Fabrication and evaluation of a highly durable and reliable chloride monitoring sensor for civil infrastructure

Subbiah Karthickab, Seung-Jun Kwonc, Han Seung Lee*a, Srinivasan Muralidharand, Velu Saraswathy*ac and Rethinam Natarajanb aDepartment of Architectural Engineering, Hanyang University, 1271 Sa 3-dong, Sangrok-gu, Ansan 426791, Korea. E-mail: ercleehs@hanyang.ac.kr; vsaracorr@cecri.res.in; Fax: +82-31-436-8169; Tel: +82-31-400-4181 bPG and Research Department of Chemistry, Alagappa Government Arts College, Karaikudi 630003, Tamilnadu, India cDepartment of Civil Engineering, Hannam University, Daejeon 306-791, Korea dCorrosion and Materials Protection Division, CSIR-Central Electrochemical Research Institute, Karaikudi 630003, Tamilnadu, India

First published on 19th June 2017

1. Introduction

Reinforced concrete structures play a significant role in the construction industry, such as in buildings, bridges, and nuclear reactors, and they involve major construction cost in the millions of dollars. When the structures are exposed to aggressive environments, the durability of the concrete is affected either due to the carbonation of the concrete cover or ingress of chloride ions.1-6 Chloride-induced corrosion is a particularly insidious problem7,8 since it leads to localized attack, which results in cracking and delamination of the concrete cover and unexpected failure of the structure, which is often catastrophic. Therefore, chloride ion monitoring is one of the most important methods to prevent the corrosion-induced damage of concrete structures in marine environments. Chloride monitoring in concrete structure involves drilling the concrete at various depths for the collection of powder samples from the steel-concrete interfacial region and analyzing the free chloride and total chloride content by potentiometric titration method using an Ag/AgCl ion selective electrode.9-12 This technique seems to be a destructive, time-consuming process and requires periodic repetition and interpretation to obtain accurate data. Furthermore, destructive methods bring additional indirect cost due to road closures and traffic delays.13,14 Hence, the civil and construction industries are searching for non-destructive techniques for chloride monitoring in concrete structures. The entire problem will be overcome by using embeddable sensors for continuous monitoring of chloride ion content, which is a simplified procedure and easily adaptable method.15 In this regard, the Ag/AgCl electrode is considered as one of the most commonly used sensors because it has a Nernstian response to the variations of chloride (Cl−) or silver (Ag+) activity and displays excellent sensitivity.16 Ag/AgCl electrodes have become well established over the last decade and have been employed for analytical applications.17 Gurusami et al. reported the use of Ag/AgCl sensors embedded in concrete as reference electrodes and good long-term results were collected over a period of about 4.5 years.16 The first attempt in concrete was made in the 1990s. Potentiometric measurement using an Ag/AgCl electrode is the standard electrochemical technique to measure the free chloride in concrete.18-22 A study carried out using Ag/AgCl for chloride monitoring in simulated concrete pore solutions revealed that at higher chloride concentration, pH has a significant influence on the potential value of the sensor.23 An Ag/AgCl wire was employed as a sensor for determination of water soluble chlorides in admixtures and aggregates for cement.24 It was reported that, according to the Nernst law, the equilibrium potential value of the Ag/AgCl electrode depends on the chloride ion activity (concentration) of the surrounding solution, and an Ag/AgCl electrode can be used to determine the chloride activity in simulated cement pore solution, mortar and concrete samples.25,26

However, studies revealed that embeddable Ag/AgCl electrodes in concrete are not stable for a long time, which is owing to the high alkalinity of the concrete. It was reported that in the pH range from 11.9 to 13.7, low concentrations of chloride cannot be accurately determined by potentiometric measurements owing to the OH− interference.22,27 Besides, in the high alkaline solution and the presence of hydroxyl ions, silver activity near the surface is determined by exchange equilibrium, as shown in eqn (1):28

2AgCl + 2OH− → Ag2O + 2Cl− + H2O (1)

The AgCl membrane becomes unstable and is turned into Ag2O partly or wholly at high pH values. By the continuous transformation of the electrode surface into Ag2O, a mixed potential is developed at the electrode/solution interface and it becomes incapable of accurately determining the free chloride content in the concrete structure.22 Therefore, to increase the stability of the Ag/AgCl electrode and to reduce the OH− ion interference in the alkaline environment, the use of a conducting, alkaline stable conjugated polymer coating on an Ag/AgCl electrode was reported.28,29

