Effect of Organic Additives on Electrodeposition Behavior of Zn from Zincate Solution Containing Potassium Hydroxide and Its Micro Structure

Abstract

Zn was electrodeposited on an Fe electrode at a current density of 50–5000 A·m⁻², charge of 4 × 10⁴ C·m⁻², and temperature of 313 K in an unagitated zincate solution containing 0.62 mol·dm⁻³ of ZnO, 4.0 mol·dm⁻³ of KOH or NaOH, and organic additives. The effects of KOH and NaOH on the deposition behavior of Zn in the solution containing the organic additives and on the microstructure of the deposits were investigated. In a solution containing a straight-chain polymer composed of a quaternary ammonium cation (PQ) and a quaternary ammonium salt with a benzene ring (QA), the current efficiency for Zn deposition in a high-current-density region (1000–5000 A·m⁻²) to produce glossy films was higher with KOH than that with NaOH. At high current densities above 1000 A·m⁻², the Zn deposition approached the diffusion limitation of ZnO₂²⁻ ions. With the addition of PQ and QA, the diffusion of ZnO₂²⁻ ions was significantly suppressed, and the degree of suppression was smaller with KOH than that with NaOH. The polarization resistance at 200 A·m⁻², which was investigated through alternating current impedance, revealed that the adsorption ability of PQ and QA onto the cathode was smaller with KOH than that with NaOH. Since the suppression effect of the additives on the Zn deposition was smaller with KOH than that with NaOH, the current efficiency for Zn deposition in the high-current-density region was larger with KOH. The upper limit of the current density needed to produce glossy films was smaller with KOH than that with NaOH, and spongy thin films were partially observed on platelet crystals obtained at high current densities in the KOH solution. The C content resulting from the additives in the deposited Zn was smaller with KOH because the adsorption ability of PQ and QA onto the cathode was smaller with KOH than that with NaOH.

 

This Paper was Originally Published in Japanese in J. Japan Inst. Met. Mater. 85 (2021) 59–66. Caption of Fig 12 is slightly changed.

Fig. 6 Current efficiency for Zn deposition from the KOH and NaOH solutions with and without additives. [● KOH without additive, ▲ KOH with PQ and QA, ○ NaOH without additive, △ NaOH with PQ and QA]

1. Introduction

The throwing power, appearance, hardness, and corrosion resistance of deposited Zn and the electrolytic voltage and current efficiency for Zn deposition depend on the kind of electrolytic solution, such as zincate,^1–6) boron fluoride,⁷⁾ chloride,^8–11) sulfate,^12–16) pyrophosphate,^17,18) and cyanide.^19,20) Cyanide and zincate solutions are used due to the excellent throwing power of alkalic solutions, but cyanide solutions have an effluent treatment problem. Therefore, the use of alkaline zincate solutions has increased from an environmental viewpoint. Since parts electroplated with Zn using zincate solutions are small and in large quantities, the increase in the electroplating rate or deposition at high current density is required. Solution additives are needed to obtain a smooth surface for Zn deposited at high current densities. Therefore, the authors selected two additives of a straight-chain polymer composed of a quaternary ammonium cation (PQ) and a quaternary ammonium salt with a benzene ring (QA), and the effects of the additives on the deposition behavior of Zn and resulting Zn microstructure were examined.^21,22)

NaOH is generally used as the principal basic agent in electrodeposition from zincate solutions. The viscosity and conductivity of the solution, the state of the water molecules of the solvent, and the activity of OH⁻ ions change according to the kind of the basic agent. Hence, the effect of additives on the deposition behavior and microstructure of Zn may differ depending on the basic agent. However, there are many ambiguities in the effects of basic agents. Therefore, in this study, KOH with lower viscosity than NaOH was selected as the basic agent, and the effect of additives on the deposition behavior and microstructure of Zn in the KOH solution was compared with that in a NaOH solution. The difference in the effect of additives between the KOH and NaOH solutions is discussed in this paper.

2. Experimental

Table 1 shows the composition of the solution 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 KOH or NaOH (4 mol·dm⁻³) in ion-exchanged water. Straight-chain polymers composed of PQ (molecule length: 20 nm) and QA were added to the electrolyte solution at concentrations of 2.9 and 0.14 g·dm⁻³, respectively. Fe sheets sized 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 galvanostatic conditions (50–5000 A·m⁻²) until the deposition charge reached 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 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 NHE as the reference.

Table 1 Solution composition and electrolysis conditions.

