Comparative H diffusion measurements were performed on Armco Fe and on Fe-Cr alloys in 0.1M NaOH solution. The results revealed a marked superiority of alternating-current techniques over direct-current ones. The lattice diffusion of H could be determined in all 3 materials on the basis of the phase difference between the input and the output currents for 1mm membranes at frequencies ranging from 10 to 40mHz. The H diffusivities which were measured at room temperature were equal to  6.2 x 10-5, 5.5 x 10-5 and 2.8 x 10-5cm2/s for annealed Armco-Fe, Fe-3%Cr and Fe-5%Cr, respectively. The Gibbs free energy which bound H atoms to Cr atoms was estimated from the latter 2 values. Depending upon the calculation method, the free energy values which were obtained ranged from -9.3 to -14.4kJ/mol. At frequencies below 10mHz, using 0.3mm membranes, the diffusivity values were too low. These were due mainly to surface effects at the entry side of the membranes.

A Study of the Diffusion and Trapping of Hydrogen in Fe-3Cr and Fe-5Cr Alloys. F.Veniali, Z.Szklarska-Smialowska: Materials Chemistry and Physics, 1986, 15[6], 545-57