Contents lists available at ScienceDirect Journal of Electroanalytical Chemistry journal homepage: www.elsevier.com/locate/jelechem ElectrochemicalnucleationandgrowthofZn-Nialloysfromchloridecitrate- based electrolyte Rabah Asseli, Mohamed Benaicha ⁎ , Sabrine Derbal, Mahdi Allam, Oualid Dilmi Energetics and Solid-State Electrochemistry Laboratory (LEES), Processes Engineering Dept., Faculty of Technology, Ferhat Abbas-Setif1 University, Setif 19000, Algeria ARTICLEINFO Keywords: Electrodeposition Zn-Ni alloy Nucleation mechanism Citrate bath Cyclic voltammetry Chrono-amperometry ABSTRACT In this work, the efects of deposition potential, [Ni 2+ ]/[Zn 2+ ] molar ratio, pH and bath temperature on the electrocrystallization mechanism of ZneNi alloy from an aqueous solution containing zinc chloride (ZnCl 2 ), nickelchloride(NiCl 2 )andtri‑sodiumcitrateascomplexingagentwerestudied.TheZn–Nialloysweredeposited fromsolutionswithdiferentmolarratios(0.25;1;2;4),pHs(1.5;3;4)andbathtemperatures(20;40;60°C). ScharifkerandHillsmodel,usedtoanalyzethepotentiostaticcurrenttransients,revealedthatinaplatingbath containinglow[Ni 2+ ]/[Zn 2+ ]molarratios(R ≤1),thenucleationmechanismwasinstantaneouswithatypical three-dimensional (3D) growth process, regardless of the bath pH and temperature. In Ni-rich electrolytes (R ≥2), the nucleation becomes more consistent with the theoretical progressive nucleation type. The nuclei numberdensityincreasedwithincreasingtheappliedcathodicpotentialandbathpHanddecreasesforbothof high [Ni 2+ ]/[Zn 2+ ] molar ratios in solution and elevated plating temperatures. The morphological, composi- tional and structural characterizations of the deposits were carried out using Field-Emission Scanning Electron Microscopy(FESEM),WavelengthDispersiveX-rayFluorescenceAnalysis(WDXRF)andX-raydifraction(XRD) respectively. The obtained results revealed that nickel content of the coatings was increased by increasing cathodic potential and [Ni 2+ ]/[Zn 2+ ] molar ratio and ranged between 5.82 and 29.3wt% (6.44–31.59at.%). ZneNi alloys coated at room temperature from an equimolar ([Ni 2+ ]/[Zn 2+ ]=1)bathwerecomposedofa mixtureoftwomajorphases:theδ-phase(Ni 3 Zn 22 )withamonoclinicstructureandtheγ-phase(NiZn 3 )withan orthorhombic structure, while for deposits obtained in Ni-rich bath, the γ-NiZn 3 phase was particularly pre- vailing. A mechanism of anomalous co-deposition of zinc with nickel is proposed and discussed. 1. Introduction In the last few decades, electrodeposited ZneNi alloys have at- tracted signifcant attention in the automotive industries due to their highthermalstabilityandexcellentmechanicalproperties.ZneNialloy depositsonironsheetscontaining10to14wt%Niprovideuptofve times better corrosion resistance than pure zinc [1,2] and have been considered as a realistic alternative to highly toxic and carcinogenic cadmium coatings in the aerospace industry [3]. Moreover, Zn-based alloys could be highly active as electrode material for hydrogen evo- lution in alkaline water electrolysis [4,5]. Diferent techniques have beenusedtoformZnNialloyssuchassol-gel[6],templatemethod[7], spray pyrolysis [8] chemical vapor deposition [9] and electroplating [10,11]. Electrodeposition is a simple and cost-efective process for deposition of high quality metals and alloys flms over large-area sub- strates with controlled thickness [12–14]. According to Brenner's clas- sifcation [15], ZneNi alloy electrodeposition is considered as anomalous co-deposition process where the less noble metal ions (Zn 2+ ) are preferentially reduced, leading to the formation of Zn-rich ZnNialloys.Uptodate,theco-depositionmechanismofzincandnickel has not been well elucidated. Various models have been proposed to explain the preferential deposition of zinc instead of nickel, which is thermodynamically more favorable. The frst attributes this phenom- enontoalocalalkalinization(neartheelectrodesurface)asaresultof H 2 evolution leading to the formation of zinc hydroxide which sup- presses the discharge of Ni 2+ ions [16]. Another mechanism has also beenproposedinvokingtheunderpotentialdeposition(UPD).Theterm “underpotential deposition” is usually used to describe a deposition process where a metal can be deposited at potentials positive to its equilibrium potential [17]. However, this model for depositing ZneNi alloys has not found much support since anomalous deposition is usuallyobservedintheoverpotentialdepositionregion[18].Although ZneNi electrodeposition has been widely studied, most of the in- vestigations have focused on the relationship between surface https://doi.org/10.1016/j.jelechem.2019.113261 Received7January2019;Receivedinrevisedform20June2019;Accepted22June2019 ⁎ Corresponding author. E-mail address: mdbenaicha@univ-setif.dz (M. Benaicha). Journal of Electroanalytical Chemistry 847 (2019) 113261 Available online 24 June 2019 1572-6657/ © 2019 Elsevier B.V. All rights reserved. T