The viscosity of the solution was calculated from the kinetic viscosity, which was evaluated by a tuning-fork vibration rheometer (amplitude: 1.2 mm), and the density of the solution. The conductivity of the solution was measured by an electrode-type conductivity meter. The alternating current (AC) impedance was measured at 200 A·m⁻² during Zn deposition. 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), and the Nyquist plots were created.

For the evaluation of the additive content in the deposited films, the concentrations of C, H, K, 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 mm ϕ, argon pressure of 600 Pa, output of 40 W, pulse frequency of 2000 Hz, and duty cycle of 0.125. The surface and cross section of the deposited films were analyzed by secondary electron and backscattered electron images using scanning electron microscopy (SEM). The backscattered electron images were obtained using an angle-selective backscattered electron detector (AsB).²³⁾ The crystal orientation of the deposited Zn was determined by using the method developed by Willson and Rogers;²⁴⁾ an X-ray diffraction (XRD) intensity of 0002 to the $11\bar{2}2$ reflection was used. The crystallite size of the deposited Zn was calculated using the Scherrer equation²⁵⁾ from the half-width of the XRD peak corresponding to the $10\bar{1}1$ reflection.

3. Results

3.1 Appearance of deposited films

Figure 1 shows the appearance of the Zn films deposited at various current densities from the KOH and NaOH solutions with and without additives. The word written in Chinese characters (nine), 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, the clearer the Chinese characters in Fig. 1, the better the gloss of the deposited films. In the KOH solution, the Zn films deposited at all the current densities from the solutions without additives and with only QA did not show gloss, while those deposited at high current densities of 2000 and 5000 A·m⁻² from the solution containing only PQ as an additive were somewhat glossy. With the addition of both PQ and QA, the films deposited at 2000 and 5000 A·m⁻² showed significant gloss. In the NaOH solution, with the addition of both PQ and QA, the films deposited at 1000, 2000, and 5000 A·m⁻² exhibited significant gloss. The gloss of the edge area of the Zn films obtained at 5000 A·m⁻² in the KOH solution disappeared, indicating that the upper limit of the current density needed to produce glossy Zn films was smaller with the KOH solution than that with the NaOH solution.

Fig. 1

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

3.2 Deposition behavior of Zn

Figure 2 shows the total polarization curve during Zn deposition in the KOH solution. The polarization curve was significantly polarized at current densities above 1000 A·m⁻² irrespective of the presence or absence of additives. The polarization was larger with the addition of only PQ and both PQ and QA as additives than that without additives. With the addition of both PQ and QA, the polarization became significant at current densities above 2000 A·m⁻².

Fig. 2

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

Figure 3 shows the total polarization curve during Zn deposition in the KOH and NaOH solutions. The total polarization curves in the KOH and NaOH solutions were almost identical in the case of the additive-free solution, but the polarization was larger with the addition of both PQ and QA than that without additives for both the KOH and NaOH solutions. At the high current densities of 2000 and 5000 A·m⁻², the polarization effect of PQ and QA was larger in the NaOH solution than that in the KOH solution. Then, the partial polarization curves for Zn deposition and hydrogen evolution were investigated.

Fig. 3

Total polarization curves for Zn deposition from the KOH and NaOH solutions with and without additives. [● KOH without additive, ▲ KOH with PQ and QA, ○ NaOH without additive, △ NaOH with PQ and QA]

Figure 4 shows the partial polarization curve for Zn deposition. In the additive-free solution, the partial polarization curve for Zn deposition in KOH was almost identical to that in the NaOH solution. In the KOH and NaOH solutions, Zn deposition was evidently polarized with the addition of both PQ and QA than that without additives. The degree of polarization became significantly high at partial current densities of Zn above 1000 A·m⁻². At partial current densities of Zn above 1000 A·m⁻², Zn deposition approached the diffusion limiting current density of ZnO₂²⁻ ions, indicating that the diffusion of ZnO₂²⁻ ions was significantly suppressed with the addition of PQ and QA. Regarding the difference in the effects of the additives between the KOH and NaOH solutions, the polarization effect of PQ and QA at partial current density of Zn above 1000 A·m⁻² was larger in NaOH solution than that in KOH solution.

Fig. 4

Partial polarization curves for Zn deposition from the KOH and NaOH solutions with and without additives. [● KOH without additive, ▲ KOH with PQ and QA, ○ NaOH without additive, △ NaOH with PQ and QA]

Figure 5 shows the partial polarization curve for hydrogen evolution. In the KOH solution, hydrogen evolution was suppressed with the addition of PQ and QA in all the potential region. In the NaOH solution, hydrogen evolution was suppressed with PQ and QA at high partial current densities above 200 A·m⁻². The degree of polarization of hydrogen evolution with PQ and QA in the NaOH solution was somewhat larger than that in the KOH solution.

