potential evolved with time due to attack of the electrode by hydroxides. The performances of the sensors largely depended on the structure and preparation of the electrodes. It is our desire to develop a practical multifunctional sensor to be able to provide more comprehensive in situ information not only on the corrosion process of steel, but also on the chemical environment of concrete, especially for the long-term monitoring of environment-induced corrosion in concrete constructions [57]. This work attempts (1) to develop a hybrid sensor composed of Ag/AgCl, IrO2 electrodes and electrochemical three electrode system to in situ monitor the Cl− concentrations and pH of concrete, and follow the corrosion process of reinforcing steel in concrete; (2) to characterize the sensitivity, stability, and response of the prepared sensors to concrete constructions; and (3) to further understand the dependence of corrosion behavior of reinforcing steel on the Cl− concentrations and pH in concrete.
2. Experimental 2.1. pH sensor The pH sensor constructed from a Ti/IrO2 electrode was prepared from H2 IrCl6 by thermal oxidation on a Ti rod surface. The Ti rod, 6 mm in diameter and 25 mm in length, was ultrasonically cleaned in EtOH and distilled water, and then treated in a boiling 10% oxalic acid solution for 20 min. A dipping solution, composed of a mixture of chloroiridic acid and isopropyl alcohol at a specific ratio, was spread on the Ti rod’s surface by a brush, dried under an infrared lamp, and then heat-treated in a muffle furnace at 450 ◦ C for 20 min. After repeating this spreading-drying-heat treating process for 20 times, the Ti rod was heat-treated at 450 ◦ C for 120 min. A compact IrO2 film was formed on the rod’s surface and used as the pH sensor. One end of the Ti rod was welded to a conducting wire, and the welding spot and side faces were sealed with an epoxy resin. Only the top of the rod was exposed. The prepared Ti/IrO2 electrode was activated in a simulated concrete pore solution of saturated Ca(OH)2 for 1 month before it was used [58]. The XRD spectra of the Ti/IrO2 electrode were obtained by a PANalytical X’Pert PRO diffractometer. The potential responses of Ti/IrO2 to pH were measured in a series of solutions adjusted to different pH values.
2.2. Cl− sensor An Ag/AgCl electrode was used as the Cl− sensor and prepared by anodic chlorination. An Ag rod (99.99% purity), 4 mm in diameter and 10 mm in length, was welded to a conducting wire, and the welding spot and part of the rod were sealed with an epoxy resin. Only about 5 mm of the length of the rod was exposed. After burnishing with an emery cloth and ultrasonic cleaning in EtOH and distilled water, the Ag rod was anodically chloridized in an HCl solution at a given current of 0.1 mA cm−2 for 24 h. An AgCl film formed on the rod’s surface. The prepared Ag/AgCl electrode was rinsed with distilled water, and was then placed in the dark. The potential responses of the prepared Ag/AgCl electrode to various Cl− concentrations were measured in saturated Ca(OH)2 solutions with different concentrations of NaCl. The potentials (vs. a saturated calomel electrode, SCE) of the pH and Cl− sensors in solution were measured by a high-resistance digital multimeter. The electrochemical measurements were performed by an Ivium electrochemical workstation.
Fig. 1. Schematic structure of MnO2 reference electrode.
S.-G. Dong et al. / Electrochimica Acta 56 (2011) 1881–1888
1883
Fig. 2. Schematic arrangement of the multifunctional sensor.
2.3. MnO2 reference electrode As shown in Fig. 1, the reference electrode was made of MnO2 powder, graphite, and a binder, mixed at a given ratio, and mounted on a stainless steel barrel 15 mm in diameter and 45 mm in length. An alkaline slurry of Ca(OH)2 was poured into the MnO2 ring, and the top was sealed with a porous hydrated cement paste, which provided an electrical contact with the surrounding environment. The stainless steel was connected to a conductive wire and the whole assembly was encapsulated in a rubber shrink tube. The half-cell potential of the MnO2 electrode was determined by an MnO2 /Mn2 O3 equilibrium potential. The filled core was composed of an alkaline slurry with a high pH corresponding to the concrete pore solution. The porous cement paste top bonded with the concrete well, so junction potentials across the cement paste were very small [26,27].
