Effect of Organic Additives on the Electrodeposition Behavior of Zn from an Alkaline Zincate Solution and Its Microstructure

Abstract

Electrodeposition of Zn was performed on an Fe electrode at a current density of 20–5000 A·m⁻² and a charge of 4 × 10⁴ C·m⁻² in an unagitated zincate solution at 313 K containing 0.62 mol·dm⁻³ of ZnO, 4.0 mol·dm⁻³ of NaOH, and organic additives. The effects of organic additives on the deposition behavior of Zn and the microstructure of the deposits were investigated. Glossy films were obtained by depositing at current densities higher than 1000 A·m⁻² from the solution containing additives of a straight-chain polymer composed of a quaternary ammonium cation (PQ) and a quaternary ammonium salt with a benzene ring (QA). The polarization curve was separated into partial polarization curves of Zn deposition and hydrogen evolution by using the galvanostatic data of Zn deposition. The overpotentials of the charge transfer of Zn deposition and that of ZnO₂²⁻ ion diffusion increased with the addition of PQ and QA. The increase in overpotential was considerable at potentials less noble than −1.5 V. Zn deposition reached the diffusion limit of ZnO₂²⁻ ions at potentials less noble than −1.5 V, indicating that the diffusion of ZnO₂²⁻ ions was suppressed considerably by PQ and QA. With the addition of PQ and QA, C, N, and H were codeposited with Zn, which demonstrated that the additives of PQ and QA were incorporated into the deposited films. Zn crystallite size decreased with increasing current density. At a high current density of 5000 A·m⁻², the crystallite size decreased with the addition of PQ and QA, and the surface of the film was smooth. The orientation index of the $\{ 10\bar{1}0\} $ plane of Zn deposited from the solution containing PQ and QA increased with increasing current density. The changes in the crystallite size and crystal orientation of deposited Zn were explained by the deposition overpotential.

 

This Paper was Originally Published in Japanese in J. Japan Inst. Metals 83 (2019) 399–406.

Partial polarization curves for Zn deposition from the solutions with and without additives and SEM images of deposited Zn. [● Additive-free, △ With QA, □ With PQ, ◆ With PQ and QA]

1. Introduction

Zn electroplating is performed from the solutions of zincate,^1–6) boron fluoride,⁷⁾ chloride,^8–11) sulfate,^12–16) pyrophosphate,^17,18) and cyanide.^19,20) The throwing power, appearance, hardness, and corrosion resistance of deposited Zn, bath voltage, and current efficiency for Zn deposition depend on the type of implemented electrolytic solution. Alkaline solutions are known for their excellent throwing power, and primarily, a cyanide solution is used. Although Zn deposited from the cyanide solution possesses superior gloss, corrosion resistance, adhesion property, and throwing power, the cyanide solution has an effluent treatment problem. Thus, the use of an alkaline zincate solution, whose electrolytic composition is simple and easy to control, has increased from the viewpoint of environmental problem. Because the parts electroplated with Zn using a zincate solution have a small complex shape and large quantities are electroplated simultaneously, the increase in the electroplating rate is required. Plating at high speed or deposition at high current density allows the transformation of deposited Zn into dendrite owing to the diffusion limitations of ZnO₂²⁻. To smooth the surface of Zn deposited at high current density, the use of an additive in the solution is essential. However, there are many ambiguities in the effect of additives on the deposition behavior and microstructure of Zn. In metal deposition, a surfactant composed of a straight-chain polymer is used as a leveling agent. Quaternary ammonium salt is a cationic surfactant, and its capability to adsorb onto the cathode increases when the cathode potential shifts to the less noble direction or when the current density increases. In this study, two additives of a straight-chain polymer composed of a quaternary ammonium cation and a quaternary ammonium salt with a benzene ring were selected, and the effect of the additives on the deposition behavior of Zn and subsequent Zn microstructure was investigated.