Fig. 5

Partial polarization curves for H₂ evolution from the KOH and NaOH solutions with and without additives. [● KOH without additive, ▲ KOH with PQ and QA, ○ NaOH without additive, △ NaOH with PQ and QA]

Figure 6 shows the current efficiency for Zn deposition. The current efficiency for Zn deposition in the KOH and NaOH solutions became maximum at 500–1000 A·m⁻² irrespective of the presence or absence of additives, and it significantly decreased with increasing current density. Zn deposition approached the diffusion limiting current density of ZnO₂²⁻ ions at current densities above 1000 A·m⁻² (Fig. 2–4); as a result, the current efficiency of Zn decreased with increasing current density in the high-current-density region. Regarding the use of a practical current density of 1000–5000 A·m⁻² to produce glossy Zn films with the addition of both PQ and QA (Fig. 1), the current efficiency of Zn from the KOH solution containing PQ and QA was higher than that from the NaOH solution. At 2000 and 5000 A·m⁻², the difference in current efficiency of Zn between the additive-free KOH and NaOH solutions was small; the decrease in the current efficiency of Zn with the addition of PQ and QA was larger in the NaOH solution than that in the KOH solution. The reason was that the suppression effect of PQ and QA on Zn deposition was larger in the NaOH solution (Fig. 4).

Fig. 6

Current efficiency for Zn deposition from the KOH and NaOH solutions with and without additives. [● KOH without additive, ▲ KOH with PQ and QA, ○ NaOH without additive, △ NaOH with PQ and QA]

The polarization resistance at 200 A·m⁻², where the charge transfer process appeared to be the rate-determining step for Zn deposition (Fig. 3, 4), was investigated by AC impedance to evaluate the adsorption ability of the additives during the deposition in the KOH and NaOH solutions. The Nyquist plots for Zn deposition are shown in Fig. 7. Figure 8 shows a magnified view of the area near the origin. The diameters of the semicircles in the Nyquist plots represent the polarization resistance during Zn deposition. The Nyquist plots are all described assuming zero resistance of the electrolyte, given that this study focuses on the polarization resistance for Zn deposition. The polarization resistance for Zn deposition, R, follows the following order irrespective of whether the solution was KOH or NaOH: R(PQ+QA) > R(PQ) ≫ R(QA) > R(none). The polarization resistance was almost identical between the additive-free KOH and NaOH solutions (Fig. 8), while R(PQ+QA) and R(PQ) were larger in the NaOH solution than those in the KOH solution (Fig. 7). That is, the suppression effect of PQ and PQ+QA on the charge transfer process of Zn deposition was larger in the NaOH solution, indicating that the adsorption ability of PQ and PQ+QA onto the cathode was larger in the NaOH solution. In this study, an inductive semicircle showing a downward loop was observed besides the capacitive semicircle irrespective of the presence or absence of additives. Such inductive semicircle reportedly occurs in successive deposition reactions through adsorption intermediate.²⁶⁾ Zn deposition from alkaline zincate solutions proceeds via a multistep reaction,²⁷⁾ and a successive deposition reaction is assumed to occur through adsorption intermediate.

Fig. 7

Nyquist plots obtained at 200 A·m⁻² in KOH (a) and NaOH (b) solutions with and without additives. [● additive-free, △ with QA, □ with PQ, ◆ with PQ and QA]

Fig. 8

Nyquist plots obtained at 200 A·m⁻² in (a) KOH and (b) NaOH solutions with and without additives. [● additive-free, △ with QA, □ with PQ, ◆ with PQ and QA]

3.3 Structure of deposited films

Figure 9 shows the SEM images of the surfaces of Zn films deposited at 2000 and 5000 A·m⁻² in the additive-free KOH and NaOH solutions. In the additive-free KOH and NaOH solutions, the nonuniform Zn platelet crystals grew largely perpendicular to the substrate. In both solutions, the size of the platelet crystals obtained at 5000 A·m⁻² was smaller than that at 2000 A·m⁻².

