Effect of Substitutional Alloying Elements on the Diffusion Coefficient of Hydrogen in α-Iron

Abstract

Diffusion of hydrogen has been studied in iron alloys with substitutional elements which act as trapping sites. With the assumption of local equilibrium for hydrogen distribution and the thermal averaging for the jump rate of the hydrogen atom, the apparent diffusion coefficient of hydrogen is derived by considering the interaction energy E_B1 between a dissolved hydrogen atom and a substitutional alloying element, and the change ΔE in the energy of the saddle point between the trapping site and the neighboring lattice site.
Diffusion coefficients of hydrogen in annealed poly-crystalline specimens of Fe-(0.50∼3.81 at%)Ni, Fe-(0.39∼4.67 at%)Cr, and Fe-(0.59∼4.63 at%)Co alloys have been measured by the electrochemical permeation method in which alcoholic solutions have been used as electrolytes at temperatures between 230 and 300 K. The experimental permeation curves were analyzed by considering the variation of the hydrogen concentration beneath the cathodic surface during the measurements. The observed temperature dependence of diffusion coefficient of hydrogen can be explained using the following values; ΔE\simeq0, E_B1\simeq8.4 kJ/mol for Fe-Cr alloys and ΔE\simeq4.2 kJ/mol, E_B1\simeq7.6 kJ/mol for Fe-Ni and Fe-Co alloys.