Direct and pulse plating of metastable Zn–Ni alloys

Abstract

Zn–Ni alloys having compositions in the range 14–18 wt-% are electrodeposited using direct current DC and pulse current PC in a cyanide-free alkaline electrolyte. Homogeneous and compact layers, suitable for corrosion protection, are obtained. Contrary to the common acceptance, X-ray diffraction spectra and differential scanning calorimetry thermal analysis show that the electrodeposited γ alloy by DC plating in electrolytes with complexant-additive agents has to be regarded as a metastable phase, whose atomic arrangement is different from that of the equilibrium γ intermetallic compound. A model for atomic distribution and the Gibbs free-energy function for the DC electrodeposited phase are discussed. It is found that PC deposited alloys from additive-free electrolytes present the same metastable behaviour attributed to the DC plated homologues in electrolytes with complexant-additive agents. The corrosion behaviour is found to be dependent on the duty cycle applied during PC deposition, as evidence of the different microstructures obtained in varying the parameters.

Keywords:

• Zinc
• Nickel
• γ phase
• Reverse pulse current
• Corrosion

Acknowledgement

This paper is based on a presentation given at the 6th European Pulse Plating Seminar, Baden, Austria, 7th March 2014.