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

摘要

Hydrogen entry into iron from NaOH solutions and from 0.05 M H{sub}2SO{sub}4 during voltammetric cycling was studied with the electrochemical permeation technique. Measurements were carried out on 3.5-μm thick membranes at 25℃. 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 H{sub}2SO{sub}4. In cathodic scans the HPR peak occurred at potentials of an oxide reduction, probably of Fe{sub}3O{sub}4 to Fe(II), whereas in anodic scans this peak appeared at potentials of the oxidation of iron to Fe(II) and to Fe{sub}3O{sub}4, and it was distinct especially at high scan rates. X-ray photoelectron spectroscopy (XPS) surface analysis indicated that the fraction of Fe{sub}3O{sub}4 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 Fe{sub}3O{sub}4 without or with Fe(II). Possibly, these species might be effective by hindering the recombination of H{sub}(ads) or by blocking adsorption sites.