Fig. 9

SEM images of the surface of Zn films deposited from the KOH and NaOH solutions without additive. [(a) KOH, 2000 A·m⁻², (b) KOH, 5000 A·m⁻², (c) NaOH, 2000 A·m⁻², (d) NaOH, 5000 A·m⁻²]

Figure 10 shows the SEM images of the surfaces of Zn films deposited at 2000 and 5000 A·m⁻² in the KOH and NaOH solutions containing PQ and QA. In both additive-containing solutions, the deposited Zn films were composed of fine platelet crystals and showed smooth surfaces. Regarding the morphological differences between the Zn films from the KOH and NaOH solutions, those deposited from the KOH solution partially exhibited spongy thin films on platelet crystals, and the tendency became significant at 5000 A·m⁻².

Fig. 10

SEM images of the surface of Zn films deposited from the KOH and NaOH solutions with PQ and QA. [(a) KOH, 2000 A·m⁻², (b) KOH, 5000 A·m⁻², (c) NaOH, 2000 A·m⁻², (d) NaOH, 5000 A·m⁻²]

Figure 11 shows the secondary electron and backscattered electron images of cross sections of the Zn films deposited at 5000 A·m⁻² in the KOH and NaOH solutions containing PQ and QA. For both solutions, the surfaces of the deposited Zn films were smooth and glossy, but a number of voids less than 100 nm in size were observed inside. There was a small difference in the number and size of voids between the films from the KOH and NaOH solutions.

Fig. 11

Secondary electron images (a), (c) and backscattered electron images (b), (d) of the cross section of Zn films deposited at 5000 A·m⁻² from the KOH and NaOH solutions with PQ and QA. [(a), (b) KOH, (c), (d) NaOH]

Figure 12 shows the crystal orientation of Zn films deposited at various current densities from the KOH and NaOH solutions. In the KOH solution, a characteristic preferred orientation was rarely observed without additives (Fig. 12(a)), while the orientation of the $\{ 10\bar{1}0\} $ plane increased in the high-current-density region of 1000–5000 A·m⁻² to show gloss with the addition of PQ and QA (Fig. 12(b)). In the NaOH solution without additives, the deposited Zn preferentially oriented to the {0001} plane at a low current density of approximately 100 A·m⁻², and {0001} decreased and $\{ 10\bar{1}1\} $ increased with increasing current density. With the addition of PQ and QA, the orientation of the $\{ 10\bar{1}0\} $ plane increased in the high-current-density region of 1000–5000 A·m⁻² to show gloss (Fig. 12(d)). The crystal orientation of hcp Zn depends on the Zn deposition overpotential and shifts 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 overpotential.^28,29) The increase in $\{ 10\bar{1}0\} $ in the high-current-density region of 1000–5000 A·m⁻² with the addition of PQ and QA into the KOH and NaOH solutions was attributed to the increase in the deposition overpotential. Regarding the crystal orientation of Zn from the KOH solution and that from the NaOH solution, a difference was observed in the low-current-density region in the additive-free solution, but this difference became small with the addition of PQ and QA.

Fig. 12

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

Figure 13 shows the crystallite size of Zn films deposited at various current densities in the KOH and NaOH solutions. The crystallite size of the deposited Zn films generally decreased with increasing current density irrespective of the presence or absence of additives. The crystallite size of the deposited Zn films evidently decreased with the addition of PQ and QA compared with the additive-free solution, but this difference was rarely observed between the KOH and NaOH solutions. Typically, when the deposition overpotential increases during metal deposition, the nucleation rate for deposition prevails over the crystal growth, thus resulting in a decreased crystallite size of the deposits.^30–35) In this study, the decrease in the crystallite size of the deposited Zn films with increasing current density irrespective of the presence of additives was attributed to the nucleation rate’s relative increase with the overpotential during deposition. The crystallite size of the deposited Zn films in the high-current-density region above 200 A·m⁻² became smaller with the addition of both PQ and QA than that from the additive-free solution; the reason is the further increase in deposition overpotential with the additives.

Fig. 13

Crystallite size of Zn deposited at various current densities in the KOH and NaOH solutions with and without additives. [● KOH without additive, ▲ KOH with PQ and QA, ○ NaOH without additive, △ NaOH with PQ and QA]

Figure 14 shows the rf-GDOES depth profile of the films deposited at 5000 A·m⁻². In the Zn films deposited from the additive-free NaOH solution, the H component from H₂O in the electrolyte was observed (Fig. 14(a)). Furthermore, C, H, K, and Na codeposited almost uniformly throughout the thickness of the Zn films deposited from the KOH and NaOH solutions containing both PQ and QA (Fig. 14(b), (c)). As for the difference in composition of the deposited films between the KOH and NaOH solutions, the C content resulting from the additives in the deposits was two times larger in the NaOH solution. K and Na codeposited on the Zn films from the KOH and NaOH solutions, respectively. Since the constituents of K and Na were not involved in the additives, these components may have resulted from the electrolyte. Fine voids were observed in the Zn films obtained from the solutions containing PQ and QA (Fig. 11), indicating that the electrolyte was incorporated into the voids.

