Electrodeposition of Zn–V Oxide Composites from a Strongly Agitated Solution without Dispersed Particles

Abstract

The electrodeposition of a Zn–V oxide composite under galvanostatic conditions from an agitated sulfate solution without dispersed particles and containing Zn²⁺ and VO²⁺ at pH 2 and 313 K was investigated. Although the V content in the deposits initially decreased with increasing current density, irrespective of the flow rate of electrolyte, a further increase in the current density resulted in an increase in the V content of the deposits. The curves, which show the relationship between the V content in the deposits and the current density, shifted to the higher-current-density region with increasing flow rate of the electrolyte. Agitation of the electrolyte decreased the V content of the deposits but reduced the segregation of V oxide. EDX point analysis of the cross-section of the deposits revealed that the V oxide concentrated at the surface of the deposits. The polarization curves in 3% NaCl solution revealed that the corrosion potential of the deposited Zn–V oxide films depended on the V content in the deposits, irrespective of the flow rate of electrolyte, and that the corrosion potential shifted toward the more noble direction with the codeposition of V oxide when the V content in the deposits was less than 2 mass%. At V contents of <4 mass%, the corrosion current density of the deposits decreased with increasing V content. The corrosion current densities of the deposits obtained from agitated solutions were smaller than those of the deposits obtained from unagitated solutions.

1. Introduction

The investigations of Zn alloy films produced by hot-dip galvanization and dry-process coating have demonstrated that the alloying elements less noble than Zn, such as Mg, Al, or Ti, are promising to improve the corrosion resistance of Zn alloy films.^{1,2,3,4,5,6,7,8,9,10)} V (with a standard single-electrode potential E⁰ = –1.186 V vs. NHE) is a considerably less noble metal than Zn (E⁰ = –0.76 V); therefore, it is expected to improve the corrosion resistance of Zn alloy films. In this previous case, the authors added VO²⁺, which hydrolyzes at a lower pH than Zn²⁺, to the electrolyte and performed the electrodeposition of the composites of Zn with V oxide in unagitated solutions to investigate the deposition behavior, structure, and polarization properties of the deposits.¹¹⁾ They observed that V was incorporated into the deposited Zn in the form of its oxide by the hydrolysis of V ions and was segregated at the edges of layered platelet crystals of Zn.

In the deposition of Zn alloys, the composition of the deposited alloy, the current efficiency, and the surface appearance have been reported to be substantially affected by the flow rate of the electrolyte because the diffusion-limiting current density of each alloy element changes with the flow rate.^12,13,14,15) When Zn–V oxide composites are electrodeposited under a high-flow-rate condition, the deposition behavior, the distribution of V oxide, and the polarization properties of the deposits are expected to be affected by the flow rate. Therefore, in this study, we electrodeposited Zn–V oxide composites under a high-flow-rate condition. The relative flow rate of the electrolyte on the cathode was altered by rotating the cylindrical cathode. The effects of the current density and the electrolyte flow rate on the content of V oxide in the deposits was investigated, and the distribution of V oxide in the deposits and the polarization properties were examined.

2. Experimental

Table 1 shows the solution composition and electrolysis conditions. The electrolytic solution was prepared by dissolving fixed amounts of high-grade ZnSO₄·7H₂O (0.52 mol/L) and VOSO₄·5H₂O (0.21 mol/L) in distilled deionized water. The pH was adjusted to 2 with sulfuric acid. A copper cylinder measuring ϕ4 cm, 2 cm in length, and 25.1 cm² was fixed with a Teflon ring and was used as cathode, as shown in Fig. 1. The electrodeposition was conducted on the cylindrical surface of a Cu cathode rotated at 500, 1000, 2000, 3000 rpm under coulostatic (10⁵ C/m²) and galvanostatic (500–7000 A/m²) conditions at 313 K. The rotation rates of 500, 1000, 2000, and 3000 rpm correspond to relative flow rates of 1.1, 2.1, 4.2, and 6.3 m/s of the electrolyte to the cathode surface, respectively. The degree of agitation is represented by the relative flow rate in the present work. Titanium mesh plated with platinum was arranged around the cathode and was used as an anode. Because VO²⁺ is oxidized to V₂O₅ on a platinum anode during electrolysis, the electrolysis was performed in a cell in which the catholyte was separated from the anolyte by filter made of Japanese paper. The deposits were dissolved from the cathode with nitric acid. Both Zn and V were quantitatively analyzed by inductively coupled plasma atomic emission spectroscopy (ICP), and the V content of the deposit and the cathode current efficiency were calculated.