2.4. Design and assembly of multifunctional sensors One end of HRB335 reinforcing steel with 8 mm in diameter, the same material as that used in concrete, was connected to a conductive wire. The welding spot and the side surface were sealed with an epoxy resin, and another cross section acted as the working area. The working area was mechanically polished by sand paper and ultrasonically cleaned in EtOH. Fig. 2 shows the schematic arrangement of the multifunctional sensor consisting of the Cl− sensor, the pH sensor, a steel working electrode, the MnO2 reference electrode, and a stainless steel thimble. All of the electrodes were assembled in and fixed to the stainless steel thimble with an epoxy resin at their designated positions. The stainless steel thimble, which was about 30 mm in diameter, was used as both a counter electrode for electrochemical measurements and a main holder for the multifunctional sensor. The top of the multifunctional sensor was covered with a thin layer (about 3 mm thick) of cement mortar with a water/cement ratio of 0.6.
2.5. Preparation of concrete sample The concrete sample was cast with 42.5# Portland cement at a cement:water:fine sand:coarse aggregate ratio of 1:0.4:1.57:2.36. The prepared multifunctional sensor was embedded in the concrete at a depth of about 10 mm. The concrete sample was cured at room temperature and 95% RH for 28 days.
2.6. Measurement of pH, Cl− concentration, and corrosion of reinforcing steel in concrete The prepared concrete sample was exposed to a cyclic condition of immersion in a 3.5% NaCl solution for 2 days and then drying for 2 days. At certain periods, the potentials of the Cl− and pH sensors were measured to calculate the Cl− concentration and pH of the concrete, respectively. At the same time, the corrosion potential, corrosion current density, and EIS spectra of the embedded steel electrode in sensor were obtained. The potential measurements of the pH and Cl− sensors, as well as the steel electrode, were carried out by a high-resistance digital multimeter relative to the MnO2 reference electrode. In order to be able to compare the data obtained with those in the literature, all measured potentials were converted and reported with respect to SCE. A three-electrode system was used to measure the corrosion current density and obtain the EIS spectra, which was performed by an Ivium electrochemical workstation. The reinforcing steel, MnO2 electrode, and stainless steel thimble acted as the working electrode, reference electrode, and counter electrode, respectively. The corrosion current density, icorr , was measured using the linear polarization method from a potential scan within Ecorr ± 15 mV at 0.167 mV s−1 , based on the following equation: icorr =
ˇa ˇc B = Rp 2.303 · Rp (ˇa + ˇc )
(1)
where Rp is the polarization resistance, and B is a constant related to the anodic and cathodic Tafel slopes. B was equal to 26 mV for the active state and 52 mV for the passive state [34,59,60]. In the present work, the icorr was obtained directly through a fitting procedure, carried out on an Ivium electrochemical measurement system. The EIS spectra of the reinforcing steel were obtained in the 105 –10−2 Hz range at 10 mV amplitude of the stimulant sinusoidal voltage. 2.7. Corrosion monitoring for a tunnel construction by the multifunctional sensor In order to further evaluate the viability of the prepared multifunctional sensor, the comprehensive corrosion data, including corrosion rate, polarization resistance, open circuit potential of the reinforcing steel, and pH and Cl− concentration of concrete, were monitored at a river tunnel concrete construction. Since the tunnel was an old concrete construction, several holes were drilled into the wall inside the tunnel, and the multifunctional sensors were inserted and fixed to the base of the holes using cement
1884
S.-G. Dong et al. / Electrochimica Acta 56 (2011) 1881–1888
Fig. 3. Time response of (a) pH sensor and (b) Cl− sensor.
mortar. The pH and Cl− concentration of the concrete, and the corrosion potential, polarization resistance, and corrosion rate of steel were monitored using the multifunctional sensor. The data were collected using a computer-assisted measurement system.
