# Corrosion Monitoring Techniques in Subcritical and Supercritical Water Environments
## 2 Non-Invasive Monitoring Techniques
### Ultrasonic Thickness Measurement  
The fundamental principle of the ultrasonic monitoring method is to generate mechanical waves or vibrations in tested samples in experimental laboratories or actual environments [8; 39], as shown in Figure 1a. Ultrasound is generated and received in different ways. The most common method of generating and receiving ultrasonic waves is piezoelectric transducers, which are transmitted to the tested material through an acoustic coupler, while electromagnetic acoustic transducers generate waves directly in the tested material [29]. Ultrasound is highly sensitive to structural damage and has high permeability to materials. The second harmonic provides valuable information about material damage conditions by measuring the amplitude ratio of the fundamental and second harmonics [40; 41]. In some countries, ultrasonic monitoring has been used as a standardized evaluation method for compressive strength and material crack depth [30]. When conducting large-scale corrosion monitoring on the high-temperature pipes of thermal power plants and oil pipelines, ultrasonic monitoring can accurately and quickly determine where corrosion occurs. Using guided ultrasound in these situations has the following advantages: fast response speed, high sensitivity in detecting minor defects, and suitability for large-scale and long-distance testing [30; 42]. Meanwhile, ultrasonic monitoring can be directly performed outside the  
\begin{table}
\begin{tabular}{c c c c c c c} \hline \hline
**Monitoring** & **Principle** & **Data Presentation** & **Advantages** & **Disadvantages** & **Application** & **Error or Accuracy** \\ \hline \multirow{6}{*}{Microwave imaging [33; 34]} & & The dielectric performance changes caused by defects or structural abnormalities are converted into readable voltage values and then & Have difficulties penetrating conductive materials; it is limited to surface correction, correcting, etc. & Determine & Accuracy better than \(1/100\) of the period \\  & Based on the interaction between increasing  
component without using probes in contact with the corrosive environment during the testing process, so it is non-invasive to closed containers [43].  
The commonly used monitoring methods based on ultrasonic testing techniques include contact piezoelectric transducers [44], lasers [45], and capacitance techniques. Contact piezoelectric transducers have low operating costs but higher requirements on the polishing degree of the contact surface to ensure good contact between the detection element (the transducer) and the sample surface. Unfortunately, this technique is unsuitable for harsh environments with high temperatures, high pressure, and strong corrosivity. Another method, the ultrasonic laser wave method, can somewhat compensate for these shortcomings. The advantage of the ultrasonic laser is that it can measure without sample contact and allows for point detection. It still has high-resolution scanning ability when working under high temperatures and pressure [46; 47]. However, this test requires a sufficiently high surface finish of the sample [48]. Therefore, the signal-to-noise ratio is high. The third ultrasonic method, the capacitive ultrasonic transducer, has high sensitivity and can measure the absolute displacement of first and second harmonics. However, sample preparation is cumbersome [30; 49].  
[PERSON] et al. [50] introduced the use of the piezoelectric pulse-echo method in corrosion monitoring. As shown in Figure 1a, the ultrasonic signal travels through the water medium, is then reflected by the specimen, and is finally received by the transducer. The typical time-domain signals from piezoelectric pulse-echo measurements of high-temperature nickel-based alloys with different corrosion layer thicknesses are displayed in Figure 1b. The signal is offset to compare the thickness of varying corrosion layers. The green line represents the reflectivity of the pipe back, while the red line represents that of the internal interface.