# Corrosion Monitoring Techniques in Subcritical and Supercritical Water Environments
## 3 Invasive Corrosion Monitoring Techniques
### Electrochemical Corrosion Potential  
When electrochemical corrosion is carried out on an isolated metal material, the cathodic and anodic reactions occur at the same potential, which is the mixed potential of these two electrode reactions coupled with each other. In corrosion electrochemistry, it is called the electrochemical corrosion potential (ECP) [90]. Since the corrosion of materials in aqueous solutions is essentially controlled by electrochemical processes, there is a specific correspondence between the ECP of materials and their corrosion behavior. Therefore, measuring ECP can be used to determine the corresponding corrosion status of materials, which can serve as a basis for adjusting environmental parameters and achieving the goal of controlling corrosion behavior. The measurement of ECP has no interference with the tested system and can perform in situ, non-destructive, long-term continuous measurement. Figure 7: Schematics of the electrical resistance sensor for corrosion measurement: (**a**) the measurement circuit and (**b**) a three-dimensional diagram of the sensitive and compensation elements Reprint with permission from Ref. [88]. 2022, MDPI. Constructing an alarm system based on the changes in ECP signals is easy and suitable for on-site corrosion monitoring. The ECP test results for two nickel-based alloys are shown in Figure 8[91]. In fact, in most studies, the OCP test results are used to roughly represent the ECP values; the ECP described in Figure 8 is the OCP test result. Figure 8a shows that the corrosion potential of 690 and 800 alloys monotonically decreases with the increase in pressure at 25 \({}^{\circ}\)C. Figure 8b shows that at a constant pressure of 10.0 MPa, the self-correction potential of 690 and 800 alloys varies with temperature at different temperatures, which can be roughly divided into two temperature influence intervals. The first temperature influence interval is from 25 \({}^{\circ}\)C to 250 \({}^{\circ}\)C, and with the increase in temperature, the self-corrosion potential of 690 alloy and 800 alloy decreases almost linearly, with the second temperature range being 250 \({}^{\circ}\)C to 300 \({}^{\circ}\)C. The average self-corrosion potential of 690 alloy and 800 alloy is very similar. The acquisition of ECP data requires using a potential meter with a high internal resistance (significantly greater than 10\({}^{6}\)\(\Omega\)) to monitor the system composed of the working material to be tested and the stable reference electrode. As for the ECP of materials, it is usually necessary for guiding the polarization curve measurement, assisting the drawing of potential-pH diagrams (E-pH), as well as determining the critical or sensitive potential ranges for local corrosion behaviors, such as pitting corrosion, crevice corrosion, and stress corrosion cracking (SCC). Based on the mutual verification relationship between these electrical corrosion data, judging whether the material will undergo corrosion problems is convenient. ECP monitoring at nuclear power plants is the most representative application. So far, ECP monitoring is the only method not requiring depressurization, current limiting, or cooling sampling to obtain in situ electrochemical information on the water chemistry and corrosion environment of advanced water-cooled nuclear reactors. Currently, ECP is mainly used as a data reference for the hydrogenation water chemistry of BWRs and PWRs and the oxygenation water chemistry of PWR secondary circuits in nuclear power corrosion monitoring [92]. Corrosion prevention and control can be achieved by adjusting the ECP below the SCC critical potential and to the stable range of the oxide film by combining the E-pH diagram [93; 94]. Due to each nuclear power plant's different operating environmental parameters, the consistency of ECP monitoring results is poor. Therefore, conducting online ECP monitoring at each nuclear power plant is necessary.