Studies on Electrochemical Deposition of Novel Zn

ISSN 2070-2051, Protection of Metals and Physical Chemistry of Surfaces, 2017, Vol. 53, No. 1, pp. 118–126. © Pleiades Publishing, Ltd., 2017.

NEW SUBSTANCES, MATERIALS AND COATINGS

Studies on Electrochemical Deposition of Novel Zn–Mn–Ni Ternary Alloys from an Ionic Liquid based on Choline Chloride1 S. Fashua, * and R. Khanb a

Department of Materials Science and Technology, Harare Institute of Technology, Harare, Zimbabwe b Department of Physics, Zhejiang University, Hangzhou 310027, China *e-mail address:[email protected] Received July 20, 2015

Abstract—Ternary Zn–Mn–Ni alloy coatings were electrodeposited for the first time from a choline chloride based ionic liquid with the aim of collecting properties of binary Zn–Mn and Zn–Ni alloys into one alloy system. The effect of electrodeposition potential on the composition and corrosion performance of the obtained ternary Zn–Mn–Ni deposits was investigated and contrasted with the characteristics of Zn–Mn and Zn–Ni deposits. Cyclic voltammetry revealed that the deposition of ternary Zn–Mn–Ni alloys behaved differently from the deposition of binary Zn–Mn and Zn–Sn alloys and that Mn deposition takes place at positive potentials in the Zn–Mn–Ni electrolyte than in the Zn–Mn electrolyte due to the presence of Ni2+ ions in the electrolyte. X-ray diffraction studies showed that the Zn–Mn–Ni ternary alloys consist of a lattice of Zn (with Mn and Ni imbedded inside) at low electrodeposition potentials and MnZn(with Ni imbedded inside) phase at high electrodeposition potentials. Chemical composition analysis show that the Mn content in the ternary Zn–Mn–Ni alloy increased with increase in electrodeposition potential, whereas Zn and Ni contents are suppressed. The corrosion tests results indicate that through addition of Ni into the Zn–Mn binary alloy, the Zn–Mn–Ni alloy tailored are more corrosion resistant than the Zn–Mn binary alloy whilst the passivation behavior is still preserved. Keywords: Electrodeposition, Deep eutectic solvent, Zn–Mn–Ni alloy, Corrosion resistance DOI: 10.1134/S2070205117010051

1. INTRODUCTION Zn alloys instead of pure Zn metal are nowadays mostly used for corrosion protection of metallic substrates from various aggressive corrosive environments. Most of these Zn alloys contain noble alloying elements like Fe, Co, Sn and Ni which are of more positive potentials than Zn such that their presence lowers the dissolution potentials of Zn in the corrosive environment. Through research, Mn (more negative potential than Zn) has been proposed as an alloying element of Zn since its alloys show a passivating behavior in a chloride environment due to formation of insoluble compounds on the corroding surface [1]. Due to their natural passivating behavior, the Zn–Mn alloys can be used without passivation processes commonly employed in other plating processes [2, 3]. Several authors have reported that, in some corrosive medium where sodium chloride (NaCl) and sulphur dioxide (SO2) are present, Zn–Mn alloys have better corrosion resistance than Zn–Fe, Zn–Co and Zn–Ni alloys [4, 5]. These Zn–Mn alloys find wide application in many industries, for steel protection in 1 The article is published in the original.

different aggressive media containing corrosion activators e.g. chloride Cl– or sulfate SO 24 − ions. In studies in this area, Boshkov et al. [6] proposed that the high corrosion resistance of Zn–Mn alloys is likely to be due to the dual protective actions of Mn: on one hand, it dissolves first as the more negative element, thereby protecting the Zn; and on the other, it ensures the formation of a corrosion product with a low solubility product, Zn5(OH)8Cl2 ⋅ H2O (ZHC), on the galvanic coating. Unfortunately, electrodeposition of Zn–Mn from aqueous solutions is a complex process. On one hand, the efficiency of the process is low, and, on the other, the process is not easy to control; particularly when high Mn contents in the alloy are required. This is mainly due to hydrogen evolution reactions at these negative potentials required for Mn reduction [7]. Researches on the electrodeposition of Zn–Mn– Ni ternary alloys are scarce. R. Kashyap et al [8] investigated the influence of variables in Zn–Mn–Ni alloy plating from a sulphate bath and observed that the proportions of Ni and Mn in the alloy increased with increase in electrodeposition current density, temperature and duration of electroplating. The electrodeposition potentials of Ni (–0.75 V vs Ag/AgCl) and

