Threshold Chloride Levels for Localized Carbon Steel Corrosion in Simulated Concrete Pore Solutions Using Coupled Multielectrode Array Sensors

Threshold Chloride Levels for Localized Carbon Steel Corrosion in Simulated Concrete Pore Solutions Using Coupled Multielectrode Array Sensors Lietai Yang,* Kuang-Tsan Chiang,, ** Hui Yu,*** Roberto T. Pabalan,**** Biswajit Dasgupta,***** and Luis Ibarra****** ABSTRACT Threshold chloride levels for localized corrosion of carbon steel material have been studied in three types of simulated concrete pore solutions: sodium hydroxide-potassium hydroxide (NaOH-KOH) with ph 11.6, calcium hydroxide (Ca[OH] 2 ) with ph 12.6, and sodium hydroxide-potassium hydroxide-calcium hydroxide (NaOH-KOH-Ca[OH] 2 ) with ph 13.3. The nonuniform corrosion rates of carbon steel were measured with coupled multielectrode array sensors (CMAS) when the chloride concentration was changed from 0.0005 mol/l to 1 mol/l in each solution. Open-circuit potentials were also measured from the coupling joint of the CMAS probes and electrodes made of rebar specimens immersed in the simulated pore solutions to verify the results from the CMAS probes. KEY WORDS: array sensor, coupled multielectrode array sensor, corrosion initiation, corrosion sensor, coupled multielectrode, multielectrode, open-circuit potential, reinforced steel bar (rebar), simulated pore solution, threshold chloride level Submitted for publication: September 6, 2012. Revised and accepted: March 18, 2014. Preprint available online: April 3, 2014, doi: http://dx.doi.org/10.5006/0830. Corresponding author. E-mail: kchiang505@gmail.com. * Corr Instruments, LLC, 7112 Oaklawn Drive, San Antonio, TX 78229. ** KC Technologies, 13104 Mystic Saddle, Helotes, TX 78023. *** Wood Group Mustang, Inc., 16001 Park Ten Place, Houston, TX 77084. **** Nuclear Waste Technical Review Board, 2300 Clarendon Boulevard, Suite 1300, Arlington, VA 22201-3367. ***** Southwest Research Institute, 6220 Culebra Road, San Antonio, TX 78238. ****** University of Utah, Department of Civil Engineering, Salt Lake City, UT 84112. INTRODUCTION It is well known that the ingress of chloride in concrete can cause corrosion of the reinforcing steel bar (rebar), leading to the premature degradation of a reinforced concrete structure. 1 Chlorides, from either the marine environments or the deicing salt applications, can compromise rebar passivity and initiate active corrosion once the chloride content at the rebar surface reaches a threshold level denoted by C th. 2 Once corrosion occurs, it can lead to reinforced concrete structure cracking and spalling. Therefore, it is critical to determine C th and understand the characteristics of reinforcement corrosion initiation. Studying embedded rebar corrosion in an actual reinforced concrete structure is labor-intensive and time-consuming. Simulated concrete pore solutions have been widely used to characterize rebar corrosion initiation and evaluate rebar materials and corrosion inhibitors. 3-7 Electrochemical methods have been used to determine rebar corrosion initiation and extent, qualitatively and quantitatively. Open-circuit potential (OCP) measurement for rebar corrosion initiation determination is sensitive, convenient, and has been standardized. 8 In this case, an absolute OCP value or an abrupt potential negative shift is generally used as a criterion of rebar corrosion initiation. However, there are controversial, experimental, absolute OCP results 9 that indicate this criterion is not unconditional, because the measured OCP values are affected by the voltage drop under certain conditions. Besides, the OCP values are also affected by mass transport (e.g., ISSN 0010-9312 (print), 1938-159X (online) 850 14/0000137/$5.00+$0.50/0 2014, NACE International CORROSION AUGUST 2014

