## 3 Reinforcement Corrosion Monitoring Techniques
### Limitations of Electrochemical Sensors for Corrosion Monitoring of RCS  
Several corrosion-monitoring sensors have been proposed for corrosion monitoring of RCS. The published papers show that when the electrochemical techniques are correctly employed, they provide fast, quantitative, and reliable information. However, most of sensors introduced so far have some limitations and constraints to reach the reliable status to monitor and assess corrosion reinforcement continuously, accurately and for long time periods. The main constraints are linked to the durability, sensitivity to electromagnetic interference and stability of the sensors over time. Each electrochemical technique has advantages and limitations. To obtain accurate and reliable information about the corrosion reinforcement over time of a particular RCS, the combination of different techniques is an essential condition. For instance, OCP measurements are found to be linked with several practical limitations such as establishing connection with reinforcement, particularly in structures with a large concrete cover. As mentioned previously, OCP measurements can only provide information of the area likely to be corroded and no assessment of the corrosion rate; e.g., OCP values are only indicative of the probability of corrosion activity of reinforcement. Furthermore, OCP measurements cannot provide reliable results with epoxy-coated reinforcement or with coated concrete surfaces and are highly influenced by moist or wetting condition of concrete and by the composition of the concrete. The analysis criteria should be carefully performed since may change for different aggressive environments [222].  
Recently, [PERSON] et al. [223] studied corrosion induced by chlorides, and the \(R_{p}\) values obtained by different methods such as potentiodynamic, potentiostatic, galvanostatic polarization, and EIS. The authors concluded that all the methods lead to the same values for \(R_{p}\). Furthermore, the authors proved that detailed information of the steel/concrete system was given by the EIS method. However, some limitations exist. EIS is a time consuming method when applied in a large range of frequencies. The interpretation of results is challenging and needs suitable EECs that are dependent on the steel surface condition. Moreover, in EIS the measured steel surface area is not easily quantifiable. For galvanostatic method the authors concluded that previous knowledge of the current density  
Figure 11: \(i_{gal}\) values of the steel working electrode vs. immersion time under the following experimental conditions: (**a**) saturated Ca(OH)\({}_{2}\) solution, pH 12.5; (**b**) saturated Ca(OH)\({}_{2}\) + CO\({}_{2}\), pH 9.5; (**c**) saturated Ca(OH)\({}_{2}\) + CO\({}_{2}\) + 3% Cl\({}^{-}\); (**d**) saturated Ca(OH)\({}_{2}\) + CO\({}_{2}\) + 3% Cl\({}^{-}\) + N\({}_{2}\) (O\({}_{2}\) depletion). For each condition, temperatures of 25, 35 and 55 \({}^{\circ}\)C have been set and the corresponding measurements have been performed [182].  
to be applied was necessary and in the case of steel in a passive state, higher polarization could destroy the sensor [223]. Concerning the potentiodynamic and the potentiostatic methods, the authors reported that the second was more advantageous since it was a faster method and the ohmic drop for the correction of the measured \(R_{p}\) value was easily obtained [223]. One of the limitations of the galvanostatic method is that the response to the pulse needs to be stabilized to provide a reliable value for \(V_{max}\). The main constraints are linked to the time necessary to achieve an equilibrium value for \(V_{max}\) which, if not achieved, may lead to deviations from the expected behavior and therefore mistakes in the assessment of the system-related parameters (\(R_{p}\) and \(C_{dl}\)), which may turn into over- or underestimation of the corrosion rate.