The Effects of Additives on the Electrodeposition of a Zn–Zr Oxide Composite from Dispersed Particle-Free Solution

Abstract

Electrodeposition of a Zn–Zr oxide composite was performed under galvanostatic conditions from an unagitated sulfate solution containing Zn²⁺ and Zr ions, as well as additives, such as NO₃⁻ ions and polyethylene glycol (PEG), at pH 2 and 313 K. The effect of these additives on the codeposition of Zr oxide and its polarization behavior, as well as the microstructure of the deposits, was investigated. The Zr content in the deposits obtained at varying current densities increased significantly with the addition of 2.0 g·dm⁻³ of NaNO₃. Zn–Zr oxide films deposited from the NaNO₃-containing solution showed a massive structure composed of fine crystals without crystalline Zn platelets, although large cracks were observed between the large crystals. EDX analysis revealed that Zr codeposited on the massive crystals as a fine concave-convex oxide. The corrosion current density of the Zn–Zr oxide films deposited from the NaNO₃-containing solution was almost the same as that of pure Zn deposits, showing that there is no improvement in corrosion resistance when Zn is codeposited with Zr oxide. Moreover, Zr content in the deposits obtained from the PEG-containing solution increased significantly along with increasing current density above 1000 A·m⁻². With the addition of 1000 mg·dm⁻³ of PEG, the crystalline Zn platelets disappeared, and the deposits were instead composed of fine mesh-like crystals with a preferred orientation of the $\{ 10\bar{1}0\} $Zn plane, resulting in a smooth surface. The cathodic current density for the reduction of dissolved oxygen on the Zn–Zr oxide films deposited from the PEG-containing solution was smaller than that of the pure Zn deposits, and as a result, the corrosion current density of the Zn–Zr oxide films was smaller than that of the pure Zn deposits. The increase in Zr content in the deposits with NO₃⁻ ions and PEG is attributed to the acceleration of the hydrolysis of Zr ions.

 

This Paper was Originally Published in Japanese in J. Japan Inst. Metals 82 (2018) 366–374.

1. Introduction

During electrodeposition of a composite, fine insoluble particles are generally suspended in a solution, and these fine particles are incorporated into the matrix metal during deposition. The films codeposited with fine particles can have properties such as wear resistance,^1–4) a lubricating ability,^5–10) and/or corrosion resistance.^11–15) However, codepositing with matrix films using fine particles is difficult because fine particles tend to gather together, both in solution and at the cathode layer during deposition. Another concern is that the particles gathered in solution will settle in the electrolytic apparatus, which can result in manufacturing problems.¹⁶⁾

On the other hand, deposition from an aqueous solution of a less noble metal, such as zinc, whose oxidation-reduction potential is low, has a side reaction where hydrogen ions are reduced to hydrogen gas and pH rises at the cathode. Some metallic ions can hydrolyze at low pH, forming a hydroxide or oxide, and coexist with the matrix ions in solution, and it’s possible for these metallic hydroxides or oxides to codeposit with matrix films.^17,18) Since this deposition technique that utilizes the hydrolysis reaction can be performed from solution without insoluble solid particles, the particles possibly codeposit in the form of ultrafine, and the fine particles don’t settle due to not suspended state in the solution, resulting in solution of manufacturing problem. Specifically, Al³⁺, Zr⁴⁺, and VO²⁺ ions, which hydrolyze at a lower pH than Zn²⁺ ions, have been added to a Zn electrolyte solution.^17–23) The results of these studies show that Al³⁺ and Zr⁴⁺ ions rarely codeposit with deposited Zn as their hydroxides and oxides.^19,22,23) In this study, Zr⁴⁺ ions were added into electrolyte, and the effect of additives to raise pH at cathode layer during Zn deposition was investigated to accelerate the hydrolysis reaction of Zr⁴⁺ ions. Nitrate ions (NO₃⁻) and polyethylene glycol (PEG) were selected as additives. NO₃⁻ ions are expected to raise the pH at the cathode layer due to the reduction reaction of “NO₃⁻ + 3H⁺ + 2e⁻ → HNO₂ + H₂O” during Zn deposition, while PEG is expected to raise the pH due to its ability to accelerate of reduction of hydrogen ions by its polarization effect on Zn deposition. The effects of NO₃⁻ ions and PEG on the content of Zr oxide, polarization behavior, and microstructure of the deposited Zn–Zr oxide composite films were investigated.

