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 cond ucted at a temperature range of 25-80 degrees C. The diffusion coeffic ient, desorption rate and absorption parameter were determined from fi tting the permeation fluxes with the new model proposed in Part I of t his series. Being independent of surface treatment? the diffusion coef ficient of hydrogen is given as follows: D = D-o exp(-Q/RT), D-o = 7.9 3 x 10(-4) cm(2)/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 valu e from the present model. The results also show that the energy barrie r for desorption is lower in group I samples compared with that in gro up 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 increasin g temperature and makes the time-lag method appropriate at elevated te mperatures. (C) 1998 Acta Metallurgica Inc. Published by Elsevier Scie nce Ltd. All rights reserved.