The diffusivity of hydrogen in nickel

Abstract

The diffusivity of hydrogen in nickel was determined at 380°C to 1000°C from nonsteady rates of hydrogen evolution from 0.3 to 0.5 in. dia. cylinders. The specimens were prepared by heating in H 2 at various temperatures above 600°C and pressures above 1 atm. Evolution rates into a vacuum were determined from 1. (1) the rate of pressure increase in a known collecting volume, and 2. (2) the pressure drop across a constriction in the gas transfer line. The measured diffusivity D varies only with temperature, indicating 1. (1) that the evolution rate is diffusion-controlled, and 2. (2) that the diffusivity is independent of both solute concentration and method of specimen preparation. The results are well represented by D = 0.00447 cm 2 sec −1 exp(−8600 cal/ RT). A combination of the present D 0 value with the theoretical expression of Wert and Zener for lattice diffusion yields a negative entropy of activation Δ S. This is explained by the hypothesis that the diffusing hydrogen particles do not perceptibly distort the metal lattice, and that Δ S is determined mainly by the change of vibrational frequency of the diffusing particles as they jump between adjacent sites. The value of Δ S calculated from available charge density data is negative and agrees sufficiently well with the experimental Δ S value to confirm strongly the hypothesis of interstitial lattice diffusion.