2. Experimental

The composition of the solution and electrolysis conditions are shown in Table 1. The electrolytic solution was prepared by dissolving fixed amounts of high-grade ZnSO₄·7H₂O (0.52 mol·dm⁻³) and Zr(SO₄)₂·4H₂O (0.1 mol·dm⁻³) in distilled, deionized water. NaNO₃ [0.4 to 2.0 g·dm⁻³ (0.0047∼0.0235 mol·dm⁻³)] and PEG (1∼1000 mg·dm⁻³) characterized by 6000 molecular weight were added to the solution. The pH was adjusted to 2 using sulfuric acid. Electrodeposition was conducted in a non-agitated solution under coulostatic (10⁵ C·m⁻²) and galvanostatic (10–5000 A·m⁻²) conditions at 313 K. A Cu sheet and a Pt sheet with the same area of 1 cm × 2 cm were used as the cathode and anode, respectively. Deposits were dissolved off the cathode using nitric acid. Zn and Zr were quantitatively analyzed using inductively coupled plasma spectroscopy, and the composition of the deposited films, current efficiency, and the partial current density for Zn deposition were calculated. The partial current density for Zn deposition was determined by multiplying the total current density by the current efficiency value of Zn. The cathodic potentials during deposition were measured against a saturated Ag/AgCl reference electrode (0.199 V vs. NHE at 298 K). In the polarization curves presented in this article, the potentials are plotted with reference to the NHE.

Table 1 Electrolysis conditions.

The surfaces and cross-sectional morphologies of the deposited films were analyzed using secondary electron images and backscattered electron images captured by a scanning electron microscope (SEM) operated at an ultra-low accelerating voltage. The backscattered electron images were obtained using an EsB (Energy Selective Backscatter Electron Detector). The element distribution of the surfaces and cross sections of the deposited films was examined with 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 Willson and Rogers.^24,25) An X-ray diffraction intensity of 0002 to the $11\bar{2}2$ reflection was used. The polarization curves of the deposited films to evaluate corrosion resistance were collected by polarizing from a less-noble potential than the corrosion potential toward the anodic-potential using a potential sweep method at 1.0 mV·s⁻¹ in an oxygen-saturated 3 mass% NaCl solution at 313 K.

3. Results and Discussion

3.1 Deposition of composite films from nitrate ion solution

3.1.1 Effect of nitrate ion addition on the deposition behavior of composite films

Figure 1 shows the Zr content in deposits obtained at 500 A·m⁻² in Zn–Zr solutions containing varying amounts of NaNO₃. The Zr content of the deposits formed in this study was calculated from the masses of Zr and Zn in the deposits using the following equation:   

\begin{align*} &\text{Zr content (mass%)}\\ &\quad = [\text{mass Zr/total mass (Zn + Zr)}] \times 100. \end{align*}

Fig. 1

Zr content in deposits obtained at 500 A·m⁻² in Zn–Zr solutions containing varying amounts of NaNO₃.

The Zr content in the deposits was almost zero at concentrations of NaNO₃ below 0.8 g·dm⁻³, gradually increased when the concentration of NaNO₃ was above 1.2 g·dm⁻³, and rapidly increased when NaNO₃ was above 2.0 g·dm⁻³.

When the concentration of Zr ions in the solution is 0.1 mol·dm⁻³, assuming that the activity coefficient of the Zr ions is 1, Zr exists in the form of ZrO²⁺ or Zr⁴⁺ at pH values less than 1.74, according to the potential vs. pH diagram for the Zr–H₂O system.²⁶⁾ When pH values are greater than 1.74, ZrO₂ is formed as a result of the hydrolytic reaction of Zr ions. In this study, no precipitation was observed in the electrolytes when a pH 2 was used. This is attributed to the activity coefficient of the Zr ions becoming less than 1.0 due to the high electrolyte concentration; consequently, the critical pH values for the hydrolysis of Zr ions increases to above pH 2.0. During the electrodeposition of Zn from aqueous solutions in this study, the pH in the cathode layer increases because of the reduction of H⁺ to H₂. When the pH in the cathode layer reaches the critical pH value for the hydrolysis of Zr ions, Zr ions are expected to be converted into an oxide, such as ZrO₂. The standard electrode potential, E⁰ for the reduction reaction of NO₃⁻ ions added into electrolyte is 0.934 V, and NO₃⁻ ions are expected to be reduced by eq. (1) during Zn deposition, resulting in an increase in pH at the cathode layer.   

