# Corrosion Monitoring Techniques in Subcritical and Supercritical Water Environments
## 3 Invasive Corrosion Monitoring Techniques
### Electrochemical Impedance Spectroscopy  
Electrochemical impedance spectroscopy (EIS) is a frequency-domain data analysis that studies the variation of electrochemical impedance with frequency by applying a small-amplitude sinusoidal AC excitation signal when the electrochemical system is under a stable DC polarization condition. EIS measurement has a lower system disturbance and a wide testing frequency range. It can obtain in situ dynamics data on electrochemical processes and electrode interface structure information related to electrochemical corrosion without damaging the system. It is suitable for online corrosion monitoring. On the other hand, there is an approximate linear relationship between disturbance and the corresponding system, which can simplify the mathematical processing of test results. As a branch of the impedance spectroscopy technique, dynamic electrochemical impedance spectroscopy can obtain richer electrochemical information and has been used in the study of material passivation films [102]. Electrochemical impedance spectroscopy contains rich information on the microscopic mechanisms and microscale process dynamics behind electrochemical corrosion behavior [103; 104]. However, most studies usually use the \"direct equivalent circuit method\" to analyze electrochemical impedance spectroscopy data, which can only obtain macroscopic values of equivalent circuit components in the corrosion system, such as resistance, capacitance, etc., and fail to provide microscale process information such as atomic-scale dynamics of alloy corrosion. Establishing an impedance mathematical model of the alloy matrix \(|\) passivation film system based on the corrosion microscale process is a crucial foundation for obtaining information on the corrosion microscale process by analyzing electrochemical impedance spectroscopy data. The Bode plots of several typical impedance spectrum test results are shown in Figure 9.  
Figure 9: Typical electrochemical impedance spectroscopy data for **(a,b)** alloy 690 at 200–300 \({}^{\circ}\)C Reprinted from Ref. [105]. 2018, Elsevier and for **(c,d)** 316L stainless steel in subcritical and supercritical water Reprinted from Ref. [106]. 2019, Elsevier. [PERSON] et al. [107] and the authors of this paper [108; 109] have employed the point defect models and the experimentally obtained EIS data to analyze the atomic scale corrosion anodic processes of some metals and alloys, such as zirconium alloy, stainless steel 304, and nickel-based alloy 600, under high-temperature conditions. For example, [PERSON] et al. [107] pointed out that the rate constant of \"the transformation from zirconium atom to the lattice zirconium within ZrO\({}_{2}\)\" at the interface of the polycrystalline zirconium matrix/passivation film at 250 \({}^{\circ}\)C is 2.42 \(\times\) 10\({}^{-15}\) mol\(\cdot\)cm\({}^{-2}\cdot\)s\({}^{-1}\), with a charge transfer coefficient of approximately 0.27. The authors' previous research found that the growth of the barrier layer of the passivation film and the thickening of the film's porous outer layer are determined by two atomic-scale processes at the interface between the passivation film and the substrate of stainless steel 304 in a PWR coolant simulation solution at 300 \({}^{\circ}\)C. They are, respectively, the process of the chromium-predominant metal atoms converting into lattice cations in chromium-rich oxides of the barrier layer and the transformation process of the iron-dominated metal atoms, which release electrons and enter the passivation film in the form of cation interstitials. The reaction rates are all approximately 10\({}^{-11}\) mol\(\cdot\)cm\({}^{-2}\cdot\)s\({}^{-1}\) and are influenced by water chemistry parameters such as pH value and dissolved oxygen content [109].