Application of Electromagnetic Ultrasonic Testing Technology in Pipeline Defects

Abstract

Pipelines, as critical carriers for energy transportation, are prone to defects such as cracks and corrosion during long-term operation. Traditional testing methods exhibit limitations in various aspects, while electromagnetic ultrasonic testing technology, leveraging its advantages of non-contact operation and couplant-free application, has emerged as a significant direction for pipeline integrity assessment. This paper analyzes the advantages of EMAT guided wave testing technology in achieving long-distance and rapid screening of pipelines, as well as the strengths of bulk wave testing technology in high-precision quantitative evaluation. It also examines the unique value of obliquely incident SV waves in the directional identification of weld defects. Furthermore, the paper discusses the potential of integrating EMAT with multiple technologies, demonstrating how multi-physical field synergy enhances detection reliability. Finally, it summarizes the remaining challenges in practical engineering applications, providing references for advancing the field toward intelligent and high-precision development.

1. Introduction

Pipelines serve as critical infrastructure in urban development, energy transmission, and industrial systems, operating continuously under harsh conditions involving high pressure, corrosive environments, and complex external mechanical loads [1]. In key sectors such as oil, natural gas, and chemical processing, the structural integrity of pipelines directly determines the safety and operational continuity of the entire system [2]. However, during manufacturing, installation, and prolonged service, pipelines inevitably develop various defects, including cracks, wall thinning due to corrosion, and geometric deformations [3,4]. Under sustained operational loading, these defects may propagate progressively, potentially leading to catastrophic failures such as leakage or rupture—resulting not only in significant economic losses but also posing severe threats to public safety and the ecological environment [5].

Depending on their specific operating conditions, pipeline configurations exhibit distinct defect types and degradation mechanisms influenced by structural design, environmental loading, and service parameters. Long-distance transmission pipelines, characterized by extensive deployment lengths, extended service cycles, and variable transported media, are prone to geometric imperfections, material loss, cracking, and weld-related flaws. These defects are primarily induced by medium-induced corrosion, cyclic stresses, and construction quality inconsistencies [6,7]. Commonly employed NDT techniques for such pipelines include magnetic flux leakage (MFL), ultrasonic testing (UT), and eddy current testing (ECT) [8,9,10]. Subsea pipelines operate under high hydrostatic pressure, aggressive seawater corrosion, and dynamic mechanical loads from waves and currents. Typical defects include delamination and cracking of concrete weight coating, degradation and disbondment of anti-corrosion coatings, depletion and potential anomalies of sacrificial anodes, and localized coating deterioration and corrosion at riser sections. The damage evolution in subsea pipelines is governed by a combination of seawater chemistry, hydrodynamic loading, and the performance of cathodic protection systems. Suitable detection methods include MFL, ultrasonic testing, ECT, and potential mapping techniques [11,12]. Buried pipelines, operating in concealed environments subjected to both soil corrosion and internal medium effects, exhibit higher unpredictability in defect initiation and progression. Common defects include cracks, and corrosion pits, as well as external coating disbondment, coating degradation, soil-induced external corrosion, and geometric deformation of the pipe body. Currently, defect identification and assessment primarily rely on MFL and conventional ultrasonic testing [13,14]. A comparative summary of different pipeline types, their characteristic defects, and applicable conventional NDT techniques is provided in

Table 1. Comparison of different types of pipeline characteristics and defects.

Pipeline Types	Operational Characteristics	Common Defect Types	Applicable Detection Techniques	References
Long-distance transmission pipeline	Long installation distance, extended service life, and complex transported media	pipeline deformation, corrosion, wall thinning, stress corrosion cracking, fatigue cracks, lack of fusion, lack of penetration	Magnetic flux leakage testing, ultrasonic testing, eddy current testing, radiographic testing	[6,7,8,9,10]
Subsea pipeline	High pressure, severe corrosion, and complex marine dynamic loads subject pipelines to sustained cyclic loading.	Concrete weight coating cracking and detachment, anti-corrosion coating damage and disbondment, sacrificial anode degradation, and corrosion	Magnetic flux leakage testing, ultrasonic testing, eddy current testing, potential mapping	[11,12]
Buried pipeline	Hidden and inaccessible condition, dual corrosion from internal and external environments, complex and variable environmental loading	Internal wall cracks and corrosion, external wall corrosion and coating disbondment, geometric deformation	Magnetic flux leakage testing, ultrasonic testing	[13,14]

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Although modern industrial systems have significantly improved pipeline quality control and periodic detection regimes, various potential defects inevitably persist in pipeline systems under the combined effects of large-scale production, complex construction conditions, and harsh operating environments, and their accurate detection remains a significant challenge. Therefore, developing efficient and reliable nondestructive testing technologies and establishing systematic pipeline integrity management solutions have become a consensus in both industrial and academic communities. Among various nondestructive testing methods, ultrasonic testing (UT) has been widely adopted due to its high sensitivity to defects such as cracks and corrosion. However, conventional piezoelectric ultrasonic testing requires couplant, limiting its application in high-temperature environments, on rough surfaces, or during detection through coatings. In this context, electromagnetic acoustic transducer (EMAT) technology—offering couplant-free operation, non-contact measurement, and compatibility with high-temperature conditions—demonstrates distinct advantages. By providing a novel technical pathway to address the aforementioned challenges, EMAT has attracted increasing attention in recent years and exhibits promising application potential in pipeline detection. This paper aims to provide a review of the principles, advances, practical applications, and future development trends of EMAT technology in pipeline defect detection.

2. Traditional Detection Methods of Pipeline Defects

The traditional pipeline detection technology is mainly divided into two categories: pipeline internal detection and pipeline external detection. In-line detection is performed by deploying intelligent pigging tools equipped with dedicated NDT sensors inside the pipeline, enabling online assessment without interrupting normal operation. Key In-line detection methods include ultrasonic testing (UT), magnetic flux leakage (MFL) testing, and eddy current testing (ECT) [15]. In contrast, for exposed pipelines such as those located on the surface or elevated structures, external detection techniques can be applied without excavation, allowing non-invasive evaluation from the outside. These methods include UT, ECT, and radiographic testing (RT) [16,17]. Based on diverse physical principles—such as acoustic wave propagation, electromagnetic induction, and radiation attenuation—these techniques collectively form a diversified technical framework for assessing pipeline integrity. The following section presents an overview of the working principles and key characteristics of five conventional NDT methods: ultrasonic testing, magnetic flux leakage testing, eddy current testing, magnetic particle testing (MT), and radiographic testing. A comparative summary of these methods is provided in

Table 2. Comparison of traditional detection methods for pipeline defects.

Detection Method	Detection Principle	Characteristics	Disadvantages	References
Ultrasonic Testing	Utilizes piezoelectric crystals or electromagnetic fields to generate ultrasonic waves that propagate into the material; defects and wall thickness are detected by analyzing reflected echo signals	High detection accuracy; capable of identifying stress corrosion cracking and deep-seated or subsurface defects; adaptable to complex geometries and high-temperature environments	Requires couplant (for piezoelectric UT); complex signal interpretation; sensitive to surface conditions such as roughness and lift-off	[18,19]
Magnetic Flux Leakage	An external magnetic field is applied to the pipeline; when defects are present, they disrupt the magnetic flux path, causing local magnetic field leakage. The leakage fields are captured by sensors to identify structural anomalies	High detection speed; suitable for large-area scanning; highly sensitive to metal loss and volumetric defects such as corrosion and pitting	Restricted to ferromagnetic materials; limited sensitivity to axial cracks; challenges in precise depth quantification; relatively low resolution for narrow or shallow defects	[20,21]
Eddy Current Testing	A coil carrying alternating current induces eddy currents. Defects disturb the distribution of these eddy currents, thereby altering the coil’s impedance. This change is measured to detect defects.	Extremely high sensitivity to surface and near-surface cracks; enables non-contact, high-speed scanning	Detection depth is relatively shallow, and results are susceptible to interference from the material’s electromagnetic properties	[22,23]
Magnetic Particle Testing	The component is magnetized, inducing a leakage field at defect sites, which attracts magnetic particles to form visible magnetic traces	Highly sensitive to surface and near-surface cracks in ferromagnetic materials; provides intuitive and visible results with low detection cost	Limited to ferromagnetic materials and detection of surface defects, with complex pre- and post-processing procedures	[24,25]
Radiographic Testing	Utilizes the attenuation of X-ray or γ-ray intensity after penetrating the pipe wall to identify internal structural defects	Results are directly visible and permanently recordable; high detection probability for volumetric defects; applicable to almost all materials	Poses radiation safety hazards; high detection cost; low efficiency; insensitive to planar cracks oriented parallel to the beam direction	[26,27]

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Ultrasonic testing is primarily categorized into piezoelectric ultrasonic testing and EMAT testing, depending on the excitation mechanism. Piezoelectric ultrasonic testing utilizes the inverse piezoelectric effect of piezoelectric crystals to generate ultrasonic waves, whereas EMAT relies on electromagnetic induction to induce eddy currents in conductive materials and subsequently excites ultrasonic waves through Lorentz forces or magnetostriction. Both methods detect and characterize defects by analyzing the echo signals generated when ultrasonic waves propagate through the material and are reflected from flaws or geometric boundaries. The core advantage shared by both techniques lies in their high detection accuracy, particularly in identifying stress corrosion cracking and subsurface or deep-seated defects, along with the capability to operate under complex surface geometries or elevated temperatures. However, piezoelectric ultrasonic testing typically requires couplant for efficient energy transfer, limiting its use in high-temperature or coated-surface applications. In contrast, EMAT operates without contact or couplant but suffers from lower transduction efficiency, higher levels of electromagnetic noise, and greater sensitivity to surface conditions such as roughness and lift-off variation [18,19].

