Optical Parameter Measurement and Characteristic Identification for Corrosion Products of Steel Materials Based on Terahertz Time-Domain Spectroscopy

Xuelei Jiang; Enen Li; Ying Xu; Tianwu Wang; Wenyu Xie

doi:10.3788/AOS222047

Abstract

Objective

Corrosion has a great impact on the strength and durability of steel materials (such as rebar and steel plate). Research reveals the absorption resonance of some iron oxides produced during steel material corrosion with terahertz (THz) electromagnetic waves. Moreover, THz waves can penetrate common coating materials. Therefore, THz spectroscopy has application potential in non-destructive testing of the early corrosion of steel materials. To investigate the optical parameters of the corrosion products of steel materials and identify the characteristics of the products, we measured the THz transmission signals of different kinds of corrosion product samples and the main component crystals Fe₃O₄, Fe₂O₃, and α-FeOOH in the samples with a THz time-domain spectroscopy (THz-TDS) system. The experimental results show that in the effective frequency range of 0-1.2 THz, the refractive indexes of the mixture samples of different corrosion products are in the range of 2.7-3.4, and those of the component crystals Fe₃O₄, Fe₂O₃,_{and α-FeOOH are 4.0, 2.7, and 2.2, respectively. The content of Fe₃O₄ in the corrosion product mixture has a substantial influence on the optical parameters, such as absorption coefficient and refractive index. Then, we built an ultra-wideband THz-TDS system based on the two-color field to further extend the effective THz measurement range to 0-10 THz. The results indicate that within 0-10 THz, no characteristic absorption peaks of Fe₃O₄ are observed. In contrast, the characteristic absorption peaks of Fe₂O₃ are located at 3.4 THz, 4.2 THz, 4.85 THz, and 5.8 THz, respectively, and those of α-FeOOH are located at 3.6 THz, 4.05 THz, 5 THz, and 5.45 THz, respectively. In addition, this THz method can identify the characteristics of Fe₂O₃ and α-FeOOH from the THz absorption spectra of the samples of different corrosion products. Suggesting that the occurrence of steel material corrosion can be determined according to the characteristic absorption peaks of Fe₂O₃ and α-FeOOH, the experimental results in this paper lay a foundation for the application of THz spectroscopy in non-destructive testing of steel material corrosion.}

Methods

First of all, we used a Terapulse 4000 instrument to determine the transmission signals of samples of different kinds of corrosion products and the main component crystals Fe₃O₄, Fe₂O₃, and α-FeOOH. Then, we built an ultra-wideband THz-TDS system to extend the effective THz measurement range to 0-10 THz. Afterwards, this ultra-wideband THz system was employed to identify the locations of the characteristic absorption peaks of Fe₂O₃ and α-FeOOH. Finally, samples of different corrosion products were measured to identify the characteristics of Fe₂O₃ and α-FeOOH in the mixtures.

Results and discussion

To investigate the optical parameters of corrosion product samples, we measured samples of different kinds of corrosion products and the main component crystals Fe₃O₄, Fe₂O₃, and α-FeOOH (Figs. 2-4). The experimental results show that the refractive indexes of the samples of the corrosion product mixtures are in the range of 2.7-3.4, and those of the component crystals Fe₃O₄, Fe₂O₃, and α-FeOOH are 4.0, 2.7, and 2.2, respectively. The content of Fe₃O₄ in the corrosion product mixture has a great influence on the optical parameters. Using the ultra-wideband THz system, we identified the locations of the characteristic absorption peaks of Fe₂O₃ and α?FeOOH. The characteristic absorption peaks of Fe₂O₃ are located at 3.4 THz, 4.2 THz, 4.85 THz, and 5.8 THz, respectively, while those of α-FeOOH are located at 3.6 THz, 4.05 THz, 5 THz, and 5.45 THz, respectively (Fig. 5). Furthermore, the characteristic absorption peaks of the Fe₂O₃ and α-FeOOH in the mixture samples were detected (Fig. 6), demonstrating that the mixture state of corrosion products does not affect the identification of the characteristics of Fe₂O₃ and α-FeOOH by THz spectroscopy.

Conclusions

We mainly studied the optical parameters of corrosion products and locates the characteristic absorption peaks of Fe₂O₃ and α?FeOOH. Since the components of the mixture samples of different corrosion products have different relative contents, the refractive indexes of the samples are in the range of 2.7-3.4. Moreover, the content of Fe₃O₄ has a great influence on the optical parameters of corrosion product mixtures. An ultra-wideband THz-TDS system based on the two-color field was built to extend the effective THz measurement range to 0-10 THz. The experimental results reveal that in the range of 0-10 THz, Fe₃O₄has no characteristic absorption peaks; the characteristic absorption peaks of Fe₂O₃ are located at 3.4 THz, 4.2 THz, 4.85 THz, and 5.8 THz, respectively, and those of α?FeOOH are located at 3.6 THz, 4.05 THz, 5 THz, and 5.45 THz, respectively. The ultra-wideband THz waves can identify the characteristic absorption peaks of the Fe₂O₃ and α?FeOOH in the corrosion product mixtures. The characteristic absorption peaks of the two corrosion products Fe₂O₃ and α?FeOOH can be used as identification marks to determine the occurrence of corrosion. The experimental results in this paper lay a foundation for the application of THz spectroscopy to the non-destructive testing of the early corrosion of steel materials.