Electrodeposition Behavior of Zn–Co Alloys from an Alkaline Zincate Solution Containing Triethanolamine

Abstract

Electrodeposition behavior of Zn–Co alloys was investigated at current densities of 2–500 A·m⁻² and a charge of 5 × 10⁴ C·m⁻² at 308 K in an unagitated zincate solution containing triethanolamine, which forms a stable complex with Co²⁺ ions. At current densities lower than 5 A·m⁻², the Zn–Co alloys exhibited normal co-deposition behavior, with the electrochemically more noble Co being preferentially deposited. By contrast, at current densities higher than 6 A·m⁻², they exhibited anomalous co-deposition behavior, with the electrochemically less noble Zn being preferentially deposited. The current efficiency for Zn–Co alloy deposition was low (about 20%) in the normal co-deposition region, while it was 95% in the anomalous co-deposition region. Also, in the anomalous co-deposition region, the partial polarization curves for Co deposition and H₂ evolution were significantly shifted to the less noble direction by the coexistence of Zn²⁺ ions, suggesting the formation of an inhibitor species that results from the presence of Zn²⁺ ions in the cathode layer. On the other hand, in the normal co-deposition region, the underpotential deposition of Zn apparently occured simultaneously with Co deposition. Zn–Co alloys are composed of stable intermetallic compounds CoZn₁₃ and Co₅Zn₂₁; therefore, the activity coefficient of Zn in the deposits appears to decrease remarkably.

1. Introduction

Electroplating of Zn–Co alloys has been the subject of many studies so far, because the corrosion resistance of Zn coatings can be improved by the co-deposition of small amounts of Co.^1,2,3,4) Although the electroplating of Zn–Co alloys is usually studied in sulfate and chloride solutions, cyanide and zincate solutions are better from the perspective of the throwing power.^5,6) However, since hydrogen cyanide is toxic, in this study, the deposition of Zn–Co alloys has been investigated in a zincate solution for environmental reasons. The deposition of Zn–Co alloys from sulfate solutions exhibits anomalous behavior, wherein the electrochemically less noble Zn is preferentially deposited over Co.^7,8,9,10,11) Although the deposition of Zn–Co alloys from chloride solutions also exhibits anomalous behavior, these characteristics gradually change from anomalous to normal with increasing chloride-ion concentrations in the solution.

Moreover, the deposition of Zn–Co alloys from zincate solutions exhibits anomalous behavior, which significantly depends on solution temperature, agitation and additives.^4,12,13,14) This indicates an ambiguity in comparison with the behavior observed in sulfate and chloride solutions, and the complexing agent used for Co²⁺ ions and the brightening agent added to the zincate solutions may affect deposition behavior. In this study, triethanolamine was added to the electrolyte as a complexing agent for Co²⁺, but no brightening agent was used to clarify the effect of the zincate solution on deposition behavior. The Zn–Co alloys were then deposited from this zincate solution, and deposition behavior was compared with that reported for conventional zincate^4,12,13,14) and sulfate^7,8,9,10,11) solutions.

