  Structural defects in materials such as vacancies, grain boundaries, and
dislocations may trap hydrogen and a local accumulation of hydrogen at these
defects can lead to the degradation of the materials properties. An important
aspect in obtaining insight into hydrogen induced embrittlement on the
atomistic level is to understand the diffusion of hydrogen in these materials.
In our study we employ kinetic Monte Carlo (kMC) simulations to investigate
hydrogen diffusion in bcc iron within different microstructures. All input data
to the kMC model, such as available sites, solution energies, and diffusion
barriers are obtained from first-principles calculations. We find that hydrogen
mainly diffuses within the interface region with an overall diffusivity that is
lower than in pure bcc-Fe bulk. The concentration dependence of the diffusion
coefficient is strongly non-linear and the diffusion coefficient may even
decrease with increasing hydrogen concentration. To describe the macroscopic
diffusion coefficient we derive an analytic expression as a function of
hydrogen concentration and temperature which is in excellent agreement with our
numerical results for idealised microstructures.