Fig. 14

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

4. Discussion

The deposition behavior and microstructure of the Zn films from the solution containing KOH as the principal basic agent were compared with those from the NaOH solution; there was only a small difference in the case of the additive-free solutions. However, with the addition of both PQ and QA, the current efficiency for Zn deposition in the practical current density region of 1000–5000 A·m⁻² to produce glossy films was higher in the KOH solution than that in the NaOH solution (Fig. 6). In the current density region above 1000 A·m⁻², Zn deposition approached the diffusion limiting current density of ZnO₂²⁻ ions, and the diffusion of ZnO₂²⁻ ions was greatly suppressed with PQ and QA; the degree of suppression was smaller in the KOH solution (Fig. 4). The polarization resistance, dE/di, at 200 A·m⁻², where the charge transfer process appeared to be the rate-determining step for Zn deposition (investigated by AC impedance), revealed that the adsorption ability of PQ and QA onto the cathode was smaller in the KOH solution (Fig. 7, 8). The current efficiency for Zn deposition in the high-current-density region to produce glossy films in the solution containing PQ and QA was higher in the KOH solution; the reason was that the suppression effect of additives on Zn deposition was smaller in the KOH solution than that in the NaOH solution.

The upper limit of the current density needed to produce glossy films was smaller in the KOH solution than that in the NaOH solution (Fig. 1), and spongy thin films were partially observed on the Zn platelet crystals obtained at high current densities in the KOH solution (Fig. 10). In addition, the C content in the deposited films resulting from the additives was smaller in the KOH solution (Fig. 14). The reason for these phenomena seemed to be the smaller adsorption ability of PQ and QA onto the cathode in the KOH solution.

The viscosities of the KOH and NaOH solutions used in this study significantly differed (1.31 and 3.01 mPa·s, respectively). The conductivities of the KOH and NaOH solutions were 516 and 400 mS·cm⁻¹, respectively. The effects of the viscosity and conductivity of the solution on the adsorption ability of PQ and QA onto the cathode are presently unknown. In addition, the coordination number of water to K⁺ ions was calculated to be 7.8 ± 0.2 and 8.3 ± 0.3 by molecular dynamics calculation and first-principles molecular dynamics simulations, respectively; the coordination number of water to Na⁺ ions was calculated to be 6.5 ± 0.2 and 5.6 ± 0.3, showing that the coordination number of water to the K⁺ ions was larger.³⁶⁾ That is, the amount of free water in the electrolyte was expected to be smaller in the KOH solution. Polymeric additives deteriorate solubility with decreasing free water in an electrolyte, resulting in a decrease in adsorption ability.^37,38) In this study, the adsorption ability of PQ and QA on the cathode was smaller in the KOH solution, and this was attributed to the smaller amount of free water in the electrolyte in the KOH solution.

5. Conclusion

The effects of additives on Zn deposition behavior from a zincate solution using KOH as the principal basic agent and on the microstructure of the deposits were investigated. In a solution containing PQ and QA, the current efficiency for Zn deposition in the high-current-density region of 1000–5000 A·m⁻² to produce glossy films was higher with KOH than that with NaOH. At high current densities above 1000 A·m⁻², Zn deposition approached the diffusion limitation of ZnO₂²⁻ ions. With the additions of PQ and QA, the diffusion of ZnO₂²⁻ ions was significantly suppressed, and the degree of suppression was smaller with KOH than that with NaOH. The polarization resistance at 200 A·m⁻², which was investigated by AC impedance, revealed that the adsorption ability of PQ and QA onto the cathode was smaller with KOH than that with NaOH. Since the suppression effect of the additives on Zn deposition was smaller with KOH than that with NaOH, the current efficiency for Zn deposition in the high-current-density region was larger with KOH. The upper limit of the current density needed to produce the glossy films was smaller with KOH than that with NaOH, and spongy thin films were partially observed on the platelet crystals obtained at high current densities in the KOH solution. The C content resulting from the additives in the deposited Zn was smaller with KOH. The reason for these phenomena was that the adsorption ability of PQ and QA onto the cathode was smaller with KOH.

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