The evolution of H from both fine- and large-grained decarburized mild steel specimens (0.002C, 0.03-0.07Si, 0.53Mn) was monitored. The grain-boundary areas of the 2 types of specimen were in the ratio of 25 to 1, but the results indicated that grain boundaries were not responsible for the anomalously low diffusion coefficients which governed H evolution from the steel. The results could be described by:

large-grained: D (cm2/s) = 1.38 exp[-35.74(kJ/mol)/RT]

small-grained: D (cm2/s) = 1.34 exp[-35.66(kJ/mol)/RT]

B.Chew, F.T.Fabling: Metal Science Journal, 1972, 6, 140-2