The effect of Cr, Cu, Mo, Ni and Si upon diffusion in the a-phase was studied at temperatures of between 160 and 430C, and at a pressure of 1atm, by using absorption methods. The diffusivity in Fe could be described by:

160-340C:     D (m2/s) = 2.38 x 10-7 exp[-9.818(kJ/mol)/RT]

270-430C:     D (m2/s) = 8.05 x 10-8 exp[-9.483(kJ/mol)/RT]

Apart from the cases of Cu and Ni, the results indicated that the H diffusivity decreased and the activation energy increased as the alloying element content was increased. This was attributed to the trapping of H atoms by the alloying elements. The trapping parameters for H in the alloys were calculated by using Oriani's approach. This revealed that the trap energy increased in the order: Si, Mo, Cr (table 37). The presence of Cu and Ni had little effect upon H trapping. The trapping characteristics of the various alloying elements could be explained in terms of chemical affinity and elastic strain effects.

K.T.Kim, J.K.Park, J.Y.Lee, S.H.Hwang: Journal of Materials Science, 1981, 16[9], 2590-6

Table 37

Trapping Energies for H in Fe Alloys

a: first-order interaction coefficient, b: trap energy