Electrodeposition Behavior of Zn-Ni Alloys Produced from Sulfate Solutions at High Current Densities
2016 Volume 57 Issue 11 Pages 1908-1914
Details
2016 Volume 57 Issue 11 Pages 1908-1914
The electrodeposition behavior of Zn-Ni alloys produced from acidic sulfate solutions was investigated from partial polarization curves obtained during alloy electrodeposition. At the current density at which the co-deposition of Zn-Ni alloys produced anomalous results, we found that Zn deposition is polarized and is affected by the bath Zn concentration and flow rate. This indicates that Zn deposition is controlled by diffusion at high current densities. Under the conditions for increased Zn deposition, Ni deposition was not suppressed even in the region of anomalous co-deposition. With a low pH bath, the Ni concentration in the deposit did not increase under a high current density because of strong suppression of Ni deposition under the low pH condition.
This Paper was Originally Published in Japanese in J. Japan Inst. Met. Mater. 79 (2015) 398–403.
Zn-Ni alloy coating has been applied to rust-preventive steel sheets for automobiles and household electrical applications and is an industrially important alloying coating system1). Moreover, its unique deposition mechanism has been studied extensively, and it is also a scientifically important alloy coating system.
Zn-iron group alloy electrodeposition, including Zn-Ni electrodeposition, has been reported to show anomalous co-deposition behavior, in which the electrochemically less noble metal Zn is preferentially deposited compared to the iron group metal. Anomalous co-deposition mechanisms have also been actively studied in Zn-Ni alloy systems, and the hydroxide suppression theory2–9) is the most influential explanation of this behavior. In addition, the polynuclear intermediate complex theory10,11), under potential deposition (UPD) theory12,13), and complex hydroxide theory14) have been also proposed. A theory that the difference of exchange current density of the formed alloy phase causes anomalous behavior15), and a theory that a variety of precipitation intermediates containing Zn are adsorbed and the adsorbed precipitation intermediate affects deposition16,17) have been proposed.
On the other hand, in industrial production of rust-preventive steel sheets, electrodeposition is performed under the conditions of high electrolytic bath flow rate and high mass transfer to improve current efficiency and eliminate the influence of the gas resulted from electrolysis.
From the point of view of productivity, electrolysis has been performed at a high current density, and the Zn-Ni alloy coating is electrodeposited under a high overpotentilal. Although studies of electrodeposition behavior under the conditions of such high bath flow rate and high current density exist18,19), there are few examples in which the basic co-deposition behavior is examined by obtaining the partial current densities of Zn and Ni and considering each changes.
In the present study, the Zn-Ni alloy electrodeposition behavior in a sulfuric acid bath, which is one type of bath used in the manufacture of anti-rust steels, is discussed under the conditions of high bath flow rate and high current density.
First, the relationships between the current density and the Ni content of deposits or current efficiency were obtained, and electrodeposition behavior was discussed by comparison of the partial current densities of Zn and Ni deposition. The co-deposition behaviors in baths with different Zn concentrations and pH levels were also compared and discussed in association with the proposed co-deposition mechanism.
In order to produce a high flow rate bath like that used under actual production conditions, electrolysis was conducted using a cell in which an anode and cathode were arranged in parallel to a bath flow between the electrodes. Figure 1 shows the outline of the experimental apparatus. A capillary was arranged from the back holes in the cathode center to measure the cathode potential without affecting the bath flow. Using this device, partial polarization curves were measured to clarify the Zn-Ni alloy co-deposition behavior.
Experimental apparatus.