2. Experimental

Table 1 shows the composition of the bath and the electrodeposition conditions used in this work. An electrolytic solution was prepared by dissolving ZnO with a purity of 99.999% (0.62 mol·dm⁻³) and reagent-grade NaOH (4 mol·dm⁻³) in ion-exchanged water. Straight-chain polymers composed of a quaternary ammonium cation (PQ) (molecule length: 20 nm) and a quaternary ammonium salt with a benzene ring (QA) were added to the electrolyte solution at concentrations of 2.9 and 0.14 g·dm⁻³, respectively. Fe sheets with sizes of 2.5 × 2 and 5 × 2.5 cm were used as the cathode and anode, respectively. Prior to electrodeposition, the cathode Fe sheet was immersed in an alkaline degreasing agent for 5 min and then washed with distilled and ion-exchanged water. Electrodeposition was performed in an unagitated solution at 313 K under the galvanostatic conditions (20–5000 A·m⁻²) until the deposition charge reached to 40 kC·m⁻². The deposited films were dissolved in a nitric acid solution, and the Zn content in the film was quantitatively analyzed by using the inductively coupled plasma spectroscopy to estimate the cathode current efficiency and the partial current density of the Zn electrodeposition. The partial current density for hydrogen evolution was determined by subtracting the partial current density for Zn deposition from the total current density. The cathode potentials during deposition were measured against a saturated Ag/AgCl reference electrode (0.199 V vs. normal hydrogen electrode (NHE) at 298 K). In the presented polarization curves, the potentials are plotted with a reference to NHE.

Table 1 Solution composition and electrolysis conditions.

The alternating current (AC) impedance was measured at 200 A·m⁻² during Zn deposition. To obtain Nyquist plots, the frequency dependence of the AC impedance and the phase difference were measured using a frequency response analyzer (±50 A·m⁻² sine wave, 10⁻¹ to 2 × 10⁴ Hz, 10 points/decade). To evaluate the content of additives involved in the deposited films, the concentrations of C, N, H, Na, Zn, and Fe in the deposits were measured by radio frequency glow discharge optical emission spectroscopy (rf-GDOES) under the following analysis conditions: diameter of 2 mmϕ, argon pressure of 600 Pa, output of 40 W, pulse frequency of 2000 Hz, and duty cycle of 0.125. The surfaces of the deposited films were analyzed using scanning electron microscopy (SEM). The crystal orientation of the deposited Zn was determined by using the method developed by Willson and Rogers;²¹⁾ an X-ray diffraction intensity of 0002 to the $11\bar{2}2$ reflection was used. The crystallite size of deposited Zn was calculated using the Scherrer equation²²⁾ from the half-width of the X-ray diffraction peak corresponding to the $10\bar{1}0$ reflection.

3. Results

3.1 Appearance of deposited films

Figure 1 shows the appearance of Zn films deposited at various current densities from the solutions with and without additives. The word (nine) written by Chinese characters, which is seen at the lower left side of the photograph, reflects the word that was written on a card placed opposite the sample. Thus, when the Chinese characters in Fig. 1 are clearly seen, it can be inferred that the deposited films exhibit high gloss. The films obtained from the solution without additives were uneven and white gray when they were deposited at low current densities of 200 and 500 A·m⁻², whereas they somewhat darkened at 1000 A·m⁻² and were rough and black at 5000 A·m⁻². In a solution containing only QA as an additive, the surface roughness of the films deposited at the high current density of 5000 A·m⁻² was somewhat improved, but the appearance was similar to that deposited from an additive-free solution. However, in a solution containing PQ alone as an additive, the films deposited at high current densities of 1000 and 5000 A·m⁻² were somewhat glossy. When a solution containing both PQ and QA was used, the gloss of the films deposited at 1000 and 5000 A·m⁻² was better than that of the films deposited from the solution containing only PQ.

Fig. 1

Appearance of Zn films deposited at various current densities from the solutions with and without additives.

3.2 Electrodeposition behavior of Zn

Figure 2 shows the total polarization curve for the Zn deposition. The slope of the total polarization curve considerably changed at current densities above 200 A·m⁻² regardless of the presence of additives. In a solution containing PQ or both PQ and QA, the total polarization curve was somewhat polarized at low current density of less than 200 A·m⁻² compared with that from an additive-free solution, whereas it was considerably polarized at a current density region above 2000 A·m⁻². However, the total polarization curve of the solution containing only QA was almost identical to that of an additive-free solution. The partial polarization curves for Zn deposition and hydrogen evolution were investigated. The results are discussed next.

