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...

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This work investigates the intergranular corrosion of a sensitized Type 430 stainless steel in 1N H2SO4 . Once the grain boundary groove is formed by dissolution of the Cr‐depleted material, a second form of localized corrosion commences within minutes and replaces the Cr‐depletion mechanism. The second‐mechanism attacks both the bulk grains (of normal Cr content) and the Cr‐depleted alloy. This is shown by groove widths that are much larger than the Cr‐depleted zone widths. Gas bubbles, deduced to be hydrogen, egress from the grain boundary grooves, indicating a sizable potential drop within the grooves and the likelihood that the second corrosion process is caused by the IR phenomenon recently found to account for crevice corrosion in iron. The IR mechanism could also account for the observed corrosion under the lacquer at the sample edges.