Atomic diffusion in alloys exposed to liquid sodium

Reasons for the observed enhancement of atomic diffusion coefficient in alloys exposed to sodium are explored. The diffusion coefficient in the presence of a chemical potential gradient is derived from a simple atomistic model. It is concluded that two mechanisms could contribute to the enhancement. (a) A non-zero gradient of partial molar enthalpy. (b) An increase in the atomic mobility due to the presence of high diffusivity paths. It is shown that the effect of (a) is not of sufficient magnitude to account for the increase in coefficient. However the results can be analysed successfully in terms of grain boundary diffusion and the coefficients are determined for iron in Ni/Cr (1.2x10-9cm2/s) at 6500C and nickel in iron (5x10-8cm2/s) at 7300C. The total absorbed diffusant is also calculated for the case of iron into Ni/Cr, and a general expression for the absorbed diffusant is given. (Auth.)