O 2 when fully immersed or close to full saturation). In addition, OCP measurement cannot be related to the quantitative rate of rebar corrosion. Therefore, the nonrecoverable drops in OCP during a continuous measurement are generally used as rebar corrosion initiation criteria in a laboratory. The linear polarization resistance (LPR) method has been widely used to characterize corrosion initiation and propagation based on the measured corrosion current density, i corr. Generally, when the i corr exceeds 0.1 μa/cm 2 (approximately corresponding to a corrosion rate of 1 μm/y), the rebar is considered under corrosion. Similar to the criterion of the absolute OCP value, the current density value of 0.1 μa/cm 2 is not an unconditional value, because it is difficult to specify rebar surface area and the constant (B value) in the LPR equation used to calculate the i corr. However, the relative sharp variation of i corr can effectively minimize the errors mentioned previously and better indicate rebar corrosion initiation. Li and Sagûës 10-13 have systematically studied C th for variable rebar surface status (e.g., sandblast, asreceived, and minor pre-rusted) in high-ph simulated pore solutions (ph = 13.6, 13.3, and 12.2) with electrochemical techniques, including OCP, electrochemical impedance spectroscopy, and cyclic polarization. Their results indicate that C th in simulated pore solutions depends strongly on ph, and when ph reaches 13.6, active corrosion cannot be sustained until [Cl ] is very high. The results also indicate that the pitting potential decreased as the surface area of tested specimens increased. Therefore, the C th acquired from laboratory specimen tests may be not conservative enough when applied to full-sized structures. Presuel- Moreno, et al., studied the cathodic prevention distribution in partially submerged reinforced concrete. 14 Hurley and Scully studied C th for Type 316LN (UNS S31653) (1) stainless steel, Type 316L (UNS S31603) stainless steel clad, 2101 LDX (UNS S32101), MMFX-2, and carbon steel rebars using potentiodynamic and potentiostatic current monitoring techniques in saturated calcium hydroxide (Ca[OH] 2 ) + sodium chloride (NaCl) solutions. 15 The results indicate that the C th for stainless steel rebar was much higher than that of carbon steel. However, surface preparation, test method, and duration of period exposed to a passivating condition prior to introduction of chloride all affected the C th obtained. Budiansky, et al., 16 and Cong, et al., 17 used coupled multielectrode arrays made of carbon steel and different types of stainless steels to investigate lateral corrosion spreading behavior of rebar alloys in simulated concrete pore solution. Each of the coupled multielectrode arrays used by the authors consisted (1) UNS numbers are listed in Metals and Alloys in the Unified Numbering System, published by the Society of Automotive Engineers (SAE International) and cosponsored by ASTM International. Trade name. of 100 flush-mounted electrodes (diameter = 250 μm) in close-packed configuration. The authors reported the spatial and temporal information on the behavior of local anodes and cathodes, and the interactions between corrosion sites can trigger or inhibit corrosion phenomena and affect corrosion. More recently, Yu, et al., 18 measured the C th in similar high-ph simulated pore solutions for sandblasted and pre-rusted bars with OCP and LPR techniques. The authors reported the variation of OCP and i corr as a function of [Cl ] and [Cl ]/[OH ]. 18 In this paper, coupled multielectrode array sensors (CMAS) were used to measure the C th in the same three simulated pore solutions that Yu, et al., 18 used. The CMAS is a recently emerged technology for corrosion monitoring. 19-27 This work demonstrated the applicability of the CMAS method in determining the C th of chloride and characterizing the corrosion of rebar materials in simulated pore solutions. EXPERIMENTAL PROCEDURES Coupled Multielectrode Arrary Sensor Probes and Corrosion Potential Electrodes In a CMAS, 19-20 multiple electrodes made of the same metal are used as the working electrodes and connected to a multichannel current-measuring device, which imposes near-zero potential among the electrodes. Therefore, all electrodes are coupled to the same electrochemical potential (the coupling joint potential) and simulate the behavior of a single piece of metal. When the metal is immersed in a corrosive liquid solution, or covered by a corrosive electrolyte, corrosion may take place on the surface of these electrodes. The corrosion is usually uneven or localized; some areas of the metal corrode more, and other areas corrode less. The areas that corrode more are referred to as the more anodic areas, and the areas that corrode less are referred to as the less anodic or the cathodic areas. Because all aqueous