# Corrosion Monitoring Techniques in Subcritical and Supercritical Water Environments
###### Abstract
The actual operation of some SCWO demonstration/industrialization plants indicates that the current equipment exposed to the transcritical water environment, such as preheaters, reactor fronts, and coolers for reaction effluents, has the highest risk of corrosion failure [13; 14], which is a common bottleneck problem that restricts the industrial promotion of various supercritical water treatment techniques, including supercritical water gasification for hydrogen production from organics, supercritical hydrothermal combustion, etc., as well as the SCWO [13; 15; 16; 17]. The SCWO treatment units constructed by HydroProcessing in Harlingen, TX, USA and Shinko Pante in Kobelco, Japan were forced to shut down due to severe corrosion in the preheater and reactor front [16]. Pitting corrosion is an essential cause of material failure in chlorine-containing subcritical/supercritical water systems [18]. A summary of the above discussion can be obtained: Advanced nuclear water-cooled reactors, (ultra) supercritical thermal power plants, as well as a series of supercritical water treatment techniques all involve highly harsh sub-/supercritical aqueous systems, which can lead to severe corrosion problems of materials [19; 20]. In sub-/supercritical water environments, changes in temperature and pressure can cause changes in water density, dielectric constant, viscosity, and other characteristics. Taking water density as an example, an increase in temperature leads to a continuous decrease in water density, a reduction in charge transfer rate, and a shift in the corrosion process within the system from electrochemical reaction influence to chemical reaction influence, and this has been discussed in detail in other papers [13; 1]. Materials used in sub-/supercritical water environments mainly include stainless steel, nickel-based alloys, and ceramics. However, nickel-based alloys with stronger corrosion resistance are the most commonly used for more complex supercritical water environments containing salt and oxygen. At the same time, ceramic materials have a risk of fragmentation and are, therefore, less commonly used [14]. Meanwhile, the application of some chromium-containing coatings, zirconia coatings, and titanium oxide coatings in subcritical/supercritical water environments has also been widely studied [21; 22; 23]. Studying and solving the corrosion problem of typical alloys in sub-/supercritical water environments is significant for promoting the development of the techniques mentioned earlier. However, the current investigation on alloy corrosion in sub-/supercritical water mainly focuses on the use of offline testing methods, which often require cutting and sampling of equipment walls, pipelines, etc., causing irreversible damage and the inability to obtain real-time corrosion data during long-term operation, resulting in significant limitations [6; 24; 25; 26; 27]. On the other hand, there is an error between the data measured under the laboratory simulation system and the actual corrosion data during equipment operation [28], so the method of in situ online monitoring of corrosion data is essential. To date, various in situ online corrosion monitoring technologies have been developed to compensate for the shortcomings of offline characterization testing techniques. Non-invasive testing techniques, such as ultrasonic monitoring, infrared thermography, etc., do not directly contact the system, avoiding damage to monitoring equipment and testing systems. However, considering that the corrosion of alloys is predominantly caused by the electrochemical processes that occur in subcritical and high-density supercritical water systems, several electrochemical in situ online monitoring technologies as invasive methods are equally significant. Therefore, this article systematically summarizes and compares the principles, advantages and disadvantages, applicable scenarios, and development progress of representative monitoring technologies from the perspectives of non-invasive and invasive methods.