Fig. 2

Total polarization curve for the Zn solutions with and without additives. [● additive-free, △ with QA, □ with PQ, ◆ with PQ and QA]

Figure 3 shows the partial polarization curve for Zn deposition. In solutions containing PQ or both PQ and QA, the deposition potential of Zn was polarized at the partial current density region of less than 200 A·m⁻² wherein the partial current density of Zn began to increase. The degree of polarization increased in the potential region less noble than −1.5 V; here, Zn deposition approached the diffusion limit of ZnO₂²⁻ regardless of the presence of an additive, and the ZnO₂²⁻ diffusion was greatly suppressed in solutions containing PQ or both PQ and QA. Diffusion of ZnO₂²⁻, which limits the current density for Zn deposition, is expected to decrease with the addition of PQ or both PQ and QA. Considering the shape of the partial polarization curve for Zn deposition, it can be concluded that the charge transfer process for Zn deposition was suppressed by the addition of PQ or both PQ and QA at the partial current density region below 200 A·m⁻².

Fig. 3

Partial polarization curves for Zn deposition from the solutions with and without additives. [● additive-free, △ with QA, □ with PQ, ◆ with PQ and QA]

Figure 4 shows the partial polarization curve for hydrogen evolution during Zn deposition. Although the effect of additives on hydrogen evolution was rarely observed at the potential region more noble than −1.5 V, hydrogen evolution was greatly suppressed by the addition of PQ or both PQ and QA in the potential region less noble than −1.5 V. As the potential decreased to less noble than −1.5 V, the partial current density for Zn deposition approached a constant value (Fig. 3), whereas it for hydrogen evolution continued to increase.

Fig. 4

Partial polarization curves for H₂ evolution from the solutions with and without additives. [● additive-free, △ with QA, □ with PQ, ◆ with PQ and QA]

Figure 5 shows the current efficiency for Zn deposition. Regardless of whether the process is performed with or without additives, the current efficiency for Zn deposition increased with increase in current density at the low current density region and became maximum at 500 A·m⁻². When the current density exceeded 1000 A·m⁻², the current efficiency for Zn deposition considerably decreased. At 2000 and 5000 A·m⁻², the current efficiency of Zn deposition noticeably decreased with the addition of PQ or both PQ and QA. However, for current density below 1000 A·m⁻², the current efficiency of Zn rarely changed with the addition of both PQ and QA, whereas it increased with the addition of QA.

Fig. 5

Current efficiency for Zn deposition from the solutions with and without additives. [● additive-free, △ with QA, □ with PQ, ◆ with PQ and QA]

Since the charge transfer process appears to be the rate-determining step for Zn deposition at 200 A·m⁻², the effect of additives on the polarization resistance for Zn deposition at 200 A·m⁻² was investigated by AC impedance. The Nyquist plots for Zn deposition are shown in Fig. 6. Figure 7 shows the magnified view of the area near the origin. The diameters of semicircles in the Nyquist plots represent the polarization resistance during Zn deposition. As can be found from Figs. 6 and 7, the degree of polarization resistance during Zn deposition, R, follows the order: R(PQ + QA) > R(PQ) ≫ R(QA) > R(none). The polarization resistance of Zn deposition was somewhat larger with the addition of QA than that without an additive (Fig. 7).