corrosion is electrochemical in nature, electrons released at the anodic areas flow to the cathodic areas. Such electron flows represent the nonuniform general corrosion (also called uneven general corrosion) 25 or localized corrosion taking place on the metal surface. The CMAS probe measures the anodic current from each electrode and uses the current from the most anodic electrode to calculate the nonuniform corrosion rate. 20-21,25-26 The surface area on each electrode was assumed as constant in the calculation; the effect of corrosion, especially localized corrosion, on the true surface area was not accounted in the calculation. It should be mentioned that the CMAS probe cannot be used in systems where the corrosion is purely uniform and all the anodic current and cathodic current flow as internal current within a single electrode. Fortunately, purely uniform corrosion is rare. For example, the internal current effect on the CMAS probe CORROSION Vol. 70, No. 8 851

TABLE 1 Chemical Composition and Nominal ph of Fresh Simulated Pore Solutions NaOH KOH Ca(OH) 2 Composition ph (g/l) (g/l) (g/l) SCS 12.6 2.0 SPS1 13.3 3.7 10.5 2.0 SPS4 11.6 0.0833 0.233 FIGURE 1. Schematic diagram of the experimental setup and the nine-electrode CMAS used in the measurements. for monitoring carbon steel and aluminum corrosion in simulated seawater was estimated with the Tafel extrapolation method to be less than 21%. 25-26 Even for the typical uniform corrosion case of carbon steel or aluminum in dilute hydrochloric acid (HCl), there were still more than 19% of the anodic currents flowing externally to the other electrodes. 25 The CMAS is essentially a special version of the coupled multielectrode arrays described by Budiansky and coworkers. 16-17 The output of most CMAS probes is a single parameter (the corrosion rate calculated from the most anodic current) as a function of time, while the output of most coupled multielectrode arrays is a map of corrosion currents (both anodic and cathodic currents from all electrodes) as a function of time. The coupled multielectrode array is a powerful but rather sophisticated tool for electrochemical and corrosion studies because typical current maps involve 25 to 100 values, and the analysis of such huge amounts of data is labor intensive. CMAS is a simple and convenient tool for corrosion monitoring; it is one of the applications of a coupled multielectrode array. Multiple commercial CMAS probes with nine electrodes were used in the experiment. The inset of Figure 1 shows the multiple three by three CMAS probes. The sensing electrodes were Type 1018 carbon steel (UNS G10180) wires with a diameter of 1 mm (exposed surface area was 0.00785 cm 2 ). The electrodes were flush-mounted in an epoxy resin, and only the end cross-sectional surface was exposed to the electrolyte during the experiment. The spacing between the electrodes was approximately 2 mm. According to the Ohmic potential field distribution calculated by Budiansky, et al., for a flush-mounted electrode of 0.15 mm diameter in a solution with a conductivity of 1 ms/cm, 16-17 the potential drop from the electrode to the solution at a distance of 2 mm is less than 500 mv if the current density is 0.1 A/cm 2. When the conductivity of the solution in the monitoring system is higher than 1 ms/cm, the potential drop caused by solution resistance is expected to be less than 0.1 mv, which is not significant, when the maximum current density from a CMAS probe is less than 0.3 ma/cm 2 (~0.35 mm/y corrosion rate). In lower conductivity systems (<1 ms/cm), the corrosion rate measured with the far-spaced CMAS probe may be lower than the actual corrosion rate if the corrosion rate is higher than 0.35 mm/y. In some of the tests, the sensing surface of the CMAS probes was polished to 320 grit and cleaned with acetone (CH 3 COCH 3 ). In other tests, the sensing surface of the CMAS probe was precorroded in a 3.5% NaCl solution or in a more aggressive simulated pore solution. Because of the large number of CMAS probes, two CMAS instruments were used in the measurements: an earlier version of a multichannel CMAS instrument that was used in previous studies 15 a commercial nanocorr field monitor Each of the instruments was connected to a computer to collect the real-time data from the CMAS instruments. To verify corrosion