Evolution and Entry of Hydrogen into Iron during Cathodic Charging in Alkaline Solution with Ethylenediaminetetraacetic Acid

Abstract

Hydrogen permeation and electrochemical impedance were measured on iron membranes cathodically polarized at a constant current in 0.1 M NaOH solution without and with ethylenediaminetetraacetic acid (EDTA). Auger electron spectroscopy and scanning electron microscopy (SEM) were used to examine the surface. During the first few hours of charging, the hydrogen permeation rate (i p ) attained a quasi-steady low level, the electrode potential (E) shifted in the negative direction, the charge-transfer resistance (R ct ) increased, and the double-layer capacitance (C d ) slightly decreased. Later i p strongly increased with simultaneously decreasing E and increasing R ct ; C d increased only in the absence of EDTA. The i p reached a maximum after about 3 days of an uninterrupted charging and then decreased with time, accompanied by reversed changes in E and R ct , and further increase in C d . Electrochemical parameters were associated mainly with the hydrogen evolution reaction (HER). They indicated that the hydrogen entry into iron increased with increasing overpotential and charge-transfer resistance of the HER. It is suggested that the rise in E and R ct resulted from the increasing hydrogen coverage θ H of the bare metal surface. The persistent increase in C d might result from the formation of a new surface layer which can be composed of disintegrated iron and its oxides. Surface oxides facilitated HER, but they hindered the hydrogen entry. Addition of EDTA strongly accelerated the hydrogen entry, mainly due to the removal of oxides. Variations in the hydrogen entry are explained in terms of θ H , surface oxides, metal disintegration, enlargement of the surface area, and exposure of bare metal.