Study of the input-side subsurface reorganization vs. the outside current density in hydrogen permeation under constant cell voltage through iron membrane according to RHC concept

In this work, hydrogen permeation tests were performed on pure iron membrane in 1 M sodium hydroxide at 298 K, subjected to hydrogen charging under “quasi-potentiostatic” polarization conditions, i.e. constant cell voltage applied between the cathode (membrane entry side) and the anode (counter electrode), which is a typical situation during metal electrodeposition or cathodic degreasing on steel in metal finishing industry.Two consecutive charging–discharging runs were carried out. Prolonged hydrogen charging under quasi-potentiostatic polarization was investigated and the change of cathodic current density (iin)chg(iin)chg and electrode potential (Ein)chg(Ein)chg as well as permeation current density (iout)chg(iout)chg were analysed. Three singularities were underlined for each experiment: (i) the curve (iin)chg(iin)chg = f  ((Ein)chg(Ein)chg), illustrating the inverse of hydrogen charge resistance RHC−1 evolution which was negative, equal to zero and then became positive; (ii) quasi-periodic instabilities during the RHC−1 zero period, probably induced by atomic reorganizing due to subsurface hydrogen insertion in the input-side; (iii) the same ratio (iout)chg/(iin)chg(iout)chg/(iin)chg = −6 × 10−5. During discharge runs, both sides of the membrane were polarized at the same potential (Ein)dischg(Ein)dischg = (Eout)dischg(Eout)dischg = −0.285 V/Hg/HgO/NaOH 1 M and the current densities, (iin)dischg(iin)dischg and (iout)dischg(iout)dischg which corresponded to the desorption rates of hydrogen, were measured. The following correlation (iout)dischg(iout)dischgvs.  (iin)dischg(iin)dischg = −6 × 10−5 was confirmed leading us to introduce the RHC−1 mirror concept to observe the input-side subsurface reorganization by the survey of its potential vs. outside current density during the hydrogen charge.Thus, this RHC−1 mirror concept showed: (i) a non-stop and irreversible progress in the subsurface reorganization during the two permeations; (ii) a probable structural evolution to a stable subsurface structure, the only condition of a real steady-state charge when (Ein)chg(Ein)chg and (iin)chg(iin)chg were stable together, either under potentiostatic or galvanostatic conditions.