following equations, respectively [61,62]: IrO2 + H+ + e → 1/2Ir2 O3 + 1/2H2 O 0 E = EIrO
2 /Ir2 O3
0 = EIrO
2 /Ir2 O3
RT
ln[H+ ] F − 0.0592 pH(vs. SHE/V, 298 K) +
3. Results and discussion
AgCl + e → g + Cl−
3.1. Characterization of pH and Cl− sensors
0 E = EAgCl/Ag +
The typical XRD spectra of the prepared Ti/IrO2 electrode and Ag/AgCl electrode were measured. By comparison to standards, it was confirmed that a tetragonal IrO2 crystal film was coated onto the Ti rod’s surface, and a cubic AgCl crystal film on the Ag rod’s surface. To evaluate the stability and sensitivity of the prepared pH and Cl− sensors, the variations of sensor potentials with time were measured in the simulated concrete pore solution. Fig. 3a shows that the potential of the pH sensor in a solution with a pH of 9.3 changes less than 3 mV within 30 min, and then rapidly shifts and becomes stable again in a few minutes when about 2 mL of 0.1 mol L−1 NaOH solution is added in the test solution. This indicates that the prepared sensor is of good stability and high sensitivity to the pH of the test solution. As shown in Fig. 3b, the potential of the Cl− sensor is very stable in a 0.01 mol L−1 NaCl solution and shows almost no change within 40 min, quickly responding to any change in Cl− concentration in the solution. This demonstrates that the prepared Cl− sensor is stable in solutions containing Cl− and is highly sensitive to variations in Cl− concentration. The principles of the Ti/IrO2 electrode as a pH sensor and the Ag/AgCl electrode as a Cl− sensor can be described through the
RT
ln[Cl− ] F 0 = EAgCl/Ag − 0.0592 lg[Cl− ](vs SHE/V, 298 K)
(2) (3) (4) (5)
According to Eqs. (3) and (5), at a given temperature, the potential of the prepared Ti/IrO2 electrode only depends on the pH of the solution, and the potential of the Ag/AgCl electrode only relies on the Cl− concentration. By measuring the potential of the sensors, the pH and Cl− concentrations of the solution can be obtained accordingly. A three-component simulated concrete pore solution [58], 0.6 mol L−1 KOH + 0.2 mol L−1 + 0.001 mol L−1 Ca(OH)2 , was prepared to calibrate the pH sensor. The pH level of the solution was adjusted by adding 0.8 mol L−1 NaHCO3 and/or diluted H2 SO4 solutions. Fig. 4a shows a good linear relationship between the potential of the pH sensor and the pH in a wide pH range of 7–14. The linear relationship can be described by the linear regression equation: E(vs. SCE/mV) = 670.3 − 58.4 pH(R2 = 0.99925)
(6)
The response slope, 58.4 mV, is close to the theoretical value of 59.2 mV at room temperature. The pH value in a normal concrete pore solution is about 13, and may drop to 8–9 when the concrete is seriously carbonized [63–65]. The linear response region of the
Fig. 4. Calibration curves of (a) pH sensor and (b) Cl− sensor.
S.-G. Dong et al. / Electrochimica Acta 56 (2011) 1881–1888
1885
Fig. 5. Variation of pH and Cl− concentration in steel/concrete interface with time.
pH sensor is sensitive enough to follow any change in the pH level in the concrete pore solution. The potential–concentration calibration curve of the Cl− sensor was measured in saturated Ca(OH)2 solutions with different concentrations of NaCl. A perfect linear relationship, as shown in Fig. 4b, shows the potential of the Cl− sensor and the logarithm of the Cl− concentration in a range of 10−4 –2 mol L−1 , as described by the linear regression equation: E(vs SCE/mV) = 11.8 − 49.1 lg[Cl− ] (R2 = 0.99939)
(7)
The slope is smaller than the theoretical value; however, the linear relationship of the sensor is good enough for measuring the Cl− concentration in concrete. 3.2. Monitoring of pH and Cl− concentrations in steel/concrete interfaces The potentials of the pH and Cl− sensors were measured during the exposure period, and the pH value and Cl− concentrations of the concrete were calculated using Eqs. (6) and (7), respectively. As shown in Fig. 5, the Cl− concentration in concrete remained at low levels during the first 40 days of exposure, and then increased gradually afterward. The increase in the Cl− concentration of the concrete was driven by the concentration difference between the external solution and the inner concrete. The Cl− concentration of concrete reached a maximum and then becomes constant after 124 days. The accumulation of the Cl− in the concrete resulted in a high Cl− concentration; the Cl− concentration in the concrete was close to that of the test solution. The pH value decreased slowly at the beginning of the experiment due to the OH− that diffused from the inner concrete to the external solution based on the pH difference between the two zones. It then exhibited a big drop after 120 days of exposure, indicating that the depth of carbonization reached the steel/concrete interface.