In this aspect, a polypyrrole polymer matrix was used because it is one of the most frequently used polymers, can be readily synthesized, and it is more stable, lower cost, and has higher conductivity when compared to other polymers; it has been employed for many applications.30,31 Pickup et al.32 constructed a polypyrrole-mercury/mercury chloride coated glassy carbon electrode as a reference electrode. Mangold et al.33 investigated a polypyrrole-silver/silver chloride as a reference electrode based on the surrounding electrolyte. The open circuit potential of this electrode strongly depends on the concentration of the supporting electrolyte. Furthermore, polypyrrole coated on a carbon electrode and polypyrrole incorporated on a metal electrode (Pt, In-SnO2 and glassy carbon) was tried as a pH sensor, glucose sensor, and biosensor. PPy and PPy-based coatings have been proved to be very effective inhibitors in the corrosion of oxidizable metals and alloys, stainless steels, mild steel, etc. Besides these, PPy films are being used in other areas, such as in sensors and bio-fuel cells.34

To date, no in-depth studies have been carried out using an Ag/AgCl electrode for in situ chloride sensing application in concrete structures. Hence, the present study aimed at developing a low cost, accurate and highly alkaline, long-term stable PPy coated Ag/AgCl solid electrode as an embeddable sensor for chloride sensing in concrete structures. The electrochemical stability of this PPy coated Ag/AgCl electrode was evaluated in simulated concrete pore solutions with different chloride ion concentrations. The chloride sensitivity of the PPy coated solid electrode was assessed against an MnO2 solid sensor in chloride contaminated concrete by embedding the sensor at different depths.

2. Materials and methods

2.1. Materials and methods

The chemicals such as a silver wire (Ag), silver chloride (AgCl) powder, pyrrole, sodium dodecyl sulfate, iron(III) chloride hexahydrate, NaOH, KOH, CaO and NaCl used were of analytical reagent grade with sufficient purity and were purchased from reputed chemical suppliers.

2.2. Testing in saturated KCl

2.3. Testing in various chloride concentrations

The Fb-Ag/AgCl and PPy-Ag/AgCl electrodes were calibrated in aqueous NaCl solutions with different concentration of chloride ions from 0.001 mole per L to 1.0 mole per L. The potentials developed by both electrodes were measured with respect to SCE. The pH of the test solution was observed as neutral. Further, the response of the Fb-Ag/AgCl and PPy-Ag/AgCl electrodes under alkaline environment was calibrated in synthetic concrete pore solution (SCPS) contaminated with chloride ions ranging from 0.001 mole per L to 1.0 mole per L.

2.4. Stability of Fb-Ag/AgCl and PPy-Ag/AgCl electrodes in SCPS

2.5. Potential measurement in various solutions

The PPy-Ag/AgCl electrode was dipped in different solutions such as distilled water and SCPS with various chloride concentrations (0.25 mole per L, 0.50 mole per L and 1.0 mole per L). The potential developed by this electrode was measured with respect to SCE. The pH of the test solutions was varied from 11 to 13.5. The potential values noted at different time durations were plotted as a graph. The reversibility behavior of PPy-Ag/AgCl electrode in low and high concentrations of chloride ion was also measured.

2.6. Electrochemical studies

The procedure mentioned in the Experimental section 2.2.2 was followed for carrying electrochemical studies, and here the working electrode taken was the PPy-Ag/AgCl electrode and the electrolyte used was SCPS and SCPS with chloride ion concentrations of 0.25 mole per L, 0.50 mole per L and 1.0 mole per L.

The same three electrode setup was used for AC-impedance measurement. The AC-impedance measurement for the PPy-Ag/AgCl electrode was carried out using an ACM Instruments (UK) field machine with a frequency range of 30 kHz to 10 mHz and an amplitude of 20 mV. Nyquist plots were recorded for all the systems studied and are presented in graph form.

