The H permeation behaviour in austenitic stainless steels (1.4301, AISI304) was studied between 25 and 85C by using the electrochemical double-cell technique. Apparent diffusion coefficients were deduced by using the current-transient plots from permeation experiments. Values which ranged from 5.8 x 10-12 to 2.7 x 10-10cm2/s were found; depending upon the condition of the material. An increase in the C-content, from 0.045 to 0.085wt%, resulted in an 8-fold decrease. This was attributed to the increased blocking of interstitial sites by the extra C atoms. Annealing resulted in slight grain growth, and caused a 50% decrease in diffusivity. This was attributed to the longer H-transport paths which were required through the grain, and the lower contribution which was made by fast diffusion paths in the larger grain-size material. Slight cold-working (5% reduction in thickness) decreased the diffusivity, whereas greater (30%) cold-working increased the diffusivity. The cold working resulted in an increase in dislocation density, and the dislocations acted as traps for H transport. This resulted in a decrease in the diffusivity values for 5% cold-worked material. Strain-induced martensite formation in the matrix occurred in more cold-worked material (10 or 30%). The presence of this body-centered cubic phase overcame the trapping effect of an increased dislocation density; thus resulting in an enhancement of H transport. On the basis of the calculated activation energy, it was concluded that the α' phase did not provide a continuous medium for H transport, but added to the overall increase in H transport.

R.K.Dayal, H.J.Grabke: Steel Research, 2000, 71[6-7], 255-60