Electrodeposition Behavior of Zn-Ni Alloy from Alkaline Zincate Solutions Containing Various Brighteners and its Microstructure

Abstract

The effect of brighteners on the deposition behavior of Zn–Ni alloys ant its microstructure was investigated. Zn–Ni alloys were electrodeposited on Cu electrode at 10–5000 A·m−2, 105 C·m−2, 308 K in unagitated zincate solutions containing various brighteners. Although the degree of suppression of hydrogen evolution was different depending on the kind of brightener, the transition current density at which the deposition behavior shifted from normal to anomalous was almost same in solutions containing brighteners. The current efficiency for alloy deposition significantly decreased with an addition of brighteners which had suppression effect on the Zn deposition. Since the brighteners more suppressed the Ni deposition than Zn deposition, the Ni content in deposited films decreased with an addition of brighteners. When the brightener of a straight-chain polymer composed of a quaternary ammonium cation (PQ) which can suppress the diffusion of ZnO22− and Ni ions in solution was added, the Ni content in deposited films increased with increasing current density at high current density region. This is attributed to that Zn which is preferentially deposited over Ni earlier reached the diffusion limitation of ZnO22− ions and Ni deposition didn’t reach the diffusion-limited current density. When both PQ and a quaternary ammonium salt with a benzene ring were added in solution, the films obtained at the diffusion-limited current density of ZnO22− ions exhibited the smooth surfaces composed of fine crystals. With an addition of brighteners to increase the overpotential for deposition, the γ-phase (intermetallic compound of Ni2Zn11) of the deposited films was formed easily.