On the Role of Buffers and Anions in NiFe Electrodeposition

Hydrogen evolution and buffering, which are important processes in deposition, have been investigated in solutions containing no Ni 2+ or Fe 2+ ions. Limiting currents for evolution are found to be significantly higher in solutions of than in and solutions at the same p H. This is shown not to be due to buffering by , but must be caused by differences in . Boric acid does not act as a buffer during H + reduction in these systems, but does reduce the overpotential for discharge. The limiting current is actually decreased in solutions by addition of ; it is postulated that this molecule can adsorb and decrease the active surface area. In solutions, has no effect on . Citric acid is an effective buffer during H + discharge. Surface buffering during evolution is not directly related to the p K of the acid; the kinetics of dissociation appear to be the crucial factor. Saccharin has little effect on the hydrogen overpotential on Ni electrodes, but causes a small concentration‐independent increase in in Cl − solutions. Overpotentials for deposition of Ni, Fe, and are lower in chloride than in sulfate solution, perhaps due to catalysis through a Cl − bridge formed between the electrode and the metal cation. Chloride thus increases efficiency. Boric acid shows no effect on linear potential sweep curves during deposition from Cl − solution, but addition of substantially increases the current efficiency. Boric acid decreases currents during linear potential sweeps in solutions, probably by decreasing the active electrode area through adsorption. The visual appearance of the deposit is improved by . The adsorption of is apparently quite important in determining deposit properties. The action of Fe 2+ in inhibiting Ni deposition is similar in and Cl − solutions. A crossover in the linear sweep curves for Ni and appears at approximately the current density where the Fe content of the deposited film is maximum.