First, coulostatic electrodeposition was conducted under a constant cathode potential and different cathode potentials. The cathode deposit at each potential was dissolved in aqueous hydrochloric acid. The Zn and Ni contents in the solution were determined by ICP emission spectroscopic analysis, and the Ni content of the deposit was calculated. Next, the partial current densities of each potential were obtained by calculating from the Zn and Ni contents of the deposits. The total current density at each potential was calculated from the time required for electrodeposition of the predetermined amount of coulombs. Because a saturated Ag/AgCl electrode (saturated KCl, 0.199 V vs. NHE, 25℃) was used as the reference electrode, the experimental results were shown in terms of the standard hydrogen electrode reference. A commercially available insoluble anode consisting of a Ti plate coated with iridium oxide was used as the anode, and a Cu plate was used as the cathode. The anode and cathode were sealed so as to expose a circular part with the diameter of 25.4 mm (1 inch) in a plate with dimensions of 35 mm × 65 mm. The details of the electroplating conditions when measuring the partial polarization curves are shown in Table 1.
| Cathode potential (V vs NHE) | −0.7～−1.6 |
| Amount of charge (C・m−2) | 9.87 × 104 |
| Cathode | Cu |
| Anode | Ti (Ir oxide coated) |
| Temperature (℃) | 50 |
| Flow rate (m・s−1) | 0.5, 2.0 |
As a basic bath, a sulfate aqueous solution of pH1.3, containing zinc sulfate, nickel sulfate and sodium sulfate was used. In addition, the bath zinc concentration was changed in the basic bath and the bath concentration was lowered to investigate the effect of the flow rate, and the bath with a different pH were also used. The bath compositions used in this study are shown in Table 2.
| A | B | C | D | mol・dm−3 | |
|---|---|---|---|---|---|
| ZnSO4・7H2O | 0.52 | 0.26 | 0.10 | 0.10 | |
| NiSO4・6H2O | 1.33 | 1.33 | 0.20 | 0.20 | |
| Na2SO4 | 0.42 | 0.42 | − | − | |
| pH | 1.3 | 1.3 | 1.2 | 0.6 |
Figure 2 shows the relationship between the current density and Ni content of deposition in baths A and B. The difference between the two baths is the Zn concentration. With each bath condition, under a low current density conditions, the Ni content decreases slightly with increasing current density. In the region where the current density is more than 10 A/dm2, the change of the Ni content becomes small and shows almost the same level of 9–12 mass%. Under a current density of 50 A/dm2 or more, the difference of the Ni content becomes significant. The increase in the Ni content depending on the current density in bath B is large compared to that in bath A. In bath A, the increase of the Ni content is small at high current densities over 100 A/dm2, and the Ni content shows a value of 13–14 mass%.
Effect of current density on Ni content of Zn-Ni alloy deposition from different Zn concentration solutions.
The Ni ratio calculated from the Zn and Ni concentrations in the bath is 70 mass% in the case of bath A and 82% in the case of bath B. The broken line in Fig. 2 is a composition reference line showing the case in which the bath composition and the Ni content of the deposit are equal (Composition Reference Line, hereinafter, CRL). If the Ni content of the deposit is located higher than this CRL, this indicates normal type electrodeposition, in which preferential electrodeposition of noble Ni has occurred. However, a Ni content lower than the CRL indicates an anomalous type of deposition, in which preferential deposition of the less noble Zn has occurred. At all the current densities in this study, the Ni contents in Fig. 2 are located lower than the CRL. Therefore, it can be said that anomalous type of deposition has occurred under these conditions. In particular, the highest Ni content of 45 mass% under the current density of 140 A/dm2 in bath B is lower than the Ni ratio of the bath composition. In other words, even under a high current density condition, anomalous type deposition has occurred.
Figure 3 shows the relationship between the current density and current efficiency. When the current density exceeds 10 A/dm2, deposition becomes remarkable and hydrogen evolution is inhibited, and a rapid increase of current efficiency occurs. Thereafter, however, current efficiency decreases gradually with increasing current density.
Effect of current density on current efficiency for Zn-Ni alloy deposition from different Zn concentration solutions.
As described below, Akiyama et al. reported that, in the electrodeposition of Zn-Iron group alloys, there is a common point in the relationship of the current density and the current efficiency or iron group metal content of deposition, and this relationship can be classified into the following four regions4).
Under low current density conditions, normal type electrodeposition occurs, in which the electrochemically noble iron group metal is deposited preferentially. In this region, the hydrogen generation current is high and the current efficiency is low (region I).
When the current density is increased, the content of the iron group metal decreases greatly and electrodeposition changes from the normal type to the anomalous type. In this region, hydrogen generation is suppressed and current efficiency increases (region II).