Fig. 6

Nyquist plots obtained at 200 A/m² in solutions with and without additives. [● additive-free, △ with QA, □ with PQ, ◆ with PQ and QA]

Fig. 7

Nyquist plots obtained at 200 A/m² in solutions with and without additives. [● additive-free, △ with QA, □ with PQ, ◆ with PQ and QA]

3.3 Structure of deposited films

Figure 8 shows the SEM images of the surface of Zn films deposited at 200 A·m⁻². In Zn films deposited from the additive-free solution (a), the platelet crystals grew to large sizes and were layered almost parallel to the substrate. The plane of the platelet crystals is the {0001} basal plane of the hcp structure of Zn, and the preferred orientation of the deposited Zn films appears to be {0001} based on the surface morphology of Zn. When only QA was added (b), Zn platelet crystals were small but were layered almost parallel to the substrate, which is similar to films deposited from the additive-free solution. However, with the addition of PQ alone (c), Zn platelet crystals were layered almost perpendicular to the substrate and formed several micrometer sized blocks. With the addition of both PQ and QA (d), the morphology of deposited Zn was almost the same as that when only PQ was added.

Fig. 8

SEM images of the surface of Zn films deposited at 200 A·m⁻² from the solutions with and without additives. [(a) additive-free, (b) with QA, (c) with PQ, (d) with PQ and QA]

Figure 9 shows the SEM images of the surface of Zn films deposited at 1000 A·m⁻² from the solutions with and without additives. The Zn films obtained from the additive-free solution (a) appeared as dendrite crystals because the ZnO₂²⁻ diffusion became the rate-determining step during Zn deposition (Fig. 3). With an addition of QA (b), clear hexagonal Zn platelet crystals grew slightly inclining to the substrate. However, with an addition of PQ (c), the morphology of the deposited Zn changed significantly, and thin small platelet crystals grew randomly perpendicular to the substrate. With the addition of both PQ and QA (d), the morphology of the deposited Zn was similar to that with the addition of PQ alone, but the morphology and size became more uniform.

Fig. 9

SEM images of the surface of Zn films deposited at 1000 A·m⁻² from the solutions with and without additives. [(a) additive-free, (b) with QA, (c) with PQ, (d) with PQ and QA]

Figure 10 shows the SEM images of the surface of Zn films deposited at 5000 A·m⁻² from the solutions with and without additives. The Zn films obtained from the additive-free solution (a) exhibited massive crystals of different sizes that were oriented perpendicular to the substrate. The Zn films obtained from the solution containing only QA (b) exhibited morphology that was similar to that obtained from the additive-free solution. However, with the addition of only PQ (c), the morphology of Zn was rather different; the surface exhibited fine granular crystals. By contrast, the Zn films obtained from the solution containing both PQ and QA (d) exhibited a smooth surface composed of small granular crystals.

Fig. 10

SEM images of the surface of Zn films deposited at 5000 A·m⁻² from the solutions with and without additives. [(a) additive-free, (b) with QA, (c) with PQ, (d) with PQ and QA]

Figure 11 shows the crystal orientation of Zn deposited at various current densities from the solutions with and without additives. The Zn films obtained from the additive-free solution (a) exhibited a preferred orientation of {0001}, and with increasing current density, the orientation of {0001} decreased, and the orientation of $\{ 10\bar{1}1\} $ increased. With an addition of only QA (b), the crystal orientation of deposited Zn was similar to that of an additive-free solution. With an addition of only PQ (c), the preferred orientation of the specific plane was rarely observed in the low current density region of less than 200 A·m⁻². However, the orientation of $\{ 10\bar{1}0\} $ increased with the increase in current density. The orientation index of $\{ 10\bar{1}3\} $ was primarily observed in the middle current density region of approximately 500 A·m⁻². With the addition of both PQ and QA (d), the crystal orientation of the deposited Zn became almost identical to that from the solution containing only PQ.

Fig. 11

Crystal orientation of Zn deposited at various current densities from the solutions with and without additives. [(a) additive-free, (b) with QA, (c) with PQ, (d) with PQ and QA, ● 0002, △ $10\bar{1}3$, □ $10\bar{1}1$, ◆ $10\bar{1}0$]

Figure 12 shows the crystallite size of the Zn films deposited at various current densities in solutions with and without additives. In general, the crystallite size of deposited Zn decreased with the increase in current density in all solutions, i.e., with and without additives. The effect of the additive on the crystallite size was unclear in the low current density region less than 200 A·m⁻², whereas the Zn crystallite size for films deposited from the solution containing only PQ or both PQ and QA decreased when compared with that from the additive-free solution at high current density region above 500 A·m⁻².

