Corrosion thickness loss monitoring using high-frequency guided ultrasonic waves

Corrosion thickness loss due to adverse environmental conditions of pipelines and marine structures can cause degradation of structural health. Monitoring in difficult to access areas can be achieved using high frequency guided waves propagating along the structure, selectively excited using standard ultrasonic angle beam transducers with single sided access. Wave propagation and mode interference depends on the thickness of the structure. At the frequency-thickness range of interest, the two fundamental Lamb wave modes are excited with slightly different wavenumbers, leading to a beating effect with energy transfer through the structure thickness. The beating effect depends on the frequency-thickness product and has been found to be very sensitive to small thickness changes. The guided wave propagation and energy transfer were visualized and predicted using 2D Finite Element simulations. Excellent agreement was found to theoretical beatlength predictions from a fit of the recorded variation of guided wave amplitude along the propagation direction. Laboratory experiments were conducted, with steel specimen wall thickness reduced by consecutive milling and using accelerated corrosion. Signal changes due to the wave mode interference were measured and the wall thickness reduction monitored from the amplitude beatlength. Good agreement with the theoretical predictions was achieved, demonstrating the sensitivity for thickness loss monitoring.