118

STUDIES ON ELECTROCHEMICAL DEPOSITION OF NOVEL Zn–Mn–Ni

119

Table 1. Electrolytes compositions Chemical

A

B

C

D

E

F

ZnCl2/M

0.40

0.40

0.40

0.40

–

–

NiCl2 · 6H2O/M

0.15

–

0.15

–

0.15

–

MnCl2 · 4H2O/M

0.70

0.70

–

–

–

0.70

H3BO3/M

0.40

0.40

–

–

–

0.40

60 1.45, 1.55,1.65

60 1.8

60 0.9

60 –

60 –

60 –

Deposition Parameters: Temperature, °C Voltage(–V)

Mn (–1.18 V vs Ag/AgCl) from aqueous electrolytes are wide apart such that co-deposition of these alloys is difficult and likely to occur in a narrow potential range. Nowadays aprotic ionic liquids have proven to be good solvents for electrodeposition of metals, alloys and semiconductors [9, 10], especially for very active elements like Mn/Al, because hydrogen evolution can be avoided. The chemical and physical properties of ionic liquids can be tailored by choosing different cations and anions, thus they can be synthesized for very specific applications [11]. Deep eutectic solvents ionic liquids have been successfully used to electrodeposit a wide range of metals/alloys [12–16]. Ionic liquids have been employed for electrodeposition of Zn–Mn alloys [17], but aqueous electrolytes are still much more used [18]. Among the Zn binary alloys, the Zn– Ni alloys were found to be of superior corrosion properties due to the strong barrier properties associated with Ni [19–23]. Zn, Ni and Mn metals all have high hardness and coatings deposited from these metals are hard enough to be used in almost all environments including aeronautic applications where hardness is of paramount importance. Thus, the present study is aimed at incorporating Ni into the binary Zn–Mn alloy such that its rate of dissolution in corrosive environments will be decreased (due to presence of Ni) and yet at the same time it will maintain its passivation (due to presence of Mn salts) behavior. In this work, electrodeposition of ternary Zn–Mn–Ni alloys was investigated from the electrolyte based on a deep eutectic solvent. Previous researches have shown that binary Zn–Mn coatings [24] from ionic liquids are sacrificial to steel but dissolves rapidly in corrosive environments forming passivating corrosion products, whereas binary Zn–Ni coatings [25] cannot offer sacrificial protection to steel to higher amounts of Ni. But, when the two binary alloys are combined, the Zn–Mn–Ni alloy coatings are expected to show dual action, dissolving slowly in corrosive environments and at the same time producing passivating corrosion products. Cyclic voltammetry (CV) was used to determine the deposition potentials of Zn–Ni, Zn–Mn and Zn–Mn–Ni alloys in-order to understand the effect of Ni addition on Zn–Mn electrodeposition. Corrosion protection properties of Zn–Ni, Zn–Mn

and Zn–Mn–Ni alloys were analyzed using potentiodynamic and linear polarization, Ecorr vs. time, and impedance techniques in a 3.5% NaCl solution. 2. MATERIALS AND EXPERIMENTAL METHODS 2.1. Electrolyte Preparation and Electrochemical Measurements ChCl [HOC2H4N(CH3)3Cl] (AR, Aladdin) and Urea (AR, Aladdin) were used as received. Sigma Aldrich chemicals were used for all experiments. The deep eutectic solvent (DES) was prepared by mixing the two components in a mole ratio of 1 choline chloride (ChCl) (Assay 99%):2 Urea (AR, Aladdin), at 70°C and stirring until a homogenous colorless liquid was formed, then metal chlorides (Assay 99.99%) were added and dissolved at same conditions to prepare different electrolytes given in Table 1 and these are the optimized formulations producing good deposits. All the measurements were performed on the potentiostat–galvanostat electrochemical analyzer CHI660e controlled by a computer and supported by software. Cyclic voltammetry experiments were carried out in a three-electrode system consisting of a platinum working electrode (1 cm2), a platinum counter electrode and a silver rod quasi-reference electrode. The working electrode was rinsed and dried before each measurement. The cyclic voltammetry experiments were performed at a temperature of 60°C and at a scan rate of 10 mVs−1. Before the cyclic voltammetry experiments, the prepared electrolytes had been dried in a vacuum oven at 60°C for two days followed by de-aeration with nitrogen (N2) to remove water and dissolved (O2). 2.2. Electrode Preparation and Electrodeposition Process The coatings were electrodeposited under potentiostatic conditions using an AutoLab potentiostat/galvanostat analyzer CHI660e. The substrate (cathode) used was the copper plate (2 cm × 4 cm in size), nickel was used as the anode and a silver rod was used as a pseudo-reference. Prior to electrodeposition,

