On the Role of Buffers and Anions in NiFe Electrodeposition

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Hydrogen evolution and buffering, which are important processes in NiFe deposition, have been investigated in solutions containing no Ni2+ or Fe2+ ions. Limiting currents for H2 evolution are found to be significantly higher in solutions of Na2SO4 than in NaCl and NaC104 solutions at the same pH. This is shown not to be due to buffering by HSO4-, but must be caused by differences in Dh +. Boric acid does not act as a buffer during H+ reduction in these systems, but does reduce the overpotential for H20 discharge. The H2 limiting current is actually decreased in Na2SO4 solutions by addition of H3BO3; it is postulated that this molecule can adsorb and decrease the active surface area. In NaCl solutions, H3BO3 has no effect on i1- Citric acid is an effective buffer during H+ discharge. Surface buffering during H2 evolution is not directly related to the pK 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 ii in Cl-solutions. Overpotentials for deposition of Ni, Fe, and NiFe 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 NiFe deposition from Cl- solution, but addition of H3BO3 substantially increases the current efficiency. Boric acid decreases currents during linear potential sweeps in SO42- solutions, probably by decreasing the active electrode area through adsorption. The visual appearance of the deposit is improved by H3BO3. The adsorption of H3BO3 is apparently quite important in determining deposit properties. The action of Fe2+ in inhibiting Ni deposition is similar in SO42- and Cl- solutions. A crossover in the linear sweep curves for Ni and NiFe appears at approximately the current density where the Fe content of the deposited film is maximum. © 1979, The Electrochemical Society, Inc. All rights reserved.


09 Dec 2019