Fig. 6. Variation of corrosion potential Ecorr of reinforcing steel embedded in concrete with time.
0.1 A cm−2 , thus proving that the steel in concrete had a moderate corrosion rate [66]. The time for the initial corrosion of the reinforcing steel is close to the time of the decrease in pH and the increase in Cl− concentration, indicating that the corrosion state and corrosion rate of steel in concrete are closely related to the pH and Cl− concentration of the concrete pore solution. Fig. 8 shows the EIS spectra, in Bode plots, for the reinforcing steel in concrete. The Nyquist plot presents a non-ideal semicircle at low frequencies. The EIS spectra, from the diameter of the semicircle and the phase angle, the total impedance in Bode plots, exhibited little change with time in the initial 124 days, but changed rapidly afterward. EIS plots are usually analyzed by an equivalent circuit of Rs (Rct Q). The Rs , Rct , and Q are the resistance of the concrete pore solution, the charge transfer resistance, and the constant phase element (CPE) of double layers of steel/concrete interfaces, respectively. The CPE can be defined using the following equation: ZCPE =
1 Y0
(jω)
−n
(8)
where Y0 is a constant, and n is the CPE-power ranging from 0 to 1. The CPE is a pure resistance and a pure capacitance if n = 0 and 1, respectively [19,20]. Based on the equivalent circuit of Rs (Rct Q), the fitting data for the EIS of the reinforcing steel in concrete are shown in Fig. 9. In the initial 124 days, the Rct was 1–10 M, the Y0 was 2.0–4.0 × 10−5 −1 sn , and the n was about 0.8, indicating
3.3. Corrosion behavior of reinforcing steel in concrete The corrosion potential, Ecorr , corrosion current density, icorr , and EIS spectra of reinforcing steel in concrete were measured and obtained at the same time as the measurements of the pH and Cl− concentrations of the steel/concrete interface were made. Fig. 6 shows that the Ecorr of the reinforcing steel in concrete exhibited a positive value (−250 to −350 mVSCE ) during the first 124 days of exposure, implying that the steel in the concrete maintained a passive state. It then shifted negatively after 124 days, close to about −550 mVSCE . The icorr of steel (see Fig. 7) was at low levels for the first 124 days, and then increased rapidly afterward to over
Fig. 7. Variation of corrosion current density icorr of reinforcing steel embedded in concrete with time.
1886
S.-G. Dong et al. / Electrochimica Acta 56 (2011) 1881–1888
Fig. 8. EIS spectra for reinforcing steel in concrete: (a) total impedance and (b) phase.
that the passive film helped prevent corrosion and that no serious corrosion occurred. This is because the pH value and Cl− concentration of the steel/concrete interface did not reach the critical values for corrosion to begin. After 124 days, large decreases in Rct (dropping to ∼10 k) and n (<0.5), and an increase in Y0 were observed, implying the appearance of corrosion on the reinforcing steel. 3.4. Dependence of the corrosion of reinforcing steel on the pH and Cl− concentration in concrete The above results show that a decrease in the pH and an increase in the Cl− concentration of the steel/concrete interface induce the initiation and acceleration of the corrosion process of reinforcing steel in concrete. When the steel/concrete interface maintains a high pH value (∼13.4) and a low Cl− concentration (∼1 mmol L−1 ), the reinforcing steel maintains a positive Ecorr and a lower icorr , indicating that the passive film on the reinforcing steel is stable in this condition. However, when the pH level dropped and the Cl− concentration increased significantly on the steel/concrete interface after 124 days, the Ecorr shifted negatively to −0.5 V and the icorr increased to 0.2 A cm−2 , implying that the reinforcing steel began to corrode. The EIS spectra also reflect the change in the surface condition of the steel in concrete, charactering as a decrease in Rct and in the diameter of the semicircle in the Nyquist plot. The exper-
Fig. 9. Fitting results for EIS spectra of reinforcing steel in concrete; (a) Rct , (b) Y0 , and (c) n.
