Diffusion of Hydrogen

Abstract

This chapter deals with the diffusion properties of hydrogen in zirconium as driven by mass, stress, and thermal gradients. The relevant diffusion equations are derived starting from the phenomenological theory of irreversible thermodynamics. A general expression for the flux of hydrogen in mass (concentration), stress, and temperature gradients is derived. The derived general equation forms the basis of specific flux relations for diffusion of hydrogen under thermal gradients to cold spots and under stress gradients to regions of elevated tensile stress. The latter provides the basis for the DHC growth equation derived in Chap. 10. The flux of hydrogen depends on the diffusion coefficient for hydrogen and, for thermal diffusion, also on the heat of transfer. Experimental results for the heat of transfer and the diffusion coefficient of hydrogen in zirconium and its alloys are summarized. A theoretical treatment is given for the effect on the diffusion rate of fast diffusion channels such as β phase stringers in α/β zirconium alloys and of hydrogen/hydride trapping at dislocations.