The influence of boric acid and sulfate ions on the hydrogen formation in Ni-Fe plating electrolytes

The current efficiency for electrodeposition of iron group metals and alloys is limited by concurrent hydrogen evolution. In the present paper the effect of dissociation equilibria in the electrolyte on the rate of the hydrogen formation is studied both experimentally and theoretically. Cathodic polarization experiments are performed in sulfate, chloride and perchlorate electrolytes of pH 3 to determine the limiting current for proton reduction. The effect of boric acid on the pH at the cathode surface is studied for sulfate electrolytes with and without nickel. Experimental results and theoretical simulations using a finite difference code were used to identify the relative importance of different electrolyte equilibria for the value of the limiting current for proton reduction and for the apparent onset of the reduction of water. The presence of boric acid leads to a release of protons in the cathodic diffusion layer and as a result, the potential region of proton reduction is extended to more negative potentials. This retards the pH increase at the cathode surface associated with the onset of water reduction. As a consequence, plating of iron group metals can be performed within a wider potential range without interference by hydroxide precipitation. (C) 2000 Elsevier Science Ireland Ltd. All rights reserved.