Kinetic rationalization of catalyst poison effects on cathodic H sorption into metals: relation of enhancement and inhibition to H coverage

Abstract

The absorption and permeation of H into transition metals under conditions of cathodic polarization or wet corrosion is commonly enhanced by adsorbed catalyst poisons which, however, decrease the coverage by adsorbed H, the intermediate in the hydrogen evolution reaction. This unexpected direction of behaviour has remained inadequately explained. Depending on the species of poison, H absorption can, in fact, be promoted or inhibited.

Effects of As-containing species on H permeation into Fe or mild-steel cathodes are encountered in the technology of electrolytic F₂ production from KF·2HF (at 85 °C). In the present work, such effects are studied comparatively vis-à-vis those at the same electrodes in aqueous NaOH, using a Devanathan–Stachurski bi-electrode membrane cell to follow H permeation currents in relation to simultaneously measured polarization behaviour on the H₂-evolution sides of the electrodes. By means of an equation-fitting procedure applied to the kinetics of all the constituent partial reactions involved, including cathodic reduction of added AsO₂⁻ to A and AsH₃, a good account of the mechanism of the effects of As-species can be given. It involves reductive desorption of As-containing species to AsH₃ (gas) coupled with increased H coverage at high overpotentials, leading to enhancement of absorption and permeation. A critical factor is the interfacial sub-surface concentration (C₀) of H in relation to the surface-coverage fraction, θ_H, of adsorbed H on the surface of the electrode.