# Corrosion Monitoring Techniques in Subcritical and Supercritical Water Environments
## 3 Invasive Corrosion Monitoring Techniques
### Electrochemical Noise  
Electrochemical noise (EN) refers to the random non-equilibrium fluctuation phenomenon of electrochemical state parameters (such as electrode potential, external measured current density, etc.) during the evolution of electrochemical dynamic systems [111]. This fluctuation phenomenon provides rich evolutionary information for the system. The electrochemical noise technique has been widely applied in industrial electrochemistry, including metal corrosion and protection, chemical power supply, and metal electrodeposition [111; 112]. As an in situ, online, and non-interference detection method, it can monitor the uniform and local corrosion of materials online and the type and strength of corrosion remotely [113]. When applying electrochemical noise techniques to monitor electrochemical corrosion in high-temperature water, attention is usually paid to corrosion potential or current noise. Monitoring the current noise usually involves connecting two working electrodes of the same material through a zero-resistance meter to monitor the coupling current between them. However, monitoring potential noise requires using reference or pseudo-reference electrodes, which face the same hardware limitations as ECP testing. In addition, the difficulty of applying EN in high-temperature water systems lies in how to reduce the internal noise of equipment to improve the accuracy and precision of data [84]. The key to successful monitoring is to use stable and reliable reference electrodes, ensure high conductivity of the test solution [114], and reduce the distance between the test electrode and the reference electrode to reduce background noise [115]. Figure 10 shows an example of EN monitoring results for Q235B steel exposed to the atmosphere of Tianjin city for 20 days [86]. When the humidity is low, the electrochemical current noise (ECN) and electrochemical potential noise (EPN) amplitudes are very small, with only a few nanoamps and millivolts, indicating a significantly lower corrosion rate. When the relative humidity increases to 65%, the fluctuation amplitude of ECN and EPN substantially increases, and the corrosion rate of Q235B steel correspondingly increases. The evolution of noise resistance (Rn) is closely related to humidity. As humidity rises, Rn values decrease accordingly; Rn increases correspondingly with the decrease in moisture [116]. On this basis, analyzing EN data and extracting feature information containing electrochemical processes is crucial. The analysis of EN data can usually be divided into two types: time-domain and frequency-domain. Time domain analysis is a visual inspection, image fitting process (transient analysis), or statistical analysis of the original EN data. The temporary shape contains basic information about electrochemical processes. Some characteristic parameters obtained from statistical analysis, such as standard deviation, root mean square value, skewness, local index, etc., can be used for qualitative evaluation of pitting, SCC, and crevic corrosion processes [117], and the obtained noise resistance can quantitatively evaluate corrosion rate [118]. Some studies use the Fast Fourier Transform (FFT) or Maximum Entropy method, converting the EN signal from a time domain to a frequency domain and obtaining the Fourier spectrum or power spectral density [119]. In 1968, [PERSON] observed, for the first time, the random fluctuation of corrosion electrode potential over time in a corrosion electrochemical system (i.e., electrochemical noise) [120]. In the last decade, the application of electrochemical noise techniques in corrosion science and related scientific fields has increasingly attracted widespread attention. In the study of the local corrosion of metals, electrochemical noise can be used to determine corrosion types and study pitting corrosion and stress corrosion cracking characteristics.