behavior measured from the CMAS probes, electrodes made of rebar specimens were placed in each of the test vessel s (Figure 1) and used for corrosion potential measurements. The rebar specimens were approximately 9 mm diameter rods machined from commercially available deformed #3 carbon steel rebar (nominal diameter of 9.525 mm). Each of the machined rods was masked with an epoxy coating, and only the bottom-end cross section and the side surface of about 2.5 cm from the bottom were exposed to the solution (total exposed surface area was 8.17 cm 2 ). Experimental Setup Similar to the experiment Yu, et al., 18 described, three kinds of simulated pore solutions (Table 1) were used as the pore solutions to determine the C th as a function of ph of the simulated pore solutions. Each of the solutions was contained in a glass vessel, which was placed on a magnetic stirrer plate. The solution was slowly stirred to maintain a full mixing. A polytetrafluoroethylene (PTFE) lid with multiple access ports 852 CORROSION AUGUST 2014

FIGURE 2. Chloride concentrations and the corrosion rates measured from the CMAS probes. for insertion of CMAS probes, corrosion potential electrodes, and reference electrodes was loosely placed on the top of each of the vessels to reduce the carbonation process by the carbon dioxide (CO 2 ) in the air. The CMAS probes, rebar specimens, and a saturated calomel reference electrode (SCE) were immersed in the solution through the access ports of the PTFE lid (Figure 1). The vessels were at room temperature (24 C to 27 C). NaCl solutions were periodically added to each of the vessels through the additional access ports on the PTFE cover to increase the chloride concentration. Because the vessels were not sealed and there was slow evaporation in each of the vessels during the experiment, deionized water was periodically added to the vessels to maintain the simulated pore solution level at a constant value. The OCP data were automatically logged for the probes connected to the commercial CMAS instrument and measured with a digital multimeter for probes connected to the earlier version of the CMAS instrument and for the rebar electrodes. RESULTS AND DISCUSSION ph 11.6 Simulated Pore Solution Figure 2 shows the chloride concentrations and the corrosion rates measured from the CMAS probes immersed in the simulated pore solution with ph 11.6. Two probes (P938 and PD#6) were used initially when the chloride concentration was low. The two probes were freshly polished before the experiment and exhibited high nonuniform corrosion rates even when the chloride concentration was low. With time, the surfaces of the electrodes passivated and the corrosion rates from both probes gradually decreased and stayed below 1 µm/y. When the chloride concentration was increased to 0.002 M, the corrosion rate from Probe PD#6 increased sharply to more than 2,000 µm/y and stayed above 2,000 µm/y. The stabilized nonuniform corrosion rate from Probe P938 remained below 1 µm/y, but started to increase approximately 30 days after the chloride concentration was increased to 0.005 M. Although the rate from PD#6 started to increase at 0.002 M, it cannot be ruled out that the rate might also start to increase when the concentration was between 0.001 M and 0.002 M. Therefore, the threshold chloride concentration from the two probes is estimated to be between 0.001 M and 0.005 M. At the later stage of the testing, Probes P934 and P935 became available from other projects, and they were also used in the present work. Both of the probes, P934 and P935, showed a high nonuniform corrosion rate when the chloride concentration was 0.005 M. The data from these two probes (P934 and P935) verified that the threshold chloride concentration is between 0.001 M and 0.005 M (or 0.003±0.002 M). Figure 3 shows the chloride concentrations and the corrosion potentials measured from the probes or electrodes immersed in the simulated pore solution with ph 11.6. The potentials, as shown in Figure 3, were measured from the coupling joint of four CMAS probes (P934, P935, P938, and PD#6) and a rebar electrode (LPR#1). According to an ASTM standard 8 and a literature report, 27 the probability of corrosion of carbon steel rebars in a reinforced concrete structure is 90% if the corrosion potential of the rebar is below 0.35 V vs. the copper sulfate electrode (or 0.280 V SCE ) and 10% if the corrosion potential of the rebar is above 0.2 V vs. the copper sulfate electrode (or 0.13 V SCE ). When chloride concentration was below 0.002 M, none of the measured corrosion potentials were close to 0.28 V SCE. Only after the chloride concentration was increased to 0.002 M, and then to 0.005 M, were low corrosion potentials near or below 0.28 V SCE observed. Therefore, the threshold chloride concentration based on the corrosion potential data from Figure 3 is also between 0.001 M and 0.005 M CORROSION Vol. 70, No. 8 853