\begin{equation} \text{NO$_{3}{}^{-}$} + \text{3H$^{+}$} + \text{2e$^{-}$} \to \text{HNO$_{2}$} + \text{H$_{2}$O} \end{equation} (1)

As a result, the Zr content in the deposits directly increased with the concentration of NaNO₃. (Fig. 1)

Figure 2 shows the current efficiency for Zn deposition at 500 A·m⁻² in Zn–Zr solutions containing varying amounts of NaNO₃. The current efficiency for Zn deposition was almost 100% in a NaNO₃-free solution, while it linearly decreased with increasing concentration of NaNO₃. At a concentration of 2.0 g·dm⁻³ it became approximately 80%. This decrease in current efficiency for Zn deposition is attributed to the fact that the rate of the reduction reaction of NO₃⁻ ions, shown in eq. (1), increases with the concentration of NaNO₃. As mentioned above, since the Zr content in the deposits increased with the concentration of NaNO₃, the concentration of NaNO₃ was fixed at 2.0 g·dm⁻³ in the following experiment.

Fig. 2

Current efficiency for Zn deposition at 500 A·m⁻² in Zn–Zr solutions containing varying amounts of NaNO₃.

Figure 3 shows the Zr content in the deposits obtained by varying current densities in Zn–Zr solutions with and without NaNO₃. The Zr content in the deposits obtained from the NaNO₃-free solution was almost zero regardless of current density, while it significantly increased with an addition of 2.0 g·dm⁻³ of NaNO₃ at all other current densities. Focusing on the current density-dependency of Zr content in deposits from the solution containing NaNO₃, Zr content significantly decreased with increasing current density in the low current density region of 100 to 1000 A·m⁻², while it increased in the high current density region above 1000 A·m⁻².

Fig. 3

Zr content in deposits obtained at varying current densities in Zn–Zr solutions with and without NaNO₃. [○ Without NaNO₃, ● With 2.0 g·dm⁻³ NaNO₃]

Figure 4 shows the current efficiency for Zn deposition at varying current densities in Zn and Zn–Zr solutions with and without NaNO₃. The current efficiency for Zn deposition from the NaNO₃-free Zn–Zr solution was approximately 95% in the low current density region below 500 A·m⁻², but it rapidly decreased at current densities above 1000 A·m⁻². The current efficiency for Zn deposition from the NaNO₃-free Zn–Zr solution was almost identical with that from NaNO₃-free pure Zn solution. In contrast, the current efficiency for Zn deposition from the Zn–Zr solution containing NaNO₃ was lower than that from the NaNO₃-free solution at all current density regions. The decrease in current efficiency with NaNO₃ is attributed to the reduction reaction of NO₃⁻ [eq. (1)]. The current efficiency for Zn deposition decreased with increasing current density in high current density regions above 1000 A·m⁻², regardless of whether NaNO₃ was present or not, which is due to reaching the diffusion limit of Zn²⁺ ions.

Fig. 4

Current efficiency for Zn deposition at varying current densities in Zn and Zn–Zr solutions with and without NaNO₃. [● Zn without NaNO₃, ▲ Zn–Zr without NaNO₃, ■ Zn–Zr with 2.0 g·dm⁻³ NaNO₃]

Figure 5 shows the total polarization curve for the Zn and Zn–Zr solutions with and without NaNO₃. The total polarization curve greatly polarized when Zr ions were present in the high current density regions, regardless of whether NaNO₃ was present or not. With the addition of NaNO₃, the total polarization curve significantly polarized from the lower current density region. Since the current density of the total polarization curve is the total amount of Zn deposition and hydrogen evolution, the effect of adding NaNO₃ on the partial polarization curve was investigated in the following experiment.