2.7. Evaluation of the PPy-Ag/AgCl electrode embedded in concrete for chloride ion sensitivity

A concrete cube with dimensions of 150 mm × 150 mm × 150 mm was cast using a 1:1.80:3.696 mix ratio of cement:fine aggregate:coarse aggregate containing a 0.55 water/cement ratio. The chemical composition of ordinary Portland cement (OPC) used is given in Table 1. Clean river sand passing through a 2.36 mm sieve and falling under zone III with a specific gravity 2.60 was used as a fine aggregate. The coarse aggregates used were crushed stone aggregates with a normal size of 19 mm and a specific gravity of 2.6. The PPy-Ag/AgCl electrodes were embedded in concrete at various depths of 5, 10, 20, 30 and 40 mm (Fig. 1) along with the MnO2 solid state reference electrode (SSRE), which was embedded very close to the PPy-Ag/AgCl electrode. After 24 h, the concrete specimens were demolded and cured for 28 days in distilled water. After curing, the concrete cubes were dried and all the four faces of the concrete cubes were sealed with epoxy, except the top and bottom surface for subjecting to the flow of chloride ions from the top surface, as shown in Fig. 1a. The top surface of the cube was surrounded with a PVC mold and glued to the sides of the concrete to prevent solution leakage. Then 3% NaCl solution was poured onto the top surface of the concrete and allowed to penetrate over a period of 90 days at room temperature. In this process, alternate wet and dry cycles were performed to accelerate the corrosion process. One cycle consists of 3 days wetting in chloride solution and 3 days drying in an open atmosphere at room temperature. The potential readings were taken during the 3rd day of wetting, after wiping the solution out. The experimental setup for carrying out the measurements is shown in Fig. 1b.

3. Results and discussion

3.1. Properties of Ag/AgCl electrode

The Ag/AgCl electrodes prepared by sintering process were rugged and electronically conductive. The polypyrrole solution prepared makes a thixotropic slurry along with NMP and PVDF and no trace of NMP was found after drying at 80 ± 10 °C.

3.2. Testing in sat. KCl

3.3. Testing in various chloride concentrations

Both the Fb-Ag/AgCl and PPy-Ag/AgCl electrodes were calibrated in different chloride concentrations to ensure the chloride sensing ability. The potentials measured with respect to SCE are plotted against various chloride concentrations ranging from 0.001 mole per L to 1.0 mole per L in Fig. 4. It was observed from Fig. 4 that both the electrodes showed similar behavior and different potentials were observed for different concentrations. Further, the measured potential decreases with increasing concentration of chloride. The chloride ion concentration decreases with the increase in potential, indicating that both the electrodes follow the Nernstian equation. (2)

Furthermore, the Fb-Ag/AgCl and PPy-Ag/AgCl electrodes calibrated in different chloride concentrations revealed a linear response with a correlation coefficient of 0.99. Hence, the result indicates that both electrodes have good sensitive behavior at different chloride ion concentrations.

Fig. 5 shows the calibration of the Fb-Ag/AgCl and PPy-Ag/AgCl electrodes under alkaline environment in synthetic concrete pore solution contaminated with chloride ions ranging from 0.001 mole per L to 1.0 mole per L. From the figure, it was found that Fb-Ag/AgCl electrode potential shows a uniform variation in the higher chloride concentration region (0.01 mole per L to 1.0 mole per L), forming a linear curve fit. At the same time, in the low concentration region (0.001 to 0.01 mole per L) Fb-Ag/AgCl electrode potential was slightly variable from the linear curve, which may be due to the interference of OH− ions,17 and this deviation could be attributed to the formation of AgOH.15 On the other hand, the PPy-Ag/AgCl electrode showed a uniform variation in potential at all the chloride ion concentrations. However, the potential value of the PPy-Ag/AgCl electrode was slightly changed at lower concentrations of chloride ions due to the higher concentration of OH− interference during the measurement. This potential deviation is very low when compared to the Fb-Ag/AgCl electrode. This minor deviation is considered as a negligible change. Hence, the straight line curve of the PPy-Ag/AgCl electrode is exhibited at all chloride ion concentrations in Fig. 5. Moreover, the PPy-Ag/AgCl electrode showed a good linear response with a correlation coefficient of 0.98. It is slightly higher than that of the Fb-Ag/AgCl electrode (0.94). Hence, a PPy-Ag/AgCl electrode has good sensible behavior and exhibits good Nernstian behavior in the high alkaline environment with low and high concentrations of chloride ions. Hence, the PPy electrode is stable in alkaline medium.39,43