In contrast, MFL testing detects defects by magnetizing the pipeline and measuring the stray magnetic field (leakage field) that occurs at flaw locations. This method is characterized by high detection speed and high sensitivity to surface-breaking defects, making it highly suitable for rapid, large-area scanning of pipelines. However, MFL is less sensitive to axial cracks, struggles with accurate depth quantification of defects, and is limited exclusively to ferromagnetic materials [20,21].

For surface-breaking crack-type defects, ECT and MT are two highly effective methods. ECT detects flaws by measuring changes in coil impedance caused by disturbances in induced eddy currents due to defects. It exhibits extremely high sensitivity to surface and near-surface cracks and enables non-contact, high-speed scanning. However, its penetration depth is limited, and the results are susceptible to interference from inherent electromagnetic properties of the material, such as conductivity and permeability [22,23]. Magnetic particle testing involves magnetizing the component so that leakage fields at defect sites attract magnetic particles, forming visible indications. This method offers exceptional sensitivity to surface and near-surface cracks in ferromagnetic materials, with intuitive results and low operational cost. Nevertheless, its applicability is restricted to ferromagnetic materials, and it is only capable of detecting surface and shallow subsurface defects. Additionally, MT requires extensive pre- and post-detection procedures, including surface preparation and demagnetization, which can be labor-intensive and time-consuming [24,25].

Additionally, radiographic testing (RT) provides a unique detection approach by analyzing the intensity attenuation of ionizing radiation (e.g., X-rays or γ-rays) after penetrating the material to generate internal images. The resulting radiographs offer direct visualization and permanent recordability, enabling high detection probability for volumetric defects such as porosity and inclusions. RT is applicable to a wide range of materials and provides excellent defect characterization capability. However, its major drawbacks include inherent radiation hazards requiring strict safety protocols, high operational costs, relatively low detection efficiency, and poor sensitivity to planar cracks oriented parallel to the direction of the radiation beam [26,27].

Currently, although these conventional methods play critical roles in their respective application domains, they still exhibit widespread limitations in terms of detection efficiency, detection accuracy, and capability for identifying complex defects. As such, they are often insufficient to fully meet the stringent safety requirements of modern pipeline industries. To address these challenges, researchers and engineers continue to pursue technological breakthroughs by developing innovative detection solutions that overcome existing shortcomings. This ongoing effort drives the evolution of pipeline nondestructive testing toward higher efficiency, greater precision, and enhanced intelligence, aiming to transcend traditional limitations and achieve comprehensive improvements in both detection performance and reliability.

3. EMAT Methods for Pipeline Defect Detection

In recent years, as pipelines evolve toward long-term operation under high temperature, high pressure, and increasingly complex service environments, conventional piezoelectric ultrasonic testing—reliant on couplant media—faces significant challenges in terms of accessibility and environmental adaptability. Under this context, Electromagnetic Acoustic Transducer (EMAT) technology, based on Lorentz force and magnetostrictive mechanisms, has emerged as a key approach for assessing pipeline structural integrity due to its distinct advantages of non-contact operation and couplant-free detection. EMAT has demonstrated irreplaceable potential in applications such as corrosion monitoring, weld detection, and wall thickness gauging. From the perspective of technological evolution, EMAT development has progressed through four sequential phases: fundamental mechanism exploration, transducer innovation, system integration, and intelligent transformation. During the mid-to-late 20th century, research primarily focused on theoretical modeling of the interaction between electromagnetic fields and elastic waves, establishing the physical foundation for generating bulk waves and guided waves. In the 21st century, advances in micromagnetic circuit design, printed circuit board coil fabrication, and high-power pulsed excitation techniques have enabled EMATs to become miniaturized, array-configured, and broadband, facilitating their transition from laboratory prototypes to field-deployable engineering solutions. More recently, the integration of multichannel EMAT arrays with robotic crawlers has significantly enhanced scanning efficiency for large-diameter pipelines. Concurrently, the application of machine learning algorithms in signal denoising, mode identification, and defect quantification marks a transformative shift toward autonomous diagnosis and intelligent decision-making, positioning EMAT at the forefront of next-generation pipeline NDT technologies [28,29,30].

Nevertheless, EMAT still faces multiple bottlenecks in engineering applications: On one hand, its relatively low signal-to-noise ratio (SNR) and significant near-surface blind zone limit its ability to detect micro-cracks and shallow subsurface defects. On the other hand, its strong dependence on material conductivity and magnetic permeability restricts its universality, while the high manufacturing costs associated with high-performance permanent magnets and precision coils hinder large-scale deployment. Furthermore, single-mode detection struggles to simultaneously meet the requirements of wide-area screening and localized high-resolution measurement, necessitating the integration of multi-physical field collaboration to achieve functional complementarity.

To review the current state of research, this paper focuses on two dominant EMAT-based methodologies—Electromagnetic Acoustic Transducer Guided Wave (EMAT-GW) and Electromagnetic Acoustic Transducer Bulk Wave (EMAT-BW)—and summarizes representative advances in pipeline defect detection from four key dimensions: excitation mechanisms, transducer design, signal processing techniques, and engineering validation. This classification not only reflects the difference in detection objectives—guided waves being primarily used for long-range, rapid screening, while bulk waves are employed for high-precision, quantitative evaluation—but also illustrates the synergistic evolution of EMAT technology across macroscopic and microscopic scales. A comparative analysis of the technical performance between these two approaches is provided in

Table 3. Comparative analysis of EMAT methods for pipeline defect detection.

Detection Method	Pipe Type	Defect Type	Application Advantages	Challenges	References
EMAT Circumferential Guided Wave	Thin-walled/small diameter tube (heat exchanger tube), oil and gas pipelines with anticorrosive coating, large diameter steel pipe (323.8 mm), non-circular pipe, T-type hanger pipe	Axial crack, circumferential weld defects (unfused), corrosion wall thickness thinning, mid-wall delamination defect, circumferential cracks in the inner wall (small diameter/special-shaped tube), hidden crack (support hanger weld zone)	The whole section is detected by one excitation, which is suitable for girth welds and circumferential defects. The non-contact SH wave can penetrate the coating, adapt to the anti-corrosion conditions, and use high-order circumferential guided waves to achieve thick-walled imaging	The multi-mode and signal are complex; the dispersion is serious and there is a detection blind area	[31,32,33,34,35,36,37,38,39,40,41,42,43]
EMAT Axial Guided Wave	Long-distance pipeline, complex coating pipeline (insulation layer, anticorrosive coating), liquid filled pipe	Axial crack (short shallow crack), surface/near-surface defects (fatigue damage), volume corrosion and wall thickness change, elbow area defects	T(0,1) is non-dispersive, which is suitable for long-distance rapid screening. It is insensitive to liquid and coating layer, adapt to harsh conditions	The T mode is insensitive to circumferential defects, and the L mode has low sensitivity to surface defects	[44,45,46,47,48,49,50,51,52,53,54,55,56,57,58,59,60]
EMAT Helical Guided Wave	Pipes with complex geometric structure (elbows, special-shaped)	Oblique cracks, localized corrosion pits and irregular geometric defects	Spiral propagation path realizes axial and circumferential synchronous scanning without dead angle of detection direction	Multipath propagation and wavefront crossing lead to severe mode mixing, the design depends on the specific pipe diameter, curvature and material parameters	[61,62,63]
EMAT Bulk Wave	High temperature pipeline (boiler tube), thick-walled/coated pipes, coarse grained material pipeline, high strength steel pipe (X80)	Internal corrosion, weld defects and micro-cracks, inclined crack, coating thickness and bonding state, residual stress and intergranular corrosion	High precision measurement, the oblique incident SV wave has unique advantages for tilt and horizontal defects by angle control. It is suitable for high temperature, rough surface and coating environment	There is a near-surface blind area. In the high temperature environment, the permanent magnet has thermal demagnetization and the coil needs thermal design. The detection speed is slow, not suitable for large-scale detection	[64,65,66,67,68,69,70,71,72,73,74,75,76]

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3.1. EMAT Guided Wave Detection Technology

Ultrasonic guided waves are a class of special elastic waves that propagate within waveguide structures with geometric boundaries. Their formation arises from the boundary constraints imposed on the wave equation in finite-sized media, leading to inherent characteristics such as dispersion and multimodal coexistence. Unlike bulk waves, guided waves can travel over long distances along slender structures like pipelines with minimal energy attenuation, making them highly effective for rapid, large-area screening and thus offering significant advantages in pipeline nondestructive testing (NDT).