Fig. 5

Total polarization curve in Zn and Zn–Zr solutions with and without NaNO₃. [● Zn without NaNO₃, ▲ Zn–Zr without NaNO₃, ■ Zn–Zr with 2.0 g·dm⁻³ NaNO₃]

Figure 6 shows the partial polarization curve for Zn deposition from Zn and Zn–Zr solutions with and without NaNO₃. The partial polarization curve for Zn deposition from the solution containing only Zn²⁺ ions was also investigated to determine the effect of the Zr ions. The partial polarization curve for Zn deposition was greatly polarized when Zr ions were present in the current density region above 200 A·m⁻², regardless of whether the solution contained NaNO₃ or not. This polarization at a current density above 200 A·m⁻² is attributed to film resistance of Zr oxide formed by Zr ion hydrolysis. The degree of polarization was somewhat larger with NaNO₃ than without it. From these results, NO₃⁻ ions are expected to accelerate the formation of Zr oxide at the cathode layer. In the Zn–Zr solution, Zn deposition was found to reach the diffusion limit of Zn²⁺ ions at its partial current density of approximately 1000 A·m⁻². As a result, the current efficiency for Zn deposition decreased with increasing current density in the high current density region above 1000 A·m⁻². (Fig. 4)

Fig. 6

Partial polarization curve for Zn deposition in Zn and Zn–Zr solutions with and without NaNO₃. [● Zn without NaNO₃, ▲ Zn–Zr without NaNO₃, ■ Zn–Zr with 2.0 g·dm⁻³ NaNO₃]

3.1.2 Effects of nitrate ion addition on the microstructure and polarization behavior of deposited composite films

Figure 7 shows the SEM images of surface deposits obtained at 1000 A·m⁻² in Zn and Zn–Zr solutions with and without NaNO₃. Deposited films of pure Zn (a) were composed of layered Zn platelet crystals with hexagonal close-packed structure, and the edges of the platelet crystals were clearly defined. The surface of the smooth plate was seen to have a lot of steps. The films (b) deposited from the NaNO₃-free Zn–Zr solution showed the layered Zn platelet crystals, as well as pure Zn films, and as with (a), many steps were observed on the smooth surface of the plates. The films (b) deposited from the NaNO₃-free Zn–Zr solution seem to have the same morphology as pure Zn films (a). On the contrary, in the Zn–2.5 mass% Zr oxide films (c) deposited from the NaNO₃ solution, the Zn platelet crystals disappeared, and films comprised of aggregates of fine crystals were deposited with large cracks frequently observed between them. Both the aggregates and cracks, shown by the arrow in Fig. 7(c), were analyzed using SEM and EDX to determine the codeposition area of Zr. The results are shown in Fig. 8. A roughness was observed on the surfaces of the aggregated crystals [① in Fig. 7(a)]. Zn, Zr, and O were detected on the aggregated crystals [Fig. 8(c)], indicating that Zr codeposited in the form of a fine, but rough oxide on the surface of the aggregated crystals of Zn. In the areas where there was cracking, Zr and O were only slightly detected [② in Fig. 8(a)] between the aggregated crystals of Zn [Fig. 8(d)], showing that Zr oxide was somewhat present even at the bottom of cracks.

Fig. 7

SEM images of surface deposits obtained at 1000 A·m⁻² in Zn and Zn–Zr solutions with and without NaNO₃. [(a) Zn without NaNO₃, (b) Zn–Zr without NaNO₃, (c) Zn–Zr with 2.0 g·dm⁻³ NaNO₃]

Fig. 8

SEM images and EDX spectra of deposits obtained at 1000 A·m⁻² from Zn–Zr solution containing 2.0 g·dm⁻³ of NaNO₃. [(a) Secondary electron image, (b) Backscattered electron image (EsB), (c) EDX of ①, (d) EDX of ②]

The element distribution of a cross section of a Zn–2.5 mass% Zr oxide deposited at 1000 A·m⁻² from the Zn–Zr solution with NaNO₃ was analyzed using EPMA. Figure 9 shows Zn (red), Zr (green), and O (blue) uniformly present throughout the depth of the deposited films, while Zr and O were detected in areas where Zn concentration was low, indicating the presence of Zr oxide. Even though Zr and O were present uniformly in the depth of the deposited films for the most part, their concentration was higher at the surface, indicated the surface was enriched with Zr oxide.