Table 1. Electrolysis conditions.

Bath composition	ZnSO4·7H2O	(mol/L)	0.52
	VOSO4·5H2O	(mol/L)	0.21
	pH		2
Operating conditions	Current density	(A/m2)	500–7000
	Amount of charge	(C/m2)	105
	Temperature	(°C)	40
	Cathode	Cu (ϕ4 cm×2 cm)
	Anode	Pt/Ti
	Flow rate	(m/s)	1.1–6.3

Fig. 1.

Apparatus for electrolysis. (Online version in color.)

The morphologies of the surface and the cross-section of deposits were observed by ultra-low-voltage scanning electron microscopy (Carl Zeiss Ultra55), and the distribution of Zn, V, and O was visualized by electron probe microanalysis (EPMA) and energy-dispersive X-ray analysis (EDX). The crystal orientation of the deposited Zn was determined using the method developed by Wilson and Rogers;¹⁶⁾ an X-ray diffraction intensity of 0002 to the 1122 reflection was used. To study the hydrolysis behavior of the hydrated metal ions in the solutions, pH titrations were performed using NaOH. A 5.0 N NaOH solution was added to a 0.05 mol/L ZnSO₄ and/or a VOSO₄ solution using a burette. The concentration of Zn²⁺ and VO²⁺ ions in the mixed solution was measured by ICP during the pH titrations. An Sb microelectrode¹⁷⁾ was fabricated to measure the pH change in the vicinity of the cathode during Zn–V oxide composite deposition accompanied by hydrogen evolution. Under a galvanostatic condition of 200 A/m² in unagitated solutions, the potential of the Sb electrode attached to a micrometer was measured at various distances between the cathode and Sb electrode. Using a pH–potential calibration curve measured in advance, we determined the pH profile near the cathode during electrolysis.

The corrosion resistance of the deposited Zn–V oxide composite films was evaluated by polarization curves. Potentiodynamic polarization curves were measured by polarizing from the less noble potential than the corrosion potential toward the anodic-potential direction using a potential sweep method at 1.0 mV/s in an oxygen-saturated 3% NaCl solution at 313 K. The corrosion current density and the corrosion potential of the deposits were calculated using a Tafel-plot extrapolation method. The polarization curves were measured using a saturated KCl, Ag/AgCl reference electrode (0.199 V vs. NHE, 298 K). Potentials were plotted with reference to the NHE.

3. Results and Discussion

3.1. Chemical State and Equilibrium Potential of Vanadium

Figure 2 shows the potential vs. pH diagram for the V–H₂O system. The concentration of V in the solution and the activity coefficient of the V ions are assumed to be 0.8 mol/L and 1, respectively. The thermodynamic data are quoted from Paurbaix.¹⁸⁾ As evident in Fig. 2, V exists in the form of VO²⁺ in the electrolyte with a pH of 2; however, at pH values greater than 2.38, V₂O₄ is formed as a result of the hydrolytic reaction of VO²⁺. In the electrodeposition of Zn from aqueous solution, the pH in the cathode layer increases as a result of the evolution of hydrogen. When the pH in the cathode layer reaches the critical pH value for the hydrolysis of VO²⁺, VO²⁺ is expected to be converted into an oxide such as V₂O₄.

Fig. 2.

Potential–pH diagram for the V–H₂O system at 298 K (a_V = 0.8).