Next, there is a region in which the iron group metal content and the current efficiency are almost constant (region III).
In the higher current density range above region III, the iron group content increases again, and current efficiency becomes low (region IV).
Comparing these experimental results and the above-mentioned four regions, it is considered that the current density region less than 10 A/dm2 corresponds to region II, 10–50 A/dm2 corresponds to region III, and the higher current density over 50 A/dm2 in bath B corresponds to region IV.
3.1.2 Comparison of total current density and Zn and Ni partial current densities in Zn-Ni alloy electrodepositionFigure 4 shows the relationship between the cathode potential and the total current density, and the partial current densities of Zn and Ni. Under the condition that the total current density is 10 A/dm2, the cathode potential is less noble than −0.8 V, which corresponds to the potential at which Zn deposition becomes possible. The current density range of 10–50 A/dm2 corresponds to region III, as described above in Fig. 2. As shown in Fig. 4, in this current density range, the deposition overpotential is greatly increased in both bath A and bath B. Also, comparing the Zn partial current densities in baths A and B in Fig. 4(b), the current density in bath A with high Zn concentration is larger than that in bath B. From these results, it can be said that, in the high current density region, the Zn ion supply near the cathode becomes the rate-limiting factor, and electrodeposition behavior is easily affected by the Zn concentration in the bath. On the other hand, as shown in Fig. 4(c), under the same overvoltage, the value of the partial current density of Ni showed almost the same value in both baths.
Total and partial polarization curves for Zn-Ni alloy deposition from different Zn concentration solutions. a) total, b) Zn (partial), c) Ni (partial).
In Zn-Fe alloy electrodeposition in sulfuric acid baths, it has been reported that the Fe deposition in a high Zn concentration bath is suppressed compared to that in a low Zn concentration bath. The suppression of Fe deposition by precipitation intermediates such as Zn(OH)2 occurred and this effect is more pronounced by increasing Zn concentration20). However, the mechanism of Zn-Ni alloy deposition is different from that of Zn-Fe alloy deposition. As shown in the comparison of baths A and B, even under a high Zn concentration condition, the effect on Ni electrodeposition behavior was small.
In Fig. 2, in the current density range over 50 A/dm2, the relationships between the current density and the Ni content were different in baths A and B. Only in bath B, the increase in the Ni content depending on the current density became obvious. In the current density range corresponding to region IV, because the concentration is low, Zn deposition first becomes diffusion-limited. As a result, Zn deposition does not proceed even if the current density is high and the iron group metal content increases4).
The results shown in Fig. 4 support this mechanism. That is, in a high overpotential region, Zn deposition is considered to become a diffusion-controlled condition earlier than Ni.
3.2 Effect of flow rate 3.2.1 Comparison of Ni content in deposits and current efficiencyNext, the effect of the flow rate on Zn and Ni electrodeposition behavior was examined by comparing the partial current polarization curves under different flow rate conditions. To observe the flow rate effects more remarkably, the lowered Zn and Ni concentration bath of Zn = 0.1 M, Ni = 0.2 M (bath C in Table 1) was used. The partial current densities of Zn and Ni were compared under the different flow rates of 0.5 m/s and 2.0 m/s.
Figures 5 and 6 show the relationship between the current density and Ni content of the deposit, and current efficiency. Compared to the result in the high concentration bath, as shown in Fig. 2, the current density range of the region II in which the Ni content starts to decrease becomes higher. However, in the current density range of 20–70 A/dm2, corresponding to region III, the Ni content shows the value of 10–15 mass%, and this value is almost the same as the results in baths A and B. Even with the low concentration bath and the large difference in flow rate conditions of 0.5 and 2.0 m/s, the effect of the flow rate on the Ni content is small in this current density region.
Effect of current density on Ni content of Zn-Ni alloy deposition with different flow rates.
In contrast, under the higher current density range corresponding to region IV, the effect of the flow rate on the Ni content becomes obvious, and the Ni content increases under the low flow rate condition of 0.5 m/s,.
Figure 6 shows the relationship between current density and current efficiency. Current efficiency is higher under the high flow rate condition; therefore, it can be estimated that the deposition of Zn and Ni is promoted under high flow rate conditions.
