Changes in hydrogen entry rate and in surface of iron during cathodic polarisation in alkaline solutions

Abstract

The hydrogen permeation rate was measured on iron membranes during cathodic polarisation under galvanostatic conditions in 0.1 M NaOH without and with a complexing agent, ethylenediaminetetraacetic acid (EDTA). Surface topography was examined ex situ by atomic force microscopy (AFM) and concentration–depth-profiles were determined by Auger electron spectroscopy (AES). The rate of hydrogen entry into iron initially strongly increased and then decreased with time. AFM images indicated a sharpening of polishing grooves and later the formation of a stalagmite-type deposit. AES showed the presence of oxygen in the surface film. The topographic features can be explained by the cathodic reduction of air-formed oxides and by deposition of iron and/or oxygen-containing species. The initial fast increase in the hydrogen entry rate might be associated with the rising surface coverage θ H and with weakening of the bonds of the adsorbed hydrogen to the metal, facilitating the H ads→H abs transfer. It is suggested that the bond weakening might also be caused by the adsorbed oxygen-containing species. The subsequent decrease of hydrogen entry can be attributed to the growth of the iron oxide deposit. In the presence of EDTA the hydrogen entry was faster and the oxide deposit was smaller than for the EDTA-free solution.