Fig. 12

Crystallite size of Zn deposited at various current densities in the solutions with and without additives. [● additive-free, △ with QA, □ with PQ, ◆ with PQ and QA]

Figure 13 shows the rf-GDOES depth profile of the films deposited at 5000 A·m⁻² from the solutions with and without additives. In the Zn films deposited from the additive-free solution (a), the H component from H₂O in the electrolyte was observed. However, with the addition of only PQ (b) or both PQ and QA (c), Na, C, N, and H codeposited almost uniformly throughout the thickness of the deposited Zn films. In the Zn films deposited from the additive-free solution (a), the profiles of Zn and Fe in the deposited Zn films exhibited inclination. This phenomenon was caused by the Fe substrate being partially sputtered during the sputtering of the deposited Zn films since the deposited Zn films were thin, and the diameter of the rf-GDOES analysis was large (2 mmϕ).

Fig. 13

Rf-GDOES depth profile of films deposited at 5000 A·m⁻² from the solutions with and without additives. [(a) additive-free, (b) with PQ, (c) with PQ and QA]

4. Discussion

The effect of additives on the Zn deposition behavior from a zincate solution and the resulting Zn microstructure is discussed as follows. By focusing on the effect of additives on the Zn deposition behavior, it was found that when only PQ or both PQ and QA were added, Zn deposition was considerably suppressed at the potentials for which the rate-determining step for Zn deposition was the diffusion of ZnO₂²⁻ (Fig. 3). This result indicates that PQ, which is adsorbed on the cathode, suppresses the transfer of ZnO₂²⁻ in the diffusion layer. The electric double layer is composed of a Helmholtz compact layer and a diffusion double layer. The thickness of the diffusion double layer (nm) is calculated with 0.3/[zc^1/2],²³⁾ where c is the concentration of the electrolyte (mol·dm⁻³), and z is the valence of ions. In this study, because 4 mol·dm⁻³ of NaOH was added as an electrolyte, the thickness of the diffusion double layer was calculated to be approximately 0.15 nm. The thickness of the Helmholtz compact layer has been reported to be 0.1–0.15 nm.²⁴⁾ The adsorption morphology of PQ on the cathode is unknown. However, because the length of the PQ molecule is 20 nm, a part of the PQ is expected to be present in the diffusion layer of ZnO₂²⁻, which is outside (beyond) the electric double layer. The suppression of ZnO₂²⁻ diffusion further increased with the addition of both PQ and QA compared with that when only PQ was added. With the addition of both PQ and QA, although the hydrogen evolution was also considerably suppressed at the high current density, it did not approach the limiting current density, which was different from the Zn deposition (Fig. 5). The hydrogen evolution from the alkaline solution proceeded according to eq. (1):²⁵⁾   

\begin{equation} \text{2H$_{2}$O} + \text{2e$^{-}$} \rightarrow \text{H$_{2}$} + \text{2OH$^{-}$} \end{equation} (1)

Because the H₂O concentration in an electrolytic solution is extremely high, the hydrogen evolution reaction does not reach the diffusion limit of H₂O. Figure 5 shows that the current efficiency during Zn deposition decreased at 2000 and 5000 A·m⁻² with and without additives, respectively, which is attributed to the diffusion limit of ZnO₂²⁻. With the addition of both PQ and QA, the current efficiency during Zn deposition further decreased more, owing to the increased suppression of ZnO₂²⁻ diffusion.