3.2. Codeposition Behavior of V Oxide

Figure 3 shows the effects of the current density and the electrolyte flow rate on the V content in the deposits. The V content in the deposits in this study was calculated on the basis of the masses of V and Zn in the deposits using the following equation:   

$$\mathrm{V}\mathrm{\hspace{0.17em} }\mathrm{c}\mathrm{o}\mathrm{n}\mathrm{t}\mathrm{e}\mathrm{n}\mathrm{t}=[\mathrm{m}\mathrm{a}\mathrm{s}\mathrm{s}\mathrm{\hspace{0.17em} }\mathrm{V}/\mathrm{t}\mathrm{o}\mathrm{t}\mathrm{a}\mathrm{l}\mathrm{\hspace{0.17em} }\mathrm{m}\mathrm{a}\mathrm{s}\mathrm{s}\mathrm{\hspace{0.17em} }(\mathrm{Z}\mathrm{n}+\mathrm{V})]\times 100$$

Fig. 3.

Effect of current density and flow rate on the V content of deposits.

The V content in the deposits decreased and then increased with increasing current density at all flow rates. The increase in the V content in the deposits with increasing current density in the high-current-density region is ascribed to an increase in the hydrolysis reaction of VO²⁺ ions because the pH in the cathode layer increases as a result of an increase in rate of hydrogen evolution with increased current density. The curves, which show the relationship between the V content in the deposits and the current density, shifted toward the higher-current-density direction with increasing flow rate. The V content in the deposits decreased with increasing flow rate in the high-current-density region. This decreased V content is caused by the suppression of the increase in pH or by the suppression of the hydrolytic reaction of VO²⁺ due to increased diffusion of H⁺ into the cathode layer at higher flow rates.

Figure 4 shows the effects of the current density and the electrolyte flow rate on the current efficiency of Zn in Zn–V oxide composite deposition. The current efficiency initially increased with increasing current density, reached a maximum, and then decreased as the current density increased further. The increase in current efficiency with increasing current density in the lower-current-density region is attributed to the increase in the overpotential for Zn deposition, whereas the decrease in current efficiency with increasing current density in the higher-current-density region is caused by reaching the diffusion limitation of Zn²⁺ ions. However, the curves that show the relationship between the current efficiency for Zn deposition and the current density shifted toward the higher-current-density direction with increasing flow rate. As a result, the current efficiency decreased with increasing flow rate at all of the experimental current densities, which is attributed to the acceleration of diffusion of H⁺ ions with increasing flow rate. At high current densities, the current density at which current efficiency began to decrease shifted toward the higher-current-density direction with increasing flow rate. This shift is caused by difficult to reach the diffusion limitation of Zn²⁺ ions with increasing flow rate. At low current densities, the V content in the deposits increased with decreasing current density at all of the experimental flow rates, as shown in Fig. 3; this increased V content is because of a decrease in the current efficiency of Zn with decreasing current density, as shown in Fig. 4.

Fig. 4.

Effect of current density and flow rate on the current efficiency for Zn deposition.

During deposition from the solution containing VO²⁺ and Zn²⁺, the pH in the vicinity of the cathode increased due to hydrogen evolution, which resulted in the formation of a metal oxide via hydrolysis. The critical pH value for V₂O₄ formation was calculated to be 3.1 on the basis of solubility product of V₂O₄ [10^–23.1 (298 K)]¹⁸⁾ in the solution containing 0.05 mol/L VO²⁺. Similarly, the critical pH value for Zn(OH)₂ formation was calculated to be 6.3 on the basis of the solubility product of Zn(OH)₂ [2 × 10^–17 (298 K)]¹⁹⁾ in the solution containing 0.05 mol/L Zn²⁺. Figure 5 shows the pH titration curves measured for a solution containing Zn²⁺ and VO²⁺ titrated with 5.0 N NaOH. In the solution containing only Zn²⁺, the increase in pH stagnated, and the solution began to form a suspension as a result of the formation of precipitates at a pH of approximately 6.5. Similarly, in the solution containing only VO²⁺, the increase in pH stagnated and the solution began to form a suspension at a pH of approximately 4.0. In the case of the solution containing both Zn²⁺ and VO²⁺, the increase in pH stagnated at pH levels of approximately 4.0 and 6.5, which is identical to the results obtained by the superposition of the two pH titration curves obtained from the solutions containing only Zn²⁺ and only VO²⁺. The pH values, which stagnated, were similar to the critical pH levels necessary for the formation of V₂O₄ and Zn(OH)₂, as calculated from each solubility product. Because the critical pH value for V₂O₄ formation is lower than that for Zn(OH)₂ formation, V₂O₄ should preferentially form at the cathode layer. Furthermore, Fig. 5 shows the concentration of Zn²⁺ and VO²⁺ ions measured during titration of the solution with NaOH. At a pH of approximately 4.0, where the increase in pH first stagnated, the concentration of VO²⁺ ions decreased, and, at a pH of approximately 6.5, where the increase in pH stagnated again, the concentration of Zn²⁺ ions decreased. These results demonstrate that the precipitations of V₂O₄ and Zn(OH)₂ occurred at pH values of 4.0 and 6.5, respectively. The concentration of Zn²⁺ ions rarely changed during the formation of the V₂O₄ precipitate, which indicate that Zn²⁺ did not coprecipitate with V₂O₄.