FIGURE 3. Chloride concentration and the OCP measured from the probes or electrodes immersed in the simulated pore solution with ph 11.6. FIGURE 4. Chloride concentrations and the corrosion rates measured from the CMAS probes immersed in the simulated pore solution with ph 12.6. (or 0.003±0.002 M), which is the same as the threshold chloride concentration derived from the CMAS probe nonuniform corrosion rate measurements (Figure 2). ph 12.6 Simulated Pore Solution Figure 4 shows the chloride concentrations and the corrosion rates measured from the probes immersed in the simulated pore solution with ph 12.6. Three CMAS probes (P937, P938, and P939) were used in the measurements. Probes P937 and P939 were freshly polished before the test and exhibited high nonuniform corrosion rates (20 µm/y) initially. With time, the surfaces of the electrodes were passivated and the corrosion rates from both probes gradually decreased and stayed below 1 µm/y. The nonuniform corrosion rate from Probes P937 and P939 increased sharply to more than 500 µm/y in two to three days after the chloride concentration was increased to 0.01 M. The nonuniform corrosion rate from Probe P938 was approximately 100 µm/y in the solution with a chloride concentration of 0.005 M initially because the probe was just taken out of the ph 11.6 simulated pore solution where the corrosion rate from the probe exceeded 2,000 µm/y, and the electrode surfaces of the probe were not cleaned before it was immersed into the ph 12.6 solution. The P938 probe simulated a precorroded specimen when it was used in the ph 12.6 simulated pore solution. The corrosion rate from Probe P938 steadily decreased to 1.7 µm/y and decreased further when more chloride was added, indicating that the ph 12.6 simulated pore solution with a chloride concentration of 0.005 M still caused the precorroded electrodes to passivate. When the chloride concentration was increased to 0.01 M, the nonuniform corrosion rate from Probe P938, like the nonuniform corrosion rate from Probes 937 and P939, sharply increased and reached 4,000 µm/y before it was removed from the ph 12.6 solution and placed in the ph 13.3 solution (as described next) for further testing. Therefore, the threshold chloride concentration for the three probes was higher than 854 CORROSION AUGUST 2014

FIGURE 5. Chloride concentration and the OCP measured from the probes or electrodes immersed in the simulated pore solution with ph 12.6. FIGURE 6. Chloride concentrations and the moving average corrosion rates measured from the CMAS probes immersed in the simulated pore solution with ph 13.3. 0.005 M, but equal to or less than 0.01 M. The estimated value is 0.0075±0.0025 M. Figure 5 shows the chloride concentrations and the corrosion potentials measured from the probes immersed in the simulated pore solution with ph 12.6. The potentials, as shown in Figure 5, were measured from the coupling joint of five CMAS probes (P934, P936, P937, P938, and P939) and three rebar electrodes (LPR#2, LPR#3, and LPR#4). When the chloride concentration was below 0.005 M, none of the measured corrosion potentials were close to 0.28 V SCE. Only after the chloride concentration was increased to 0.005 M and then to 0.01 M were low corrosion potentials near or below 0.28 V SCE observed. As a matter of fact, all of the stabilized corrosion potentials were lower than 0.5 V SCE when the chloride concentration was higher than 0.01 M. Therefore, the threshold chloride concentration based on the corrosion potential data from Figure 4 is also between 0.005 M and 0.01 M (or 0.0075±0.0025 M), which is the same as the threshold chloride concentration derived from the CMAS probe nonuniform corrosion rate measurements in ph 12.6 solution (Figure 