# Corrosion Monitoring Techniques in Subcritical and Supercritical Water Environments
## 2 Non-Invasive Monitoring Techniques
### Microwave Imaging  
Microwave non-destructive testing is based on the interaction between microwaves and dielectric materials. The frequency range of employed microwaves is generally 5-50 GHz. At this frequency range, defects or structural abnormalities in the material can cause enough changes in dielectric property. With the help of corresponding analysis software, this dielectric difference can be converted into readable voltage values, which can then be restored to images of internal defects and structures through specific algorithms. The schematic diagram of the microwave detection is shown in Figure 4a, taking one steel plate covered by coating as an example. The near-field of an open waveguide illuminates the incident signal. It is transmitted into layered media (the coatings and the steel plate substrate) and reflected by conductive plates. The ratio of these two signals gives the effective reflection coefficient of the tested sample [61]. The microwave transmitter inside the energy emission device emits the selected microwave signal within a specific frequency range, ensuring the microwave propagates in the tested dielectric material. Different dielectric characteristics will reflect changes in the material structure or internal defects. These subtle differences can cause changes in the amplitude and phase of the reflected microwaves. The receiving sensor within the measurement unit can recognize and collect the performance parameters of the reflected waves and perform real-time analysis and calculation, ultimately displaying real-time images of the dielectric material's internal structure and defect morphology, as given in Figure 4b,c. Microwaves can effectively penetrate dielectric materials, and any small discontinuity in the material can cause changes in its dielectric properties. Microwaves are very sensitive to changes in dielectric properties, so they have a very high detection sensitivity for any slight defects in dielectric materials [62]. Figure 4: Microwave reflection and transmission for coated metal (**a**) and the changes in amplitude (**b**) and phase (**c**) microwave detection Reprinted from Ref. [61]. 2013, Elsevier. Figure 3: Moisture detected under the upper hinge of the rudder test sample Reprint with permission from Ref. [54]. 2014, MDPI. The microwave non-destructive monitoring supports remote detection, estimates defects' physical size and direction, and is easy to operate [63; 64]. Microwave NDT techniques may be the unique solution in some situations (such as high temperature applications) [61]. Near-field open waveguide non-destructive microwave imaging seems to be one of the most promising techniques for detecting the presence of a particular layer or internal defects in layered structures [61; 65]. In recent years, microwave non-destructive testing (MNDT) has significant potential in determining the properties of concrete, as it is highly effective in detecting pores, cracks, unevenness, and defects [33]. The main application of microwave imaging techniques in metal corrosion monitoring is as follows: a waveguide probe with primary mode excitation is used to irradiate structures with electromagnetic waves in the proper frequency range. The amplitude and phase difference between the reflection coefficients of non-coated and coated samples are related to the tested layer thickness and dielectric constant. Generally, there is a significant difference in attenuation and reflection signals between the defect and non-defect areas. The differential characteristics recorded in the reflection signal are employed to detect and determine the defect size of the tested sample. The calculation of reflection coefficients for various media mainly includes deriving the forward and backward propagating electric and magnetic field components in each layer based on the known incident field and applying appropriate boundary conditions at each interface. These complex reflection coefficients refer to the complex number of spectral changes in their phase and amplitude.