# Corrosion Monitoring Techniques in Subcritical and Supercritical Water Environments
## 2 Non-Invasive Monitoring Techniques
### Infrared Thermography Method  
Temperature is one of the most common indicators to measure the health status of equipment and components [32; 51; 52], and based on this, the infrared thermal (IRT) imaging technique has been developed. It provides a reliable, fast, direct evaluation method with a broad response range. It uses infrared imaging systems (such as infrared cameras) to record electromagnetic waves emitted by objects and establish a clear relationship between temperature and material corrosion defects [53]. The advantages of IRT are as follows: as a non-invasive technique, IRT does not affect the target [51; 54] and can safely monitor very high temperatures or dangerous objects [54]; IRT provides two-dimensional images for comparison of different regions of the target [54]; IRT is an online (real-time) monitoring technique that can achieve large-scale real-time monitoring (with the help of advanced image processing techniques) [32; 54; 55]. Thermal imaging techniques can be divided into two categories: active and passive. In active thermal imaging, in addition to a thermal imager, an external excitation source is also needed to stimulate the thermal evolution inside the object during heating or cooling. On the contrary, passive thermal imaging does not require an external stimulus source because the object is at a temperature naturally different from the background (such as the human  
Figure 1: Schematic diagram of the device and principle for ultrasonic measurement (**a**) and typical results (**b**) Reprinted from Ref. [50]. 2018, Elsevier. body). Passive thermal imaging is more commonly considered a qualitative method for identifying abnormal temperature patterns. Worthy of special description, it is the particular controlled requirements of the active thermal imaging method, including the amount and form of stimulation, that enable it to allow for not only the identification of defects but also quantitative analysis of anomalies, such as the characterization of the physical and thermal characteristics of defects. The excitation mechanisms in active imaging techniques mainly include optical thermography, laser thermography [56], induction thermography [57], vibrothermography [58], and microwave thermography [59; 60]. Metal defect size, defect depth recovery/reconstruction, and defect thermal characteristics, such as thermal diffusivity, are considered quantitative IRT methods for evaluating corrosion and metal loss [32]. During the analysis and processing of corrosion data measured by IRT, thermal contrast calculation is the most commonly used method to improve the visibility of subsurface defects. Among them, absolute temperature contrast is achieved by assigning color or intensity to each infrared energy level corresponding to electromagnetic flux or exact temperature (through radiometry), converting images obtained using infrared cameras into visible images (heat maps) [54]. Absolute thermal comparison requires consideration of two critical assumptions. Firstly, we need to understand the sound field's position within the field of view of an infrared camera; secondly, we need to consider that thermal stimulation is uniform on the sample. To address the testing limitations brought by these two hypotheses, [PERSON] et al. proposed a modified version of absolute thermal contrast, named differential absolute contrast (DAC). In addition, the analysis methods for IRT results also include temperature signal reconstruction (TSR) using temperature and contrast derivatives, statistical techniques, matrix factorization, phase-sensitive techniques, and artistic intelligence-based techniques. Please refer to [32] for detailed information. As for the IRT mechanism exhibited in Figure 2, the total radiation received by the camera comes from three typical sources: emission from the target object (E\({}_{\text{obj}}\)), emission and reflection from the surrounding environment (E\({}_{\text{refl}}\)), and emission from the atmosphere (E\({}_{\text{atm}}\)).