Deposition of Nanocrystalline Zinc-Nickel Alloys by D.C. Plating in Additive Free Chloride Bath

Abstract

Nanocrystalline zinc-nickel alloy coatings were electro-deposited in chloride bath containing ZuCl 2 (50-200 gL -1 ), NiCl 2 .6H 2 O (50-200 gL -1 ) and H 3 BO 3 (40 gL -1 ) at45°C. The initial alloy composition, as investigated by voltammetry, chronopotentiometry or EDX analysis, exhibited a pronounced dependence on the composition of plating bath. However, the alloy composition tended to approach an upper limit of 18 atomic percent when deposits grew thicker, for most of the plating bath compositions. The process was clearly the anomalous codeposition, which was considered to be due to the formation of an adsorbed layer of Zn(OH) 2 containing nickel cations, as an intermediate product, whose composition determines the composition of the electro-deposited alloy. The electro-deposited coatings comprised of single-phase structure of γ-phase (Nι 5 Zn 21 ) for about 12-18 atomic percent nickel and a mixture of γ- and η-phases (hcp) for nickel atom percent below 8. The grain size decreased from about 50nm to 20nm with decrease in nickel content away from the stoichiometric composition of Ni 5 Zn 21 along with a decrease in hardness from about 450KHN to 150KHN. The decrease in hardness with a decrease in grain size seems attributable to the grain boundary sliding as predominant deformation mechanism. Anodic dissolution of the alloys involved pronounced dezincification, forming α-phase (fcc) that dissolved at the end.