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.