Study of the electrochemical co-reduction of Cu2+ and Zn2+ ions from an alkaline non-cyanide solution containing glycine

https://doi.org/10.1016/j.jelechem.2014.04.020Get rights and content

Highlights

  • A new alkaline non-cyanide solution for electrodeposition of Cu–Zn alloy.

  • The composition of Cu–Zn alloy is function of applied potential and electrolyte composition.

  • The Cu–Zn alloys obtained showed a golden color.

Abstract

The electrochemical deposition of Cu–Zn alloy was studied in weakly alkaline glycine solutions at pH 10.0 and room temperature. This study was realized by cyclic voltammetry, chronoamperometry, scanning electron microscopy, energy dispersive X-rays and atomic absorption spectrometry. Voltammetric studies displayed three cathodic and anodic peaks, respectively, for Cu–Zn alloys. ALSV profiles showed different oxidation peaks for dissolution of the deposits of Cu–Zn alloy and these had shapes which depend on the conditions of electrodeposition. The dissolution peaks do not coincide with peak potential for the dissolution peak of pure copper and zinc. The physical and chemical characterization of these coatings showed to be interrelated, meaning that the chemical composition, cathodic current efficiency and surface morphology of Cu–Zn alloys obtained were found to be depending on the electrolyte composition and applied potential. AAS analysis revealed the presence of mainly copper, which suggests that the process of formation of Cu–Zn alloy can be classified as normal (irregular) solution.

Introduction

As early as 1841 [1] the electrodeposition of brass was discovered. The chief applications of Cu–Zn alloy deposits are for used for decorative purposes, protection of steel, promotion of rubber adhesion to steel and other metals [2]. Cyanide has been conventionally used as the complexing agent in Cu–Zn electrolytes [3], [4], [5], despite its high toxicity and the need of a rigorous maintenance and control of its solutions. The problem of disposal of cyanide waste and the decomposition of the bath during operation which necessitates frequent addition of cyanide are the major problems in cyanide baths. Brenner [5] made an extensive literature survey on non-cyanide plating baths before 1963. Since then, several investigations have shown the possibility of Cu–Zn alloy deposition from non-cyanide baths. Efforts were made to deposit Cu–Zn alloy from thiocyanate [6], sulfate [7], oxalate [8], triethanolamine [9], glycerol [10], thiosulfate [11], tartrate [12], pyrophosphate [13], citrate [14], sorbitol [15], glucoheptonate [16], ethylenediamine [17], polyphosphate [18], EDTA [19], d-mannitol [20], trilonate [21], pyrophosphate–oxalate [22], nitrilotriacetic acid [23] and recently from ionic liquid baths [24]. However, the non-cyanide bath has not become commercially operated, probably because each bath has some disadvantage compared to the cyanide baths. For example the deposits from the non-cyanide bath may not be of right color or cannot be deposited directly on steel due to the formation of an immersion deposit. The most outstanding characteristics of the cyanide baths are the high stabilities of Cu(I)–cyanide complexes that bring the deposition potential of Cu and Zn close together. In addition, brass deposits from cyanide baths have uniform color, that is, they possess a uniform composition on a irregular substrate where nonuniform current density distribution is unavoidable. These baths are operated at higher current densities than those where the maximum Zn content of the deposits occurs, however, the zinc content of the deposits obtained is not decreased in these industrial practices.