4). ph 13.3 Simulated Pore Solution Figure 6 shows the chloride concentrations and the corrosion rates measured from the probes immersed in the simulated pore solution with ph 13.3. Three CMAS probes (P934, P938, and P940) were used in the measurements. Probe P940 was freshly polished before the experiment and was used from the very start of the experiment when the chloride concentration was low. Except for the initial 6 days when the chloride concentration was below 0.5 mm, the nonuniform corrosion rate from Probe 940 was below 1 µm/y, which is the lower detection limit of the earlier version of the CMAS instrument used in the experiment. Probes P934 and P938 were precorroded in the high-chloride ph 12.6 simulated pore solution (Figure 5), and the corrosion rates from these two probes were stabilized below 10 µm/y for P934 or below 1 µm/y for P938. When the chloride concentra- CORROSION Vol. 70, No. 8 855

FIGURE 7. Chloride concentration and the OCP measured from the probes or electrodes immersed in the simulated pore solution with ph 13.3. FIGURE 8. Comparison of the threshold chloride concentrations derived from the present work with the threshold chloride concentrations reported by previous investigators. 2-3,5,12,14 tion reached 0.5 M, the nonuniform corrosion rates from all three probes increased to more than 50 µm/y. The corrosion rates from the two precorroded probes continued to increase to approximately 100 µm/y after the chloride concentration was increased to 1 M. However, the probe with clean electrodes was passivated in the low-chloride solutions for more than 4 months and appeared to be stabilized at 6 µm/y to 10 µm/y (after the initial increase to 50 µm/y when chloride concentration was 0.5 M) when the chloride concentration was 1 M, indicating that 1 M of chloride was still not aggressive enough for the clean probe in the ph 13.3 simulated pore solution. Based on the nonuniform corrosion rates from the two precorroded probes and the initial relatively high corrosion rate from the clean probe when the chloride concentration was 0.5 M, the threshold chloride concentration in the ph 13.3 solution is estimated to be from 0.1 M to 1 M (or 0.55±0.45 M). Figure 7 shows the chloride concentrations and the corrosion potentials measured from the CMAS probes and from the rebar electrodes immersed in the simulated pore solution with ph 13.3. The potentials as shown in Figure 6 were measured from the coupling joint of four CMAS probes (P934, P935, P938, and P940) and two rebar electrodes (LPR#2 and LPR#5). Except for Probe P940, none of the measured corrosion potentials were below 0.28 V SCE when the chloride concentration was below 0.5 M. After the chloride concentration reached 0.5 M to 1 M, the corrosion potentials started to drop below 0.28 V SCE. There were two data points from Probe P940 that were lower than 0.28 V SCE ( 0.288 V and 0.321 V, respectively) when the chloride concentration was 0.05 M. These data points were probably anomalies. Based on the majority of corrosion potential data shown in Figure 7, the threshold chloride concentration is also between 0.1 M and 1 M (or 0.55±0.45 M), which is the same as the threshold chloride concentration derived from the CMAS probe nonuniform corrosion rate measurements in the ph 13.3 solution (Figure 7). Figure 8 shows the comparison of the threshold chloride concentrations derived from this work and the threshold chloride concentrations previous investigators reported. The data from the present work were based on both nonuniform corrosion rate measurements from multiple CMAS probes, OCP measurements from the coupling joints of the multiple CMAS probes, and multiple rebar electrodes. The data from the present work were scattered, especially when the ph was 13.3. However, they are within the scattering range of the data reported in the literature. The corrosion rate measurements from CMAS probes are continuous and easy to conduct. The corrosion rate from a CMAS probe is the rate caused from the nonuniform corrosion, including localized corrosion (such as pitting corrosion). The CMAS probes can be easily made into mini probes, and they can be easily inserted into small but deep boreholes to evaluate the effect of corrosion, especially localized corrosion, 856 CORROSION AUGUST 2014

