Fiber optic hydrogen sensors with the advantages of intrinsic safety, anti-electromagnetic interference and without electric spark have been widely used in the field of hydrogen sensing. The development of hydrogen-sensitive materials with good performance is the key of obtaining high-performance optical hydrogen sensors. Tungsten oxide (WO₃) thin film has been usually used in hydrogen sensing due to its stable chemical properties and high oxygen vacancy diffusion coefficient. However, when measuring hydrogen in the environment of hydrogen production from offshore wind power and gas transmission in buried hydrogen pipelines, water molecules and chloride ions in the environment cover or occupy the active sites on the surface of WO₃ thin films which hinders the contact with hydrogen and reduces the stability of sensing films. Therefore, it is necessary to improve the salt spray-resistant performance of WO₃ hydrogen sensitive films.

WO₃ thin film is modified by perfluorodecyl trimethoxysilane (FAS) and silane coupling agent (KH-570) ethanol aqueous solution which improves the hydrophobicity and resistance to chloride ion penetration of the thin films. First, WO₃ thin film is prepared by the sol-gel method and spin-coated on glass substrates. Then, FAS ethanol solution is used for fluorination hydrophobic treatment to enhance the hydrophobicity of the WO₃ thin film. Meanwhile, the different factor levels in the fluorination hydrophobic treatment process are investigated by orthogonal experiments to determine the optimal hydrophobic process parameters. Subsequently, on the basis of the above experimental results, KH-570 ethanol aqueous solution is utilized to modify the WO₃ thin film, and by studying the mass fraction ratio of water and ethanol during the hydrolysis of KH-570, the hydrolysis products can be better grafted onto the thin film to improve its resistance to chloride ion penetration. Finally, a transmission hydrogen sensing system is built, and the effects of repeatability, long-term stability on the salt spray-resistant performance of the modified WO₃ thin film are studied.

Double modification treatment of WO₃ thin films using FAS and KH-570 can maintain good hydrogen sensitivity in salt spray environment. The different factor levels in the fluorination hydrophobic treatment process are investigated by orthogonal experiments, and the contact angle of the FAS-WO₃ film is obtained from the nine groups of orthogonal experiments, with the highest contact angle measured at 120° for scheme #5 (Fig. 7). Meanwhile, before and after fluorinated hydrophobic treatment of the WO₃ film, the sensitivity and response time of the film in scheme #5 are minimally affected by the fluorination process, with sensitivity decreased by 0.05% and response time increased by 13 s (Fig. 8). In order to verify the above conclusion, three thin film characterization methods of scanning electron microscope (SEM), X-ray energy dispersive spectrometer (EDS), and Fourier transform infrared spectroscopy (FTIR) are used to test and analyze them (Fig. 9). It is observed that white particles are uniformly distributed on the surface of the FAS-WO₃ film, which is found by EDS mapping that the white particles are mainly composed of Si, O, and F. A sharp peak at 1030 cm?1 in the FAS-WO₃ film corresponds to the stretching vibrations of the —CF₂— and —CF₃— groups in FAS, attributed to the uniform network structure formed by the added FAS covering the surface of the WO₃ film. Additionally, at a water-ethanol mass fraction ratio of 5∶1, the sensitivity and response time of the film remain relatively stable as the increase of duration in the 90% salt spray environment, demonstrating the best salt spray resistance (Fig. 10). The infrared spectrum of the FAS-WO₃ film modified with KH-570 shows characteristic peaks at 1712 cm?1 and 1638 cm?1, corresponding to the stretching vibrations of C=C and C=O, confirming the successful grafting of KH-570 with the film (Fig. 12). Accordingly, the contact angle of the modified thin film is 117°, compared to 47° for the unmodified thin film (Fig. 13). The sensitivity of the thin film is 15.01% with response time of 40 s and repeatability of 98.65% in 20 consecutive tests at 5% hydrogen volume fraction. The thin film also exhibits good hydrogen-sensitive properties after 7 d of storage in a high humidity and high salt environment, and there is no deterioration of the hydrogen-sensitive properties after 90 d in air (Figs. 16,17, and 18).

FAS ethanol solution and KH-570 ethanol aqueous solution are used to modify the WO₃ film to improve the salt spray resistance characteristic. The results show that immersing in 0.02 mol/L FAS ethanol solution for 2 h followed by heat treatment at 200 ℃ for 3 h is the optimal hydrophobic process, achieving a contact angle of 120°. When the mass fraction ratio of water to ethanol is 5∶1, the hydrolysis product of KH-570 can be better grafted onto the film to improve the chloride ion permeation resistance of the film. The repeatability of the modified sensitive film can reach 98.65% at a hydrogen volume fraction of 5%, with good repeatability and stability against hydrogen.