Effect of near-surface pH on electrodeposition of nickel

Near-surface pH was measured during Ni electrodeposition from NiCl2 solutions. As NiCl2 concentration was increased, the usually observed increase of surface pH was suppressed due to hydrolysis of Ni+2 at low potentials and due to precipitation of hydroxides at high cathodic potentials. The role of boric acid was examined both during H2 evolution in NaCl and during Ni deposition. In NaCl solution, boric acid does not affect the H+ discharge limiting current, however it decreases the overpotential for the H2O decomposition reaction. Due to the high pK, values of boric acid, hydrolysis proceeds in the potential region of water decomposition. In general, a boric-acid-containing Ni solution gives lower surface pH values than a solution containing only NiCl2. Furthermore, the current efficiency of electrodeposition increases with boric acid in the Ni solution. Thus, it is suggested that boric acid works with the Ni species either as a catalyst or as a weak complexing agent, resulting in an enhanced Ni discharge rate.