THz wave reflected from metal surface covered by invisible paint can be detected, because THz wave possesses both characteristics of radio wave and light. When the paint layer is not transparent in visible light region, the reflection of THz wave possibly detects the corrosion products underneath the paint layer. In this study, the reflection-absorption imaging of THz wave was applied to detection of corrosion products on hot-dip galvanizing steel sheets which was covered by epoxy resin. The specimen of this study was hot-dip galvanizing steel sheets with scratch damage on the surface, and it was corroded by 5wt% NaCl solution spraying before the measurement. Frequency tunable THz light was generated by difference frequency generation from a GaP crystal. Large absorption was observed at area around the scratch zone at 2.0 THz and 2.8 THz imaging. From the comparison of the THz imaging with XRF mapping, zinc hydroxylchloride and zinc chloride was estimated as the corrosion products. It was found that the THz wave technique is very available for non-destructive novel corrosion diagnosis of metal surface covered with invisible insulators or painted films.
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