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the electrolyte was bubbled with nitrogen for 5 min in order to remove oxygen. Zn–Ni, Zn–Mn and Zn– Ni–Mn alloy deposition was carried at a temperature of 60°C using electrolyte compositions given in Table 1. The electrodeposition results reported here were done at 60°C because such conditions were good in producing homogeneous, bright and compact coatings. Prior to electrodeposition the cathodes were electropolished for about 5 minutes in a solution of 80% (v/v) phosphoric acid, 10% (v/v) methanol and 10% (v/v) deionised water. Electropolishing was necessary to remove all the surface impurities and any mechanical damage from the copper cathode substrate. The cathodes were then rinsed in ethanol, flowing deionised water and dried. All experiments were carried out in duplicate and the reproducibility of these measurements was found to be satisfactory. The design allowed the anode and cathode to be parallel to each other at a gap distance of 3 cm and perpendicular to the cell base to ensure uniform current distribution over the surface of the electrodes. Constant temperature was maintained by circulating water in the jacketed cell in which the deposition was carried out. Ternary Zn–Mn–Ni alloys of different compositions were obtained by potentiostatic electrodeposition at voltages of –1.45 V, –1.55 V and –1.65 V. The durations of deposition were adjusted to get deposits of the same thickness by maintaining the same total charge density of 6 C cm–2 in all cases. The final deposits were sequentially rinsed with ethanol and deionized water and dried. 2.3. Materials Characterization Energy Dispersive Analysis using X-rays (EDAX) was used to analyze the chemical distribution of the elements in the final deposits. To ensure accuracy of the element distributions, EDAX was done at three points on the surface of the substrate and averaged. The accuracy of measurements for the equipment used was rated at ±1 wt %. The surface morphology and the microstructure of the coatings were analyzed using Scanning Electron Microscopy (SEM) integrated with the EDAX. The crystalline structure of the coatings were determined by using an X-ray diffraction (Rigaku D/max 2550PC) at scanning rate and step of 4°/min and 0.02°, respectively. The size of crystal grains was calculated with the Scherrer equation [26]:

kλ , β cos θ where D is the granularity of the crystal grain, k is the Scherrer constant (0.89), λ is the wavelength of the X-rays, β is the full width at half maximum (FWHM) of the diffraction peaks, and is the angle of diffraction. D=

2.4. Electrochemical Characterization A variety of electrochemical techniques were used to evaluate the corrosion properties of the coatings.

The coated substrate (1 cm2) was used as the working electrode and a platinum plate was used as the counter electrode while a silver/silver chloride (Ag/AgCl (3 M KCl)) was used as the reference electrode. Prior to each experiment, the specimens were immersed into the 3.5 wt % NaCl solution (which simulates the marine environment) for 4 h until the open circuit potentials (EOCP) were stable. EIS measurements were made with the amplitude of 10 mV (peak to zero) in the frequency range from 10 kHz to 0.1 Hz and after that EOCP were recorded from the same system for a time period of 13800 s. Potentiodynamic polarization studies (Tafel plots) were carried out by polarizing the working electrode from the OCP to more positive potentials in anodic direction at a scan rate 1 mV/s. The corrosion current densities (icorr) were determined by extrapolating the straight-line section of the anodic and cathodic branches of the Tafel curves in the vicinity of the corrosion potential (±25 mV) using the software installed in the instrument. The polarization resistance (Rp) measurements were carried out in a potential range of ±10 mV around the open circuit potential at a scan rate of 0.2 mV/s. For each measurement, a fresh portion of electrolyte (100 mL) and a new working electrode were used.

