KINETIC RATIONALIZATION OF CATALYST POISON EFFECTS ON CATHODIC H-SORPTION INTO METALS - RELATION OF ENHANCEMENT AND INHIBITION TO H-COVERAGE

The absorption and permeation of H into transition metals under condit ions of cathodic polarization or wet corrosion is commonly enhanced by adsorbed catalyst poisons which, however, decrease the coverage by ad sorbed H, the intermediate in the hydrogen evolution reaction. This un expected direction of behaviour has remained inadequately explained. D epending on the species of poison, H absorption can, in fact, be promo ted or inhibited. Effects of As-containing species on H permeation int o Fe or mild-steel cathodes are encountered in the technology of elect rolytic F-2 production from KF . 2HF (at 85 degrees C). In the present work, such effects are studied comparatively vis-a-vis those at the s ame electrodes in aqueous NaOH, using a Devanathan-Stachurski bi-elect rode membrane cell to follow H permeation currents in relation to simu ltaneously measured polarization behaviour on the H-2-evolution sides of the electrodes. By means of an equation-fitting procedure applied t o the kinetics of all the constituent partial reactions involved, incl uding cathodic reduction of added AsO2- to As-0 and AsH3, a good accou nt of the mechanism of the effects of As-species can be given. It invo lves reductive desorption of As-containing species to AsH3 (gas) coupl ed with increased H coverage at high overpotentials, leading to enhanc ement of absorption and permeation. A critical factor is the interfaci al sub-surface concentration (C-0) of H in relation to the surface-cov erage fraction, theta(H), of adsorbed H on the surface of the electrod e.