Transient hydrogen permeation through iron after potential jumps from cathodic to anodic polarisation in NaOH solution

Abstract

This study is aimed at achieving a better understanding of the enhanced entry of hydrogen into iron upon renewal of its surface. The hydrogen permeation rate (HPR) through a 35-μm thick iron membrane was measured in 0.1 M NaOH at 25 °C with the electrochemical technique at the acquisition rate of 10 4 s −1. Fast potential jumps from cathodic to anodic polarisation caused a transient HPR rise which occurred when anodic current was decaying. Partial cathodic and anodic currents were estimated using the HPR vs. i c 1 / 2 dependence. They showed that hydrogen evolution induced an acceleration of anodic processes. It was evaluated that HPR continued to rise till the thickness of an assumed Fe(OH) 2 film was about 0.9–6.5 nm depending on the potential. The HPR rise can be ascribed to acidification and to the catalytic effect of Fe–O species on the evolution of hydrogen and its entry. It is suggested that the enhanced hydrogen entry upon renewal of the metal surface results from an accelerated anodic oxidation of the metal rather than accelerated hydrogen evolution directly on the bare surface.