Characterization of the Ag/AgCl electrode is a necessary step toward its application as a chloride sensor in a highly alkaline medium, such as concrete. The nucleation and growth of AgCl on Ag in 0.1 M HCl was verified through cyclic voltammetry. Ag anodization was performed at current densities, determined by potentiodynamic polarization in the same (0.1 M HCl) medium. The morphology and microstructure of the AgCl layers were evaluated via electron microscopy, while surface chemistry was studied through energy-dispersive spectroscopy and X-ray photoelectron spectroscopy. At current density above 2 mA/cm2, the thickness and heterogeneity of the AgCl layer increased. In this condition, small AgCl particles formed in the immediate vicinity of the Ag substrate, subsequently weakening the bond strength of the Ag/AgCl interface. Silver oxide-based or carbon-based impurities were present on the surface of the sensor in amounts proportional to the thickness and heterogeneity of the AgCl layer. It is concluded that a well-defined link exists between the properties of the AgCl layer, the applied current density and the recorded overpotential during Ag anodization. The results can be used as a recommendation for preparation of chloride sensors with stable performance in cementitious materials.@en

The stability and reproducibility of an Ag/AgCl sensors’ response in an alkaline medium are important for the application of these sensors in cementitious materials. The sensors’ response, or their open circuit potential (OCP), reflects a dynamic equilibrium at the sensor/environment interface. The OCP response in an alkaline medium is affected by the presence of hydroxide ions. The interference of hydroxide ions leads to inaccuracies or a delay in the sensors’ response to a certain chloride content. In this article, the potentiometric response (or OCP evolution) of the chloride sensors is measured in model solutions, resembling the concrete pore water. The scatter of the sensors’ OCP is discussed with respect to the interference of hydroxide ions at varying chloride concentration in the medium. The deviation of the sensor’s response from its ideal performance (determined by the Nernst law) is attributed to dechlorination of the AgCl layer and the formation of Ag2O on the sensor’s surface. Results from the surface XPS analysis of the AgCl layer before and after treatment in alkaline medium confirm these observations in view of chemical transformation of AgCl to Ag2O.@en

The need for an accurate determination of the chloride threshold value for corrosion initiation in reinforced concrete has long been recognized. Numerous investigations and reports on this subject are available. However, the obtained chloride threshold values have always been, and still are, debatable. The main concern is linked to the methods for corrosion detection and chloride content determination in view of the critical chloride content itself. In order to measure the chloride content, relevant to the corrosion initiation on steel, destructive methods are used. These traditional methods are inaccurate, expensive, time consuming and noncontinuous. Therefore, the application of a cost-effective Ag/AgCl ion selective electrode (chloride sensor) to measure the chloride content directly and continuously is desirable. The advantage would be an in situ measurement, in depth of the concrete bulk, as well as at the steel/concrete interface. The aim of this work was to evaluate the importance of the sensor’s properties for a reliable chloride content measurement. The main point of interest with this regard was the contribution of the AgCl layer and Ag/AgCl interface within the process of chloride content determination in cementitious materials. The electrochemical behavior of sensors and steel, both embedded in cement paste in a close proximity, hence in identical environment, were recorded and outcomes correlated towards clarifying the objectives of this work. The main point of interest was to simultaneously detect and correlate the time to corrosion initiation and the critical chloride content. The electrochemical response of steel was monitored to determine the onset of corrosion activity, whereas the sensors’ electrochemical response accounted for the chloride content. For evaluating the electrochemical state of both sensors and steel, electrochemical impedance spectroscopy (EIS) and open circuit potential (OCP) measurements were employed. The results confirm that determination of the time to corrosion initiation is not always possible and straightforward through the application of OCP tests only. In contrast, EIS is a nondestructive and reliable method for determination of corrosion activity over time. The obtained results for corrosion current densities for the embedded steel, determined by EIS, were in a good agreement with the sensors’ half-cell potential readings. In other words, the sensors are able to accurately determine the chloride ions activity at the steel/cement paste interface, which in turn brings about detectable by EIS changes in the active/passive state of steel. The electrochemical response was supported by studies on the morphology and surface chemistry of the sensors, derived from electron microscopy (ESEM) and X-ray photoelectron spectroscopy (XPS). It can be concluded that the accuracy of the sensors, within detection of the time to corrosion initiation and critical chloride content, is determined by the sensors’ properties in terms of thickness and morphology of the AgCl layer, being an integral part of the Ag/AgCl sensors. @en