Effect of current density on current efficiency for Zn-Ni alloy deposition with different flow rates.
Figure 7 shows the relationship between the cathode potential and total current density and partial current densities of Zn and Ni. Under the high potential condition, the partial current densities of Ni and Zn are affected by the flow rate, and under the higher flow rate conditions, the partial current density becomes large. In Zn-Fe alloy electrodeposition from an acidic sulfate solution, it has been reported that Zn deposition increases but Fe deposition decreases under a high flow rate condition20). The high flow rate condition promotes Zn deposition and an increase of precipitation intermediates such as Zn(OH)2, and this intermediate is assumed to decrease the deposition sites of Fe electrodeposition. Even though this is an alloy electrodeposition process, Fe is deposited by the same mechanism as that in single Fe deposition, and this decrease in deposition sites suppresses Fe deposition20).
Total and partial polarization curves for Zn-Ni alloy deposition with different flow rates. a) total, b) Zn (partial), c) Ni (partial).
As shown above in section 3.1.2, the increase of the Zn concentration does not affect Ni deposition. Under the high flow rate condition, in which Zn deposition increases, Ni deposition is not suppressed. Unlike Zn-Fe alloy electrodeposition, in Zn-Ni alloy electrodeposition, Ni deposition is not suppressed even under conditions which promote Zn deposition.
In Fig. 5, the difference in the Ni content at current densities over 20 A/dm2 is small, but the Ni content under the low flow rate condition is slightly high. The behavior of the Ni content increase under the low flow rate condition is the same as in other reported results18). From Fig. 7(a), the cathode potential corresponding to the total current density of over 20 A/dm2 is the range which is less noble than −1.0 V. As shown in Fig. 7(b) and (c), the Zn and Ni partial current densities are already affected by the flow rate in this potential range. Anomalous co-deposition occurs in this region, and the Zn partial current density is higher than that of Ni. However, the effect of the flow rate on Zn deposition is significant because of the low bath concentration, and this is assumed to cause the high Ni content under the low flow rate condition.
As shown in Fig. 5, in the high current density range over 70 A/dm2, the effect of the flow rate becomes significant. From Fig. 7(a), this current density range corresponds to the cathode potential range less noble than −1.6 V. The Zn and Ni partial current densities at a cathode potential of around −1.6 V are both affected by the flow rate, as shown in Fig. 7(b) and (c). The difference of the Ni partial current density seems to be small as the cathode potential become less noble. In the case of bath C, the Zn concentration is 0.1 M, which is very small compared to baths A and B. Because of this low concentration, Zn deposition first becomes diffusion-limited, and suppression of Ni deposition by Zn starts to decrease. As the cathode potential becomes less noble, this suppression effect on Ni deposition become small, and as a result, the Ni content becomes high under the low flow rate condition.
3.3 Effect of pH 3.3.1 Comparison of Ni content in deposits and current efficiencyFigure 8 shows the relationship between the cathode current density and the Ni content deposited in the different pH baths, i.e., baths C and D, under the 0.5 m/s flow rate condition. Even under a very small pH condition of 0.6, Zn deposition occurs preferentially. In the current density range of 20–60 A/dm2, the Ni content becomes 11–17 mass%. The difference of the Ni content in the pH0.6 and pH1.2 baths is small. On the other hand, in the high current density range, the Ni content increases, and under the high pH condition, it becomes higher than under the low pH condition.
Effect of current density on Ni content of Zn-Ni alloy deposition from different pH solutions.
Figure 9 shows the relationship between current density and current efficiency. In the low pH case, hydrogen generation increases, and this obviously causes the low current efficiency. From Figs. 8 and 9, the current density region II, in which normal electrodeposition changes to the anomalous type, can be seen even in the low pH bath. In this region, suppression of hydrogen generation begins, and current efficiency starts to increase. At higher current density, region III, in which the Ni content and current efficiency become constant, also exists.
Effect of current density on current efficiency for Zn-Ni alloy deposition from different pH solutions.
