Detection of rust in coated steel plates using terahertz waves

The applicability of terahertz (THz) waves to the detection of rust in coated steel plates has been examined. Spectral intensity measurements of reflected light from a sample coated with epoxy resin show that optical coherence tomography using near-infrared light and THz waves can detect the hidden rest region. This region can be reproduced by so-called THz imaging without contact. Since rust exhibits strong absorption in the frequency range from 1 to 5 THz, the absorption coefficient can be used to estimate the rust state with high sensitivity. In contrast, neither X-ray fluorescence nor Fourier transform infrared spectroscopy can detect the rust, because of the strong X-ray and IR peaks of the coating. Electro-chemical impedance measurement, which is one of the promising diagnostic methods, judges the water blocking ability of the coating. The impedance value and THz absorption due to rust shows good correlation until the amount of water in the film reaches its maximum.