A theoretical study is presented of the effects of bubbles attached to the surface of a gas‐evolving electrode, with emphasis on their influence on the local current distribution and on the potential drop at the electrode. The mathematical model accounts for the combined influence of (i) ohmic obstruction within the electrolyte, (ii) area masking on the electrode surface, which raises surface overpotential by increasing the effective current density, and (iii) decreased local supersaturation, which lowers the concentration overpotential. The electrolytic transport is described by potential theory, and the dissolved gas is assumed to obey steady‐state diffusion within a concentration boundary layer. The coupled field equations are solved numerically using the boundary‐element method. The model is applied to hydrogen evolution in potassium‐hydroxide solution. For gas evolution in the Tafel kinetic regime, the current distribution is nearly uniform over the unmasked electrode area, and the increase in surface overpotential is the dominant voltage effect. However, outside the Tafel regime (e.g. on cathodes of greater catalytic activity) the current density is strongly enhanced near the bubble‐contact zone, and the supersaturation‐lowering effect is quite strong, largely offsetting the ohmic and surface‐overpotential effects. Proceeding from a set of base conditions, we perform a systematic examination of attached‐bubble effects, their relative importance, and their dependence on system variables.
Agitation effects on
normalNiFe
electrodeposition have been systematically investigated using a rotating ring‐disk electrode. Alloy compositions and bath current efficiencies have been shown to vary widely over the range of plating current densities and rotation speeds used. An analysis of partial currents as a function of electrode potential has shown that the Fe deposition reaction is mass‐transport controlled at sufficient cathodic potentials, and that the diffusivity of the Fe2+ ion is the same for both the reduction to Fe0 and the oxidation to Fe3+. In agreement with the observations of Dahms and Croll, the Ni deposition reaction is inhibited when Fe is codeposited. This inhibition effect, manifest as a cathodic shift in the Ni polarization curve, increases in magnitude with increased agitation. The dependence of Ni inhibition on agitation is seen only under conditions at which Fe deposition is mass‐transfer influenced, which suggests that Ni inhibition is primarily dependent on the flux of Fe2+ to the electrode surface. The rate of hydrogen evolution during
normalNiFe
deposition shows the same dependence on potential and agitation as it does in a Ni2+‐ and Fe2+‐free bath on a
normalNiFe
substrate.
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