Based on the relationship between particle vibration direction and propagation path, guided waves in pipes are categorized into three types: circumferential guided waves, axial guided waves, and helical guided waves, each exhibiting distinct physical properties and application scenarios, enabling targeted and efficient detection of specific defect types. First, circumferential guided waves are divided into circumferential shear-horizontal (SH) waves and circumferential Lamb-type waves. Due to its vibration direction being perpendicular to circumferential cracks, it possesses a high detection capability for common welding defects in the girth weld area, such as circumferential cracks and lack of fusion. Particularly in the detection of girth welds in subsea and high-temperature/high-pressure pipelines, circumferential guided waves effectively overcome the blind spots of conventional axial guided waves in detecting circumferential defects, thereby enhancing the safety assessment of critical joint areas. Second, axial guided waves include longitudinal modes (L(0,2)), torsional modes (T(0,1)), and flexural modes (F). The T(0,1) mode, characterized by circumferential particle motion, demonstrates strong immunity to environmental interference and is well suited for complex service conditions involving supports, insulation layers, or coatings. The L(0,2) mode, with particle displacement parallel to the propagation direction, is highly sensitive to axial cracks and wall thinning. Although flexural modes possess lower energy and are prone to dispersion, they offer certain capability for detecting localized geometric anomalies. These axial guided wave modes are widely applied in rapid detection of long-distance transmission and buried pipelines for axial defects. Lastly, helical guided waves represent an emerging wave type in recent years, overcoming the limitations of traditional two-dimensional planar wave propagation. Through tailored excitation using spiral coil configurations and bias magnetic fields, helical waves propagate along a spiral trajectory around the pipe circumference while advancing axially, enabling volumetric coverage of the entire pipe wall in a single scan. This facilitates true full-circumference and full-length detection with three-dimensional sensitivity, supporting volumetric imaging and precise localization of complex defect morphologies. Helical guided wave technology exemplifies the evolution of guided wave detection toward intelligent, high-resolution, and comprehensive structural health monitoring [77]. Figure 1 shows three guided waves and modes in the pipeline.

In summary, guided waves with different propagation paths exhibit complementary capabilities, forming a synergistic detection framework. Circumferential guided waves focus on critical zones such as girth welds, axial guided waves enable rapid long-range screening, and helical guided waves provide advanced three-dimensional volumetric detection. When integrated with EMAT, these guided wave modes operate without couplant and remain functional under harsh environments—including high temperatures and coated surfaces—offering robust technical support for integrity assessment across diverse pipeline systems, including long-distance transmission, subsea, and buried pipelines.

3.1.1. EMAT Circumferential Guided Wave Detection Technology

Electromagnetic Acoustic Transducer (EMAT) circumferential guided wave technology is one of the key approaches for full-circumference, non-contact detection of pipelines. By exciting ultrasonic guided waves that propagate along the circumferential direction, this method enables efficient detection of typical defects such as axial cracks and corrosion-induced wall thinning, demonstrating significant application potential in in-service detection of critical infrastructure including long-distance oil and gas pipelines and nuclear primary coolant piping. Current research efforts are focused on addressing four major technical challenges: signal decoupling difficulties caused by multimodal dispersion, limited defect quantification capability, poor transducer directivity, and insufficient adaptability to complex structural geometries. To tackle these issues, the research community has gradually established a systematic development pathway centered on “wave mechanics analysis—selective mode excitation—unidirectional beam steering—system integration and experimental validation.”

The Circumferential Lamb wave (CL wave) is a type of ultrasonic guided wave propagating along the pipe circumference, with particle motion perpendicular to the axial direction and confined within the radial-circumferential plane. It can be conceptualized as a Lamb wave propagating in a curved plate, where its dispersive characteristics are significantly influenced by pipe curvature and wall thickness. Severe multimodal coexistence complicates signal interpretation and defect feature extraction. To address modal interference, Li Mingliang et al. [31] conducted a systematic investigation into frequency-dependent modal selectivity based on modal expansion theory. Their experimental results demonstrated that, under fixed coil configuration, specific excitation frequencies enable preferential generation of targeted modes: for instance, 0.87 MHz predominantly excites the A₀ mode, 1.15 MHz simultaneously generates A₀ and S₀ modes, while 1.85 MHz and 2.06 MHz correspond to the A₁ and S₁ modes, respectively. This work provides a theoretical foundation for achieving high signal-to-noise ratio (SNR) and single-mode dominant CL wave generation in thin-walled pipes. Building upon effective modal control, researchers have further explored the application of circumferential Lamb waves in quantitative defect evaluation. Li Ziming et al. [32] developed a three-dimensional finite element model and employed advanced signal processing techniques—including frequency-wavenumber (f-k), spatial-frequency-wavenumber (x-f-k), and local wavenumber analysis—to reveal the multiple interaction mechanisms between the CL₀ mode and mid-wall delamination defects. The findings were experimentally validated using a fully non-contact system combining EMAT excitation with laser Doppler vibrometer reception. Results showed that the method not only reliably identifies the presence of defects but also achieves defect imaging with localization errors less than 10%, offering a viable solution for high-precision nondestructive evaluation of seamless pipelines.

Although wavenumber analysis methods have enhanced defect identification capability, conventional EMATs suffer from bidirectional wave propagation due to structural symmetry, resulting in complex echo signals and difficulty in determining defect azimuth. To address this, Shen Xianqing et al. [33] designed a unidirectional EMAT with dual-coil-pair excitation based on Huygens’ principle, utilizing a phase interference mechanism to enhance the clockwise wave packet (gain up to 2.14 times) while suppressing the counterclockwise component (attenuated to 0.37 times). Both simulation and experimental results demonstrate that this configuration significantly improves the identifiability of defect echoes, offering a new approach for precise defect localization under full circumferential coverage. Zhang Xu et al. [34] addressed the issues of low detection frequency and poor resolution in traditional periodic permanent magnet EMAT (PPM-EMAT) by proposing a high-frequency unidirectional circumferential Lamb wave method. By optimizing the coil and magnet configuration, high-amplitude, low-dispersion CLamb waves were successfully excited in the high-frequency-thickness product regime, it possesses a high detection capability for common welding defects in the girth weld area, such as circumferential cracks and lack of fusion, enabling efficient detection of near-surface defects. Experimental results show that the excitation efficiency of this EMAT is more than ten times higher than that of conventional PPM-EMAT, with significantly improved signal-to-noise ratio and resolution, as well as the ability to distinguish between inner and outer wall defects. Combined with SH₀ mode detection, it has the potential to establish a multi-modal integrated comprehensive detection system. In the intelligent quantification of the depth and contour of pipeline corrosion defects, Garcia-Gomez et al. [35] utilized multi-frequency Lamb wave excitation and intelligent acoustic signal processing technology, combined with evolutionary algorithms for feature selection and neural network modeling. This approach significantly improved the accuracy of quantitative defect depth assessment, achieving a root mean square error below 1.48 mm in testing. This provides a novel methodological framework for the automated and high-precision detection of defects with complex geometric morphologies.

Compared to Lamb waves, circumferential shear horizontal waves (SH waves) are more suitable for long-range and high-stability detection due to their particle motion parallel to the axial direction, simpler governing equations, and advantageous characteristics of the zero-order mode (SH₀), such as non-dispersive behavior, concentrated energy distribution, and strong immunity to interference. Based on these advantages, researchers have developed an electromagnetic acoustic internal detection system utilizing the Lorentz force mechanism, combined with equidistant spatial sampling and multi-mode analysis techniques, enabling dynamic identification and precise localization of various defects such as through-holes and corrosion. Gautam et al. [36] proposed a novel EMAT configuration composed of V-shaped coils and periodic permanent magnets. By arranging conductors at a 45° angle, constructive interference and beam focusing are achieved, significantly enhancing energy concentration and directivity, outperforming traditional racetrack coil designs. This provides a new technical solution for efficient and accurate detection of circumferential defects such as axial cracks in pipelines. In reconstructing the thickness profiles of complex corrosion morphologies, Thon et al. [37] optimized the magnetic circuit and coil configuration of a periodic permanent magnet EMAT. By leveraging the cut-off frequency characteristics of higher-order Shear Horizontal guided waves, they successfully reconstructed the circumferential thickness distribution of a steel pipe with a diameter of 323.8 mm, achieving quantitative detection of wall thickness losses of up to 50%. This advancement promotes the engineering application of EMAT technology in high-resolution imaging for localized corrosion. Wang Fubin et al. [38] investigated the propagation characteristics of SH waves in pipelines with protective coatings, establishing a waveguide model for coated structures and analyzing the energy distribution of four SH modes within the 0–300 kHz frequency range. The results show that Modes 2 and 3 exhibit dominant energy confinement within the pipe wall in the 80–160 kHz range, making them suitable for internal detection. In contrast, Mode 1 tends to leak energy into the coating layer, while Mode 4 operates at excessively high frequencies limited by transducer bandwidth. This study provides theoretical guidance for optimal mode selection in multilayered pipeline detection.