3. RESULTS AND DISCUSSION 3.1. Cyclic Voltammetry 3.1.1. Pure Zn, Ni and Mn ions electrodeposition. In order to investigate the electrochemical behavior of the electrolytes (Table 1) on a platinum electrode, cyclic votammograms were performed at a scan rate of 10 mV/s. Figure 1a shows voltammograms measured for pure Zn, Mn and Ni metal electrolytes at a temperature of 60°C. In the cathodic scan the following peaks are observed, which can be ascribed to the deposition of pure metals (Zn at a potential around –1.39 V, Ni at potential around –0.67 V and Mn has no defined peak potential). Regarding the anodic processes, one can see an oxidation peak at a potential of about ‒0.51 V for Zn dissolution, and at around +0.30 V for Ni dissolution and there was no distinct peak for Mn metal. Figure 1a show that the current densities of Ni and Mn when compared to Zn were very small showing that Ni/Mn kinetics are slow under these conditions. A similar electrodeposition behavior for Ni was reported by Alfantazi et al. [23] and it was also shown that Ni reduction from Zn–Ni bath is controlled by diffusion. 3.1.2. Zn–Mn, Zn–Ni and Zn–Mn–Ni electrodeposition. Figure 1b shows the cyclic voltammetry curves for Zn–Mn and Zn–Ni binary alloys recorded at a temperature of 60°C on a platinum electrode. Here, the reduction peaks for Zn–Mn and Zn–Ni binary alloys are positioned at potentials of (–1.40 V, –1.63 V) and (not defined), respectively, where the more noble potentials corresponds to electrodeposi-


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Current density (i, A cm–2)

OZn

0.004

ONi

0.002 0 –0.002 RNi

–0.004 –0.006 –0.008

0.1Ni 0.4Zn

–0.010

0.7Mn

–0.012

0.5 0 –1.5 –1.0 –0.5 Potential (E vs Ag wire, V)

1.0

(b) 0.012 OZnMn OZnNi1

OZnMnNi

0.008

OZnNi2

0.004 0 –0.004

RZnMnNi1 RZnMnNi2

–0.008 –0.012

0.4Zn–0.7Mn RZnMn2

RZnMn1

RZnNi RZnMnNi3

–2.0 –1.5 –1.0 –0.5 0 0.5 Potential (E vs Ag wire, V)

1.0

Fig. 1. Cyclic voltammograms of (a) 0.4M Zn, 0.7M Mn and 0.1M Ni, and (b) Zn–Mn, Zn–Ni and Zn–Mn–Ni at a temperature of 60°C on a platinum electrode using silver reference electrode at a scan rate of 10 mV/s (vs Ag/AgCl).

0 Current density (A cm–2)

3.2. Surface Morphology and Elemental Compositions To understand the influence of electrodeposition potential (which results in metal ions reduction) on electrodeposition behavior in Zn–Ni, Zn–Mn and Zn–Mn–Ni films during electrodeposition, the individual current(i)–time(t) curves from the electrolytes (Table 1) for 10 min at different deposition potentials were obtained, and are shown in Fig. 2. The currents increased as the negativity of the potential increased, as shown in Fig. 2. These behaviors reveal that the deposition potential resulted in the decrease of the electrolyte metal ions and resulting current density, causing substantial nucleation and grain growth in the deposited films. Hosseini et al. [27] suggested that the deposition current decreases when electroplating time is prolonged, owing to the diffusion process during a constant deposition potential period. Of important to note is that the current density of Zn–Mn–Ni alloys are higher than those of Zn–Mn and Zn–Ni alloys. This is related to higher electrolyte conductivity of Zn–Mn–Ni electrolyte. The current densities for electrodeposition of Zn–Mn–Ni alloys increase with increase in electrodeposition potentials and this is related to increased transport kinetics of electroactive species from the bulk electrolyte. The typical EDAX spectra for the Zn–Mn–Ni ternary alloy electrodeposited at a potential of –1.55 V and temperature of 60°C is shown in Fig. 3 and show

(a)

