Monitoring the Electrochemical Response of Chloride Sensors Embedded in Cement Paste
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Monitoring the Electrochemical Response of Chloride Sensors Embedded in Cement Paste F. Pargar1, D.A. Koleva1, E.A.B. Koenders1, K. van Breugel1 1 Faculty of Civil Engineering and Geosciences, Delft University of Technology, Section of Materials and Environment, Stevinweg 1, 2628 CN Delft, The Netherlands. ABSTRACT This work presents the electrochemical behavior of Ag/AgCl electrodes (chloride sensors) in cement paste environment, monitored over a period of 180 days via open circuit potential (OCP) readings and electrochemical impedance spectroscopy (EIS). The EIS response indicates modification of the sensors’ morphology, in particular alteration of the AgCl layers, as a result of continuous chloride penetration into the bulk matrix towards the vicinity of the sensor/cement paste interface. A gradual shift to more cathodic OCP values and stabilization at approximately 1mVSCE to 2mVSCE was observed at the end of the test, reflecting chloride content of 820mM to 930mM in the pore solution surrounding the sensors, which differs 5-10% from the chloride concentration in the external solution. The water soluble chloride content in the cement pate, as destructively measured wet chemically by Volhard method and photometry, was in the range of 1100mM - 1300mM i.e. about 30-50% more than the chloride concentration in the external solution. This difference of maximum 50% in the recorded chloride levels is attributed to the fact that the sensors “read” the average amount of free chloride at the interface sensor/cement paste, while the destructively measured water soluble chloride reflects the average free (with possible contribution of physically bound chloride) in the total volume of analyzed cement paste. It can be concluded that for the conditions of this experiment, more reliable free chloride content is measured via the sensors’ readings. Hence, if chloride thresholds for corrosion initiation are to be determined, the sensors’ readings will be more representative and accurate if compared to destructive water soluble chloride determination. INTRODUCTION Among the techniques available to measure the free chloride content, chloride ion selective electrodes (i.e. Ag/AgCl sensors) can be used for continuous monitoring of the free chloride activity in the proximity of the steel surface and/or in the bulk concrete matrix. These electrodes are sensitive mainly to chloride ions and according to the Nernst equation exhibit a certain electrochemical potential that depends on the chloride ion activity (concentration respectively) in the environment. Hence, by measuring the potential of a Ag/AgCl electrode (or sensor) versus another stable and known half-cell (reference electrode), the Cl- ion activity in the solution can be calculated based on the sensor potential reading and implementing mean activity coefficients; thus chloride activity can be calculated as relevant chloride concentration [1]. It should be mentioned that a measurement error of already a few millivolts adversely affects the accuracy of this technique. In other words
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