With the maturation of theoretical models and transducer designs, research focus has shifted from component-level development to system-level integration. Tian Shaojun et al. [39] developed a complete internal detection system for pipeline electromagnetic acoustic circumferential guided waves, integrating an embedded host, FPGA-based high-speed data acquisition module, equidistant spatial sampling triggering technique, and dedicated software platform. The system supports dynamic scanning, real-time data storage, and multi-mode analysis, and successfully detected a Φ12.7 mm through-hole and a 25.4 mm × 25.4 mm × 0.8 mm square groove on a Φ377 mm test specimen, marking a significant step toward system integration and practical application. As detection targets expand to include irregular-shaped pipes and small-diameter tubes, traditional guided wave modes face adaptability challenges. To address this, Deng Pen et al. [40] proposed a pitch-catch detection method based on end-surface waves for inspecting inner circumferential cracks in small-diameter and non-circular pipes. Using a 1.5 MHz EMAT to excite surface waves on a flat end surface, when the chamfer size matches the wavelength (size-to-wavelength ratio of 1), transmission efficiency reaches 78%, with propagation distance exceeding 2680 mm. Multiple machined notches were successfully identified in pulse-echo mode. This method avoids the difficulties associated with curved surface coupling and extends the applicability of guided wave detection. Fang Zhou et al. [41] proposed a helical-propagation CL wave detection scheme, utilizing a magnetostrictive patch transducer to generate helical CL1 waves with energy concentrated within a 10–30° conical angle. This configuration offers good detection capability for defects that are short in axial extent but have limited circumferential spread. Experiments achieved an axial positioning accuracy of 1.25%, and defect axial dimensions were evaluated through multi-angle statistical analysis, effectively compensating for blind zones in conventional guided wave methods. This work provides a new approach for comprehensive detection and assessment of various defect types in pipelines. Zhang Xingjun et al. [42] introduced an electromagnetic ultrasonic testing method based on circumferential shear horizontal guided waves (CSH₀) combined with waveform subtraction techniques, effectively addressing the challenge of detecting hidden cracks obscured by weld echoes in T-type support and hanger regions. The method can detect non-penetrating cracks as small as 5 mm × 1 mm, offering a non-contact, high-interference-rejection solution for in-service NDT of complex-structure pipelines. Additionally, the thickness evaluation method based on the cut-off frequency effect of higher-order SH guided waves has gradually gained attention. Thon et al. [43] designed a periodic permanent magnet EMAT that, by exciting multiple SH modes and leveraging their cut-off behavior in thickness reduction regions, enables the evaluation of minimum remaining thickness over long-distance paths. This approach achieves a resolution of 2 mm on a 10 mm aluminum plate, providing a non-contact, full-field scanning supplementary method for localized corrosion assessment.

In summary, electromagnetic acoustic circumferential guided wave technology has achieved systematic breakthroughs in selective mode excitation, unidirectional propagation control, adaptability to complex structures, and system integration. However, significant challenges remain for practical deployment. For instance, there is a lack of universal detection models under complex operational conditions, and protective coatings can interfere with wave energy distribution; sensors inherently suffer from low transduction efficiency, resulting in poor signal-to-noise ratio on rough pipe surfaces, while simultaneously facing trade-offs between wear resistance and miniaturization; furthermore, the systems have not yet undergone extensive field validation over long distances, with practical barriers still existing in terms of passability, stability, and intelligent interpretation of multi-mode signals. Future research should focus on developing more realistic theoretical models, advancing high-performance and robust transducer designs, and conducting large-scale field trials to promote this technology toward becoming a mature and routine method for pipeline nondestructive testing.

3.1.2. EMAT Axial Guided Wave Detection Technology

Pipeline axial guided wave detection technology, as an important branch of ultrasonic guided wave applications, is primarily based on three fundamental propagation modes: longitudinal mode L(0,m), torsional mode T(0,m), and flexural mode F(n,m). Among them, the axisymmetric longitudinal and torsional modes are widely used in practical industrial detections due to their simple excitation mechanisms, regular sound field distribution, and easily interpretable signals. Current research focuses on key issues such as multi-mode selection and optimization, improvement of excitation efficiency, and quantitative defect evaluation. Researchers are continuously advancing this technology toward higher precision and stronger adaptability through approaches including establishing accurate propagation models, innovating transducer designs, and developing multi-mode fusion analysis methods.

The torsional mode (T-mode) features particle vibration perpendicular to the wave propagation direction. Among the three basic modes, it has attracted significant attention due to its non-dispersive characteristic, making it particularly suitable for detecting surface and near-surface defects in pipelines and plates, as well as for applications sensitive to defect orientation. Muraveva et al. [44] conducted theoretical research on EMAT excitation of torsional guided waves in pipelines, providing a theoretical foundation for parameter matching in multi-sized pipelines. The study established a mathematical model for torsional wave propagation that considers excitation parameters, geometric dimensions, and viscoelastic properties, revealing that the amplitude of angular displacement attenuates with increasing pipe diameter, wall thickness, and frequency, and indicating an optimal EMAT aperture length of approximately 1/4 wavelength. To further improve the excitation efficiency and control accuracy of the torsional mode, several innovative transducer designs have been proposed. Hill et al. [45] introduced a novel EMAT structure based on a periodic permanent magnet array and a serpentine coil, which achieves efficient excitation of the T(0,1) mode through in-phase circumferential forces. By employing a dual-PPM configuration, the operational bandwidth is extended up to 150 kHz, significantly enhancing spatial resolution. Yang Lijian et al. [46] addressed the problem of complex signals and difficult defect localization caused by bidirectional wave propagation in conventional electromagnetic acoustic guided wave systems by designing a unidirectional T(0,1) mode EMAT. Based on a mathematical model combining magnetic field, displacement, and wave superposition, this design uses a dual-coil excitation scheme. With adjacent conductor spacing set to λ/4 and excitation delay time set to T/4, unidirectional ultrasonic wave propagation is achieved. Experimental results show that the backward-propagating wave is effectively suppressed, yielding a signal-to-noise ratio as high as 5.88 dB, while simplifying echo signal interpretation and improving defect localization accuracy.

To further enhance detection sensitivity, the development of focused EMAT designs has emerged as a new trend. Huang Songling et al. [47] proposed a novel electromagnetic acoustic transducer design for exciting the T(0,1) torsional guided wave mode in pipelines. This design employs a sector-shaped periodic permanent magnet (PPM) array combined with a racetrack coil structure, generating a focused circumferential Lorentz force to achieve point-focusing of guided waves. Experiments show that the signal amplitude at the focal point increases by nearly 70%, significantly enhancing the response to localized defects. Similarly, Liu Dengrong et al. [48] developed an insert-type magnetostrictive torsional guided wave EMAT suitable for internal detection of ferromagnetic pipes. Based on the Wiedemann effect, this design uses an internal rectangular permanent magnet and arc-shaped coil configuration to generate a static bias magnetic field in the circumferential direction and a dynamic axial magnetic field. The transducer effectively excites the T(0,1) mode at an excitation frequency of 0.16 MHz, with a group velocity measurement error of only 2.6%, and demonstrates superior sensitivity to short, shallow axial cracks compared to longitudinal guided waves. Sen Deng et al. [49] proposed a non-contact axial stress measurement method for hollow cylindrical steel components based on the non-dispersive T(0,1) guided wave mode. By establishing an equivalent acoustic propagation model and using EMAT, they innovatively introduced the LMS-Gabor algorithm to precisely extract nanosecond-level time-of-flight variations, significantly improving the sensitivity and noise immunity of stress measurements.

Although theoretical research on the torsional mode has become relatively mature, geometric discontinuities such as bends and tees in practical pipeline structures significantly affect guided wave propagation characteristics. Heinlein et al. [50] conducted a systematic study using 3D finite element simulations to investigate the reflection behavior of torsional T(0,1) mode guided waves at defects located in pipe bends. The results show that the reflection coefficient of small defects varies significantly with position: the intrados side exhibits a region of low detectability (minimum reflection as low as 10% of that in straight pipe), while the extrados side shows up to a fourfold enhancement. The study further reveals that the reflection amplitude at a defect is approximately proportional to the square of the von Mises stress generated by the incident wave at that location—a relationship applicable to both circumferential cracks and corrosion-like defects—helping to explain weak reflections in bend regions and providing guidance for detection path planning. As detection requirements extend from external attachment to internal scanning, the application of higher-order modes has gradually advanced. Nurmalia et al. [51] proposed an internal array EMAT technique based on the T(0,2) mode, using phase variation rather than amplitude attenuation for quantitative defect assessment. Their work revealed the competing mechanisms between mode conversion and wave diffraction, demonstrating that phase information offers greater potential for accurate quantification.

The longitudinal mode (L-mode), characterized by particle vibration parallel to the direction of wave propagation, possesses features such as long-distance axial propagation, low dispersion, and high penetration capability. It is well suited for rapid screening and quantitative evaluation of internal defects in large-scale pipeline structures, demonstrating complementary advantages over the torsional mode (T-mode) in non-contact electromagnetic ultrasonic testing. In recent years, L-mode excitation and reception techniques based on electromagnetic acoustic transducers (EMATs) and magnetostrictive effects have continued to advance, with research focus gradually shifting from basic functionality to system performance optimization and enhanced adaptability to complex environments.

In terms of integrated modeling of excitation, propagation, and reception, Sun Pengfei et al. [52] developed a finite element model that incorporates the entire process, including dynamic magnetic field coupling, magnetostrictive effects, and coil induction. This model, for the first time, accounts for electromagnetic reciprocity during reception and the non-uniformity of the static bias magnetic field within the simulation framework. By accurately predicting the voltage response of the receiving coil, the reliability of the model in signal amplitude quantification was validated, providing a high-fidelity numerical platform for optimizing sensor structural parameters. Sheng et al. [53] further addressed liquid-filled pipeline applications by combining dispersion curve analysis with attenuation modeling, and designed a multi-segment distributed coil to control the spatial distribution of the dynamic magnetic field. Experiments showed that the defect reflection coefficient exhibits a good linear relationship with cross-sectional area loss, with axial positioning error controlled within 3.84%. This study provides an effective non-contact method for quantitative defect detection in liquid-filled pipelines.