Fig. 5.

pH titration curves for the Zn–V solution and the concentration of VO²⁺ and Zn²⁺ in Zn–V solutions. (Online version in color.)

The pH in the vicinity of the cathode was measured using an Sb microelectrode in the unagitated solutions containing VO²⁺ and Zn²⁺. Figure 6 shows the pH in the vicinity of the cathode during deposition at 200 A/m². The pH of the cathode surface increased to 5.3, which is substantially higher than the pH of the bulk solution containing only Zn²⁺, whereas the pH increased to only 3.8 in the solution containing both VO²⁺ and Zn²⁺. In the solution containing only Zn²⁺, the pH appeared to increase to the critical pH necessary for the formation of Zn(OH)₂ because of hydrogen evolution during electrolysis. In contrast, in the solution containing both VO²⁺ and Zn²⁺, the pH stagnated at a pH lower than the critical pH for the formation of Zn(OH)₂, which is attributed to buffer action by the hydrolysis of VO²⁺ at a pH of approximately 4.0. As previously mentioned, during deposition from the solution containing both VO²⁺ and Zn²⁺, the pH in cathode layer increased due to hydrogen evolution; however, the pH did not reach the critical pH for formation of Zn(OH)₂ because of buffer action by the hydrolysis of VO²⁺, which resulted in the formation of V₂O₄ only in the cathode layer. We hypothesize that Zn²⁺ ions are reduced to Zn via the V₂O₄ layer without the formation of Zn(OH)₂ intermediate, and V₂O₄ is then incorporated into the resulting deposits.

Fig. 6.

pH profiles in the vicinity of the cathode during Zn–V and Zn deposition.

3.3. Structure of the Deposits

Figure 7 shows SEM images of pure Zn and Zn–V oxide composites deposited at 5000 A/m² from unagitated solutions and at a flow rate of 2.1 m/s. Pure Zn deposited from an unagitated solution and at a flow rate of 2.1 m/s consisted of layers of large Zn platelet crystals with a hexagonal structure (hcp); the edges of the platelet crystals were clear, as shown in Figs. 7(a) and 7(c). In contrast, SEM images of the Zn–V oxide composite deposited from an unagitated solution and at a flow rate of 2.1 m/s showed smooth (b) and granular crystals (d) at the voids of the Zn platelet crystals; the clear Zn platelet crystals were not observed.

Fig. 7.

SEM images of deposits obtained at 5000 A/m² from solutions with and without VO²⁺ ions.