In the literature [25] is reported that the simultaneous reduction of two or more cations on the cathode surface can only be achieved if their reduction potential values are similar. The reduction potentials of metal ions with different standard potentials can be approximated by varying their concentrations in solution. This way, high quality metallic alloy coatings are usually obtained by using complexing agents, which diminish the activity of the nobler metallic ion in the solution and allow for their simultaneous deposition [25]. Brenner [5] has listed five types of deposition system: (1) Regular solutions under diffusion control, where uncomplexed metal ions and two metals of widely differing nobility; (2) Irregular solution under cathode potential control. Static potential affected by complexing alone, e.g. cyanide bath for copper–zinc alloys; (3) Equilibrium solutions where at low current densities the bath metal concentrations give the deposit metal directly, e.g. lead–tin alloys from acid baths; (4) Anomalous solutions in which the less noble metal deposits preferentially, e.g. iron, cobalt or nickel; (5) Induced solutions in which a metal can be co-deposited as an alloy although it will not deposit singly, e.g. molybdenum or tungsten with iron group metals. The first three are classed as normal systems in that the proportions of metal deposited may be estimated on the basis of the polarization curves of the individual metals. Several papers have described the electrodeposition of different kinds of alloy by means of complexing agents [5], [6], [7], [8], [9], [10], [11], [12], [13], [14], [15], [16], [17], [18], [19], [20], [21], [22], [23], [24], [25], [26], [27], [28], [29], [30]. The glycine has been used as a complexing agent in the electrodeposition of Zn–Ni [31], Cu–Co [32], Zn–Co [33], Zn–Co–Cu alloys [34] and more recently by our research group to obtain Zn–Co alloy [35]. These studies show that the deposits obtained from alkaline bath containing glycine are of high quality. In sum, this complexing agent is nontoxic, easily obtained and, upon degradation, effluent treatment is easier.

These types of solutions represent an attractive medium due to its ability to form complexes with different metal cations in weakly alkaline electrolytes. In order to develop an alternative Cu–Zn plating bath that avoids the use of cyanide, we have studied an alkaline bath using glycine as the complexing agent for copper and zinc ions. This choice of bath resides in the fact that we recently have published that the presence of glycine has led to acceptable copper and zinc deposits [36], [37]. In this light, we undertook a study of the electrochemical reduction of Cu2+ and Zn2+ ions on nickel substrate from an alkaline non-cyanide bath containing glycine (where H2NCH2COO is the anion form of glycine in solution, denoted as: G for brevity). The present work takes into account the solution chemistry as it seeks to elucidate the influence of zinc/copper ratio and deposition conditions on the alloy composition, cathodic current efficiency, surface morphology and brightness of the Cu–Zn electrodeposits. Nickel electrode was utilized because the brass deposits at industrial level contain an underlayer of nickel.

Section snippets

Experimental

The electrochemical reduction of Cu2+ and Zn2+ ions on nickel electrode was carried out in a conventional three-electrode cell from the solutions shown in Table 1 at 25 °C. All solutions were prepared using analytic grade reagents (provided by Sigma–Aldrich Company) with ultra pure water (Millipore-Q system) and were deoxygenated by bubbling N2 for 20 min before each experiment. The working electrode (WE) was a nickel electrode provided by Sigma–Aldrich, made of a nickel rod embedded in Teflon.

Thermodynamic study

The thermodynamic description of the complexes formed under our experimental conditions was obtained from our studies recently published on the electrodeposition of copper and zinc from copper–glycine and zinc–glycine complexes [36], [37]. As discussed in our previous studies, under these conditions (Table 1) the thermodynamic calculations show that CuG2 and ZnG3- complexes are the predominant species for copper and zinc in solution at pH 10, respectively. For the experimental copper-zinc mixed

Conclusions

It has been shown that Cu–Zn alloys with a range of compositions were successfully obtained on nickel substrates from an alkaline non-cyanide bath containing glycine at pH 10.0 and room temperature. The composition of Cu–Zn alloy could be controlled through the deposition potential and/or the Zn(II)/Cu(II) ratio in solution. The effect of Zn(II)/Cu(II) ratio in solution on the cathodic current efficiency was studied and was founded that increasing Zn content in the bath improves the cathodic

Conflict of interest

There is no conflict of interest.

Acknowledgments

This work was supported by the Centro de Investigación & Desarrollo Tecnológico en Electroquímica (CIDETEQ) and also by Departamento de Ecomateriales & Energy of the Facultad de Ingeniería Civil (UANL) through the project CONACYT-FOINS 75/12 “Fotosíntesis Artificial”.

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