To address the issues of weak and non-uniform bias magnetic field in large-diameter pipelines, Ma Hongwei et al. [54] proposed a magnetic circuit optimization strategy based on Ansoft Maxwell, introducing the variance of magnetic field distribution as a quantitative index for uniformity. The study revealed that magnetic field uniformity increases logarithmically with the number of magnetic circuit units, while the average magnetic field strength at the central cross-section rises approximately linearly. After comprehensive trade-off analysis, five magnetic circuit units were determined as the optimal configuration for an 88 mm outer diameter pipe, achieving a balance between high field strength and high uniformity, thus providing a replicable design paradigm for engineering deployment. Similarly, Wu Wentao et al. [55] developed an insert-type Lorentz force EMAT for small-diameter nickel-copper alloy tubes, targeting applications in confined spaces. By optimizing the axial length ratio between the annular permanent magnet and the circumferential coil (determined to be 3:1 as optimal), a uniform radial static magnetic field was successfully established. Experimental results showed that, when exciting the L(0,1) mode at 40 kHz, the maximum signal amplitude was achieved using a coil with 0.2 mm wire diameter. Furthermore, a multi-array configuration demonstrated significant signal-to-noise ratio improvement when exciting the L(0,2) mode at 180 kHz, extending its application potential in restricted environments such as heat exchanger tube bundles.

To address special operating conditions such as high temperature and helical structures, novel designs of internal transducers continue to emerge. Wang Libo et al. [56] proposed a compression-type magnetic flux concentrating EMAT suitable for heat exchanger tubes in high-temperature gas-cooled reactor steam generators. By using a permanent magnet–pure iron composite structure, the vertical component of the magnetic field in the coil region is enhanced while horizontal interference fields are effectively suppressed, enabling selective excitation of the low-dispersion L(0,1) mode. Simulation and experimental results verified that pure guided wave signals can be generated at 50 kHz, successfully detecting a Φ5 mm through-hole and a circumferential groove defect of size 20 mm × 1.5 mm × 1.2 mm, providing a high-precision solution for long-range online detection of helical tubes. Cong Ming et al. [57] proposed a compact longitudinal mode EMAT based on a chain of permanent magnets, which enhances reception sensitivity by integrating a solenoid test coil in series, achieving effective excitation and reception of the L(0,2) mode. The device is small, lightweight, and easy to install, significantly improving field detection efficiency. Furthermore, researchers have begun to focus on the intelligence of sensor systems and the evaluability of transduction efficiency. Sun Yong et al. [58] established a physical mapping model between bias magnetic field strength and received signal amplitude based on the magnetostrictive energy conversion mechanism. They proposed a method to indirectly reconstruct the internal bias field distribution by measuring the axial leakage magnetic field on the pipe’s outer surface. By calculating the transduction efficiency using weighted signal amplitudes, they achieved quantitative evaluation of the sensor’s operational status and identification of its optimal working point, providing a theoretical foundation for intelligent design of magnetostrictive guided wave testing systems.

As detection requirements evolve toward higher precision and full-dimensional sensing, the integration of multi-modal fusion and emerging sensing technologies has become a key development direction. Willey et al. [59] proposed a guided wave tomography method based on high-order helical modes, which extends the virtual array aperture and enhances ray path coverage density through multiple circumferential passes. Combined with a temperature compensation algorithm, this method was experimentally validated, achieving depth inversion of irregular corrosion defects in an 8-inch steel pipe over a temperature range from room temperature to 175 °C in a controlled setting, with a maximum error of less than 0.04 mm. This work demonstrates the stability of EMAT for short-term imaging in high-temperature environments and its potential for future continuous monitoring applications. Okada et al. [60] innovatively combined high-temperature superconducting quantum interference device (SQUID) sensors with magnetostrictive guided waves for inspecting STPG370 pipes with thick insulation layers (20 mm). Simulations confirmed that full axial magnetization can be achieved externally, and by using differential pickup coils together with SQUID gradiometers to receive L(0,2) mode signals, uniform circumferential sensitivity is obtained without the need for sensor rotation. This approach enables identification of circumferential defects from outside the insulation layer, opening a new pathway for NDT of corrosion under insulation (CUI).

Pipeline axial guided wave detection technology is primarily dominated by longitudinal L(0,m) and torsional T(0,m) modes, which excel, respectively, in long-range screening of internal defects and identification of surface damage. The T-mode, benefiting from its non-dispersive nature, is well suited for high-precision stress measurement and defect localization, with current research focusing on unidirectional excitation, focused transducer design, and wave propagation control in complex structures. The L-mode, through optimization of transducer configuration and magnetic field distribution, has achieved improved excitation efficiency and signal interpretability, extending its applicability to challenging environments such as high temperature and small-diameter pipes. As detection requirements advance, the integration of multi-modal fusion and advanced sensing technologies is driving the development of more precise, non-contact detection methods. In the future, combined with intelligent modeling and multi-technology synergy, this field is expected to accelerate toward a new stage of automation and online monitoring.

3.1.3. EMAT Helical Guided Wave Detection Technology

Electromagnetic acoustic helical guided wave is a significant technological breakthrough in the field of pipeline nondestructive testing in recent years. Unlike conventional circumferential or axial guided waves, helical guided waves propagate along a spiral path at a specific angle relative to the pipe axis. This unique propagation mode enables multi-directional scanning under a single excitation, significantly enhancing spatial sensitivity to asymmetric defects such as oblique cracks and corrosion pits. It demonstrates distinct advantages in defect orientation identification and high-resolution imaging.

In theoretical modeling of helical guided wave propagation characteristics and research on directional controllability, Li Mengqi et al. [61] developed an S-shaped internal EMAT for quantitative detection of pipeline oblique cracks using helical guided waves. Through finite element simulations and experimental system validation, this transducer was shown to excite helical guided waves with different helix angles, exhibiting higher sensitivity to oblique cracks. Compared to the conventional L(0,2) axial guided wave, the crack echo signal amplitude increased by approximately 78%, achieving high-resolution detection of pipeline cracks.

In terms of sensor structure innovation, Zhe Wang et al. [62] developed a two-layer coil-sectorial pole composite structure EMAT, which excites spiral Lamb waves with large divergence angles based on the Lorentz force mechanism, effectively expanding the circumferential coverage. Experimental results show that the design enables simultaneous signal reception at multiple circumferential angles, achieving combined axial and circumferential defect localization, outperforming traditional meander-line coil configurations. Zhou Jinjie et al. [63], from the perspective of wave propagation mechanisms, pointed out that helical guided waves are essentially Lamb waves generated by a point source on a curved surface, exhibiting multi-path propagation and wavefront crossing characteristics in the near field. Array experiments verified that this method provides multi-angle incidence information, contributing to improved spatial resolution in guided wave tomography.

In summary, helical guided wave technology has achieved substantial progress in wave propagation control, transducer design, and imaging potential, owing to its multi- directional scanning capability. However, its multi-path propagation characteristics lead to complex signals and difficulties in echo interpretation, while its adaptability across different pipe diameters and materials remains to be fully validated. Future research should focus on developing intelligent signal processing algorithms to interpret complex echoes, optimizing sensor array configurations to enhance detection efficiency, and advancing the technology toward standardization and practical deployment, ultimately enabling large-scale industrial applications.

In electromagnetic ultrasonic guided wave technology, circumferential, axial, and helical guided waves constitute the three fundamental propagation forms, each encompassing multiple guided wave modes with distinct vibrational characteristics. To detect different types of pipeline defects, it is necessary to select the appropriate guided wave mode based on the geometric features and spatial distribution of the defects, thereby achieving efficient and precise detection.

Circumferential guided wave modes A₁ and S₁ are typically effectively excited within 200–500 kHz, demonstrating high sensitivity to circumferential weld defects and delamination, and are commonly used for localized inspection of thin-walled or small-diameter pipes. Higher-order shear horizontal waves SH₂ and SH₃ concentrate energy more within the pipe body in the 100–300 kHz frequency range, making them suitable for detecting internal and external wall defects in pipes with anti-corrosion coatings. For thicker coatings, the SH₃ mode can be selected or the frequency can be reduced to enhance penetration. The axial guided wave longitudinal mode L(0,2) is typically excited within 100–300 kHz, featuring low group velocity dispersion and strong axial propagation capability, making it suitable for long-distance rapid screening of volumetric defects such as axial cracks and corrosion-induced wall thinning. The torsional mode T(0,1) maintains non-dispersive characteristics in the 50–200 kHz frequency range, exhibits high sensitivity to surface and near-surface axial cracks, and is less affected by liquids and coatings, making it commonly used for defect screening in coated pipelines. Higher-order torsional modes T(0,2) and T(0,3) can be excited at 200–500 kHz, offering high resolution for localized thickness variations and minor defects. However, due to higher attenuation, their effective propagation distance is limited, making them suitable for localized fine inspection. Helical guided waves are typically excited within 150–400 kHz. Their spiral propagation path enables three-dimensional volumetric coverage, effectively detecting defects in complex regions such as bends and supports, and providing unique advantages in identifying inclined cracks.