2. Experimental

Table 1 shows the composition of the zincate solution and the electrolysis conditions for Zn–Co alloy deposition. The electrolytic solutions were prepared by dissolving reagent-grade ZnO (0.15 mol·dm⁻³), CoSO₄·6H₂O (0.016 mol·dm⁻³), N(CH₂CH₂OH)₃ (0.16 mol·dm⁻³), and NaOH (2.5 mol·dm⁻³) in distilled and deionized water. In some experiments, deposition was conducted from pure Co or Zn solutions to avoid the presence of CoSO₄·6H₂O or ZnO, respectively, from the standard solution described above. Electrodeposition was performed in unagitated solutions under coulostatic (5 × 10⁴ C·m⁻²) and galvanostatic (2–500 A·m⁻²) conditions at 308 K. Copper (1 × 2 cm²) and platinum (1 × 2 cm²) sheets were used as the cathode and anode, respectively. Cathode deposits were dissolved using nitric acid, and the Zn and Co contents of the deposited Zn–Co alloys were quantitatively analyzed by inductively coupled plasma spectroscopy. The Co content in the deposit and the cathode current efficiency for Zn and Co deposition were calculated. The partial current densities for Zn and Co deposition, as well as that for H₂ evolution, were determined by multiplying the total current density by each current efficiency value. The current efficiency for H₂ evolution was calculated by subtracting the current efficiency values for Zn and Co deposition from 100%. All potentials were plotted with reference to the normal hydrogen electrode (NHE), and the cathode potentials were measured using a saturated KCl, Ag/AgCl reference electrode (0.199 V vs. NHE, 298 K). The chemical states of Zn and Co in the deposits obtained at 5 and 20 A·m⁻² were investigated by X-ray photoelectron spectroscopy (XPS) after sputtering for 10 min with Ar gas. The morphology of the 3-μm-thick deposits was observed by scanning electron microscopy (SEM), and the structure of the alloys was examined by X-ray diffraction (XRD).

Table 1. Solution compositions and electrolysis conditions.

ZnO (mol·dm3)	0.15	Current density (A·m2)	2–500
CoSO4·7H2O (mol·dm3)	0.016	Temperature (K)	308
N (CH2CH2OH)3 (mol·dm3)	0.16	Amount of charge (C·m2)	5×104
NaOH (mol·dm3)	2.5	Cathode	Cu (1×2 cm2)
		Anode	Pt (1×2 cm2)
		Quiescent bath

3. Results and Discussion

3.1. Deposition Behavior of Zn–Co Alloys

Figure 1 shows the total polarization curve and partial polarization curves for Zn and Co during the deposition of the Zn–Co alloy. In the total polarization curve, the current density gradually increased, starting from −0.95 V, and the cathode potential rapidly shifted to the less noble direction at current densities above 5 A·m⁻². A second rapid increase in the current density was observed at approximately −1.27 V. A partial current density for Zn deposition was detected even at potentials between −0.95 and −1.0 V, and the cathode potential apparently shifted rapidly to the less noble direction; it increased at approximately −1.27 V in a manner similar to that observed in the total polarization curve. The rate of increase in the partial current density for Zn deposition slowed down at current densities above 500 A·m⁻², which is attributed to the approach to the diffusion limitation of Zn²⁺ ions. On the other hand, the partial current density for Co deposition gradually increased, starting from −0.95 V, and the cathode potential rapidly shifted to the less noble direction at current densities above 0.2 A·m⁻². Then, a second rapid increase in the partial current density for Co deposition was observed at approximately −1.27 V, and it became almost constant at approximately 8 A·m⁻². This is attributed to the approach to the diffusion limitation of Co²⁺ ions. The equilibrium potential for Zn deposition was calculated to be −1.27 V, assuming that the deposit is pure Zn.

Fig. 1.

Polarization curves for Zn–Co alloy deposition from a zincate solution.

Figure 2 shows the effect of the current density on the Co content in the deposits and the current efficiency for Zn–Co alloy deposition. The broken line in the figure shows the composition reference line (CRL) of Co, which means that the Co content in solution is identical to that in the deposits. If the Co content in the deposits is greater than the CRL, normal co-deposition occurs, wherein the electrochemically more noble Co is preferentially deposited over Zn. In contrast, if the Co content in the deposits is lesser than the CRL, anomalous co-deposition occurs, wherein the less noble Zn is preferentially deposited over Co. As can be seen in Fig. 2, the Co content in the deposits significantly changed at approximately 5 A·m⁻². At current densities below 3 A·m⁻², the Co content in the deposits was approximately 85 mass% above the CRL, exhibiting normal co-deposition, whereas at current densities above 6 A·m⁻², the Co content was below the CRL, showing anomalous co-deposition. The current density at which the Co content in the deposits changed notably was approximately 5 A·m⁻². At this value, the cathode potential in the total polarization curve rapidly shifted in the less noble direction, as shown in Fig. 1. Also, as can be seen in Fig. 2, the current efficiency for Zn–Co alloy deposition was approximately 20% in the normal co-deposition region, reaching a minimum value at 5 A·m⁻² and then dramatically increasing with the current density until a large decrease was observed at 500 A·m⁻². The rapid increase in the current efficiency observed above 6 A·m⁻² is caused by the fact that the cathode potential reaches the equilibrium potential for Zn deposition (−1.27 V), whereas the decrease at 500 A·m⁻² is attributed to the approach to diffusion limitation of the Co²⁺ and Zn²⁺ ions, as can be seen from the polarization curves in Fig. 1.

