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

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 ( i ) the hydrogen surface coverage and surface concentration; ( ii ) the hydrogen adsorption, absorption, discharge and recombination rate constants; and ( iii ) the h.e.r. coverage‐dependent transfer coefficient, α, and the exchange current density , 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 the permeation flux and the 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 ( i ) the mechanics of the discharge step involve a (proposed) selvedge reaction, and ( ii ) surface hydrogen coverages are relatively high as in the presence of poisons ( e.g. , or ). Some of the existing literature data for hydrogen permeation in iron and nickel in acid and alkaline solutions are successfully analyzed.