3.2. EMAT Bulk Wave Detection Technology

Electromagnetic acoustic transducer (EMAT) bulk wave detection technology, as a non-contact and couplant-free nondestructive testing method, excites longitudinal and shear waves within materials through Lorentz force or magnetostrictive mechanisms. Leveraging its strong penetration power and capability to detect internal defects, EMAT bulk waves are widely applied in pipeline wall thickness measurement, weld defect detection, and structural health monitoring of high-temperature in-service components. Compared with guided waves, bulk waves are less influenced by boundary reflections and dispersion effects, making them more suitable for localized, high-resolution detection and quantitative evaluation. Current research focuses on multi-modal cooperative excitation, improved high-temperature adaptability, signal enhancement techniques, and innovations in imaging algorithms, aiming to address key technical challenges such as mode interference, low signal-to-noise ratio (SNR), and insufficient robustness under varying environmental conditions. Figure 2 shows the excitation mechanism of EMAT bulk wave.

Transducer structure design is central to achieving efficient bulk wave excitation. In recent years, dual-mode EMATs have become a research hotspot. A typical configuration is the two-layer butterfly coil, which, under a spatially modulated bias magnetic field, generates longitudinal waves primarily in the central segment where the horizontal magnetic field dominates, while transverse waves are preferentially excited in the side regions exposed to vertical magnetic fields, enabling simultaneous excitation of both shear and longitudinal waves. Although the Lorentz force is the primary driving mechanism, in ferromagnetic materials, the coupled effects of magnetostrictive forces and magnetization forces must also be considered, as they offer potential for controlling wave mode distribution and energy allocation. To further enhance longitudinal wave excitation efficiency, Xiao Fei et al. [64] proposed an optimized permanent magnet configuration, incorporating an additional horizontally magnetized center magnet between the poles of a saddle-shaped magnet. This design significantly strengthens the horizontal magnetic field in the central region of the coil. Since the Lorentz force is proportional to the square of the magnetic field strength, this optimization leads to a substantial increase in longitudinal wave signal amplitude. Experiments demonstrate that the transducer can operate stably at temperatures up to 750 °C, successfully enabling continuous wall thickness monitoring of high-temperature pipelines. This advancement not only breaks through the conventional temperature limitations of EMATs but also reduces the need for probe switching due to thermal variations by integrating dual-mode capability, significantly improving the versatility and engineering practicality of the detection system. However, the long-term operational stability required for practical industrial applications (on the order of months or years) remains challenged by factors such as thermal demagnetization and material aging, constituting a key focus of current research.

For thickness measurement, shear waves are uniquely advantageous in sensing thickness variations owing to their wave propagation properties, making them particularly suitable for remaining wall thickness assessment and fatigue damage evaluation. For the quantitative detection of corrosion-induced wall thickness in pipelines, Tian et al. [65] proposed a joint denoising algorithm for electromagnetic ultrasonic signals based on Empirical Mode Decomposition and Singular Spectrum Analysis. This method confines the calculation error of pipeline wall thickness within 0.19 mm, effectively enhancing the identification accuracy and quantitative reliability of corrosion defects. In the high-precision assessment of coating thickness and bonding quality, Malikov et al. [66] employed an electromagnetic ultrasonic bulk wave pulse-echo technique combined with Short-Time Fourier Transform and a Convolutional Neural Network. This approach achieved non-contact thickness measurement and intelligent classification of bonding states for high-attenuation epoxy coatings. The method demonstrated a measurement error of no more than 1.1% for coatings thinner than 1.5 mm, with defect identification accuracy exceeding 99%. This provides a reliable technical pathway for rapid health monitoring of in-service coating structures. However, their application is limited by low signal-to-noise ratio (SNR) and beam divergence, which affect measurement accuracy. To address these issues, researchers have focused on improving shear wave excitation efficiency and signal quality through coordinated optimization of finite element simulations and high-power excitation systems. Xu Lijun et al. [67] developed a non-contact pipe thickness detection system based on electromagnetic ultrasonic shear waves for accurate measurement of remaining pipe wall thickness. By combining orthogonal experiments with finite element simulation, they optimized key parameters of the printed circuit board spiral coil, including number of turns, trace width, and spacing. The study confirmed that the number of turns and trace width are the primary factors influencing echo peak amplitude and SNR, respectively. The fabricated transducer was tested on curved aluminum pipes, showing highly focused beam profiles with negligible influence from curvature. The thickness measurement error was less than 0.2%, demonstrating strong industrial applicability. Zhang Shuang et al. [68] developed a high-power excitation system integrated with a parameter-optimized EMAT, achieving ultra-high precision thickness measurement with resolution down to 0.01 mm on an aluminum pipe with 200 mm diameter and 5 mm wall thickness. The system operates without surface polishing or couplant, exhibiting excellent adaptability for field applications. However, the study also noted that signal distortion caused by inductive impedance and delays in data processing remain key factors limiting real-time performance. In the early-stage non-destructive evaluation of intergranular corrosion, electromagnetic ultrasonic technology demonstrates significant potential. Cieslik et al. [69] successfully identified microstructural changes in AISI 304 stainless steel sensitized at 700 °C for 0.5 h using the EMAT shear wave testing technique. A signal amplitude variation of 0.1 demonstrates the technique’s effectiveness in detecting grain boundary carbide precipitation. This provides a rapid and non-destructive approach for safety assessment in high-temperature service pipeline weld zones.

In addressing the challenges of residual stress measurement in coated pipelines, Luo et al. [70] systematically investigated the effects of coating thickness and surface roughness on the electromagnetic ultrasonic shear wave birefringence method. Their study confirmed that, when the coating thickness does not exceed 1.9 mm, the stress measurement error can be consistently controlled within 50 MPa. This provides an experimental basis for the engineering-level detection of in-service pipeline stress states without the need for surface treatment. In enhancing the transduction efficiency of EMAT, material innovation plays a particularly crucial role. Liang et al. [71] proposed a magnetostrictive EMAT with a plasma-sprayed Fe₃O₄ coating, which combines a high magnetostrictive coefficient with oxidation resistance, significantly improving the generation and reception efficiency of ultrasonic signals. Experiments showed that the amplitude of the received signals increased by factors of 1.1 × 10⁴ and 2.2 × 10³ compared to traditional stainless steel and carbon steel samples, respectively. This provides a new transducer design approach for high-temperature, long-term structural health monitoring.

The above studies demonstrate that whether through structural optimization or excitation enhancement, the core objective is to maximize shear wave energy output and receiving signal stability. These systematic efforts have enabled electromagnetic ultrasonic shear wave technology to achieve sub-millimeter and even micrometer-level thickness measurement accuracy under non-contact conditions, effectively distinguishing between wall thinning and early-stage fatigue damage, thus exhibiting outstanding potential for quantitative detection.

Electromagnetic ultrasonic oblique incidence technology is a non-contact detection method based on the Lorentz force mechanism. Its key advantage lies in controllable wave mode conversion and adjustable detection angle. Compared to conventional shear waves that are primarily sensitive to vertical defects, this technique exhibits higher sensitivity to inclined or horizontally oriented flaws, generating strong reflected signals through evident mode conversion when encountering such defects. By optimizing probe design, a pitch-catch tandem configuration can be achieved, enabling effective detection of internal defects oriented at an angle to the surface. The performance of this method is mainly influenced by frequency, as well as magnet and coil configurations; optimizing these parameters significantly enhances signal amplitude, focusing accuracy, and detection sensitivity. These characteristics make the technology uniquely valuable in complex detection scenarios such as weld detection and flaw detection in coarse-grained materials, allowing for precise localization and characterization of micro-defects within a certain depth range.

Qu Zhengyang et al. [72] proposed a shear vertical wave EMAT using a horizontal magnetization structure, which effectively expands the ultrasonic incidence angle through optimized magnetic field configuration and significantly suppresses head wave interference. Results show that under incidence angles greater than 40°, this horizontal magnetization structure notably enhances signal amplitude and SNR, effectively eliminating signal discontinuities and interference, thus providing a feasible technical solution for large-angle scanning detection. Zhang Jin et al. [73] established finite element models under flat and convex surface conditions to investigate the influence of curvature radius, coil turns, and initial angle on the acoustic field. The results indicate that on convex surfaces, the main lobe peak increases by 22.76% and the main lobe width decreases by 10.56%; coil gain increases significantly within 28 turns and then tends to saturate; smaller curvature radii are more favorable for beam focusing. This study provides theoretical support for EMAT design in thick-walled components. Qian Shengjie et al. [74] investigated oblique incidence SV-wave EMAT technology for detecting buried defects in high-temperature equipment. Based on the propagation mechanism of SV waves, they proposed a defect localization algorithm suitable for multi-reflection paths and validated it on test specimens containing artificial notches and welding flaws at an elevated temperature of 80 °C. Experiments demonstrated accurate detection of 1–3 mm deep notches, with lateral and depth positioning errors less than 0.5 mm and 0.8 mm, respectively. This method offers a reliable solution for the rapid online inspection of high-temperature equipment. Liu Yang et al. [75] proposed a non-contact detection method based on electromagnetic ultrasonic oblique incidence SV waves for high-temperature weld detection in steel pipes. The study optimized the magnetic attachment structure and coil parameters of the EMAT, and employed advanced signal processing techniques including wavelet threshold denoising, matched filtering, and time-frequency joint analysis, significantly improving SNR and defect recognition accuracy under high-temperature conditions. Experiments showed 100% detection rate for typical weld defects such as cracks, porosity, and slag inclusions at room temperature, at temperatures up to 300 °C, this work provides an effective technical pathway for the rapid online inspection of industrial high-temperature equipment. In addition to oblique incidence SV waves, focused Electromagnetic Acoustic Transducers (PF-EMAT) have demonstrated significant advantages in the detection of weld defects. Nakamura et al. [76] developed a point-focused EMAT for SV waves, where SV waves excited by multiple concentric linear sources superimpose in-phase at the focal point, significantly enhancing the signal-to-noise ratio and spatial resolution. The study systematically compared defect detection capabilities at frequencies ranging from 1.1 to 3.0 MHz and found that a 2 MHz PF-EMAT could clearly detect cracks with depths as shallow as 0.05 mm in stainless steel plates, achieving a spatial resolution of 4 mm.