Figure 8 shows EPMA images of Zn–V oxide composites deposited at 5000 A/m² and at flow rates of 1.1, 2.1, and 4.2 m/s. The backscattered electron images (a) and characteristic X-ray images of Zn and V (b, c) in deposits with a V content of 7.3 mass% obtained at a flow rate 1.1 m/s revealed that the V concentration was high in areas of low Zn concentration and that V was segregated at the boundaries of Zn platelet crystals. In addition, segregation of oxygen (d) was observed, and its position coincided with the position of segregated V (c), indicating that V codeposited in its oxide form with Zn. In the case of the deposit obtained at a flow rate 2.1 m/s, the V (g) concentration was high in areas of low Zn concentration and at the boundaries of Zn platelet crystals; however, the distribution was more uniform than in the deposit formed at a flow rate of 1.1 m/s. Because the V concentration at the boundaries of the Zn platelet crystals was substantial, the crystals at the boundaries of the Zn platelet crystals observed in the SEM images presented in Fig. 7(d) appear to be V oxide. A further increase in the flow rate to 4.2 m/s decreased the V content to 1.8 mass% and resulted in a more uniform distribution of V. The increase in flow rate caused the uniform distribution of V, which suggests that the hydrolysis reaction of VO²⁺ occurs more uniformly at the cathode layer when the solution is agitated.

Fig. 8.

EPMA images of deposits obtained at 5000 A/m² and at various flow rates from a solution containing VO²⁺ ions. (Online version in color.)

Figure 9 shows the crystal orientation of pure Zn deposited at various current densities and at a flow rate of 2.1 m/s. The preferred orientation of the Zn was along its {1013} plane, and the orientation of the {0001} basal plane was significantly observed at all of the investigated current densities. The inclination of the {1013} to the {0001} basal plane of hcp is smaller than that of the other planes to {0001}. When the preferred orientation of deposited Zn is along the {1013} plane, the basal plane of Zn platelet crystals are likely to be parallel to the substrate, which corresponds to the SEM images shown in Fig. 6(c).

Fig. 9.

Crystal orientation of Zn deposited at a flow rate of 2.1 m/s and at various current densities [●(0002), ▲(1012), ■(1013), □(1122), ×(1120)].

Figure 10 shows the crystal orientation of Zn in Zn–V oxide composites deposited at various current densities and at a flow rate of 2.1 m/s. The preferred orientation of Zn in the Zn–V oxide composites was the {1122} plane, and the orientation of the {1013} and {0001} planes was smaller than that of the pure Zn deposited at all of the investigated current densities. This result indicates that the basal plane of Zn platelet crystals grew inclining to the substrate. Codeposited V oxide appears to suppress the lateral growth of Zn for orientation of the {0001} plane.

Fig. 10.

Crystal orientation of Zn in Zn–V oxide deposited at a flow rate of 2.1 m/s and at various current densities [●(0002), ○(1010), ▲(1012), ■(1013), ◆(1122), ×(1120)].

Figure 11 shows SEM images of the cross-sections of a Zn–V oxide composite and pure Zn deposited at 5000 A/m² and at a flow rate of 2.1 m/s. A thin layer was observed at the surface of the Zn–V oxide composite (a), which was not observed in the case of pure Zn (b). Because this thin layer was darker than the internal layer or was lower in mass, we presumed it was V oxide. We used EDX to analyze the surface of the Zn–V oxide composite (a) visible in the cross-section. Figure 12 shows the EDX spectrum of the surface of the Zn–V oxide composite deposited at 5000 A/m² and at a flow rate of 2.1 m/s. V was detected, which indicates the presence of V oxide at the surface of the deposits. V was not detected by EDX in the middle or bottom along the thickness direction of the deposits, which indicates that V oxide was concentrated at the surface of the deposits. The mechanism by which V oxide concentrates at the surface of the deposits is unknown, and further investigation is required.

Fig. 11.

SEM images of the cross-section of deposits obtained at 5000 A/m² and at 2.1 m/s from the solutions with and without VO²⁺ ions.

Fig. 12.

EDX spectrum of the cross-section of deposits obtained at 5000 A/m² and at 2.1 m/s from the solutions containing VO²⁺ ions.

3.4. Polarization Properties of Deposits

Figure 13 shows the polarization curves of the deposits with V contents of 2.1 and 0 mass% in 3% NaCl solution. The corrosion potential of the deposit with a V content of 2.1 mass% was more noble than that of a deposit of pure Zn. The current densities for both the anode and cathode reactions were lower in the deposit with a V content of 2.1 mass% than in the deposit of pure Zn; as a result, the corrosion current density of the deposit with a V content of 2.1 mass% was lower. Because the cathode reaction is the reduction of dissolved oxygen in 3% NaCl solution, the reduction of dissolved oxygen appears to be suppressed in the deposits with a V content of 2.1 mass%. The V oxide may represent a diffusion barrier for the dissolved oxygen.

