Theoretical evaluation of the role of crystal defects on local equilibrium and effective diffusivity of hydrogen in iron

Hydrogen diffusion and trapping in ferrite is evaluated by quantum mechanically informed kinetic Monte Carlo simulations in defective microstructures. We find that the lattice diffusivity is attenuated by two to four orders of magnitude due to the presence of dislocations. We also find that pipe diffusivity is vanishingly small along screw dislocations and demonstrate that dislocations do not provide fast diffusion pathways for hydrogen as is sometimes supposed. We make contact between our simulations and the predictions of Oriani's theory of effective diffusivity', and find that local equilibrium is maintained between lattice and trap sites. We also find that the predicted effective diffusivity is in agreement with our simulated results in cases where the distribution of traps is spatially homogeneous; in the trapping of hydrogen by dislocations where this condition is not met, the Oriani effective diffusivity is in agreement with the simulations to within a factor of two.