Fe-C-Coated Single-Mode–Multimode–Single- Mode Optical Fiber Sensor for Steel Corrosion Monitoring

Fe-C-coated single-mode–multimode–single-mode (SMS) optical fiber sensor is fabricated and characterized for steel corrosion monitoring in 3.5 wt.% NaCl solution. The multimode fiber (MMF) is a none core fiber and has a length of 30 mm. A 100-nm-thick layer of the gold film is first sputter deposited on the surface of the SMS fiber and then the Fe-C layer is electroplated with a thickness of around 20 $\mu \text{m}$ . The microstructure of both the inner gold film and the outer Fe-C layer is characterized by scanning electron microscopy (SEM) coupled with energy-dispersive X-ray spectroscopy. The corrosion evolution of the Fe-C layer over time in 3.5 wt.% NaCl solution is measured with open circuit potential (OCP), linear polarization resistance (LPR), and electrochemical impedance spectroscopy (EIS). In the meantime, the change in the transmission spectrum is recorded with an optical spectra analyzer during the corrosion test. The shift in the characteristic wavelength of the Fe-C-coated SMS fiber sensor is extracted and correlated with the corrosion-induced mass loss of the Fe-C layer. Regression analysis demonstrates that the corrosion-induced mass loss of the Fe-C layer exponentially increases with an increase in the characteristic wavelength of the Fe-C-coated SMS optical fiber sensor.