0.006

Current density (i, A cm–2)

tion of more noble elements. The Zn–Mn alloy consists of a single oxidation peaks at potentials of –0.46 V. This oxidation peak is likely to be of pure Zn, since it was shown that Mn sometimes does not have a noticeable dissolution peak due to its rapid dissolution in the electrolyte [24]. The oxidation peaks for Zn–Ni electrolyte are positioned at potentials of –0.36 V and +0.18 V and these are likely to be for dissolution of pure Zn and pure Ni metals, respectively. The voltammogram curve for the Zn–Mn–Ni electrolyte is also included in Fig. 1b. In this figure, it can be observed that when three metallic ions, Zn, Mn and Ni are present in the electrolyte, the voltammogram consists of three distinct reduction peaks. The reduction peaks for Zn–Mn–Ni are positioned at (–0.68 V, –0.98 V, –1.55V). These are for the reduction of Ni, Zn and Mn metals, respectively. Comparison of this curve with the Zn–Mn voltammogram show that the reduction peaks of Mn are shifted in the positive direction in presence of Ni2+ ions in the electrolyte. This shows that a Zn–Mn–Ni ternary alloy can be electrodeposited at low potentials (underpotential deposition) in presence of Ni2+ ions in the electrolyte. From Fig. 1b, oxidation peaks for Zn–Mn–Ni electrolyte are positioned at +0.11 V and +0.14 V. These are likely to be for Zn and Ni stripping, respectively. Thus, from the analysis of the voltammogram corresponding to the Zn– Mn–Ni system, the electrodeposition was carried out at three potentials of –1.45 V, –1.55 and –1.65 V.

121

–0.02

Zn–Mn–Ni–1.45V Zn–Mn–Ni–1.55V Zn–Mn–Ni–1.65V Zn–Mn Zn–Ni

0

100

200

300 400 Time/s

500

600

Fig. 2. Variation of the electrodeposition current density vs. time curves during electrodeposition from Zn–Ni, Zn–Mn and Zn–Mn–Ni electrolytes at a temperature of 60°C on copper substrate (2 × 4 cm) where silver reference and Ni anode were used as electrodes.


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ZnKa NiKb MnKa ZnKb MnKbNiKa

OKa AuMa

0

1

2

3

4 5 6 Energy, keV

7

8

9

Mn 6

87

Ni Ni

4

86 1.5 1.6 Electrodeposition Potential/Ag vs. AgCl(–V)

10

Fig. 3. EDAX of the typical Zn–Mn–Ni film electrodeposited from Zn–Mn–Ni electrolyte at a potential of –1.55V and a temperature of 60°C on a copper substrate (2 × 4 cm).

Zn

Fig. 4. Variation of Zn, Mn and Ni contents in the ternary alloy with electrodeposition potentials of –1.45 V, –1.55 V, and –1.65 V.

MnZn(112) MnZn(112)

A

10 μm

10 μm

(b)

Intensity, a.u.

Zn–Mn–Ni–1.65V

(a)

Film composition/wt %

0.6

8

B

MnZn(112)

1.3

88

Mn

Cu(220)

1.9

Zn

MnZn(112)

2.5

10

(Cu, MnZn)(111)

ZnLa

Film composition/wt %

KCnt 3.1

Cu(200)

122

Zn–Mn–Ni–1.55V

C Zn–Mn–Ni–1.45V

(c)

10 μm

Fig. 5. Scanning electron micrographs (SEM) of Zn– Mn–Ni films electrodeposited on a copper substrate (2 × 4 cm) at a temperature of 60°C at electrodeposition potentials of (a) –1.45 V, (b) –1.55 V, and (c) –1.65 V.

that Zn, Mn and Ni are co-deposited from this electrolyte (Table 1) under these conditions. Figure 4 shows the chemical composition analysis of the Zn– Mn–Ni ternary alloy deposits as a function of electrodeposition potential. It is notable that Zn and Ni content are negatively correlated with the deposition potential, while Mn content tends to increase with increase in potential since it is the most active metal. The effect of electroplating potential on the surface morphology of the Zn–Mn–Ni alloy coatings is shown in Fig. 5. From the analysis of these SEM images, the morphologies of all films are characterized by micropores or cavities and these cavities are significantly severe at higher electrodeposition potentials. The grain size decreases with increase in electrodeposition potentials. This is due to rapid nucleation rate at

20

30

40

50 2θ/degree

60

70

80

Fig. 6. XRD spectrum for Zn–Mn–Ni alloys deposited at potentials of –1.45V, –1.55V, and –1.65V.

