Abstract
Steel sheet with an insulator to prevent corrosion is used for various purposes including in building and car manufacture. Terahertz waves, for which insulators are highly permeable and metal surfaces are highly reflective, have been studied in order to establish a new inspection technology for these steel plates. In our previous research, spectroscopic measurements in the 1.0-4.0 THz range, generated by a GaP crystal, were carried out in order to collect information on the infrared activity of the metal corrosion products formed on Zn-Al hot-dip galvanized steel sheet. In the previous work, the infrared activity of Fe-based corrosion products was not examined. To examine these products, we conducted THz spectroscopy on goethite (α-FeOOH) in the range from 8.4 to 11.0 THz, generated by a GaSe crystal. The results of Attenuated Total Reflectance (ATR) FTIR spectral measurements and molecular vibration calculations were analyzed, on the basis of which the natural vibration modes of α-FeOOH in the THz frequency range were assigned.
Highlights
Terahertz waves have a number of useful characteristics
They are highly permeable in non-polarized materials; second, they are highly reflective at metal surfaces; third, they can be used to obtain finger print spectra for molecular bonding; and fourth, they are safe to use as they are harmless to human tissue
THz spectroscopy has the advantage of having high resolution, and there is no need for a large interferometer or a long optical path such as needed for Fourier Transform Infrared Spectroscopy (FTIR) spectroscopy
Summary
Terahertz waves have a number of useful characteristics. First, they are highly permeable in non-polarized materials; second, they are highly reflective at metal surfaces; third, they can be used to obtain finger print spectra for molecular bonding; and fourth, they are safe to use as they are harmless to human tissue. THz spectroscopy can be used to inspect the condition of steel substrates covered by an insulator/coating. Metal corrosion products can be qualitatively and quantitatively measured through the absorption of THz waves at the natural vibration frequencies of the molecules in these products. In the 1990s, THz spectroscopy was conducted using THz waves generated using a photoconductive antenna [1]. Our research group has performed spectroscopic analysis on α-FeOOH at frequencies of 1.0-4.0 THz, which is the range of THz waves generated by GaP crystals. The infrared activity of α-FeOOH was not obtained in this frequency range. In this study, spectroscopic analysis of α - FeOOH was carried out using a GaSe crystal, which can generate THz waves in a higher frequency region than GaP crystals. Calculations of the vibration frequencies of single molecules were conducted enabling us to assign the natural vibration modes of α-FeOOH
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