Fig. 13.

Polarization curves of the Zn–V oxide composite in 3% NaCl solution. (Online version in color.)

The corrosion potentials of the deposits, with different V contents, obtained at various flow rates were determined from the polarization curves. Figure 14 shows the corrosion potentials of the deposits. The corrosion potentials of deposits changed depending on the V content in the deposit, irrespective of the flow rate, and were shifted toward the noble direction as the V content was increased to 2 mass%, whereas they were shifted toward the less-noble direction as the V content was increased beyond 2 mass%. Because anodic polarization curves for Zn dissolution are shifted toward the noble direction with the codeposition of V oxide when the V content is less than 2 mass%, the corrosion potentials of the deposits appears to shift toward the noble direction. This shift in the anodic polarization curve is ascribed to a barrier effect of V oxide, which has a low conductivity, on the dissolution of Zn. Although the reason for the corrosion potentials shifting in the less-noble direction at V contents greater than 2 mass% is currently unknown, it can be explained if the anodic reaction is assumed to change from “Zn→Zn²⁺+2e^–” to “Zn+H₂O→ZnO+2H⁺+2e^–.”

Fig. 14.

Relationship between the V content and the corrosion potential of the Zn–V oxide composite in 3% NaCl solution.

Figure 15 shows the corrosion current densities determined from the polarization curves of the deposits. The relationship between the corrosion current density and the V content in the deposits exhibited dispersion; however, the corrosion current density decreased with increasing V content in deposits up to 4 mass%. This decreased current density is attributed to a suppression of both anodic and cathodic reactions by the barrier effect of V oxide. We have reported that the corrosion current density of deposits obtained from unagitated solutions exhibit extensive dispersion and that a clear relationship between the V content in deposits and the corrosion current density was not observed, as shown in Fig. 16.¹⁵⁾ A comparison of the corrosion current densities in Figs. 15 and 16 reveals that the corrosion current density was lower in deposits from agitated solutions than in deposits from unagitated solutions. Because the distribution of V oxide in deposits was more uniform in deposits from agitated solutions, the uniformity of the V oxide may affect the corrosion current density.

Fig. 15.

Relationship between the V content and the corrosion current density of the Zn–V oxide composite in 3% NaCl solution.

Fig. 16.

Relationship between the V content and the corrosion current density of the Zn–V oxide composite in 3% NaCl solution.¹⁵⁾

4. Conclusion

Electrodeposition of Zn–V oxide composites was performed from a strong agitated sulfate solution containing Zn²⁺ and VO²⁺, and the deposition behavior, the structure, and the polarization properties of the deposits were investigated. Although the V content in deposits decreased with increasing current density irrespective of the flow rate of the electrolyte, a further increase in current density brought about the increase in V content in the deposits. The curves, which show the relationship between the V content in the deposits and the current density, shifted to a higher-current-density region with increasing flow rate of the electrolyte. Agitation of the electrolyte decreased the V content of the deposits but reduced the segregation of V oxide. EDX point analysis of the cross-sections of the deposits revealed that the V oxide concentrated at the surface of the deposits. The pH in the vicinity of the cathode, as measured by an Sb microelectrode, was approximately 4.0, which is close to the critical pH for the formation of V₂O₄. The polarization curves in 3% NaCl solution revealed that the corrosion potential of the deposited Zn–V oxide films depended on the V content in the deposits, irrespective of the flow rate of the electrolyte, and that the corrosion potential shifted toward the more noble direction with the codeposition of V oxide when the V content in the deposits was less than 2 mass%. The corrosion current density decreased with increasing V content in deposits up to 4 mass%. The corrosion current densities of the deposits obtained from agitated solutions were smaller than those of the deposits obtained from unagitated solutions.

Acknowledgment

This work was supported by JSPS Grant-in-Aid for Scientific Research (B) Grant Number 26289274.

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