THz Spectroscopic Study of Iron Oxide Layers within Steel-Reinforced Structural Components
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THz Spectroscopic Study of Iron Oxide Layers within Steel-Reinforced Structural Components D. F. Plusquellic1, V. Provenzano2, S. G. Chou1 1. Physical Measurement Laboratory, 2. Material Measurement Laboratory, National Institute of Standard and Technology, Gaithersburg, MD20899 Abstract In this study, we use THz spectroscopy in the energy range between 45 GHz to 3000 GHz (1.5 cm-1 and 100 cm-1) as a non-destructive diagnostic tool to characterize the corrosion by-products (rust) on aged steel structural components that are usually embedded in concrete. The THz radiation has been shown to penetrate concrete with extinction coefficients between (1.0 to 2.3) x 10-2 GHz-1 cm-1, depending on the composition and the moisture content of the concrete. The previously reported antiferromagnetic resonance (AFR) near 140 GHz in iron oxide composites was found to be less than our current detection sensitivity (fractional absorbance sensitivity of >10%). However, a strong transition centered near 725 GHz (24 cm-1) has been observed for the first time. This feature has appeared in reflection from several different samples of mildly corroded steel plates and has been tentatively attributed to a broad phonon density of states commonly referred to as the “Boson peak” found in disordered materials. By taking advantage of this strong transition and a powerful excitation sources in the AFR region, we expect THz spectroscopy and imaging to be an effective diagnostic tool with broad applications in corrosion diagnostics and inspection. Introduction: The detection and repair of corroded steel structural components hidden inside rebarreinforced concrete infrastructure presents a major challenge for a number of industries in the coming decade. Even though a number of non-invasive detection techniques such as ultrasonics, infrared, eddy current and microwave technologies have been under development 1, the sensitive detection and quantification of corrosion remains a challenge. Recent development in Mossbauer spectroscopy has suggested the presence of antiferromagnetic compounds that absorb microwave radiation in the range between 50 GHz150 GHz. Since microwave radiation has shown effective penetration of concrete material, the antiferromagnetic absorption feature could potentially be exploited for non-invasive rust identification and quantification through concrete. In this work, we systematically conduct spectroscopic studies in the energy range from (45 to 3000) GHz to evaluate the extinction profile of concrete as well as the spectra for various synthetic iron oxide compounds and real-life rust samples. The predicted antiferromagnetic resonance transition is in the sub-mm frequency region from (30 to 150) GHz but been found to be weaker than our current detection sensitivity. However, new instrumentation is currently under development to overcome the limited power and spectral resolution. Furthermore, a strong transition around 725 GHz (24 cm-1) has been observed in reflection mode from multiple mildly corroded steel plate samples with a
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