Accelerated entry of hydrogen into iron from NaOH solutions at low cathodic and low anodic polarisations

Hydrogen entry into iron from NaOH solutions and from 0.05 M H2SO4 during voltammetric cycling was studied with the electrochemical permeation technique. Measurements were carried out on 35-μm thick membranes at 25 °C. Accelerated entry of hydrogen was revealed by peaks of hydrogen permeation rate (HPR) which occurred at low cathodic and low anodic polarisations in NaOH solutions, but not in 0.05 M H2SO4. In cathodic scans the HPR peak occurred at potentials of an oxide reduction, probably of Fe3O4 to Fe(II), whereas in anodic scans this peak appeared at potentials of the oxidation of iron to Fe(II) and to Fe3O4, and it was distinct especially at high scan rates. X-ray photoelectron spectroscopy (XPS) surface analysis indicated that the fraction of Fe3O4 in the surface film during cathodic scans was at the potential of HPR peak significantly larger than that at a nobler potential. It is suggested that the occurrence of HPR peaks can be due to a partial removal of surface layers and to the formation of species promoting the hydrogen entry, supposedly Fe3O4 without or with Fe(II). Possibly, these species might be effective by hindering the recombination of Hads or by blocking adsorption sites.