High-resolution axial scanning of pipeline corrosion via local resonance: A non-contact ZGV-EMAT approach

Detecting early-stage inner wall corrosion in small-diameter thin-walled pipelines remains challenging due to the coupling limitations of ultrasonics and the skin depth restrictions of eddy current testing. This study proposes a non-contact methodology utilizing Zero-Group-Velocity (ZGV) modes excited by a permanent magnet Electromagnetic Acoustic Transducer (EMAT). Dispersion analysis identifies the axisymmetric longitudinal L(0,4)-ZGV mode as the optimal candidate for 6061 aluminum alloy pipes with 32 mm inner diameter and 5 mm wall thickness. A specialized EMAT with 10 mm coil width is optimized to selectively excite this mode, generating a localized standing wave. Experimental validation on pipes subjected to HCl etching for 2 to 12 h demonstrates that the ZGV resonance frequency exhibits a monotonic upward shift from 618.7 to 623.3 kHz, correlating with progressive structural degradation. The non-contact approach eliminates mass-loading effects inherent to bonded piezoelectric transducers, ensuring frequency stability and enabling free axial translation for continuous scanning with 5 mm step size. The method exhibits statistically equivalent sensitivity to inner and outer wall defects across three etching durations, overcoming the near-surface dead zone of conventional ultrasonics. Comparative analysis reveals that the ZGV-EMAT method achieves a detection sensitivity approximately one order of magnitude higher than that of the L(0,2) guided wave benchmark under the tested conditions. These results demonstrate the feasibility of this approach for early-stage corrosion characterization in laboratory conditions, with further validation required for in-situ industrial deployment across diverse pipe materials and corrosion mechanisms.