## 3 Method Proposed for Corrosion Evaluation
### Laboratory Analysis on the Corrosion Detection  
The sulfate-based electrode was built by electrochemical sulfation of a silver wire with a diameter of 0.6 mm. Complete sulfation of the wire surface was achieved by imposing a constant current density of 10 \(\mu\)A cm\({}^{-2}\) for 24 h in 0.1 M sulfuric acid solution. The process was conducted in a two-electrode cell, where the silver wire acted as the working electrode and a platinum mesh as the counter electrode.  
The sensor was tested in an aqueous solution at pH 12 (KOH), with different Na2 SO4 concentrations, ranging from 0.2 to 1 M. The Open Circuit Potential (OCP) was measured in a two-electrode cell, where AgSO4 wire acted as the working electrode and Hg/HgO as the reference electrode. All electrochemical tests were conducted using three different electrodes obtained in the same conditions. In addition, OCP measurements at different sulfate concentrations were repeated three times for each electrode. All the electrochemical measurements were carried out using a Parstat mod 2273 potentiostat/galvanostatic.  
The sensor was electrochemically characterized by OCP tests to assess its sensitivity to variations in sulfate concentration. The tests were carried out in alkaline to simulate a condition similar to concrete pH. The aqueous solution contained different concentrations of sulfate ions. Regarding the duration of the tests, 100 s was the time required for OCP value stabilization, and, for this reason, it was set as the duration of each test. Figure 11A shows OCP measurements for a single electrode at different sulfate concentrations. These curves were used to construct the calibration line of the sensor. In particular, the value of stationary OCP of the electrode was reported in comparison with the concentration of sulfate ions present in the solution in Figure 11A.  
The aforementioned calibration line, together with the standard deviation related to each point, are reported in Figure 11B. The very low standard deviation values led us to conclude that the sulfate sensor had excellent reproducibility. Specifically, the equation of the sensor calibration line was extrapolated by linear regression, and is given as:  
\[\begin{array}{l}OCP\;[\mathrm{mV}]=b\;Log\;C+E_{0}\\ b\;[\mathrm{mV}]=-31.64\pm 1.56\\ E_{0}\;[\mathrm{mV}]=272.3\pm 15.33\end{array}\]  
where \(b\) represents the slope of the curve, and \(E_{0}\) is the intercept. The values of \(b\) and \(E_{0}\) are the averages of the values obtained from the tests performed using three different electrodes. These values are consistent with Nernstian behavior, and they are, therefore, a sign of good functioning of the sensor. These results, indeed, demonstrate the good sensitivity and reproducibility of the manufactured sensors.  
However, the authors specify that research on sensor optimization for sulfates is still ongoing. In particular, the possibility to make the sensor directly selective for chloride ions will be investigated by surrounding the sensitive part with a solid-state membrane selective for sulfate ions, which can work inside the concrete. The membrane will consist of a homogeneous mixture of sulfides and sulfates concerning various metals, in order to obtain an ion-selective system for sulfate ions. Furthermore, a simpler system will be also tested, in which the sensing element of the sensor will be coated with montmorillonite. This mineral has the property of preventing the diffusion of chloride ions. Finally, both modified sensors will be tested both in liquid solution and inside concrete.