FEM simulation supported evaluation of a hydrogen grain boundary diffusion coefficient in MgH2

- FEM simulations show influence of grain boundaries on hydrogen diffusion in magnesium.
- Large scatter of Mg-H diffusion coefficients in literature explained by grain boundary diffusion.
- Knowledge of the grain boundary microstructure is essential to evaluate diffusion measurements.

The diffusion coefficient data of hydrogen in the Magnesium-hydrogen system shows a large scatter, their trends extrapolations vary at room temperature between 10−12 m2/s and 10−29 m2/s. At room temperature the hydrogen diffusion coefficient in MgH2 is, thus, uncertain by about 17 orders of magnitude. This may be partially attributed to grain boundaries contributing to the measured diffusion coefficient. In this paper we use finite-element (FEM) simulations to evaluate the influence of the grain boundary diffusion on the measured total diffusion depending on the difference of the grain boundary (DGB) and volume (DV) diffusion coefficients, as well as on the grain size. These results will be compared to Harrisson's analytical solutions. When the diffusion coefficients differ by more than DV < 10−3·DGB, Harrison's diffusion regime C becomes the best way to describe the total diffusion. The results are used to re-interpret literature data on hydrogen diffusion in MgH2 from this grain boundary contribution point of view. At 300 K, a hydrogen grain boundary diffusion coefficient ranging from DGB = 10−17 m2/s to DGB = 10−20 m2/s, depending on the individual type of sample in MgH2, results from the data evaluation.