Fig. 2.

Effect of current density on the alloy composition in the deposit and current efficiency for Zn–Co deposition from a zincate solution.

Figure 3 shows the partial polarization curves for Co deposition from both pure Co and Zn–Co alloy solutions. In both cases, the partial current density of Co rapidly increased at almost the same potential (approximately −1.0 V), so that no effects of the presence of Zn²⁺ ions on Co deposition were observed at current densities below 0.2 A·m⁻². However, at partial current densities above this value, the cathode potential for Co deposition from a Zn–Co alloy solution shifted to the less noble direction compared to the equilibrium potential for Zn deposition, exhibiting a polarization larger than that from the pure Co solution.

Fig. 3.

Partial polarization curves for Co deposition from pure Co and Zn–Co alloy alkaline solutions.

Figure 4 shows the partial polarization curves for H₂ evolution from pure Co and Zn–Co alloy solutions. In both cases, H₂ evolution began at almost the same potential (approximately −0.95 V), and no effects of the presence of Zn²⁺ ions on H₂ evolution were observed at cathode potentials more noble than −1.0 V. However, upon increasing the total current density further, the cathode potential for H₂ evolution shifted to the less noble direction compared to the equilibrium potential for Zn deposition because of the coexistence of Zn²⁺ ions, which results in a polarization greater than that observed from the pure Co solution. H₂ evolution behavior from pure Co and Zn–Co alloy solutions showed trends similar to that observed for Co deposition, as can be seen in Fig. 3. In other words, H₂ evolution and Co deposition were significantly suppressed at high current densities by the coexistence of Zn²⁺ ions. To confirm that Co deposition is not affected by the presence of Zn²⁺ ions at low current densities, the effect of these ions on the current efficiency of this process was investigated. As can be seen in Fig. 5, at low current densities of approximately 3 A·m⁻² (at which normal co-deposition occurs), the current efficiency for Co deposition did not decrease, even under the coexistence of Zn²⁺ ions, suggesting that there is no effect of the presence of these ions on Co deposition.

Fig. 4.

Partial polarization curves for H₂ evolution from pure Co and Zn–Co alloy alkaline solutions.

Fig. 5.

Current efficiencies for Co deposition from pure Co and Zn–Co alloy alkaline solutions.

Figure 6 shows the partial polarization curves for Zn deposition obtained from pure Zn and Zn–Co alloy solutions. Focusing on the initiation of Zn deposition, it can be seen that Zn began to deposit at approximately −1.27 V (i.e., the equilibrium potential for pure Zn deposition) when it was obtained from the pure Zn solution, but it already began to form at potentials about 0.3 V nobler than the equilibrium potential when it was deposited from the Zn–Co alloy solution. This result suggests that the underpotential deposition of Zn may occur in the Zn–Co alloy solution.

Fig. 6.

Partial polarization curves for Zn deposition from pure Zn and Zn–Co alloy alkaline solutions.

The partial polarization curve for Zn deposition is slightly affected by the addition of triethanolamine.^15,16) However, because the degree of polarization is low, triethanolamine (as a ligand) appears to marginally affect the complexation of Zn²⁺ ions, and therefore also on the Zn deposition process.

