# Corrosion Monitoring Techniques in Subcritical and Supercritical Water Environments
## 3 Invasive Corrosion Monitoring Techniques
### Research Platform of In Situ Electrochemical Corrosion Testing
from Xi'an Jiaotong University (XJTU) developed a new generation of multi-functional electrochemical research platforms, as shown in Figure 14[135]. It comprises a high-pressure gas injection circuit, a low-pressure gas dissolution control circuit, a sub-/supercritical hydrochemical research device, and a sub-/supercritical water primary circuit. By comparison, this system directly introduces the target gas into the reactor through a liquid gas tank, which can effectively increase the upper limit of the dissolved gas concentration in the solution and is suitable for studying the effect of high-concentration liquefied gas on the electrochemical corrosion behavior of materials. Meanwhile, high concentrations of dissolved gases are beneficial for electrochemical catalytic synthesis. On the other hand, the combination of wire-through-type high-temperature sealing components, high-temperature insulation sealing materials, threaded connections, and welding can achieve sealing and insulation between the electrode and the reactor under high-temperature and high-pressure conditions. The hanging connection method improves the flexibility of the electrode and makes it less susceptible to mechanical impact damage. Figure 14: Photo show of a multi-functional sub-/supercritical water electrochemical research platform built in XJTU. (**a**) the main view of the research platform, (**b**) autoclave, (**c**) Reactor base and electrode arrangement. Figure 13: High-temperature potentiodynamic curves for Alloy 690 at 300 \({}^{\circ}\)C (**a**) in different pH solutions without Zn, (**b**) in solutions with 1500 ppm B, 2.3 ppm Li, and several Zn concentrations Reprinted with permission from Ref. [134]. 2011, Elsevier. [PERSON] and [PERSON] et al. [136] obtained the electrochemical corrosion data of nickel-based alloy 690 samples in simulated primary coolants of PWRs at temperatures of 200-300 \({}^{\circ}\)C. The simulated primary coolant is a lithium borate solution with a dissolved oxygen concentration of 400 ppb, 2000 ppm B, and 2 ppm Li. As shown in Figure 15, as the experimental temperature increases, the open circuit potentials of alloy 690 decrease from \(-0.553\) V\({}_{\mathrm{SHE}}\) to \(-0.783\) V\({}_{\mathrm{SHE}}\). On the other hand, the current density is independent of the formation potential of the passivation film at the same temperature. In addition, the study also tested the electrochemical impedance spectrum at open circuit potentials, and the results showed that the absolute value of impedance decreased with increasing temperature, indicating a decrease in overall corrosion resistance. This is consistent with the current behavior of passivation. In summary, the current research apparatus used for electrochemical testing in high-temperature and high-pressure water systems are mainly the continuous flow platforms established by scholars represented by [PERSON]. New platforms are upgraded and rebuilt based on them. These continuous platforms solve the problems of solution contamination and salt deposition in sequential batch platforms. They are often used to simulate the corrosion problem of the primary circuit of PWRs. However, due to the issue of electrode sealing insulation, it is challenging to apply it to tests above 300 \({}^{\circ}\)C. The newly established platform developed by [PERSON]'s team at Xi'an Jiaotong University based on [PERSON]'s design concept can meet the higher temperature requirements of 550 \({}^{\circ}\)C and achieve a wide adjustable content range of interested dissolved gases in the system [137]. In addition, the research platform based on tube furnaces designed by [PERSON] et al. introduces a new electrode installation and sealing method, which can further increase the operating temperature to 700 \({}^{\circ}\)C [138].