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

Abstract

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, ~, 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 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 dischargerecombination 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., H2S or As2OD. Some of the existing literature data for hydrogen permeation in iron and nickel in acid and alkaline solutions are successfully analyzed.Hydrogen is one of the most damaging species in metals, causing hydrogen assisted cracking, blisters, and similar phenomena. Hydrogen entry into metals from aqueous solutions has been studied extensively by hydrogen permeation experiments of thin samples using the DevanathanStachurski cell (1-7). Some studies carried out on iron in acid and alkaline solutions (2, 4, 6) reveal a coupled discharge-recombination mechanism for the hydrogen evolution reaction (h.e.r.) with diffusion of absorbed hydrogen into the metal being rate controlling for the permeation process. The models based on this mechanism predict a square-root relationship between the hydrogen charging current and the steady-state hydrogen permeation flux or current (2). In all of these models, it has been assumed that the hydrogen permeation flux is negligible. It has also been assumed that the hydrogen coverage (Os) is quite low, although the coverage itself is unknown. However, especially when poisons such as H2S or As~O3 are present in the solution, both the permeation current and the hydrogen coverage can be appreciable (8,9). If the steady-state permeation current (i~) is proportional to the square root of the charging current (and the Tafel slope is -120 mV decade -~ and d~l/d log i~ is -240 mV decade -~ where ~1 is the overpotential for the h.e.r.), the h.e.r, is generally considered to follow a coupled diseharge-reeombinati0n mechanism. Apart from these considerations which have not produced operative models yielding the relevant parameters, e.g., 0s, the current models also do not explain the mechanics of the intermediate reaction between adsorption and absorption of hydrogen in metals, which is probably correctly assumed in many eases, but perhaps not in all, to be in equilibrium at the cathode surface.T...

Evaluation of Hydrogen Absorption into Iron by Alternating Current Responses in an Electrochemical Hydrogen Permeation Cell

“…19) As reported in the literature, 7,20,21) the relationship between the surface coverage of adsorbed H atoms, θ, and electrode potential, E, can be expressed by Eq. (7).…”

FEM Analysis for Sinusoidal Perturbation of Hydrogen Permeation into a Steel Sheet

“…7) They are considered to be always in an equilibrium state. During hydrogen permeation measurement with a DS cell, therefore, hydrogen concentrations c entry and c exit at the entry side interface and the exit side interface, respectively, in a mass-transport controlling process can be expressed as follows: where z is a coordinate in the direction of sheet thickness.…”

Development of Novel Process for the Deposition of Nanostructured Ternary Alloys and Composites for Replacement of Cadmium Coatings

“…Several models have been published in literature to explain the hydrogen entry into metals and to evaluate the kinetic and permeation parameters required to explain the process [53][54][55][56][57]. The Tafel slope and dri/dlog(ia) relationship that is obtained for any particular system determines the selection of an appropriate model that can be used to evaluate the kinetics parameters for hydrogen permeation.…”

“…The objective of this work is to determine the hydrogen permeation properties of the Ni-Zn-P deposits and compare it with that of Zn-Ni and Cd deposits obtained from commercial baths. Several models have tried to explain hydrogen entry into metal substrates and to evaluate kinetic parameters of hydrogen evolution and permeation based on experimental data [53][54][55][56][57][58]. The most significant of them is the model proposed by Iyer et al [56].…”

“…Several models have tried to explain hydrogen entry into metal substrates and to evaluate kinetic parameters of hydrogen evolution and permeation based on experimental data [53][54][55][56][57][58]. The most significant of them is the model proposed by Iyer et al [56]. However, an important assumption used in this model is the low hydrogen coverage, and cannot be applied when the hydrogen coverage is high.…”