# Enhanced Nondestructive Testing Using Pulsed Eddy Current and Electromagnetic Ultrasonic Techniques
## 1 Introduction  
Sewater is a potent medium for corrosion owing to its high salt concentration. Notably, ships spend the majority of their time at sea, where they are continuously exposed to corrosive conditions. Marine propulsion support systems, generator support systems, and auxiliary systems contain numerous metal components critical to ship operations. For example, seawater pipelines are essential for cooling main and auxiliary machines, firefighting, ballasting, and cleaning operations. These components play a crucial role in the stable operation of marine power plants and the normal functioning of auxiliary machinery and other equipment. The combination of humid, high-temperature environments on ships frequently leads to corrosion-related issues. The most common corrosion conditions can be divided into four types: contact corrosion, electrochemical corrosion, stress corrosion, and cavitation corrosion. Contact corrosion occurs when two different metals are present in a seawater pipeline. The interaction of these metals in the seawater electrolyte leads to electromagnetic corrosion, resulting in electrochemical corrosion. Impact corrosion refers to the electrochemical corrosion of the metal surface of seawater pipelines. Stress corrosion refers to damage to the metal in seawater pipelines under the combined effects of seawater and tensile stress, making cracks more likely to occur and posing a threat to ships. Cavitation corrosion refers to the formation of internal bubbles in a state of fluid turbulence, which then collapse under high internal pressure, leading to an impact on surrounding metal pipelines. This, combined with other forms of corrosion, results in deformation and damage in seawater pipelines. It is a comprehensive type of corrosion where mechanical failure is the primary condition, supported by other corrosion factors [1]. Corrosion of metal components can cause significant disruptions to onboard life and, in severe cases, compromise the safety of the crew and the structural integrity of the ship. An analysis by a British company identified that corrosion accounts for 33% of equipment failures within ships [1]. Utilizing portable metal damage detection equipment allows thecrew to regularly inspect crucial equipment, prevent failures, reduce maintenance times, and ensure smooth operation of the ship's systems. Most current metal damage detection tools employ nondestructive testing technologies. These methods enable the identification of surface or internal defects in materials without compromising their integrity or functionality. Nondestructive testing technologies operate by detecting variations in physical characteristics such as heat, sound, light, and electricity, which reflect changes in the material's structure [2]. Common nondestructive testing techniques include eddy current testing, X-ray testing, magnetic flux leakage testing, magnetic particle testing, piezoelectric ultrasonic testing, and electromagnetic ultrasonic testing [3]. In the 1950s, [PERSON] et al. conducted theoretical and experimental studies on the use of pulsed eddy current measurements to determine the thickness of metal protective coatings in nuclear reactors [4]. Their research established a correlation between the zero-crossing point, the lift-off point, and the thickness of the tested part, where the amplitude of the lift-off point could indicate the thickness of the protective layer. The pioneering work of [PERSON] et al. laid a foundational basis for further exploration into pulsed eddy current detection technology. [PERSON] et al. designed a transverse pulsed eddy current probe to detect circumferential cracks in heat exchanger pipes [5]. [PERSON] proposed a quantitative detection method for natural fatigue cracks in aviation aluminum alloy based on pulse eddy current technology and a neural network algorithm, achieving high accuracy [6]. [PERSON] et al. studied the effect of coil placement on pulsed eddy current detection [7]. [PERSON] et al.