high electrodeposition potentials producing fine grains [28]. 3.3. Crystal Structure The three reflections at (111), (200) and (220) in Fig. 6 correspond to the copper substrate. X-ray diffraction studies in Fig. 6 show that Zn phase is observed for the Zn–Mn–Ni alloy at low electrodeposition potential of –1.4 5V. This shows that Mn and Ni may exist as solid solutions in a Zn matrix since their presence in the alloy was detected by EDAX analysis. As the electrodeposition potential increases, the Zn phase disappears and a MnZn13 phase appears (–1.55 and –1.65 V). Using the Bragg’s equation and the XRD pattern of diffraction angles (phase shifts at


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3.4. Corrosion Performance of Deposits The corrosion resistances of Zn–Mn, Zn–Ni and Zn–Mn–Sn alloys were investigated to establish their corrosion kinetics and properties. A variety of corrosion testing techniques were employed to confirm the reliability of the results and obtain more information on corrosion performances. The corrosion resistances of the electrodeposited Zn–Ni, Zn–Mn and Zn–Mn–Ni coatings in 3.5wt % NaCl solution were investigated and are shown in the potentiodynamic polarization plots in Fig. 7a. It is obvious from the summarized data (Table 2) of corrosion behavior that the corrosion potential of these deposits strongly depends on the alloy composition. The corrosion potential of the Zn–Ni alloy (–0.36 V) was much more positive than that of steel (Ecorr = ‒0.63 V), this is due to the barrier properties of Ni and these coatings cannot sacrificially protect steel. These Zn–Ni alloys showed no passivation behavior, and have the smallest corrosion currents when compared to Zn–Mn and Zn–Mn–Ni alloys. On the contrary, the corrosion potential of Zn–Mn alloy (–1.19) was the most negative with highest corrosion currents, and this is due to the most negative corrosion potential of Mn metal. They were also passivating as typical of Zn–Mn alloys [30]. The corrosion potentials and current values of ternary Zn–Mn–Ni alloys were between those of Zn–Ni and Zn–Mn alloys (Table 2) and they were sacrificial to steel. In addition they showed a passivation behavior due to the presence of Mn metal. As the Ni content increases, the corrosion potentials of the ternary Zn–Mn–Ni alloy coatings shift into the positive direction from –1.11 V to –1.03 V. The corrosion currents decreased with increase in Ni content

10–1

(a)

log j (Acm–2)

10–3 10–5 10–7 10

–9

Zn–Mn–Ni–1.45V Zn–Mn–Ni–1.55V Zn–Mn–Ni–1.65V Zn–Mn Zn–Ni

10–11 –1.4 –1.2 –1.0 –0.8 –0.6 –0.4 –0.2 Potential (Ag vs. AgCl(V))

j/μAcm–2

positions A, B and C), the lattice constants of the alloys were calculated and found to be increasing from 2.13 Å for Zn–Mn–Ni alloy electodeposited at –1.45 V to 2.11 A for Zn–Mn–Ni alloy electrodeposited at ‒1.65V, proving that Mn/Ni exists as solid solution in a Zn matrix. These phase compositions are in agreement to those of the Zn–Mn–Ni equilibrium phase diagram [29]. On the basis of the Scherer equation [23], the crystallite sizes of the Zn–Mn–Ni coatings were determined. The average crystallite sizes computed at (1 1 2) peaks for the Zn–Mn–Ni films are summarized in Table 2. It is noticed that with increase in electrodeposition potential, the grain size decreases from 72 to 42 nm. The trend of decreasing crystallite size with increasing electrodeposition potential can be best explained by an increased nucleation rate caused by the higher electrodeposition overpotential. At higher deposition potentials, the activation energy of nucleation decreases leading to an increased nucleation rate. Consequently when the nucleation mechanism dominates the growth process, a large number of nuclei are generated on the substrate, and the growth of nuclei and crystallites are strongly impeded [28].

123

0.8 0.6 0.4 0.2 0 –0.2 –0.4 –0.6 –0.8 –1.0 –1.12

0

(b)

Zn–Mn–Ni–1.45V Zn–Mn–Ni–1.65V Zn–Mn–Ni–1.55V

–1.10 –1.08 –1.06 –1.04 –1.02 Potential (Ag vs. AgCl(V))

Fig. 7. Tafel polarization plots (a), and linear polarization curves (b), of Zn–Mn–Ni films in 3.5 wt % NaCl (using Ag/AgCl).

