High-resolution in situ ultrasonic imaging platform for studying localized corrosion morphology

Localized corrosion poses significant threat to structural integrity. Understanding the kinetics of localized corrosion-induced material morphology changes is crucial to realizing effective corrosion prevention and control and safe structural design. However, the mainstream research tools in this area suffer from various limitations, including inability to examine substrates that are covered by corrosion products (e.g., in situ confocal laser scanning microscopy and in situ optical microscopy) and susceptibility to non-Faradaic current-induced accuracy loss (e.g., wire beam electrode and scanning vibrating electrode technique). In this paper, we introduce an ultrasonic technique that can carry out real-time 3D imaging of localized corrosion-induced material morphology changes in a direct, non-intrusive, and corrosion product-immune manner, circumventing the limitations of the existing research tools. The dedicated experimental specimens for the technique are prepared by permanently installing, on a substrate, a large-scale piezoelectric transducer patch. During an experiment, different zones of the transducer patch are activated in a round-robin fashion to generate ultrasonic waves and acquire reflections. The instantaneous morphologies of the substrate are then reconstructed by processing, via tomography, the reflection signals acquired. A series of localized corrosion experiments were conducted to assess the performance of the technique. The results ascertained the accuracy of the technique, and revealed that the axial resolution of the technique can reach as high as 0.005% of the thickness of the substrate used, and that the lateral resolution of the technique can be enhanced by as simple as exciting ultrasonic waves more closely to each other.