iment was conducted in accelerated conditions, including a thinner concrete layer, and also dry and wet cycles, so that the initial time for corrosion to occur was shorter than usual. In the same way, the corrosion behavior of steel is affected by changes in the chemical environment of the concrete. The multifunctional sensor is helpful in deducing the environmental corrosion conditions and the corrosion process of steel in concrete. It is promising that the sensor becomes a powerful tool in evaluating the service life of concrete constructions.
S.-G. Dong et al. / Electrochimica Acta 56 (2011) 1881–1888
1887
Fig. 10. Variations of pH and Cl− concentration in a tunnel concrete construction.
Fig. 12. Variations of polarization resistance (Rp ) and corrosion rate of reinforcing steel in a tunnel concrete construction.
3.5. Corrosion monitoring for a real concrete construction in the field
term monitor pH and Cl− concentration, the most pivotal corrosion species in concrete. The multifunctional sensor is promising to become a powerful tool for monitoring of chemical environment and corrosion of steel in concrete.
Fig. 10 shows a typical result of variations of pH and Cl− concentrations in concrete for a real tunnel construction. The pH in the concrete pore maintained at levels higher than 11.5. The Cl− concentration was always lower than 0.006 mol L−1 , implying that the Cl− concentration in the concrete was very low. As shown in Fig. 11, the Ecorr of the reinforcing steel stayed positive, at about −0.25 VSCE , indicating that the steel in the concrete was in a passive state. Fig. 12 shows that the polarization resistance of steel was large, at about 35 k cm2 , and the corrosion rate was very small, less than 0.007 mm a−1 . In other words, the results demonstrate that the passive film on the reinforced steel remains stable and its corrosion is negligible due to the high pH and low Cl− ion concentration in the concrete, implying that the tunnel construction is safe. Comparing to the previously developed sensors, the multifunctional sensor in this work, integrated with the pH/Cl− sensors, working electrode, reference electrode and counter electrode, is able to perform a more accurate and comprehensive measurement of chemical environment and corrosion behavior of steel in concrete. It is demonstrated in both laboratory measurement and monitoring for a real concrete construction, that the prepared Ti/IrO2 and Ag/AgCl electrodes in this work are suitable to long-
4. Conclusion (1) A multifunctional sensor was developed in this work not only for in situ detection of the pH and Cl− concentration in concrete, but also for determining the corrosion behavior of reinforcing steel in concrete. (2) It was demonstrated that the prepared Ti/IrO2 pH sensor and Ag/AgCl Cl− sensor are of very good potential responses, sensitivity, and stability in a wide pH and Cl− concentration range in solution. They are suitable for in situ and long term monitoring the pH and Cl− concentrations in concrete. (3) The multifunctional sensor may become a powerful tool for in situ and non-destructive monitoring both the corrosion of reinforcing steel in concrete and the chemical environment of the concrete. The corrosion potential, corrosion current density, and EIS behavior of the reinforced steel are closely related to the pH and Cl− concentration in concrete. (4) Corrosion of reinforcing steel occurs after 124 days of exposure, corresponding to a distinct increase in Cl− concentration and decrease in pH, indicating a critical condition of chemical environment to corrosion of reinforcing steel exists in the concrete. (5) The availability of the prepared multifunctional sensor has been demonstrated by the field corrosion monitoring of a real tunnel construction. It is promising to become a powerful tool for providing comprehensive and accurate information on the environment-induced corrosion of reinforcing steel in concrete constructions. Acknowledgements This work has been supported by the National Natural Science Foundation of China (50731004) and Technology Support Programs of China (2007BAB27B04). References
Fig. 11. Variation of corrosion potential of reinforcing steel in a tunnel concrete construction.