and this is despite the fact that the morphology coarsens with increase in Ni content i.e. this occurs at low electrodeposition potentials. The polarization resistance of Zn–Mn, Zn–Ni and Zn–Mn–Ni alloy coatings are presented in Fig. 7b and summarized in Table 2 Table 2. Corrosion characteristics, obtained by polarization of films in 3.5 wt % NaCl solution (using Ag/AgCl reference electrode), and the average grain sizes of the Zn– Mn–Ni deposits Film Zn–Mn–Ni–1.45V Zn–Mn–Ni–1.55V Zn–Mn–Ni–1.65V Zn–Mn Zn–Ni

icorr, Ecorr, mV μA cm–2

K, Ω cm2

Avg grain size, nm

1031 1062 1110 1190 357

7.84 4.68 1.96 – –

72 57 42 – –

1.45 2.43 3.14 2.90 0.48

Rp aPolarization resistance from linear polarization curves.


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1120

3.2 3.0 2.8 2.6 2.4 2.2 2.0 1.8 1.6 1.4

1100 V i

1080 1060 1040

Ecorr/Ag vs. AgCl(–mV)

icorr/μAcm–2

124

1020 1.40 1.45 1.50 1.55 1.60 1.65 1.70 Electrodeposition potential (Ag vs. AgCl(–V))

Fig. 8.Variation of Zn–Mn–Ni alloys corrosion potentials (Ecorr) and currents (icorr) with electrodeposition potentials of –1.45 V, –1.55 V and –1.65 V.

–0.6 OCP/Ag vs AgC(V)

–0.7 Zn–Mn–Ni–1.45V Zn–Mn–Ni–1.55V Zn–Mn–Ni–1.65V Steel

–0.8 –0.9 –1.0 –1.1 –1.2 –1.3 –1.4 0

2000

4000

6000

8000 Time/s

10 000 14 000 12 000

Fig. 9. Open circuit potentials vs. time variation in 3.5 wt % NaCl (using Ag/AgCl reference electrode) of steel and Zn– Mn–Ni coatings electrodeposited on a copper substrate (2 × 4 cm) at a temperature of 60°C.

and show that the corrosion resistance of alloys (Zn– Mn, Zn–Ni and Zn–Mn–Ni) increased with increase in alloy Ni content since Ni offers barrier pro-

tection to the coating. Figure 8 shows the variation of the electrodeposited Zn–Mn–Ni alloys corrosion potentials and corrosion currents with increase in elecrodeposition potentials. With decrease in electrodeposition potential, both the corrosion potentials and the corrosion currents increases and this is attributed to an increase of the less noble metal Mn in the alloy. Open circuit potential (OCP) measurements of the coatings for a period of 13800s after establishment of stable OCPs are shown in Fig. 9. The initial OCP values slightly changed to more negative potential due to the exposure of zinc/manganese rich phase to the test solution and become stable for the entire testing period. It can be seen that, there is not much fluctuation in the initial OCP behavior with time indicating the high protective nature of these coatings [31]. With increase in ternary alloy Mn content, the open circuit potentials shifts to more negative values and this is likely to be due to increase in the active element, Mn. However, the results should be interpreted with caution since OCP measurements are qualitative measurements and do not quantify corrosion rates. Electrochemical impedance spectroscopy (EIS) was used to evaluate the barrier properties of Zn–Ni, Zn–Mn and Zn–Mn–Ni coatings and to determine the polarization resistances without modifying the surface. Figure 10 presents a comparison of Nyquist responses obtained for Zn–Ni, Zn–Mn and Zn– Mn–Ni coatings. A qualitative analysis of these results shows that the diameters of the semi-circles increased from Zn–Mn, Zn–Mn–Ni to Zn–Ni alloy coatings. The equivalent circuit shown as an insert in Fig. 10 was used to fit the EIS results, and the parameters of the equivalent circuit elements obtained by simulation using the regression calculation software ZSIMPWIN are presented in Table 3. The experimental data and the calculated data matched well, which indicates that the equivalent circuit was suitable for simulation. Rs is the resistance of the electrolyte between the electrodes determined by the conductance of the NaCl solution and according to these results the solution resistance of all coatings shows no significant difference; Rp is the polarization resistance; and Q is the constant phase element. Table 3 show that the polarization resistances

Table 3. Impedance parameters, obtained from the impedance spectra in 3.5 wt % NaCl solution (using Ag/AgCl reference electrode), of the electrodeposited Zn–Mn–Ni films Film Zn–Mn–Ni–1.45V Zn–Mn–Ni–1.55V Zn–Mn–Ni–1.65V Zn–Mn Zn–Ni