To compare the deposition behavior of Zn–Co alloys obtained from sulfate and zincate solutions, the dependence of the alloy composition in the deposits and the current efficiency for Zn–Co alloy deposition on the current density in a sulfate solution is shown in Fig. 7.⁹⁾ Although in this case, normal co-deposition was observed at lower current densities, while evolution of H₂, which is the most electrochemically noble, preferentially occurred, which resulted in a current efficiency of approximately zero. On the other hand, at the higher current densities required to obtain practical Zn–Co alloys with high current efficiency, a specific anomalous deposition was observed.^7,8,9) To explain this anomalous co-deposition behavior in a sulfate solution, the hydroxide suppression mechanism, which is described in points (1) and (2), has been proposed:^7,8,9) (1) Co-deposition from hydrated ions proceeds by a multistep reduction mechanism by the adsorption of the intermediate species CoOH, which contains a hydroxyl group. The adsorption sites for CoOH are limited. (2) Zn(OH)₂ species, which result from the increase in pH at the cathode layer caused by H₂ evolution during electrolysis, adsorb on the cathode and obstruct the available sites for CoOH_ad, thus suppressing Co deposition. In the case of the zincate solution studied herein, normal co-deposition occurred at lower current densities, whereas anomalous co-deposition was observed at increasing current densities. This behavior is similar to that observed in the sulfate solution, as shown in Fig. 2. However, in the zincate solution, the current efficiency for Zn–Co alloy deposition was not close to zero in the normal co-deposition region (i.e., at lower current densities), which is clearly different from that observed in the sulfate solution, where the current efficiency for alloy deposition was approximately zero at low current densities. The reason for this is that the sites available for CoOH_ad adsorption are limited, and Zn(OH)₂ species present in the medium can further suppress Co deposition by blocking the adsorption sites for CoOH_ad. On the other hand, since Co deposition is not affected by the coexistence of Zn²⁺ ions at lower current densities in the zincate solution (as shown in Figs. 3 and 5), it appears that either the sites for Co deposition are not restricted in this case or no inhibitors for Co deposition, such as Zn(OH)₂, can be formed. This means that points (1) and/or (2), described above for the sulfate solution, do not apply in this case, and therefore, the current efficiency for alloy deposition at lower current densities is greater in the zincate solution than in the sulfate solution. However, with increasing current densities, anomalous co-deposition behavior, similar to that observed from the sulfate solution, was also observed in the zincate solution. Co deposition and H₂ evolution were considerably suppressed by the coexistence of Zn²⁺ ions at higher current densities, as shown in Figs. 3 and 4. Since Zn(OH)₂ species formed at the cathode layer suppress both Co deposition and H₂ evolution in sulfate solutions,^7,8,9) and considering that in this study, the cathode potential rapidly shifted to the less noble direction at current densities above 5 A·m⁻² (Fig. 1), some type of inhibitor for Co deposition and H₂ evolution resulting from the presence of Zn²⁺ ions may also be formed at current densities higher than 5 A·m⁻² in the zincate solution. The minimum current efficiency for alloy deposition, observed at approximately 5 A・m⁻² (as shown in Fig. 2), is attributed to a significant suppression of the Co deposition process by an inhibitor species resulting from Zn²⁺ ions present in the medium. For the deposition of Zn from a zincate solution, the single-step, two-electron reaction described by Eq. (1) and the multistep reaction described by Eqs. (2) and (3) have been reported on the basis of interfacial impedance measurements:^17,18,19)   

$${\text{Zn(OH)}}_{\text{4}}^{\text{2-}}+\text{2e}\to \text{Zn}+{\text{4OH}}^{-}$$(1)

  

$${\text{Zn(OH)}}_{\text{4}}^{\text{2-}}\to {\text{Zn(OH)}}_2+{\text{2OH}}^{-}$$(2)

  

$${\text{Zn(OH)}}_2+2e\to \text{Zn}+{\text{2OH}}^{-}$$(3)

Assuming that Zn deposition proceeds by the multistep reaction described by Eqs. (2) and (3) at higher current densities, the mechanism of anomalous co-deposition can be explained by the suppression of Co deposition, caused by the formation of Zn(OH)₂ species at the cathode layer.

Fig. 7.

