# Corrosion Monitoring Techniques in Subcritical and Supercritical Water Environments
## 3 Invasive Corrosion Monitoring Techniques
### Research Platform of In Situ Electrochemical Corrosion Testing  
To investigate the electrochemical corrosion of alloys in high-temperature and high-pressure water systems, it is necessary to establish a research platform for simulating experimental environments. [PERSON] and his group have extensively researched high-temperature electrochemical corrosion [127, 128, 129, 130]. They have designed the circulating flow loop with a hydrodynamic autoclave vessel, as shown in Figure 11, which contains the storage tank, a high-pressure positive displacement pump (2 L/h capacity pulsating diaphragm pump), a hydraulic accumulator, a regenerative heat exchanger (cooled by freshwater), and so on. The circulating flow loop with a hydrodynamic autoclave can continuously update the solution in the reactor, keep the simulated solution in the testing area fresh, and avoid the interference of impurities and salt deposition during long-term experimental processes. Due to the existence of circulating loops, the reactor's design requirements and production costs are reduced. In addition, the circulating flow loop with a hydrodynamic autoclave designed in the research platform is consistent with most engineering practical application scenarios. The reference electrode is the core component of the electrochemical research platform, which mainly includes various reference electrodes such as hydrogen electrodes, YSZ electrodes, metal/metal oxide electrodes, external pressure balance reference electrodes, etc., especially the YSZ electrode, which has been successfully applied in high-temperature conditions exceeding 500 \({}^{\circ}\)C [1,106]. These contents are extensive and complex and will be further explored in another review article. Figure 11: Once-through recirculating flow loop with hydrodynamic autoclave vessel Reprinted from Ref. [129]. 2010, Elsevier. Based on the research platform established by [PERSON]'s group, researchers from the Institute of Metals (IMR) in the Chinese Academy of Sciences have developed a high-temperature electrochemical research platform that can monitor and control water quality, as shown in Figure 12. Two refreshed autoclaves were connected in series in the loop, with autoclave 1 arranged in the upstream section. Both autoclaves are made of stainless steel and have a 2 L volume. Autoclave 1 is lined with pure Ni, while autoclave 2 is not. These two autoclaves could be used collectively or solely during the exposure experiments. The inlet and outlet water chemistry parameters, namely dissolved oxygen concentration (DO) and conductivity, were monitored with METTLER TOLEDO Thornton Model 3X7-210 (METTLER TOLEDO, Zurich, Switzerland) dissolved oxygen sensors and Model 240-201 conductivity sensors, respectively. A computer with Labview 8.5 software was used to control temperature and DO. Temperature fluctuations in both autoclaves were controlled well within \(\pm\)0.5 \({}^{\circ}\)C, and DO fluctuation in inlet water was within \(\pm\)10 ppb (by weight) [131; 132; 133]. [PERSON] et al. investigated the corrosion electrochemical characteristics of nickel-based alloy 690 using the research platform, as shown in Figure 13[134]. Figure 13a shows potentiodynamic polarization curves for Alloy 690 at 300 \({}^{\circ}\)C in 1500 ppm B and different concentrations of Li solutions without Zn injection. As the pH value increases, the electrochemical corrosion potential sharply decreases, and the passivation current density slightly decreases. This indicates that the 690 alloy forms an oxide film in all pH solutions studied and exhibits good corrosion resistance in alkaline solutions. Figure 13b shows the polarization curves of 690 alloy in solutions with different zinc concentrations at 300 \({}^{\circ}\)C, indicating an increase in zinc concentration enhances corrosion resistance. The passivation current density is \(10^{-4}\) A\(\cdot\)cm\({}^{-2}\). Figure 12: Schematic diagram of the high-temperature and high-pressure water loop in IMR Reprinted with permission from Ref. [131]. 2017, Elsevier. [PERSON] et al.