Fig. 9

EPMA images of the deposit’s surface obtained at 1000 A·m⁻² from Zn–Zr solution containing 2.0 g·dm⁻³ of NaNO₃. [(a) SE image, (b) Elemental mapping of (a)]

Figure 10 shows the polarization curves collected in a 3 mass% NaCl solution for deposits obtained at 1000 A·m⁻² for Zn and Zn–Zr solutions with and without NaNO₃. Comparing the polarization curves of films deposited from the NaNO₃-free Zn–Zr solution with that of pure Zn films, the anodic current density was almost identical, while the cathodic current density of films deposited from the Zn–Zr solution was somewhat larger than that of pure Zn films. On the other hand, the corrosion potential of Zn–2.5 mass% Zr oxide films deposited from the solution containing NaNO₃ significantly shifted in a less noble direction. The anodic current density of the Zn–2.5 mass% Zr oxide films had an almost constant at corrosion potential to approximately −0.84 V, but it rapidly increased at a more noble potential than −0.84 V. The corrosion current density of Zn–2.5 mass% Zr oxide films was almost the same as that of pure Zn films, showing no improvement in corrosion resistance with codeposition of Zr oxide. This seems to be due to the existence of cracks in the deposited films.

Fig. 10

Polarization curves in 3 mass% NaCl solution for deposits obtained at 1000 A·m⁻² in Zn and Zn–Zr solutions with and without NaNO₃. [(a) Zn without NaNO₃, (b) Zn–Zr without NaNO₃, (c) Zn–Zr with 2.0 g·dm⁻³ NaNO₃]

3.2 Deposition of composite films from the solution containing PEG

3.2.1 Effect of PEG addition on the deposition behavior of composite films

Figure 11 shows the Zr content in the deposits obtained at 1000 A·m⁻² in Zn–Zr solution containing varying amounts of PEG. Zr content in the deposits was approximately 0.1 mass% and almost constant at PEG concentrations in solution between 1 to 100 mg·dm⁻³; however, it rapidly increased when the PEG concentration rose above 100 mg·dm⁻³ to be approximately 0.9 mass% at a PEG concentration of 1000 mg·dm⁻³.

Fig. 11

Zr content in deposits obtained at 1000 A·m⁻² in Zn–Zr solution containing varying amounts of PEG.

Figure 12 shows the Zr content in deposits obtained at varying current densities in Zn–Zr solutions with and without PEG. With the addition of PEG, the Zr content in the deposits significantly increased at current densities above 1000 A·m⁻². In fact, the effect of PEG addition on the Zr content in the deposits was remarkable in the current density region above 1000 A·m⁻².

Fig. 12

Zr content in deposits obtained at varying current densities in Zn–Zr solutions with and without PEG. [○ Without PEG, ● With 1000 mg·dm⁻³ PEG]

Figure 13 shows the current efficiency for Zn deposition at varying current densities in Zn and Zn–Zr solutions with and without PEG. The current efficiency for Zn deposition from the Zn–Zr solution with PEG increased with current density at first, reaching a maximum at 500 A·m⁻² before then decreasing as the current density was further increased. The current efficiency for Zn deposition decreased with the addition of PEG throughout the current density regions. The degree of decrease was particularly large in the low current density region.

Fig. 13

Current efficiency for Zn deposition at varying current densities in Zn and Zn–Zr solutions with and without PEG. [● Zn without PEG, ▲ Zn–Zr without PEG, ■ Zn–Zr with 1000 mg·dm⁻³ PEG]

Figure 14 shows the total polarization curve of the Zn and Zn–Zr solutions with and without PEG. Both curves for the Zn and Zn–Zr solutions were significantly polarized with PEG addition. The effect of the addition of PEG on the partial polarization curve for Zn deposition was investigated in the following experiment.

Fig. 14

Total polarization curve in Zn and Zn–Zr solutions with and without PEG. [● Zn without PEG, ▲ Zn–Zr without PEG, ■ Zn–Zr with 1000 mg·dm⁻³ PEG, ◆ Zn with 1000 mg·dm⁻³ PEG]

Figure 15 shows the partial polarization curve for Zn deposition from Zn and Zn–Zr solutions with and without PEG. The partial polarization curve for Zn deposition from the solution containing Zn²⁺ only was investigated to determine the role of Zr ions. The partial polarization curve for Zn deposition from the Zn–Zr solution was greatly polarized with the addition of PEG. The partial polarization curve for Zn deposition from the Zn single solution was polarized with PEG to the same degree as that of the Zn–Zr solution, showing that Zn deposition was significantly suppressed with the addition of PEG regardless of the presence of Zr ions. In a solution containing PEG, when the partial current density for Zn deposition was approximately 1000 A·m⁻², the cathode potential was greatly polarized to the potential region less noble than −1.2 V. When the cathode potential was polarized to the potential region less noble than −1.2 V, eq. (3) has been reported to occur more often than that of eq. (2) for the production of hydrogen gas.²⁷⁾ Since the concentration of H₂O in solution is considerably larger than that of H⁺, the pH at the cathode layer is easy to increase if eq. (3) becomes the main reaction.   

