# Corrosion Monitoring Techniques in Subcritical and Supercritical Water Environments
## 4 Conclusions  
Subcritical/supercritical water is widely employed in energy and environmental fields. The extremely high temperature and high-pressure hydrochemical conditions, coupled with the possible existence of corrosive species such as dissolved oxygen and various inorganic salts, pose significant challenges to developing, designing, and safely operating various subcritical/supercritical water-related equipment due to corrosion. Studying and controlling the corrosion behavior of typical materials in sub-/supercritical water systems is the key to solving the corrosion problem, especially when performing in situ corrosion monitoring. It helps capture real-time corrosion information and promptly adjust relevant equipment parameters for corrosion control. It also provides accurate data references for optimizing and improving various corrosion prediction models.  
This paper first comprehensively introduces the fundamentals, advantages and disadvantages, and applications of several non-invasive in situ monitoring technologies, such as ultrasonic measurement, infrared thermal imaging, microwave monitoring, eddy current detection, and acoustic emission, which can be used in a wide temperature range and can be employed as portable corrosion monitoring instruments in practical engineering  
Figure 15: Change of open circuit potential with temperature (**a**), variation of the passive current densities with typical anodic potentials and temperatures (**b**), and Bode diagrams (**c**) for alloy 690 in lithium borate solutions Reprinted with permission from Ref. [136]. 2019, Elsevier.  
applications. Additionally, considering the importance of electrochemical corrosion in the study of corrosion mechanisms and prediction of corrosion behavior in sub-/supercritical water systems, invasive in situ electrochemical testing methods, including electrochemical corrosion potential, electrochemical impedance spectroscopy, and electrochemical noise, are also discussed in detail. The typical existing electrochemical research platform for sub- and supercritical aqueous systems can achieve in situ online water chemistry monitoring and regulation. Some new electrode installation methods, such as using pyrophyllite for sealing and the hanging connection, may elevate the operating temperature up to 700 \({}^{\circ}\)C, providing fresh ideas for upgrading and renovating existing mainstream research platforms.  
Z.B. and Y.L. contribute equally. Conceptualization, Y.L. and Z.B.; Investigation, Z.B., L.X., Q.Z. and S.D.; writing--original draft preparation, Y.L., Z.B. and Y.Z.; writing--review and editing, Y.L., P.G. and Z.Y.; visualization, Z.B. and L.X.; supervision, Y.L.; funding acquisition, Y.L. and D.X. All authors have read and agreed to the published version of the manuscript.  
This work is supported by the National Natural Science Foundation of China (22378322, 22008190), the Key Research and Development Program of Shaanxi (2024 GX-YBXM-425), the China Postdoctoral Science Foundation (2022M72526), the Fundamental Research Funds for the Central Universities (xtr052022009), and the National Key Research and Development Program of China (2021 YFE0104900).  
The authors declare no conflicts of interest.