To further enhance defect characterization capability, electromagnetic ultrasonic C-scan technology integrates high-performance transducers with precision scanning platforms to achieve three-dimensional visualization and reconstruction of defect morphology, distribution, and depth. This technique operates without couplant and is particularly suitable for rough surfaces, high-temperature environments, or difficult-to-access conditions. Ma Sitong et al. [78] applied electromagnetic ultrasonic C-scan technology to internal corrosion detection in pipelines. They successfully obtained accurate information on corrosion area, depth, and 3D morphology, achieving corrosion quantification and visual imaging with 0.1 mm accuracy, verifying its reliability in pipeline safety assessment. Sarris et al. [79] conducted fatigue damage detection using shear wave C-scan, revealing that the sensitivity of shear waves to fatigue damage is 2.17 times higher than that of longitudinal waves. High repeatability was achieved in scanning both flat plates and pipe specimens, enabling effective differentiation between wall thinning and fatigue evolution processes, thus providing a new solution for industrial life assessment.

This section highlights several key trends in bulk wave technology: the functional integration achieved through dual-mode EMAT design, the expansion of high-temperature applicability via magnetic circuit optimization, and the high-precision defect imaging and characterization enabled by oblique incidence SV waves combined with C-scanning. However, these advancements also introduce new trade-offs. Multi-modal integration may compromise the optimal performance of individual functions, extreme high-temperature environments pose severe challenges to the long-term reliability of permanent magnets and coils, and high-precision imaging often comes at the cost of reduced detection throughput. Therefore, the core focus of the next phase of research lies in optimizing the overall performance balance among integration, environmental resilience, and inspection efficiency.

The above content provides a review of the research progress and engineering practices of technologies such as electromagnetic ultrasonic guided waves and bulk waves in the field of pipeline nondestructive testing. These techniques have demonstrated promising application prospects in corrosion monitoring, crack identification, wall thickness measurement, and structural integrity assessment, owing to their notable advantages, including non-contact excitation, no need for coupling agents, high-temperature resistance, and adaptability to complex working conditions.

Table 4. Classification of Validation Levels for Representative EMAT Pipeline Detection Technologies.

Research Type	Validation Object	Core Technology	References
Simulation	Established a numerical model based on wavenumber analysis method	Three-dimensional finite element model for interaction between circumferential Lamb waves and delamination defects	[32]
	Investigated the influence of curvature radius on the acoustic field	Finite element analysis of the acoustic field of line-focused oblique incidence SV waves on a convex surface	[73]
	Optimized the design parameters of the integrated probe	Finite element model of an EMAT-PEC composite probe	[80]
Laboratory Testing	Artificial thin-walled pipe specimen, frequency tuning experiments	Frequency-dependent modal selective excitation of circumferential Lamb waves	[31]
	Flat plate and tubular test specimens, performance comparison with traditional coil designs	Novel EMAT design using V-shaped coils	[36]
	Specimen containing artificial notches and welding flaws, tested on an 80 °C heating platform	Defect localization algorithm for oblique incidence SV waves	[74]
Pipe Spool Testing	8-inch steel pipe with a 1.5D bend section, artificial defects	Reflection behavior of T(0,1) guided waves from defects located in pipe bends	[50]
	8-inch Schedule 40 steel pipe, artificial corrosion defect, tested from room temperature to 175 °C	Guided wave tomography based on high-order helical modes	[59]
	45 steel pipe, tested during the cooling process from 750 °C in a high-temperature furnace	High-temperature wall thickness measurement performance of a dual-mode EMAT	[64]
Field Trial	Industrial high-temperature steel pipe welds, tested in a 300 °C environment	Detection of weld defects in high-temperature steel pipes using electromagnetic ultrasonic SV waves	[75]
	In-service oil and gas pipeline, long-range corrosion screening and localization	Pipeline corrosion screening using a combined guided wave and EMAT technique	[81]

summarizes and classifies the validation levels of representative EMAT pipeline detection technologies.

4. EMAT Composite Detection Technology

With the increasing diversification of pipeline material types, large changes in cross-section structure and harsh surface conditions in industrial environments, single detection mode has limitations in sensitivity, penetration depth and environmental adaptability. Especially under strong noise interference, weak defect signals are easy to be masked, and the coexistence of multiple defect types leads to feature confusion, which further aggravates the decline of detection rate and recognition reliability. In this context, the composite testing technology that utilizes the complementary advantages of multi-physical field synergy and multi-mode fusion has become a key way to overcome the bottleneck of a single method. The following chapters will summarize and analyze the construction mechanism, key technical bottlenecks and potential solutions related to typical composite technology systems.

Proposal and Application of Composite Detection Technology

In the field of pipeline defect detection, single NDT techniques often face limitations in sensitivity, detection depth, spatial resolution, and environmental adaptability due to inherent physical constraints. For example, while EMAT offers significant advantages such as non-contact operation, no need for couplant, and suitability for high-temperature environments, it suffers from a near-surface detection blind zone and relatively low sensitivity to small surface cracks. To overcome these limitations, researchers have increasingly explored composite detection technologies that deeply integrate EMAT with other NDT methods in recent years. By combining sensor integration, cooperative excitation mechanisms, and multi-source signal fusion, these hybrid approaches enable comprehensive defect identification—from surface to subsurface, and from large-scale screening to localized high-resolution detection. Such composite techniques are now recognized as a key direction for advancing the safety assessment of complex industrial equipment.

Currently, typical detection technologies integrated with electromagnetic ultrasound include: pulsed eddy current (PEC), phased array ultrasonic testing (PAUT), ultrasonic guided waves, magnetic flux leakage (MFL), and alternating current field measurement (ACFM). These hybrid configurations achieve multi-level synergy in hardware architecture, excitation methods, and data interpretation, significantly enhancing the comprehensiveness and reliability of detection. A comparative analysis of electromagnetic ultrasonic composite detection technologies for pipelines is provided in

Table 5. Comparison of electromagnetic ultrasonic composite detection technology in pipeline.

Composite Technology	Detection Principle	Characteristics	References
EMAT + PEC	Using the same probe for time-division excitation, low-frequency signals generate pulsed eddy currents to detect surface cracks, while high-frequency signals excite ultrasonic waves through the Lorentz force mechanism to probe internal defects	It solves the inherent detection blind area of single method technology, but puts forward strict requirements for signal separation technology	[80,82]
EMAT + PAUT	EMAT generates ultrasonic waves in metals through electromagnetic induction, while phased arrays employ multi-element probes to achieve dynamic beam focusing and steering	The combination of efficient screening and accurate imaging significantly improves the detection efficiency and reliability, but the system is complex and expensive	[83,84]
EMAT + Ultrasonic Guided Wave	EMAT enables high-precision thickness measurement by exciting bulk waves through electromagnetic induction, and facilitates low-frequency, long-distance rapid scanning by generating guided waves via the magnetostrictive effect or electromagnetic induction	It is suitable for the location and quantitative detection of large area corrosion and axial crack, but it is necessary to remove the insulating layer in the detection of high temperature pipeline	[81,85]
EMAT + MFL	It conducts magnetic MFL testing using a permanent magnet and yoke, while simutaneously employing the same magnetic circuit along with a high-frequency coil to excite ultrasonic waves for internal defect detection	A single probe realizes dual physical field detection, which solves the problem of insufficient detection depth of internal defects, but the integrated design is complex and the MFL still has blind spots for defects parallel to the magnetization direction	[86]
EMAT + ACFM	ACFM quantitatively detects surface cracks through magnetic field distortion, while EMAT employs high-frequency excitation to generate bulk waves, precisely identifying the depth and dimensions of subsurface defects based on the time-domain characteristics of echoes	The surface defect and subsurface defect information can be obtained at the same time, and the detection accuracy is high. However, it is mainly suitable for aluminum tubes at present, and its adaptability in ferromagnetic materials and complex industrial environments needs to be verified	[87]

.