\begin{equation} \text{2H$^{+}$} + \text{2e$^{-}$} \to \text{H$_{2}$} \end{equation} (2)

  

\begin{equation} \text{2H$_{2}$O} + \text{2e$^{-}$} \to \text{H$_{2}$} + \text{2OH$^{-}$} \end{equation} (3)

Therefore, when PEG was added to solution, the hydrolysis reaction of Zr ions occurred more readily, resulting in an increase of Zr in deposits. The effect was remarkable at current densities above 1000 A·m⁻² (Fig. 11), and this is attributed to the increase in rate of hydrogen evolution or the increase in OH⁻ formation.

Fig. 15

Partial polarization curve for Zn deposition in Zn and Zn–Zr solutions with and without PEG. [● Zn without PEG, ▲ Zn–Zr without PEG, ■ Zn–Zr with 1000 mg·dm⁻³ PEG, ◆ Zn with 1000 mg·dm⁻³ PEG]

3.2.2 Effect of PEG addition on the microstructure and polarization behavior of deposited composite films

Figure 16 shows the SEM images of the deposit’s surface obtained at 1000 A·m⁻² in Zn–Zr solutions containing varying amounts of PEG. The films (a) deposited from the solution containing 1 mg·dm⁻³ of PEG were comprised of crystalline Zn platelets whose basal plane was layered almost parallel to the substrate and showed the same morphology as deposited films of pure Zn [Fig. 7(a)]. When the concentration of PEG was increased to 10 mg·dm⁻³ (b), the size of the crystalline Zn platelets became small, and the crystals were layered somewhat inclining towards the substrate. When the concentration of PEG was increased to 100 mg·dm⁻³ (c), the thin crystalline Zn platelets randomly deposited perpendicularly with respect to the substrate. Further increase of PEG concentration to 1000 mg·dm⁻³ (d) caused the Zn platelets to disappear, resulting in deposits comprised of fine mesh-patterned crystals with a smooth surface.

Fig. 16

SEM images of surface of deposits obtained at 1000 A·m⁻² in Zn–Zr solutions containing various amounts of PEG. [PEG concentration, (a) 1 mg·dm⁻³, (b) 10, (c) 100, (d) 1000]

Figure 17 shows the secondary electron image, backscattered electron image, and EDX spectrum of surface of deposits obtained at 1000 A·m⁻² from the Zn–Zr solution containing 1000 mg·dm⁻³ of PEG. In the backscattered electron image (SEM-EsB image), a lighter element is depicted as being darker. The black area shown by ① in Fig. 17 was partially observed in the backscattered electron image (b), while Zn, Zr, and O were detected in the black area by EDX (c), showing that Zr oxide exists in the deposit.

Fig. 17

SEM images and EDX spectrum of surface deposits obtained at 1000 A·m⁻² in Zn–Zr solutions containing 1000 mg·dm⁻³ of PEG. [(a) Secondary electron image, (b) Backscattered electron image (EsB), (c) EDX of ①]

Figure 18 shows the crystal orientation of Zn in the deposits obtained at 1000 A·m⁻² in Zn–Zr solutions containing varying amounts of PEG. The preferred crystal orientations of the deposited pure Zn films were the {0001} and $\{ 10\bar{1}3\} $ planes. The preferred orientation {0001} means that the basal plane of the Zn platelets approaches being almost parallel to the substrate. In the deposition from Zn–Zr solutions, when the concentrations of PEG were 0 and 1 mg·dm⁻³, the orientation of the {0001}Zn plane was strong. This result corresponds to the surface SEM images shown in Fig. 16(c). With the increasing PEG concentration above 10 mg·dm⁻³, the orientation of the {0001}Zn plane significantly decreased as the orientation of the $\{ 10\bar{1}0\} $ plane increased. The preferred orientation of the $\{ 10\bar{1}0\} $ plane of Zn means that the basal plane of the Zn platelets approaches being perpendicular to the substrate, which corresponds to the surface SEM images shown in Fig. 16(c). The crystal orientation of the deposited Zn has been previously reported to depend on the overpotential for deposition.^28–30) Pangarov calculated the relative values of a two-dimensional nucleation site for various crystal planes.^31,32) By assuming that two-dimensional nuclei with the smallest nucleation site were formed at a given crystallization overpotential, he examined the overpotential dependence of the preferred orientation of various metals deposited from aqueous solutions. According to Pangarov, the preferred orientations of hcp Zn shift from the {0001} plane to the $\{ 10\bar{1}1\} $, $\{ 11\bar{2}0\} $, and $\{ 10\bar{1}0\} $ planes (in that order) as the Zn deposition overpotential increased progressively. In the present study of Zn–Zr oxide composite deposition, both the decrease in orientation of the {0001} plane and increase in orientation of the $\{ 10\bar{1}0\} $ plane with an addition of PEG is attributed to an increase in overpotential for the deposition with PEG. (Fig. 13)

