Effects of absorption and desorption on hydrogen permeation - II. experimental measurements of activation energies

Electrochemical permeation experiments on fully-annealed-commercially-pure iron samples with two group samples, group I samples with plasma cleaning and pre-sputtering and group II samples without it, were conducted at a temperature range of 25-80°C. The diffusion coefficient, desorption rate and absorption parameter were determined from fitting the permeation fluxes with the new model proposed in Part I of this series. Being independent of surface treatment, the diffusion coefficient of hydrogen is given as follows: D = D₀ exp(-Q/RT), D₀ = 7.93 × 10^-4 cm²/s, Q = 7340 J/mol. If the diffusion activation energy was evaluated by the time-lag model, the diffusion activation energy for group II samples is 22 970 J/mol, three times higher than the value from the present model. The results also show that the energy barrier for desorption is lower in group I samples compared with that in group II samples. The activation energy of desorption for each of the two group samples is higher than the activation energy of diffusion. As a direct result, the ratio of drift velocities increases with increasing temperature and makes the time-lag method appropriate at elevated temperatures.