Diffusion and trapping of hydrogen in carbon steel at different temperatures

The presence of interstitial hydrogen in steel produces embrittlement, which poses a severe risk to the integrity of structural components. Although real steel, as a multi-phase material with crystal defects at several scales is too complex a system to be modelled utilizing ab-initio calculations, mechanisms of hydrogen (H) diffusion in metals have attracted interest and have been widely described in the literature. Here, we study from first-principles (DFT code CASTEP) the role of phonons on the diffusion of hydrogen at different temperatures in a bcc iron lattice (Fe16H) via a calculation of Helmholtz's free energy, which has been fed into COMSOL for finite element calculations. The diffusion coefficient of hydrogen between 250 K and 700 K was obtained and, the increment in the diffusion barrier at the higher temperatures, traditionally attributed to a transition regime, is now explained by the contribution of phonons. The effect of different traps sites on hydrogen diffusion is studied by using the FEM model.