The permeation of hydrogen in steel in the presence of acid gases is not a simple phenomenon as the
steel may contain trapping sites and also because the permeation may be governed by surface
reactions associated with corrosion. Recently, hydrogen permeation experiments carried out at
corrosion potential have shown a constant flux for various membrane thicknesses in the range 0.05 –
0.8 mm. This is in apparent contradiction with the Fick's laws of diffusion, predicting a diffusion flux
inversely proportional to the thickness of the membrane. Such results confirm the possibility for
permeation to be governed by surface reaction and not by the diffusion through steel: this situation
corresponds to a thin membrane case.