Ultrasonic guided wave-based defect detection in extra-long, small-diameter, thin-walled solar heat-absorbing tubes with heterogeneous salt films

We developed a salt film surface simulation method that integrates stochastic processes and time-series modelling to address the challenges of ultrasonic guided wave (UGW)-based inspection in solar heat-absorbing tubes, where heterogeneous salt films develop owing to prolonged exposure to high temperatures, intense radiation and molten salt corrosion in tower-based solar thermal energy storage systems. A multilayer heterogeneous pipeline model comprising a substrate, coating and salt film was established using COMSOL Multiphysics® finite element software to systematically investigate the effects of salt film deposition on defect echo characteristics. The simulation results revealed that salt films considerably attenuated echo signal amplitudes and reduced the wave propagation velocity. In particular, severe interference was observed in the detection of structural discontinuities, such as weld seams. To address spatial constraints in practical field inspections, we developed a magnetostrictive sensor for dry-coupled UGW excitation and reception. Our study clarifies the mechanisms of signal attenuation and mode aliasing induced by heterogeneous salt films, offering both theoretical insights and technological innovations for defect detection in multilayer pipeline structures. Moreover, these findings can pave the way for improved safety evaluation and maintenance efficiency in solar thermal systems, providing critical support for the reliability and sustainability of energy infrastructure.