Impedance characterization of the activation of iron surface for hydrogen entry from alkaline solution

Simultaneous hydrogen permeation and impedance measurements were carried out on iron membranes cathodically polarized at a constant current in a NaOH solution. A prolonged cathodic polarization of iron made the metal surface considerably more active for the hydrogen entry. The hydrogen permeation rate (i(p)) reached a maximum after about three days of an uninterrupted charging and then decreased with time. As i(p) rose, the electrode potential (E) shifted in the negative direction, the charge transfer resistance (R(ct)) increased, and the differential double layer capacitance(C-d) decreased. The rise in i(p) and R(ct) with the accompanying drop in E and C-d can be attributed to the reduction of the air-formed oxides and disintegration of the superficial metal layer leading to the increase of the surface area and the formation of a new surface layer. The reverse changes in i(p), R(ct), E and C-d can be due to the thickening of the newly formed layer and its chemical and/or structural changes. This layer was probably composed of disintegrated iron and hydrated iron oxides.