[1] A. Bentur, S. Diamond, N.S. Berke, Steel Corrosion in Concrete: Fundamentals and Civil Engineering Practice, E & FN Spon, London, 1997. [2] J.P. Broomfield, Corrosion of Steel in Concrete, E & FN Spon, London, 1997. [3] E. Meck, V. Sirivivatnanon, Cement Concrete Res. 33 (2003) 1113. [4] J.A. Gonzalez, S. Feliu, P. Rodriguez, Corrosion 53 (1997) 65.
1888
S.-G. Dong et al. / Electrochimica Acta 56 (2011) 1881–1888
[5] ASTM C 876-91, Standard Test Method for Half-Cell Potentials of Uncoated Reinforcing Steel in Concrete. America Society of Test and Materials, 1991. [6] B. Elsener, Constr. Build. Mater. 15 (2001) 133. [7] G. Baronio, M. Berra, L. Bertolini, T. Pastore, Cement Concrete Res. 26 (1996) 683. [8] T. Parthiban, R. Ravi, G.T. Parthiban, Adv. Eng. Softw. 37 (2006) 375. [9] C. Andrade, J.A. Gonzalez, Mater. Corros. 29 (1978) 515. [10] S. Feliu, J.A. González, M.C. Andrade, V. Feliu, Corros. Sci. 29 (1989) 105. [11] K.R. Gowers, S.G. Millard, J.S. Gill, R.P. Gill, Br. Corros. J. 29 (1994) 25. [12] B. Elsener, Corros. Sci. 47 (2005) 3019. [13] Y.S. Choi, J.G. Kim, K.M. Lee, Corros. Sci. 48 (2006) 1733. [14] M.F. Montemor, A.M.P. Simoes, M.M. Salta, Cement Concrete Compos. 22 (2000) 175. [15] C. Andrade, M. Keddam, X.R. Nóvoa, M.C. Pérez, C.M. Rangel, H. Takenouti, Electrochim. Acta 46 (2001) 3905. [16] A. Carnot, I. Frateur, S. Zanna, B. Tribollet, I. Dubois-Brugger, P. Marcus, Corros. Sci. 45 (2003) 2513. [17] I. Mohamed, O. Masayasu, Constr. Build. 20 (2006) 458. [18] J. Zhang, P.J.M. Monteiro, Cement Concrete Res. 31 (2001) 603. [19] B. Zhao, J.H. Li, R.G. Hu, R.G. Du, C.J. Lin, Electrochim. Acta 52 (2007) 3976. [20] G.F. Qiao, J.P. Ou, Elctrochim. Acta 52 (2007) 8008. [21] A. Nagiub, F. Mansfeld, Corros. Sci. 43 (2001) 2147. [22] A. Legat, Electrochim. Acta 52 (2007) 7590. [23] S. Muralidharan, V. Saraswathy, A. Madhavamayandi, K. Thangavel, N. Palaniswamy, Electrochim. Acta 53 (2008) 7248. [24] S. Muralidharan, T.H. Ha, J.H. Bae, Y.C. Ha, H.G. Lee, D.K. Kim, Measurement 40 (2007) 600. [25] S. Muralidharan, T.H. Ha, J.H. Bae, Y.C. Ha, H.G. Lee, K.W. Park, D.K. Kim, Sens. Actuators B 113 (2006) 187. [26] S. Muralidharan, T.H. ha, J.H. Bae, Y.C. Ha, H.G. Lee, K.W. Park, D.K. Kim, Mater. Lett. 60 (2006) 651. [27] S. Muralidharan, V. Saraswathy, K. Thangavel, N. Palaniswamy, Sens. Actuators B 130 (2008) 864. [28] G.S. Duffo, S.B. Farina, C.M. Giordano, Electrochim. Acta 54 (2009) 1010. [29] J.H. Yoo, Z.T. Park, J.G. Kim, L. Chuang, Cement Concrete Res. 33 (2003) 2057. [30] Z.T. Park, Y.S. Choi, J.G. Kim, L. Chung, Cement Concrete Res. 35 (2005) 1814. [31] J. Gulikers, Constr. Build. 11 (1997) 143. [32] M. Raupach, P. Schiel, NDT&E Int. 34 (2001) 435. [33] B. Elsener, Cement Concrete Compos. 24 (2002) 65. [34] W.J. McCarter, Ø. Vennesland, Constr. Build. 18 (2004) 351. [35] M.J. Correia, E.V. Pereira, M.M. Salta, I.T.E. Fonseca, Cement Concrete Compos. 28 (2006) 226.
