Abstract
The diffusion of electrolytic hydrogen through membranes of pure iron, and two iron-carbon alloys has been investigated. In the initial experiments diffusion coefficients for electrolytic hydrogen in iron membranes have been obtained using a refined time-lag technique. Results showed an initial fast diffusion followed by a secondary slower stage. The initial stage yields diffusion coefficients which agree closely with extrapolated high temperature values (2 to 5.5 x 10-5 cm2 sec-1); the second stage yields considerably lower coefficients (1 to 3 x 10-6 cm2 sec-1) which vary with membrane thickness. Results indicate the formation of a barrier to hydrogen flow as diffusion proceeds. Calculation of the effective barrier thickness enables the correction of apparent coefficients reported in the literature and quantitatively predicts the anomalous thickness effect. Further experiments using a more sophisticated experimental technique yield electrochemical constants for the hydrogen discharge and entry mechanism (transfer coefficients alpha1=3/4 and alpha2 =1/2). Overpotential measurements indicate that the barrier may be identified with the formation of adsorbed hydrogen on the membrane surface. Reaction of oxygen with the adsorbed layer is responsible for occurrence of the secondary diffusion effect. The temperature dependence of the diffusion coefficient and permeability has been determined as: D= 4. 91 x 10 -4 exp (-1450/RT) cm2 sec and D = 1.3x10-3 exp (-5600/RT) cm2 sec-1, respectively. Diffusion coefficients determined the conditions of polarisation were found to vary with the overpotential and this therefore yields data on forced diffusion (mu=-4. 25 x 10-4 cm2/sec/volt). From the value of mobility an attempt has been made to calculate the charge of the diffusing species Permeation results indicate that in the absence of an adsorbed layer, the electrolytic hydrogen enters the metal In the same elementary form (or act) in which it is discharged. The mechanism was found to change with time and with the overpotential. Adsorption and forced diffusion allows a satisfactory explanation of the detailed variation of diffusivity and permeation with overpotential.