## 2 The Process Plant Inspection and Maintenance System
### Potential Non-Destructive Testing Techniques for On-Stream Inspection
Another technique that is often used for corrosion detection is AET, which is based on the principle of the generation and propagation of elastic waves in materials [78; 79]. AET can provide information about the location and size of defects, making it suitable for detecting localized corrosion. However, AET is less suitable for high-temperature environments and requires materials that can generate and propagate elastic waves, which may not be the case for all materials used in processing plants. Table 4 provides a comparison of different NDT techniques, including their advantages, limitations, and suitability for process-plant applications and high-temperature surfaces. This table allows the reader to easily compare and contrast the different techniques. Comparative studies have affirmed the advantages of PAUT over traditional UT, [PERSON] et al. [14] found that PAUT's scanning speed covered a 30 mm width in a single scan. Similarly, [PERSON] et al. [13] reported that corrosion mapping using PAUT offers good detectability and comprehensive results through A-, B-, and C-scan displays. These studies underscore the superiority of PACM in terms of efficiency and accuracy for corrosion detection and analysis. Further comparisons by researchers such as [PERSON] et al. [80] highlighted PAUT's capacity to display a range of thicknesses simultaneously, distinguishing it from UT, which  
\begin{table}
\begin{tabular}{p{56.9 pt} p{113.8 pt} p{113.8 pt} p{113.8 pt} p{113.8 pt} p{113.8 pt}} \hline \hline
**Technique** & **Advantages** & **Limitations** & **Suitable for Process Plant** & \multicolumn{1}{p{113.8 pt}}{**Suitable for High-Temperature Surfaces**} \\ \hline \multirow{6}{*}{Phased Array Corrosion (PACM)} & • & Rapid and accurate results & • & Requires specialized equipment and & & \\  & & Three-dimensional defect information & • & & & \\  & & Safe for simultaneous on-site use with other trades & • & & & Yes (with limitations) \\  & & Ability to operate at elevated temperatures & • & & Reduced productivity in spot thickness measurements measurements & Yes & Yes \\  & & Effective in detecting general corrosion & • & Limited sensitivity to localized corrosion & & & \\  & & Widely used for inspecting welds and detecting (UT) & • & Limited sensitivity to localized corrosion in thin-walled structures & • & & \\  & & & & &  
provides thickness information simultaneously. In pursuit of increased productivity, [PERSON] et al. (2015) adopted a HydroFORM scanner, which yielded favorable results compared to UTG. However, while several studies have lauded UTG's high-temperature capabilities of UTG and explored PAUT for enhanced scanning speed and result display, most experiments have been conducted at ambient temperatures. [PERSON] et al. (2016) extended the PAUT corrosion mapping experiment to 150 \({}^{\circ}\)C by using a Dual Linear Probe. This effort revealed challenges, such as couplant selection, scanner choice, velocity alteration due to temperature changes, and the risk of probe damage from prolonged contact with hot surfaces. The field of corrosion detection and analysis benefits from the high-temperature functionality of the UTG technique, and the advancements introduced by PAUT overcome the limitations of the traditional UT methods. The capacity of the latter for rapid scanning, three-dimensional defect representation, and enhanced coverage makes it a promising solution for improved corrosion assessment, even in challenging environments.