[36] M.A. Climent-Llorca, E. Viqueira-Pérez, M.M. López-Atalaya, Cement Concrete Res. 26 (1996) 1157. [37] M.F. Montemor, J.H. Alves, A.M. Simões, J.C.S. Fernandes, Z. Lourenc¸o, A.J.S. Costa, A.J. Appleton, M.G.S. Ferreira, Cement Concrete Compos. 28 (2006) 233. [38] G. de Vera, M.A. Climent, C. Antón, A. Hidalgo, C. Andrade, J. Electroanal. Chem. 639 (2010) 43. [39] H.S. Lee, S.W. Shin, J.M. Ahn, Y.C. Kim, Y.T. Kho, Mater. Corros. 54 (2003) 229. [40] S.Y. Li, Y.G. Kim, S. Jung, H.S. Song, S.M. Lee, Sens. Actuators B 120 (2007) 368. [41] M. Blumentritt, K. Melhorn, J. Flachsbarth, M. Kroener, W. Kowalsky, H.H. Johannes, Sens. Actuators B 131 (2008) 504. [42] M. Ghandehari, C.S. Vimer, NDT&E Int. 37 (2004) 611. [43] C.K.Y. Leung, NDT&E Int. 34 (2001) 85. [44] Z. Zhang, F. Ansari, Sens. Actuators A 126 (2006) 107. ´ N. Simon, Cement Concrete Compos. 22 (2000) 115. [45] B. Gliˇsic, [46] A. Norris, M. Saafi, P. Romine, Constr. Build. 22 (2008) 111. [47] L.F. Li, A.A. Sagüés, N. Poor, Cement Concrete Res. 29 (1999) 315. [48] L. Cáseres, A.A. Sagüés, S.C. Kranc, R.E. Weyers, Cement Concrete Res. 36 (2006) 492. [49] L.F. Li, J. Nam, W.H. Hartt, Cement Concrete Res. 35 (2005) 277. [50] M. Castellote, C. Alonso, C. Andrade, P. Castro, M. Echeverria, Cement Concrete Res. 31 (2001) 233. [51] Y.H. Liao, J.C. Chou, Mater. Chem. Phys. (2009) 542. [52] S.A.M. Marzouk, Anal. Chem. 75 (2003) 1258. [53] M. Castellote, C. Andrade, C. Alonso, Corros. Sci. 44 (2002) 2409. [54] B. Elsener, L. Zimmermann, H. Böhni, Mater. Corros. 54 (2003) 440. [55] U. Angst, B. Elsener, C.K. Larsen, Ø. Vennesland, Cement Concrete Res. 39 (2009) 1122. [56] P. Castro, A.A. Sagüés, E.I. Moreno, L. Maldonado, J. Genesca, Corrosion 52 (1996) 609. [57] R.G. Du, R.G. Hu, R.S. Huang, C.J. Lin, Anal. Chem. 78 (2006) 3179. [58] C.J. Kitowski, H.G. Wheat, Corrosion 53 (1997) 216. [59] J.P. Broomfield, K. Davies, K. Hladky, Cement Concrete Compos. 24 (2002) 27. [60] M.F. Montemor, A.M.P. Simões, M.G.S. Ferreira, Cement Concrete Compos. 25 (2003) 491. [61] K. Kinoshita, M.J. Madou, J. Electrochem. Soc. 131 (1984) 1089. [62] D. Ammann, Ion-selective Microelectrodes: Principles, Design and Application, first ed., Springer, New York, 1986. [63] V. Kumar, Corros. Rev. 16 (1988) 317. [64] M. Moreno, W. Morris, M.G. Alvarez, G.S. Duffó, Corros. Sci. 46 (2004) 2681. [65] O.A. Kayyali, M.N. Haque, Mag. Concrete Res. 47 (1995) 235. [66] S¸. Erdo˘gdu, T.W. Bremner, I.L. Kondratova, Cement Concrete Res. 31 (2001) 861.