With the addition of only PQ or both PQ and QA, Zn deposition was suppressed even in the low current density region less than 200 A·m⁻² (Figs. 3 and 6). Tafel’s linear relationship was completed at the low current density less than 200 A·m⁻², which indicates that the charge transfer process is the rate-determining step for Zn deposition. In this region, because Zn deposition was suppressed with the addition of PQ, PQ appears to have a suppression effect on the charge transfer process during Zn deposition. However, by comparing the degree of polarization during Zn deposition at the low current density less than 200 A·m⁻² with that at the high current density above 1000 A·m⁻², the polarization effect with PQ was found to be larger at the high current density (Fig. 3). This result shows that the addition of PQ suppresses both the charge transfer and diffusion processes of ZnO₂²⁻ during Zn deposition, and the effect of suppression on the diffusion process of ZnO₂²⁻ is large. The added PQ is a polymer that is composed of a quaternary ammonium cation, and its capability to adsorb on the cathode increases with a change in the cathode potential to a less noble value.^26–28) Therefore, at high current density region (i.e. at less noble potential), the suppression of ZnO₂²⁻ diffusion appears to increase owing to the increase in the adsorption capability of PQ. However, it is unclear why the polarization effect during Zn deposition was larger with the addition of both PQ and QA than that with the addition of only PQ. PQ is a straight-chain polymer, whereas QA is a single quaternary ammonium salt. Some compounds of PQ and QA may form at the diffusion layer of ZnO₂²⁻ and become a three-dimensional obstacle for ZnO₂²⁻ diffusion. Large molecular weight PQ has many adsorption sites and appears to adsorb on large areas of the cathode; in contrast, QA is a small molecular weight compound and is expected to adsorb on limited areas. Therefore, QA appears to adsorb at the gaps between PQ and thus potentially increases the coverage of additives. However, the mechanism is unknown, and further investigations are required.

The effect of additives on the microstructure of deposited Zn films was investigated, and Zn films were deposited from the solution containing both PQ and QA at 5000 A·m⁻² wherein the diffusion overpotential of ZnO₂²⁻ increased and the films exhibited a smooth surface that was composed of fine crystals [Fig. 10(d)]. Thus, the deposited Zn films were glossy (Fig. 1). In general, during metal deposition, when the deposition is performed at diffusion-limited current density, the concentration of metal ions on the cathode surface becomes zero, which increases the surface roughness.²⁹⁾ However, in this study, a smooth surface was obtained owing to the effect of additives regardless of the deposition near the diffusion-limited current density. When the overpotential (i.e., the difference between the equilibrium and deposition potentials) increases during metal deposition, the nucleation rate for deposition prevails over the crystal growth, thus resulting in a decreased crystallite size of the deposits.^29–34) In this study, the crystallite size of the deposited Zn films decreased with increasing current density in all cases, i.e., with and without additives. This phenomenon was caused by the nucleation rate increasing with increase in the overpotential during deposition. At high current density above 500 A·m⁻², with the addition of PQ or both PQ and QA, the crystallite size of deposited Zn films decreased when compared with that formed in an additive-free solution, which is attributed to the further increase in overpotential with additives during deposition. The relative increase in nucleation rate (owing to the increase in the diffusion overpotential of ZnO₂²⁻) and the suppressing effect of additives (which are preferentially adsorbed on the convex site of the cathode³⁵⁾) on the deposition appear to decrease the crystallite size of the deposited Zn films, and the surface becomes glossy.

With the addition of PQ or both PQ and QA, C, N, and H codeposited almost uniformly throughout the thickness of the Zn films. This phenomenon occurred because PQ and QA were incorporated into the deposited Zn films. The decrease in the crystallite size of the deposited Zn with the addition of PQ may occur because the adsorbed molecule, which is composed of C and N, codeposits and becomes the origin of nucleation, or it suppresses the crystal growth of Zn, in addition to increasing the overpotential with PQ during deposition. The content of Na in the deposits increased with the addition of PQ, which results from the contamination of the electrolytic solution as Na is excluded in the additives.

Pangarov calculated the relative values of two-dimensional nucleation work for various crystal planes.^36,37) By assuming that two-dimensional nuclei with the smallest nucleation work were formed at a given crystallization overpotential, he examined the overpotential dependence of the preferred orientation of various metals deposited from aqueous solutions.^36,37) According to Pangarov, the preferred orientation of hcp Zn shifted from the {0001} plane to the $\{ 10\bar{1}1\} $, $\{ 11\bar{2}0\} $, and $\{ 10\bar{1}0\} $ planes (in that order) with increasing Zn deposition overpotential. In this study, without an additive and with the addition of QA, the preferred orientation of Zn films deposited at low current density region was {0001}, and the orientation of $\{ 10\bar{1}1\} $ increased with the increase in current density (Fig. 11), which is attributed to an increase in the overpotential for deposition. With the addition of PQ or both PQ and QA, the orientation of $\{ 10\bar{1}1\} $ increased with the increase in current density. This phenomenon occurred because the overpotential for deposition further increased compared with that in an additive-free solution.