3.4. Stability of Fb-Ag/AgCl and PPy coated Ag/AgCl electrodes in SCPS

3.5. Stability of PPy coated Ag/AgCl electrode in various test solutions

Fig. 11 shows the potentiometric response of the PPy-Ag/AgCl electrode in different test solutions such as distilled water, chloride-free synthetic concrete pore solution (SCPS) and chloride-contaminated synthetic concrete pore solution. The PPy-Ag/AgCl electrode showed stable average potential values of 121.0 ± 2 mV vs. SCE in distilled water and 119.5 ± 2 mV vs. SCE in SCPS. This potential difference between distilled water and SCPS was 1.5 mV vs. SCE. Furthermore, there was no significant change in the electrode potential of the PPy-Ag/AgCl electrode. Therefore, the PPy coating can prevent OH− ion interference when measuring the potential of the PPy-Ag/AgCl electrode in SCPS. Upon the addition of chloride ions (such as 0.25 mole per L, 0.50 mole per L and 1.0 mole per L) in SCPS, the measured potential values were 28.3 ± 2 mV, 12.3 ± 2 mV and −3.3 ± 2 mV, respectively. The relation between electrode potential and chloride concentration is given in Table 3. As noted earlier, the potential values decrease with increasing chloride ion concentration. This direct relationship enables us to use the PPy-Ag/AgCl electrode for chloride sensing application in concrete structures.

Since the PPy-Ag/AgCl electrode exhibits excellent stability and reversibility in SCPS and different chloride concentration solutions, its response times with low and high concentration of chloride are essential to validate the chloride ion sensing ability.

The response time graph of the PPy-Ag/AgCl electrode at 0.001 mole per L and 1.0 mole per L is given in Fig. 12. It was observed from Fig. 12 that the time taken to reach a steady potential value from 0.001 mole per L to 1.0 mole per L chloride was 90 seconds; it is interesting to note that the rapid response may be due to the high chloride ion concentration and the presence of hydrophilic quaternary PPy48 for quick interaction with chloride ions. The time taken to reach a steady potential value is 360 seconds for the reverse (1.0 mole per L to 0.001 mole per L) measurement. These results indicate that the concentration of chloride ions was very low when compared to OH− ions and the internal OH− ions accelerated the recovery time but they do not influence the chloride sensing properties.

3.6. Electrochemical behaviours of PPy-Ag/AgCl electrode

Fig. 13 shows the potentiodynamic polarization of the PPy-Ag/AgCl electrode in SCPS with different chloride concentrations. The corresponding corrosion kinetic parameters are given in Table 4. The Ecorr value of the PPy-Ag/AgCl electrode was 103 mV vs. SCE in SCPS. In the presence of 0.25 mole per L, 0.50 mole per L and 1.0 mole per L of chloride, the Ecorr values of the electrode were 26.08, 11.63 and −2.8 mV (vs. SCE), respectively. The corrosion current density of the PPy coated Ag/AgCl electrode was 0.1529 mA cm−2 in SCPS. Upon the addition of 0.25 mole per L, 0.50 mole per L and 1.0 mole per L of chloride, the Icorr values observed were 0.3123, 0.4112 and 0.6277 mA cm−2, respectively. The addition of chloride in SCPS increased the corrosion current density of the PPy-Ag/AgCl electrode.

Fig. 14 shows the Nyquist curves for the PPy-Ag/AgCl electrode in SCPS containing chloride ion concentrations of 0.25 mole per L, 0.50 mole per L and 1.0 mole per L. The impedance parameters are given in Table 5. The Rct value of the PPy-Ag/AgCl electrode was 11.81 Ω cm2. The Rct values noted were 9.152, 2.266 and 2.162 Ω cm2 in the presence of chloride at 0.25 mole per L, 0.50 mole per L and 1.0 mole per L, respectively. It was also observed that the increase in chloride ion concentration decreases the charge transfer resistance (Rct) of the PPy-Ag/AgCl electrode. The Cdl value of the PPy coated Ag/AgCl electrode was 0.4033 × 10−1 F cm−2. In the presence of 0.25 mole per L, 0.50 mole per L and 1.0 mole per L of chloride, the Cdl values observed are 0.58163×10−1, 1.6560×10−1 and 1.7410×10−1 F cm−2, respectively.

