Characteristic identification of steel corrosion products based on terahertz transmission spectroscopy

Corrosion of steel materials (such as rebars and steel plates) usually occurs under coating materials. It is, with the existing non-destructive testing (NDT) technologies, difficult to accurately determine the corrosion of steel materials. The terahertz (THz) electromagnetic waves can penetrate the non-polar materials, and theoretically experience an absorption resonance with some corrosion products crystals (i.e., iron oxides). The THz spectroscopy has, therefore, a great application potential to characteristically identify the corrosion of steel materials. In the present study, the optical parameters and, especially, the positions of the characteristic absorption peaks of the corrosion products were systematically investigated and identified using the THz system. The experimental results showed that the refractive index of the different corrosion product samples was in the range of 2.7-3.4 in the effective frequency range of 0 THz -1.2 THz, and the content of Fe3O4 had a large influence on the optical parameters in the corrosion product mixtures. To measure the characteristic absorption peaks of the corrosion products, the effective frequency range was extended to 0 THz -10 THz by an assembled broadband THz system. Within this range, the positions of the characteristic absorption peaks were identified as 3.4 THz, 4.2 THz, 4.85 THz, and 5.8 THz for Fe2O3, and as 3.6 THz, 4.05 THz, 5 THz, and 5.45 THz for alpha-FeOOH. For Fe2O3 and alpha-FeOOH, the intensities of the absorption peaks at 4.85 THz and 3.6 THz, respectively, showed a linear relationship with the mass content. Furthermore, the THz spectroscopy could accurately identify Fe2O3 and alpha-FeOOH in the corrosion product mixtures by comparing with the results from IR spectroscopy and Raman spectroscopy. This means that the occurrence of steel corrosion could be detected according to the characteristic absorption peaks of Fe2O3 and alpha-FeOOH. The experimental results provide a basis for the application of THz spectroscopy in the NDT of the corrosion of steel materials.