on the performance of the rebar materials in reinforced concrete structures. In the present studies, the threshold chloride values from the CMAS corrosion rates were the same as the threshold values from the OCP measurements. Compared to the OCP method, the CMAS methods provided the quantitative localized corrosion rates, which may be used for remaining life prediction. However, the OCP method can only be used to predict the probability for the corrosion of rebar. CONCLUSIONS v Relatively long-term measurements for the nonuniform corrosion rates of rebar materials were conducted using multiple CMAS probes in three types of simulated pore solutions. NaCl was progressively added into the simulated pore solutions. The threshold chloride concentrations that start to cause the CMAS probes to corrode in each of the simulated pore solutions were determined. The OCP was also measured from the coupling joint of the CMAS probes and electrodes made of rebar specimens immersed in the simulated pore solutions at different chloride concentrations. The threshold chloride concentrations from both methods agree well with each other. This work proved that the CMAS probe is a highly reliable and convenient method to determine the threshold chloride concentration in the simulated pore solutions. Because of the size of the CMAS mini probe, it may also be used in small boreholes filled in the reinforced concrete structures in the field. ACKNOWLEDGMENTS This work was supported by Southwest Research Institute Internal Research and Development Projects R8090 and R8084. The authors would like to thank G. Norman and B. Derby for laboratory assistance. The authors acknowledge the technical review of P. Shukla, editorial review of L. Mulverhill, programmatic review of S. Mohanty, and the assistance of L. Gutierrez in preparing this paper. REFERENCES 1. V.Ø. Gjørv, Cem. Concr. Res. 9, 2 (1979): p. 229-238. 2. D.A. Hausmann, J. Mater. Prot. (1967): p. 19-23. 3. V.K. Gouda, Br. Corros. J. 5, 2 (1970): p. 198-203. 4. S. Goni, C. Andrade, Cem. Concr. Res. 20 (1990): p. 525-539. 5. W. Breit, Mater. Corros. 49, 6 (1998): p. 539-550. 6. C.J. Kitowski, H.G. Wheat, Corrosion 53, 3 (1997): p. 216-226. 7. J. Xu, L. Jiang, J. Wang, Constr. Build Mater. 23 (2009): p. 1,902-1,908. 8. ASTM C876-91, Standard Test Method for Half-Cell Potentials of Uncoated Reinforcing Steel in Concrete (West Conshohocken, PA: ASTM International, 1991). 9. H. Yu, X. Shi, W.H. Hartt, B. Lu, Cem. Concr. Res. 40, 10 (2010): p. 1507-1516. 10. L. Li, A.A. Sagüés, Effect of Chloride Concentration on the Pitting and Repassivation Potentials of Reinforcing Steel in Alkaline Solutions, CORROSION/99, paper no. 567 (Houston, TX: NACE International, 1999). 11. L. Li, A.A. Sagüés, Effect of Material Surface Condition on the Chloride Corrosion Threshold of Reinforcing Steel in Alkaline Solutions, CORROSION/2000, paper no 801 (Houston, TX: NACE International, 2000). 12. L. Li, A.A. Sagüés, Corrosion 57, 1 (2001): p. 19-28. 13. L. Li, A.A. Sagüés, Corrosion 58, 4 (2002): p. 305-316. 14. F.J. Presuel-Moreno, S.C. Kranc, A.A. Sagüés, Corrosion 61, 6 (2005): p. 548-558. 15. M.F. Hurley, J.R. Scully, Corrosion 62, 10 (2006): p. 892-904, doi: http://dx.doi.org/10.5006/1.3279899. 16. N.D. Budiansky, F. Bocher, H. Cong, M.F. Hurley, J.R. Scully, Corrosion 63, 6 (2007): p. 537-554. 17. H. Cong, F. Bocher, N.D. Budiansky, M.F. Hurley, J.R. Scully, J. ASTM Int. 4, 10 (2007). 18. H. Yu, K.T. Chiang, L. Yang, Constr. Build. Mater. 26 (2012): p. 723-729. 19. L. Yang, Multielectrode Systems, in Corrosion Monitoring Techniques, ed. L. Yang (Cambridge, U.K.: Woodhead Publishing, 2008). 20. L. Yang, N. Sridhar, O. Pensado, D. Dunn, Corrosion 58 (2002): p. 1004. 21. L. Yang, N. Sridhar, C.S. Brossia, D.S. Dunn, Corros. Sci. 47 (2005): p. 1,794-1,809. 22. A. Anderko, N. Sridhar, L. Yang, S.L. Grise, B.J. Saldanha, M.H. Dorsey, Corros. Eng., Sci. Technol. 40 (2005): p. 33-42. 23. K.T. Chiang, L. Yang, Corrosion 64 (2008): p. 805-812. 24. K.T. Chiang, L. Yang, Corrosion 66 (2010): p. 095002. 25. L. Yang, K.T. Chiang, P.K. Shukla, N. Shiratori, Corrosion 66 (2010): p. 115005. 26. L. Yang and K.T. Chiang, J. ASTM Int. 6, 3 (2009): Paper ID JAI101253. 27. P. Schiessl, Corrosion Monitoring in Concrete, in Corrosion Monitoring Techniques, ed. L. Yang (Cambridge, U.K.: Woodhead Publishing, 2008). CORROSION Vol. 70, No. 8 857