# Corrosion Monitoring Techniques in Subcritical and Supercritical Water Environments
## 2 Non-Invasive Monitoring Techniques
### Acoustic Emission  
Acoustic emission (AE) refers to the phenomenon where a material emits transient sound waves locally due to the rapid release of energy. The deposition of corrosion products, the rupture of passivation films, and the initiation and propagation of cracks all generate AE signals with different characteristic parameters [38; 74]. Therefore, based on this information, the occurrence time, starting sites, and severity of corrosion damage can be determined. The application of acoustic emission techniques in electrochemical corrosion research has a history of more than 40 years, with existing research mainly focusing on monitoring acoustic emission signals during the electrochemical corrosion processes to determine the corrosion locations and degrees. Actually, during the electrochemical corrosion process, the generation and movement of hydrogen gas, stress changes of the metal surface, deposition of corrosion products, rupture of passivation films or salt layers, and oxygen reduction may all be sources of acoustic emission. Meanwhile, it is necessary to establish the relationship between acoustic emission signals and electrochemical corrosion rate, corrosion potential, etc. Based on this, it is possible to identify the characteristic acoustic emission signals in the electrochemical corrosion process and clarify the corresponding physical correspondence [75]. The application of AE in the on-site measurement of small-diameter drilling processing status is shown in Figure 6.  
However, the difficulty in AE monitoring under normal and high-temperature conditions is how to analyze the signal. The diversity of AE sources and the suddenness and uncertainty of signals pose significant challenges to analyzing AE data and establishing corresponding relationships between AE signals and corrosion behavior. In addition, attenuation of sound signals, reflection of sound waves, mode conversion, structural correlations or discontinuities, and interference from background noise can all hinder the analysis of  
Figure 5: A typical schematic of the PEC system (**a**), as well as the measured values and fitted lines of PV(Bnorm) for uncoated and coated corrosion (**b**) Reprinted with permission from Ref. [35]. 2019, Elsevier and from Ref. [73]. 2012, IEEE. Figure 6: Experimental setup and the AE measurement system mounted on a small automatic drilling machine Reprint with permission from Ref. [76]. 2024, MDPI. AE signals [77]. Currently, the commonly used AE analysis method is parameter analysis. Various signal processing techniques, such as wavelet analysis, modern spectral analysis, and neural network analysis, have been successfully applied. The development of modal AE theory and related techniques has provided a new way to further explain AE signals' physical meaning [75]. AE has been used to monitor the coupled environmental cracking process in high-temperature and high-pressure water environments [78; 79]. [PERSON] et al. [80] believe that compared to room temperature, the oxide film formed on the surface of materials in high-temperature and high-pressure water environments is thicker and that the amplitude of AE signals generated by crack initiation and propagation is relatively higher, making it possible to monitor cracks. [PERSON] et al. [81] investigated the sensitivity of various materials to SCC in chloride-containing environments at 300-320 \({}^{\circ}\)C and 70-80 MPa via AE. They indicate that the combination of the event count and amplitude of AE signals can determine cracks' initiation and instability. [PERSON] et al. [82] conducted a similar study on nickel-based alloy 600 in a simulated PWR environment (290-330 \({}^{\circ}\)C) but used characteristic parameters such as the AE signal's amplitude, energy, and rise time. [PERSON] et al. [78] and [PERSON] et al. [79; 83] studied the SCC process of solid solution and sensitized 304 SS in high-temperature and high-pressure water.