As mentioned previously, with the addition of both PQ and QA, the crystallite size of the deposited Zn decreased, and the surface smoothed. In previous work, straight-chain polymer was reported to lower roughness on an area larger than 400–600 nm, and a brightening agent with a small molecular weight has a smoothing effect at the atomic level.^35,38) In this study, a synergistic effect that involves a macroleveling action with PQ and a microsmoothing action with QA may take place.

5. Conclusion

The effects of organic additives on the deposition behavior of Zn and the microstructure of the deposits were investigated. The films deposited at current densities higher than 1000 A·m⁻² from the solution containing additives of a straight-chain polymer composed of a quaternary ammonium cation (PQ) and a quaternary ammonium salt with a benzene ring (QA) were glossy. The polarization curve was separated into the partial polarization curves of Zn deposition and hydrogen evolution by using the galvanostatic data of Zn deposition. The overpotentials of the charge transfer of Zn deposition and that of ZnO₂²⁻ diffusion increased with the addition of PQ and QA. The increase in overpotential was considerable at potentials less noble than −1.5 V. Zn deposition reached the diffusion limit of ZnO₂²⁻ at the potential less noble than −1.5 V, which indicated that ZnO₂²⁻ diffusion was considerably suppressed by PQ and QA. With the addition of PQ and QA, C, N, and H were codeposited with Zn, thus demonstrating that PQ and QA were incorporated in the deposited films. The size of the Zn crystallite decreased with increasing current density. At a high current density of 5000 A·m⁻², the crystallite size decreased with the addition of PQ and QA, and the film exhibited a smooth surface. The orientation index of the $\{ 10\bar{1}0\} $ plane of Zn deposited from the solution containing PQ and QA increased with the increase in current density. The change in the crystallite size and crystal orientation of the deposited Zn was related to the deposition overpotential.

