Abstract
The hydrogen diffusion process in ferritic steel during thermal desorption tests was simulated using the finite element method based on the two-dimensional diffusion-trapping coupled model. This model was first verified by experimental data to obtain a physically meaningful combination of trap/lattice parameters. Then, the effect of specimen geometry was studied by varying the height of cylindrical specimens with other parameters fixed at constant values. Simulation of desorption spectra with different specimen geometries indicates that the measurement of hydrogen concentration is not affected by the change in specimen geometry due to the mass conservation law, for original thermal desorption spectra (TDS), which are, however, unlikely to be detected in traditional experiments due to the necessity of specimen transfer procedures. Considering the hydrogen escape during rest time (specimen preparation/transfer/evacuation), the measured TDS curves are expected to be strongly dependent on the specimen geometry. The effect of specimen geometry on desorption spectra is more pronounced for smaller specimens, resulting in the dramatic decrease in peak flux and the increased error of Kissinger method in the determination of trap deactivation energy. The present study may contribute to better understanding and more reliable interpretation of the TDS curves by considering the size effect.
Highlights
Hydrogen is one of the most promising clean energy sources due to its features of zero emission and no pollution
The diffusion-trapping coupled partial differential equation was solved by analogy to the stabilized convection–diffusion model in COMSOL software
The mass conservation law holds for different specimen geometries, it is expected that the measured hydrogen concentration is dependent on the specimen size, especially for the small disk-shaped specimens
Summary
Hydrogen is one of the most promising clean energy sources due to its features of zero emission and no pollution. Prevention of hydrogen-assisted failures requires in-depth understanding of the hydrogen diffusion process and its interaction with local microstructures and stress fields [4,5]. The hydrogen trapping behavior in materials is thereby an important factor because it slows down the diffusion and affects the level of diffusive hydrogen at lattice sites. The popular method for precisely measuring the trapping parameters of hydrogen for a specific material is thermal desorption spectroscopy (TDS), during which the hydrogen atoms in trap sites gradually escape with the increase in temperature, diffuse out of the specimen through lattice sites and are detected by the instrument.
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