Fig. 18

Crystal orientation of Zn deposited at 1000 A·m⁻² in Zn–Zr solutions containing various amounts of PEG. [Orientation of Zn, ● {0002}, ▲ $\{ 10\bar{1}3\} $, ■ $\{ 10\bar{1}1\} $, ◆ $\{ 10\bar{1}0\} $]

Figure 19 shows the polarization curves in 3 mass% NaCl solution for the deposits obtained at 1000 A·m⁻² in Zn and Zn–Zr solutions with and without PEG. The polarization curve of the films deposited from the PEG-free Zn–Zr solution was almost identical with that of the deposited pure Zn films, but the corrosion potential of the Zn–0.9 mass% Zr oxide films deposited from the solution containing PEG significantly shifted to a less noble direction. The corrosion current density of Zn–0.9 mass% Zr oxide films was smaller than that of deposited pure Zn films because the cathodic current density for the reduction reaction of dissolved oxygen on Zn–0.9 mass% Zr oxide films was smaller than that on pure Zn films. In the case of Zr-free solutions, the cathodic current density for the reduction reaction of dissolved oxygen somewhat decreased with the addition of PEG. The cathodic current density on Zn–0.9 mass% Zr oxide films was smaller than that on pure Zn films, which seems to be due to the fact that Zr oxide was contained in deposited films and the roughness of the film’s surface was changed by codeposition of Zr oxide; however, the details are currently unknown and further investigation is required.

Fig. 19

Polarization curves in 3 mass% NaCl solution for deposits obtained at 1000 A·m⁻² in Zn and Zn–Zr solutions with and without PEG. [(a) Zn without PEG, (b) Zn–Zr without PEG, (c) Zn–Zr with 1000 mg·dm⁻³ PEG, (d) Zn with 1000 mg·dm⁻³ PEG]

4. Conclusion

Electrodeposition of a Zn–Zr oxide composite was performed in a sulfate solution without insoluble solid particles, and the effects of the addition of NO₃⁻ ions and PEG on the content of Zr oxide, polarization behavior and microstructure of the deposited Zn–Zr oxide composite films were investigated. Zr content in the deposits obtained at the different current densities significantly increased with an addition of 2.0 g·dm⁻³ of NaNO₃. Zn–Zr oxide films deposited from the solution containing NaNO₃ were a massive structure composed of fine crystals without crystalline Zn platelets, and between these massive crystals were numerous large cracks. EDX analysis revealed that Zr codeposited on the massive crystals as a fine concave-convex oxide. The corrosion current density of the Zn–Zr oxide deposited from the solution containing NaNO₃ was almost the same as that of the pure Zn deposits, showing no improvement in corrosion resistance with codeposition of Zr oxide. On the other hand, Zr content in the deposits obtained from the solution containing PEG significantly increased at current densities above 1000 A·m⁻², and continued to increase as current density was increased. With an addition of 1000 mg·dm⁻³ of PEG, the crystalline Zn platelets disappeared, and the structure of the deposits changed from massive crystals to aggregates of fine mesh-like crystals with the preferred orientation $\{ 10\bar{1}0\} $Zn plane, resulting in a smooth surface. The cathodic current density for the reduction reaction of dissolved oxygen on the Zn–Zr oxide films deposited from the solution containing PEG was smaller than that of the pure Zn deposits; as a result, the corrosion current density of Zn–Zr oxide films was smaller than that of pure Zn films. The increase in Zr content in the deposits made with NO₃⁻ ions and PEG is attributed to the acceleration of the hydrolysis reaction of Zr ions.

Acknowledgement

This work was supported by JSPS KAKENHI Grant Number JP18H01753 and a Research Promotion Grant in 2017 from JFE 21st Century.

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