Ab-initio approach to study hydrogen diffusion in 9Cr steels

We calculate the equilibrium energies and migration barriers of Fe, Cr and H interstitial defects in α-FeX(X=Cr)$\mathrm{\alpha }\mathrm{-}\mathrm{FeX}(\mathrm{X}=\mathrm{Cr})$. We use the ab-initio electronic structure code, SIESTA, coupled to the monomer method to find activated states (or migration barriers), in order to study atomic defects diffusion. Ab-initio calculations reveal that in the presence of Cr the H migration barriers are higher than in pure α-Fe$\mathrm{\alpha }\mathrm{-}\mathrm{Fe}$. On the other hand, our permeation tests on 9%Cr–91%Fe alloys reveal a permeation coefficient 10 times lower and a diffusion coefficient 200 times lower than in pure, annealed iron. Focusing on our experimental results, we explore very simple model of new H trapping sites and possible migration paths that can explain the experimental observations.