To address the near-surface detection blind zone of EMAT, researchers have proposed combining it with PEC technology. The EMAT-PEC hybrid detection technique uses the same coil to alternately excite high-frequency and low-frequency signals: high-frequency signals generate ultrasonic waves for detecting internal defects or measuring wall thickness, while low-frequency signals induce pulsed eddy currents for identifying surface cracks. Chen Tao et al. [80] developed a combined EMAT-PEC detection method, enabling simultaneous detection of inner and outer wall defects in pressure pipelines. By establishing a finite element model and fabricating an integrated probe, experiments demonstrated that the method can effectively detect a 1 mm-wide, 2 mm-deep outer wall crack and a 2 mm-thick inner wall loss, with a thickness measurement error of only 1.3%. The sensitivity of PEC detection improved by 19% compared to conventional methods, effectively eliminating the blind zones inherent to each individual technique. To further mitigate signal interference and blind zone issues, Hu Peng et al. [82] proposed a time-shared dual-frequency excitation strategy. Through frequency optimization, the SNR of EMAT was improved by 3.5 dB, and the effective detection depth of PEC increased from 2 mm to 3 mm. This approach achieved concurrent enhancement of both modalities, offering a new solution for optimizing composite detection systems.

In thick-walled structure detection, the integration of EMAT and PAUT demonstrates unique advantages. The former provides non-contact excitation capability, suitable for high-temperature and high-speed online detection, while the latter enables high-resolution imaging through beam steering and focusing functions. Tu Zhenyue [83] applied this composite detection technique to the detection of thick-walled pipes in ultra-supercritical boilers, overcoming the technical limitations of conventional radiographic testing caused by excessive wall thickness. Wang Lei et al. [84] further applied the method to defect diagnosis in pipe bend regions, successfully identifying slight internal surface corrosion and a subsurface defect with a depth of up to 6.1 mm. By combining the results from phased array scanning, the defect was determined to be an internal crack evolved from residual slag inclusion formed during manufacturing. Research indicates that this hybrid approach offers strong complementarity in defect localization, morphology reconstruction, and failure origin tracing.

To meet the demand for rapid detection of large-scale pipeline structures, the EMAT and ultrasonic guided wave composite detection technology—employing a “screening-to-target” or “combined point-surface” strategy—is gaining increasing attention. Luo Rong et al. [81] applied ultrasonic guided waves for preliminary screening of corrosion over hundreds of meters in oil and gas pipelines, capable of identifying regions with cross-sectional area loss as low as 1.2%. Subsequently, electromagnetic ultrasonic testing was used to perform localized thickness measurement and long-term monitoring at identified anomaly locations, forming a closed-loop detection process. This approach operates without shutdown, balancing detection efficiency and economic feasibility. Building on this, Tang Zhifeng et al. [85] developed an integrated hybrid transducer that uses a spiral coil to excite high-frequency shear waves for high-precision local thickness measurement (error < 0.1 mm), while simultaneously employing a meander-line coil to generate low-frequency circumferential Lamb waves for full-circumference rapid scanning. The detection sensitivity for axial cracks and hole-type defects reached equivalent cross-sectional area losses of 2.93% and 3.13%, respectively. This architecture effectively mitigates the high miss-rate of conventional ultrasonic thickness gauging and the poor quantification capability of guided waves, making it suitable for online monitoring and life assessment of industrial pipelines.

To address the challenge of simultaneous detection of surface and internal defects in ferromagnetic materials, the EMAT and MFL composite detection technology achieves concurrent detection and defect classification by sharing a common magnetic circuit. Tang Qin et al. [86] developed an integrated MFL-EMAT sensor that utilizes a shared permanent magnet circuit to combine an MFL module (for surface defect detection) and an EMAT module (for internal defect detection) within a single probe. Due to the significant frequency difference between the two signals, they can be separated via filtering and acquired in parallel. Experiments show that the system can reliably detect surface cracks as narrow as 0.5 mm and internal defects with depths up to 16.7% of the specimen thickness. This approach overcomes the dual limitations of EMAT’s near-surface blind zone and MFL’s limited detection depth, providing a robust, multi-level diagnostic solution for ferromagnetic components.

For the detection and quantification of surface and subsurface defects, the composite detection technology of EMAT and ACFM realizes synchronous detection and quantification. Li Wei et al. [87] developed a dual-modal detection system that uses magnetic field distortion signals to reconstruct the geometric parameters of surface defects, while simultaneously employing ultrasonic bulk wave echoes to determine the location and depth of subsurface defects. Test results show that the measurement error for surface defect dimensions is less than 10%, and the depth estimation error for subsurface defects is below 7%, demonstrating high detection accuracy. This method provides a feasible approach for simultaneous identification of multi-level defects in non-ferromagnetic materials such as aluminum.

In summary, electromagnetic ultrasonic composite detection technology, through multi-physical-field synergy, multi-functional integration, and multi-source information fusion, has effectively expanded the detection capabilities beyond those of individual methods, enabling a transition from “surface–internal” to “local–global” and from “qualitative–quantitative” detection. With continuous advancements in sensor integration, signal processing algorithms, and intelligent diagnostic systems, these hybrid techniques are poised to play a critical role in high-demand industries such as oil and gas pipelines, nuclear power equipment, and aerospace, providing key technical support for the safe and reliable operation of industrial systems.

5. Summary and Outlook

This paper elaborates on the application of electromagnetic ultrasonic testing technology in pipeline defect detection. Addressing the typical defects in pipelines under different working conditions—such as long-distance, subsea, and buried pipelines—it compares and analyzes the limitations of traditional nondestructive testing methods, including piezoelectric ultrasound, magnetic flux leakage, and eddy current testing. It highlights the significant potential of EMAT technology, leveraging its advantages such as non-contact operation, no need for couplants, and adaptability to high temperatures and complex environments, in achieving accurate identification and assessment of various pipeline defects. The paper provides a detailed review of the technical principles, research progress, and typical transducer designs—such as unidirectional excitation, focused, and composite EMATs—in the two major technical systems of electromagnetic ultrasonic guided waves (including circumferential, axial, and helical guided waves) and bulk waves, along with their specific applications in detecting cracks, corrosion, and other defects. It also explores the unique value of oblique-incident shear vertical (SV) wave technology in weld detection and targeted identification of inclined defects, and through comparisons, reveals the complementary nature of different guided wave modes in terms of detection range and accuracy. Furthermore, the paper discusses composite testing solutions combining EMAT with multiple technologies such as pulsed eddy current, phased array ultrasound, and magnetic flux leakage, emphasizing the importance of multi-physics synergy and multi-source information fusion in overcoming the limitations of single-technique approaches and achieving comprehensive defect detection. Overall, electromagnetic ultrasonic technology has emerged as a key direction for enhancing pipeline structural integrity assessment, demonstrating broad application prospects. However, there remain several unresolved issues and challenges.

Currently, EMAT pipeline inspection involves multiple modalities, including guided waves (circumferential, axial, helical) and bulk waves. In practical applications, phenomena such as modal aliasing, dispersion effects, and mode conversion are prone to occur. Particularly in structurally discontinuous regions of pipelines, such as welds, elbows, and coated surfaces, signals exhibit complex superposition characteristics due to multi-path propagation and energy competition. Traditional signal analysis methods struggle to effectively interpret such composite signals and lack the capability for synergistic interpretation and fusion-based diagnosis of multimodal information. As a result, an adaptive and transferable intelligent diagnostic framework has yet to be established. Additionally, the detection performance of EMAT in real industrial environments is significantly influenced by multiple environmental factors. Coatings and rough surfaces can cause ultrasonic energy to dissipate in non-pipe media or undergo mode conversion, thereby reducing defect signal response and signal-to-noise ratio. In high-temperature environments, permanent magnets face the risk of thermal demagnetization, and coils require specialized thermal design, with long-term stability still pending verification. Differences in the electromagnetic properties of materials also affect transduction efficiency, particularly in special materials such as high-strength steels and coarse-grained alloys. Furthermore, irregularities in pipeline structures, such as elbows and supports, alter acoustic field propagation characteristics, leading to inspection blind zones or signal misinterpretation.

Future research should focus on developing intelligent systems capable of organically integrating guided wave rapid screening with bulk wave precise quantification. Leveraging deep learning and multimodal signal fusion technologies, a unified interpretation model for multimodal signals should be established to achieve automated defect recognition, classification, and quantitative assessment, thereby constructing a true closed-loop “screening-diagnosis-evaluation” detection system. Second, the research emphasis should shift from general-purpose transducer designs to condition-specific solutions. This includes developing high-temperature-resistant EMATs with optimized magnetic circuit layouts and cooling structures, coating-adaptive transducers capable of selective mode excitation, and adaptive probe designs, such as insert-type or arched structures, for small-diameter, bent, or irregularly shaped pipelines. Systematically studying the coupling effects of environmental factors and establishing quantitative performance prediction models will be key. Additionally, efforts should be directed toward addressing the engineering challenges of EMAT composite detection technologies. This includes establishing hardware-level standards for signal separation and acquisition in hybrid systems such as EMAT-PEC and EMAT-MFL to reduce cross-interference, developing standardized data fusion and processing software frameworks, and constructing integrated embedded inspection systems. Furthermore, large-scale, long-term field trials should be conducted under complex operating conditions, such as buried or subsea pipelines, to accumulate reliability data and provide support for formulating industry application guidelines.