Non-Destructive Evaluation of Hidden Defects Beneath the Multilayer Organic Protective Coatings Based on Terahertz Technology

Tu, Wanli, Shuncong Zhong, Manting Luo, and Qiukun Zhang. "Non-Destructive Evaluation of Hidden Defects Beneath the Multilayer Organic Protective Coatings Based on Terahertz Technology." Frontiers in Physics 9 (2021): 304.

Abstract
An organic protective coating system plays an important role for the corrosion protection of offshore metallic structures. It is of practical importance to detect possible coating defects and evaluate coating performance for corrosion degradation monitoring. Reliable defect identification can provide timely and effective maintenance to avoid serious consequences. This work investigated various multilayer organic coating systems by terahertz pulse imaging technology and aimed to explore the inspection ability of terahertz non-destructive testing (THz NDT) of protective coatings. Several types of defective coating samples were measured. The comparison of measurements obtained for sites with and without defects was provided. The changes in the signals caused by the presence of defects were explained. Structural analysis, quantitative evaluation, and defect identification were carried out in detail. The results of measurements showed that corrosion defects, paint bulge defects, and paint detachment defects can be distinguished and identified in combination with the causes of defects. It can provide effective technical guidance for terahertz technology to be gradually extended to engineering applications.


for full paper see https://www.frontiersin.org/articles/10.3389/fphy.2021.676851/full

 In this work, measurement is performed using a commercial THz-TDS system, TeraPulse 4000 (TeraView Ltd., Cambridge, United Kingdom). The system works with a reflection imaging module (RIM), and the stable terahertz spectrum ranges from 0.1–4 THz. The diameter of the terahertz spot at the focal point is about 0.2 mm. The scanning step is set at 0.1 mm with the consideration of measurement effect and measurement speed, and it scans approximately 16 pixels per sec. The achieved signal-to-noise ratio can be up to 90 dB.

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