Effect of current density on the composition and current efficiency for Zn–Co alloy deposition from sulfate solutions⁹⁾ (ZnSO₄ 0.5 mol·dm⁻³, CoSO₄ 0.5 mol·dm⁻³, pH 3, 313 K).

In the case of Zn–Co alloy deposition from a zincate solution, deposition behavior becomes anomalous in solutions containing triethanolamine,^12,13) diethanolamine,¹²⁾ tetraethylenepentamine,⁴⁾ and hydroxy carboxylic acid¹⁴⁾ as complexing agents for Co²⁺ ions. In this study, when triethanolamine was added to the electrolyte as the complexing agent, anomalous deposition behavior was observed, confirming the trend reported in the literature. However, at lower current densities, normal deposition behavior was observed, suggesting the formation of some type of inhibitor species for Co deposition (resulting from Zn²⁺ ions present in the medium) at current densities above a specific value. The effects of Zn²⁺ ions on the partial polarization curves for Co deposition and H₂ evolution were also investigated. It was found that the cathode potentials for these two processes were significantly affected by the coexistence of Zn²⁺ ions, thus proving the formation of an inhibitor for Co deposition and H₂ evolution resulting from the Zn²⁺ species.

3.2. Structure of the Deposited Zn–Co Alloys

In this study, the equilibrium potential for pure Zn deposition was −1.27 V. However, in the case of Zn–Co alloy deposition, the partial current density for Zn deposition was detected at approximately −0.95 V, as shown in Figs. 1 and 6. This result indicates the apparent underpotential deposition of Zn. An XPS analysis was conducted to confirm whether Zn in the deposits obtained at potentials more noble than the equilibrium potential of Zn (−1.27 V) is in the metallic state. Figure 8 shows the XPS spectra of the Zn component in deposits obtained at 5 and 20 A·m⁻² at cathode potentials of −1.0 and −1.27 V, respectively. The Co contents in these deposits were 90 and 8 mass%, respectively, and the XPS peaks for Zn corresponded to metallic Zn (after sputtering for 10 min with Ar gas), suggesting that Zn was deposited in the metallic state. As mentioned above, an apparent underpotential deposition of Zn was observed at lower current densities in the case of the Zn–Co alloy formation from a zincate solution. Similar Zn underpotential depositions have occurred from the formation of Zn–Ni alloys^15,16) from zincate solutions and Zn–iron-group metal alloys²⁰⁾ from sulfate and chloride solutions under conditions that decreased the inherent overpotential for the deposition of the iron-group metals.

Fig. 8.

XPS of the Zn component in Zn–Co alloys deposited at (a) 5 and (b) 20 A·m⁻² from a zincate solution.

Figure 9 shows the XPS spectra of Co in deposits obtained at 5 and 20 A·m⁻². The Co peaks corresponded to metallic Co (after sputtering for 10 min with Ar gas), suggesting that Co was deposited in the metallic state.

Fig. 9.

XPS of the Zn component in Zn–Co alloys deposited at (a) 5 and (b) 20 A·m⁻² from a zincate solution.

Figure 10 shows the XRD pattern of a deposit obtained at 5 A·m⁻². This deposit, which was formed at a more noble potential than the equilibrium potential of Zn, exhibited peaks that result from the intermetallic compounds CoZn₁₃ and Co₅Zn₂₁ and from single-phase Co. Since the Co content in the deposit obtained at 5 A·m⁻² was 90 mass%, this deposit seems to be composed of the intermetallic compounds CoZn₁₃ and Co₅Zn₂₁ and of single-phase Co. The four peaks for single-phase Co, which correspond to the (1010), (1011), (1020), and (1022) Co planes (in increasing order of the diffraction angle) are shown in Fig. 10.

Fig. 10.

X-ray diffraction pattern of a Zn–Co alloy deposited at 5 A·m⁻² from a zincate solution.

