Detection of rust in coated steel plates using terahertz waves

Abstract

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.

High‐Speed Underfilm Corrosion Imaging Using a Terahertz Camera

“…shows that degree of degradation at corrosion spots cannot be distinguished in such moving measurement. THz absorption coefficient of common iron rust (γ‐FeO(OH): lepidocrocite) is generally around 310 cm −1 , and therefore corrosion of 75 μm or thicker cannot be distinguished with single‐digit dynamic range. On the other hand, degree of degradation at every corrosion site can be distinguished with two‐digit S/N ratio obtained by combinatorial use of TDS system or GS according to the current study.…”

“…2 shows that degree of degradation at corrosion spots cannot be distinguished in such moving measurement. THz absorption coefficient of common iron rust (γ-FeO(OH): lepidocrocite) is generally around 310 cm −1 [11], and therefore corrosion of 75 μm or thicker cannot be distinguished with single-digit dynamic range.…”

“…Terahertz (THz) waves feature high permeability in organic coating films [4][5][6], while it is strongly absorbed by metal rust [7][8][9][10][11][12]. These optical properties can be utilized for early nondestructive and quantitative detection of rust on the inner side of opaque coating films through calculation of two-dimensional (2-D) distribution of THz reflected intensity [13,14].…”