Hydrogen Sulfide Effect on Hydrogen Entry into Iron—A Mechanistic Study

Abstract

The recently developed I-P-Z model is modified in order to analyze the observed enhanced permeation of hydrogen that occurs in the presence of hydrogen sulfide during cathodic hydrogen charging of iron. The modification accounts for the fact that the energy of adsorption becomes coverage dependent at the higher coverages and affects the hydrogen evolution reaction (HER) in the presence of H2S. Charging experiments were performed on Ferrovac E–iron membranes 0.5 mm thick using a Devanathan–Stachurski cell in deaerated, pre-electrolyzed solutions made from 0.1 M HClO4 and 0.1 M NadO4 with pH values of 1 and 2. The transfer coefficient, α, exchange current density, i0, thickness-dependent absorption-adsorption rate constant, k″, recombination rate constant, k3, surface hydrogen coverage, θH, and discharge rate constant, k1o were obtained by application of the model to the experimental results. As a result, the role of H2S has been clarified. While θH is increased in the presence of H2S, the overpotential, η, is decreased consistent with an observed increase in a, and the increased H entry is found to be the result of a decreased k3 as well as the increased α. In addition, a very important relationship has been derived that will enable the calculation of the absorption, kabs, and adsorption, kads, rate constants from the electrochemical permeation results for different membrane thicknesses.