3.7. Standardization of PPy-Ag/AgCl electrode with respect to MnO2

The PPy-Ag/AgCl electrode was calibrated with an MnO2 electrode. The fabrication of the MnO2 solid state reference electrode (SSRE) was discussed elsewhere.49 The stable potential value of the MnO2 electrode was +206 ± 6 mV in SCPS and chloride contaminated SCPS.50 The potential of the PPy-Ag/AgCl electrode with respect to MnO2 in SCPS containing low chloride concentrations ranging from 0.001 to 0.01 mole per L was measured and is plotted in Fig. 15a. The figure shows that there was a significant change in the potential observed with the increasing chloride ion concentration in SCPS. Further, the measured potential decreases from −106 mV to −153 mV with increasing chloride ion concentration ranging from 0.001 to 0.01 mole per L in SCPS. In addition, the potential vs. high chloride ion concentration ranging from 0.01 to 1.0 mole per L is shown in Fig. 15b. Here also, the potential values decrease from −174 mV to −213 mV with increasing chloride ion concentrations ranging from 0.01 to 1.0 mole per L.

The PPy-Ag/AgCl electrode has a good linear relationship between the potential and the logarithmic chloride ions activity is termed as the Nernstian equation. Furthermore, the correlation coefficient R2 values are 0.98 and 0.99 at low (0.001 to 0.01 mole per L) and high (0.01 to 1.0 mole per L) concentrations of chloride ions in SCPS, respectively. However, a slight deviation (R2 = 0.98) from linearity is observed at low chloride ion concentrations when compared with high chloride ion concentrations. The slight deviation of potential is due to the OH− interference during the measurement. This minor deviation is considered as a negligible change. Hence, a straight line curve for the PPy-Ag/AgCl electrode was exhibited at all chloride ion concentrations in Fig. 15a and b. From this study, it is inferred that the PPy-Ag/AgCl electrode has a stable potential and is suitable for application in long-term chloride ion monitoring of civil infrastructures.

3.8. Evaluation of PPy-Ag/AgCl electrode embedded in concrete

Fig. 16 shows the chloride migration profile of the PPy-Ag/AgCl electrode embedded at 5, 10, 20, 30 and 40 mm depths of concrete. The potentials vary with increasing depth. The impact of the free Cl− ions concentration at lower depth (5 and 10 mm) is greater than at the higher depth of concrete (20 mm to 40 mm). This is because initially the concrete is dry and porous in nature, which allows more chloride ions to penetrate through the pores of the concrete. The chloride penetration at 5 and 10 mm depth is rapid during the first 5 days, showing the increase in potential, and after that the potential decreases; after 17th day the potential increases again and it reached the maximum at the 30th day. The decrease in potential may be owing to the complexation of chlorides with the cement hydration products, leading to the formation of Friedel's salt,51 which caused a decrease in the free chloride content.

4. Conclusions

The following conclusions were drawn from the above study:

• A solid state alkaline chloride electrode was prepared and fabricated for chloride sensing application in concrete infrastructure.

• The presence of the conducting polymer prevented the formation of Ag2O in the PPy-Ag/AgCl electrode, which is comparable with the Fb-Ag/AgCl electrode and has good alkaline stability.

• In addition to alkaline stability, the chloride sensing capability of PPy-Ag/AgCl electrode was confirmed by HCP and polarization studies. It illustrates that the PPy coating did not affect the electrochemical behavior of the Ag/AgCl electrode.

• The PPy-Ag/AgCl electrode possesses a rapid, reversible characterization in chloride solution. For example, from 0.001 to 1.0 mole per L of chloride the time taken to reach the steady state potential is 90 s, and the same is 360 seconds for the reverse measurements (from high concentration to low concentration of chloride ions).

• The HCP of the PPy-Ag/AgCl electrode was 120 mV vs. SCE. The electrode clearly distinguished the difference in chloride ion concentrations ranging from 0.001 to 1.0 mole per L per ppm and thereby the electrode has an excellent chloride sensing ability.

• Further calibration of the PPy-Ag/AgCl electrode was evaluated with respect to MnO2 under various depths in chloride contaminated concrete and the results showed that the PPy coated electrode has excellent chloride sensing ability.

• The data collected from the above tests reveal that the fabricated electrode is more stable in an alkaline medium and can be used as a chloride monitoring sensor in civil infrastructures.

Acknowledgements

This research was supported by the Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT, & Future Planning (No. 2015R1A5A1037548).

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