Analysis of Hydrogen Evolution and Entry into Metals for the Discharge-Recombination Process

摘要

A mechanistic model has been developed which for the first time considers the effect of hydrogen entry into a metal on the kinetics of the hydrogen evolution reaction (h.e.r.). The model enables computation of: 1) the hydrogen surface coverage and surface concentration; 2) the hydrogen adsorption, absorption, discharge and recombination rate constants; and 3) the h.e.r. coverage-dependent transfer coefficient, alpha and the exchange current density io, from a knowledge of the steady state hydrogen permeation current, cathodic charging current, hydrogen overvoltage and hydrogen diffusivity. The model predicts a linear relationship between permeation flux and square-root of the hydrogen recombination flux and provides an analytical method to determine the cathodic potential range for operation of a coupled discharge-recombination mechanism of the h.e.r. With modifications the model can treat permeation data for which 1) the mechanics of the discharge step involve a (proposed) selvedge reaction; and 2) surface hydrogen coverages are relatively high as in the presence of poisons (e.g., H2S or As2O3). Some of the existing literature data for hydrogen permeation in iron and nickel in acid and alkaline solutions are successfully analyzed. Keywords: Electrochemistry; Hydrogen permeation model; Surface coverage; H.E.R. rate constants; Selvedge reaction; Exchange current; Hydrogen overvoltage; Hydrogen absorption and adsorption rate constants. (edc)