Rs, Ω cm2

Rp, kΩ cm2

Q–Y, Ω–1 cm–2 S–n

n

Error, %

6.45 6.45 7.46 – 7.49

7.21 5.62 2.07 – 24.56

8.6 E-5 7.9 E-5 6.5 E-5 – 1.20 E-5

0.77 0.74 0.88 – 0.87

3.87 4.12 4.51 3.56

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–Imaginary Z II/ohm cm–2

10 000 8000 6000 4000 2000

0

Rs

Q Rp Zn–Mn–Ni–1.45V Zn–Mn–1.6V Zn–Mn–Ni–1.55V Zn–Ni–0.9V Zn–Mn–Ni–1.65V

4000 8000 12 000 I Rea; Z /ohm cm–2

16 000

8 72 68 64 60

7 6 5

56 52 48 3 44 2 40 1.40 1.45 1.50 1.55 1.60 1.65 1.70 Electrodeposition voltage/–V vs Ag/AgCl

4

Grain size/nm

Polarization resistance (kΩ cm2)

Fig. 10. Nyquist plots in 3.5 wt % NaCl (using Ag/AgCl reference electrode) of Zn–Ni, Zn–Mn and Zn–Mn–Ni films electrodeposited on copper substrates (2 × 4 cm) at a temperature of 60°C.The equivalent circuit used for fitting the impedance spectra is also inserted in the figure.

Polarization resistance (kΩ cm2)

Fig. 11. Variation of electrodeposited Zn–Mn–Ni alloys polarization resistances Rp and grain sizes with electrodeposition potentials of –1.45 V, –1.55 V and –1.65 V.

8 7

Linear polarization Impedance

6 5 4 3

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values of ternary Zn–Mn–Ni alloy coatings were between those of binary Zn–Ni and Zn–Mn alloy deposits. Figure 11 represents the variation of polarization resistance of ternary Zn–Mn–Ni alloy coatings versus the grain size and electrodeposition potential. According to this figure, the corrosion resistance of the ternary Zn–Mn–Ni alloy coatings increases with a decrease in electrodeposition potential and grain size. Figure 12 compares the polarization resistance values of different Zn–Mn–Ni alloy coatings calculated using impedance analysis and linear polarization techniques. The polarization resistance values obtained using EIS analysis are in close agreement with those obtained using linear polarization studies and this shows that both methods were reliable in measuring corrosion performances of the coatings. 4. CONCLUSIONS Ternary Zn–Mn–Ni alloy coatings were electrodeposited for the first time from a choline chloride based ionic liquid with the aim of collecting properties of binary Zn–Mn and Zn–Ni alloys into one ternary alloy system. The conclusions from this work can be summarized as: 1) In presence of Ni2+ ions in the Zn–Mn electrolyte, Mn deposition takes place at positive potentials than in the pure Zn–Mn electrolyte. 2) Chemical composition analysis show that the Mn content in the ternary Zn–Mn–Ni alloy increased with increase in electrodeposition potential, whereas Zn and Ni contents are suppressed. 3) X-ray diffraction studies showed that the Zn– Mn–Ni ternary alloys consists of an expanded lattice of Zn (with Mn and Ni embedded inside) at low electrodeposition potentials and MnZn13 (with Ni embedded inside) phase at high electrodeposition potentials. 4) The corrosion tests results indicate that through addition of Ni to the Zn–Mn alloy, the sacrificial Zn– Mn–Ni alloy formed is of low corrosion rates than binary Zn–Mn alloy and the corrosion products are is still appreciably passivating. Fig. 9. Open circuit potentials vs. time variation in 3.5 wt % NaCl (using Ag/AgCl reference electrode) of steel and Zn–Mn–Ni coatings electrodeposited on a copper substrate (2 × 4 cm) at a temperature of 60°C.

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ACKNOWLEDGMENTS

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This author would like to thank Mr M. Tozvireva in assisting to proof read this article.

–1.45V –1.55V –1.65V Electrodeposition voltage/–V vs Ag/AgCl Fig. 12. Comparison of polarization resistance Rp values of Zn–Mn–Ni alloys deposited at various potentials (–1.45 V, –1.55 V and –1.65 V) calculated from impedance analysis and linear polarization techniques.

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