REFERENCES

• 1)   H.  Nezu,  S.  Fujii,  N.  Kaneko and  M.  Ofuchi: J. Met. Finish. Soc. Jpn. 32 (1981) 17–22.
• 2)   Y.  Tsuru,  Y.  Nakagawa,  M.  Matsunaga and  K.  Hosokawa: J. Met. Finish. Soc. Jpn. 34 (1983) 376–383.
• 3)   J.O’M.  Bockris,  Z.  Nagy and  D.  Drazic: J. Electrochem. Soc. 120 (1973) 30–41.
• 4)   R.D.  Naybour: Electrochim. Acta 13 (1968) 763–769.
• 5)   P.J.  Sonneveld,  W.  Visscher and  E.  Barendrecht: Electrochim. Acta 37 (1992) 1199–1205.
• 6)   Y.  Tsuru,  M.  Matsunaga and  K.  Hosokawa: J. Met. Finish. Soc. Jpn. 35 (1984) 196–200.
• 7)   M.  Kawasaki: J. Met. Finish. Soc. Jpn. 20 (1969) 621–627.
• 8)   H.  Noguchi: J. Finish. Soc. Jpn. 52 (2001) 456–461.
• 9)   D.J.  Mackinnon,  J.M.  Brannen and  V.L.  Lakshmanan: J. Appl. Electrochem. 10 (1980) 321–334.
• 10)   I.  Tomov,  Chr.  Cvetkova,  V.  Velinov,  A.  Riesenkampf and  B.  Pawlik: J. Appl. Electrochem. 19 (1989) 377–382.
• 11)   H.  Noguchi,  I.  Kotani and  T.  Suzuki: J. Met. Finish. Soc. Jpn. 34 (1983) 554–559.
• 12)   H.  Nakano,  S.  Oue,  Y.  Hamaguchi,  S.  Kobayashi and  H.  Fukushima: ISIJ Int. 49 (2009) 1769–1775.
• 13)   H.  Nakano,  K.  Araga,  M.  Iwai and  K.  Miki: Tetsu-to-Hagané 85 (1999) 806–813.
• 14)   H.  Nakano: Trans. Nonferrous Met. Soc. China 19 (2009) 835–841.
• 15)   H.  Nakano,  S.  Oue,  S.  Taniguchi,  S.  Kobayashi and  H.  Fukushima: ISIJ Int. 48 (2008) 634–639.
• 16)   H.  Nakano,  S.  Oue,  T.  Miki,  S.  Kobayashi and  H.  Fukushima: Tetsu-to-Hagané 92 (2006) 501–506.
• 17)   A.  Nakagawa and  H.  Enomoto: J. Met. Finish. Soc. Jpn. 20 (1969) 109–114.
• 18)   Y.  Imai: J. Met. Finish. Soc. Jpn. 18 (1967) 313–318.
• 19)   T.  Hayashi and  T.  Ishida: J. Met. Finish. Soc. Jpn. 11 (1960) 638–643.
• 20)   K.  Okubo,  S.  Suyama and  S.  Toba: J. Met. Finish. Soc. Jpn. 30 (1979) 641–645.
• 21)   K.S.  Willson and  J.A.  Rogers: Tech. Proc. Amer. Electroplaters Soc. 51 (1964) 92–95.
• 22)   T.  Watanabe: J. Finish. Soc. Jpn. 40 (1989) 280–286.
• 23)  H.H. Bauer, translated by R. Tamamushi and G. Satoh: Electrodics, (Tokyo Kagaku Dojin, Tokyo, 1982) p. 67.
• 24)  T. Oki: Kinzoku Denkikagaku, (Kyoritsu Shuppan, Tokyo, 1987) p. 38.
• 25)   M.  Stern: J. Electrochem. Soc. 102 (1955) 609–616.
• 26)   H.  Nakano,  S.  Oue,  S.  Hisano,  S.  Kobayashi and  H.  Fukushima: ISIJ Int. 47 (2007) 1029–1033.
• 27)   H.  Nakano,  S.  Oue,  S.  Kobayashi,  H.  Fukushima,  K.  Araga,  K.  Okumura and  H.  Shige: Tetsu-to-Hagané 90 (2004) 801–806.
• 28)   H.  Nakano,  S.  Oue,  S.  Hisano,  S.  Kobayashi and  H.  Fukushima: Tetsu-to-Hagané 93 (2007) 39–43.
• 29)   R.  Winand: J. Appl. Electrochem. 21 (1991) 377–385.
• 30)   K.  Kashida,  S.  Oue and  H.  Nakano: Mater. Trans. 58 (2017) 1418–1426.
• 31)   H.  Nakano and  H.  Fukushima: Tetsu-to-Hagané 88 (2002) 236–242.
• 32)   K.  Fuchi,  S.  Oue,  Y.  Kikuchi,  S.  Akamatsu,  Y.  Kashiwa and  H.  Nakano: Mater. Trans. 59 (2018) 1767–1776.
• 33)   F.  Yang,  W.  Tian,  H.  Nakano,  H.  Tsuji,  S.  Oue and  H.  Fukushima: Mater. Trans. 51 (2010) 948–956.
• 34)   K.  Fuchi,  S.  Oue,  Y.  Kikuchi,  S.  Akamatsu,  Y.  Kashiwa and  H.  Nakano: J. Japan Inst. Met. Mater. 82 (2018) 281–288.
• 35)  Electroplating Technology for Future Generation, (Denki Mekki Kenkyukai, Nikkan Kogyo Shinbunsya, Tokyo, 2004) pp. 7–8.
• 36)   N.A.  Pangarov: J. Electroanal. Chem. 9 (1965) 70–85.
• 37)   N.A.  Pangarov: Electrochim. Acta 7 (1962) 139–146.
• 38)  S. Haruyama: Electrochemistry for Surface Engineer, (Maruzen, Tokyo, 2001) p. 152.