Figure 11 shows the XRD pattern of a deposit obtained at 20 A·m⁻². This deposit exhibited peaks resulting from single-phase Zn and the intermetallic compounds CoZn₁₃ and Co₅Zn₂₁. Since the Co content in the deposit obtained at 20 A·m⁻² was 8 mass%, this deposit seems to be composed of the intermetallic compounds CoZn₁₃ and Co₅Zn₂₁ and of single-phase Zn.

Fig. 11.

X-ray diffraction pattern of a Zn–Co alloy deposited at 20 A·m⁻² from a zincate solution.

According to the XRD analysis, during Zn–Co alloy deposition from a zincate solution, Zn is most probably deposited in the form of the stable intermetallic compounds CoZn₁₃ and Co₅Zn₂₁ at potentials more noble than the equilibrium potential for pure Zn deposition (i.e., −1.27 V).

E_M^eq, which is the equilibrium potential for metal deposition, is expressed by the Nernst Eq. (4).   

$${E_{\text{M}}}^{\text{eq}}={E_{\text{M}}}^{\text{0}}+\text{(}RT/nF\text{)ln(}{a}_{\text{Mn+}}/a_{\text{M}}\text{)}$$(4),

where E_M⁰ is the standard single electrode potential of the metal M; R is the gas constant; T is the absolute temperature of the solution; n is the charge number of M; F is the Faraday constant; a_Mn+ is the activity of the Mⁿ⁺ ions; and a_M is the activity of the deposited M:   

$$a_{\mathrm{M}}=f_{\mathrm{M}}{X}_{\mathrm{M}}$$(5),

where f_M and X_M are the activity coefficient and mole fraction of the metal M, respectively.

Assuming that the activity coefficient of Zn in the deposits is f_Zn=10⁻¹⁰ at a Zn mole fraction of 0.1 in the deposited Zn–Co alloy, the equilibrium potential for Zn deposition E_Zn^eq is calculated to be −0.95 V, suggesting that the apparent underpotential deposition of this component can be explained thermodynamically. During Zn–Co alloy deposition, the activity coefficient of Zn in the deposits is assumed to markedly decrease by the formation of the stable intermetallic compounds CoZn₁₃ and Co₅Zn₂₁.

Figure 12 shows SEM images of deposits obtained at 5 and 20 A·m⁻². The deposit obtained at 5 A·m⁻² corresponded to the apparent underpotential deposition of Zn and consisted of platelet crystals that are significantly inclined perpendicular to the substrate surface. Since the crystal orientation of single-phase Co is (1010), (1011), (1020) and (1022), as shown in Fig. 10, the platelet crystals [(0001) Co basal plane of hcp structure] appeared to be significantly inclined perpendicular to the substrate surface. On the other hand, the deposit obtained at 20 A·m⁻² consisted of aggregates of granular crystals with diameters below 1 μm, and these granular crystals are in turn composed of further smaller granular crystals.

Fig. 12.

SEM images of Zn–Co alloys deposited at (a) 5 and (b) 20 A·m⁻² from a zincate solution.

4. Conclusion

Deposition behavior of Zn–Co alloys was investigated from a zincate solution containing triethanolamine as the complexing agent for Co²⁺. Normal co-deposition, wherein the electrochemically more noble Co was preferentially deposited, occurred at low current densities below 5 A・m^–2, whereas anomalous co-deposition, wherein the electrochemically less noble Zn was preferably deposited, was observed at current densities above 6 A・m^–2. The current efficiency for alloy deposition was low (approximately 20%) in the normal co-deposition region and increased to 95% in the anomalous co-deposition region. The partial polarization curves for Co deposition and H₂ evolution were significantly affected by coexisting Zn²⁺ ions in the anomalous co-deposition region. This indicates the formation of an inhibitor species resulting from the presence of Zn²⁺ ions in the cathode layer. In the normal co-deposition region, the underpotential deposition of Zn apparently occurred simultaneously with Co deposition. Zn–Co alloys are composed of stable intermetallic compounds CoZn₁₃ and Co₅Zn₂₁; therefore, the activity coefficient of Zn in the deposits appears to decrease remarkably.

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