Figure 10 shows the relationship between the cathode potential and the total current density and the partial current densities of Zn and Ni. Under the low pH bath condition, Ni and Zn deposition are strongly suppressed. On the other hand, as shown in Fig. 10(a), the total current density in the low pH bath becomes high, indicating that the hydrogen generation current has increased under the low pH condition.
Total and partial polarization curves for Zn-Ni alloy deposition from different pH concentration solutions. a) total, b) Zn (partial), c) Ni (partial).
As shown in Fig. 8, in the current density range of 20–60 A/dm2, the difference of the Ni content is small. From Fig. 10, this current density condition corresponds to the potential range less noble than −1.0 V.
In Fig. 8, the difference of the Ni content becomes significant in the high current density range over 70 A/dm2, and under the high pH condition, the Ni content is high. This behavior, i.e., a significant difference in the Ni content under the high current density condition, is the same as the results in Figs. 2 and 5. In Figs. 2 and 5, the Zn suppression conditions, such as the low Zn concentration and low flow rate, result in an increasing Ni content. However, in the high pH bath, as shown in Fig. 10 (b), Zn deposition increases. This point is different from the cases in Figs. 2 and 5. From these results, in addition to the Zn diffusion limitation under the high current density condition described in sections 3.1.2 and 3.2.2, it is also necessary to consider other factors influencing the Ni content.
From the comparison of the Ni partial current density in the baths with different Zn concentrations, as shown in Fig. 4(c), the effect of the Zn concentration on Ni deposition is small.
Comparing the Ni suppression levels in Figs. 7(c) and 10(c), the suppression level seems to be more significant in Fig. 10(c). Figure 7(c) shows the flow rate effect, and Fig. 10(c) shows the pH effect. Therefore, under these experimental conditions, it can be said that the suppression of Ni deposition by low pH is more significant. Under the low pH0.6 condition, even with a high current density condition, in which Zn deposition is diffusion-limited, the Ni content does not increase because of the high Ni suppression effect of the low pH condition.
3.4 Comparison of Zn-Ni alloy deposition mechanism and results of this experimentA hydroxide suppression mechanism has been proposed as the most influential theory to explain the anomalous electrodeposition of a Zn-Ni alloy from an acidic sulfate bath2–9). This mechanism is described as follows. As a result of an increase of pH near the cathode due to hydrogen generation, Zn(OH)2 is generated and adsorbed on the cathode. The adsorbed Zn(OH)2 closes the adsorption sites of the intermediate of Ni electrodeposition, NiOHad, and this suppresses Ni electrodeposition.
In the comparison of changing the bath and changing the flow rate, the electrodeposition behavior in which first Zn deposition becomes diffusion-limited and the Ni content increases at high current density can be explained by this mechanism. The sites blocked by Zn(OH)2 decrease under the diffusion-limited condition, and NiOHad adsorption sites increase. Under the high current density conditions in Figs. 2 and 5, the increase of the Ni content is suppressed in the case of high Zn concentration and high flow rate conditions. This behavior is explained as follows. Under both conditions, supply of Zn ions to the cathode is promoted, and the decrease of blocked sites by Zn(OH)2 is suppressed. On the other hand, under the lower current density range, corresponding to region III (10–50 A/dm2 in Fig. 2, 20–70 A/dm2 in Fig. 5), the difference of the Ni content depending on changes in the Zn concentration and flow rate is small. In addition, as shown in the comparison of the Ni partial current densities in Figs. 4 (c) and 7(c), Ni deposition is not suppressed even under the condition of increasing Zn deposition. In this region, the hydroxide suppression mechanism is effective and Ni deposition sites are blocked by Zn(OH)2. However, even in the case of Zn deposition promotion conditions under a high Zn concentration or high flow rate, the change in the blocked sites is small.
As described in 3.1.2, in Zn-Fe alloy electrodeposition, Fe deposition is suppressed under the condition of an increase in Zn deposition. In this case, unlike Zn-Ni alloy deposition, the Fe deposition blocking effect is assumed to be more significant. In addition, in Zn-Fe alloy electrodeposition, the following mechanism of single Fe electrodeposition is also suggested to occur even in the co-deposition process21). From this mechanism, the change of the intermediate FeOH+ concentration is assumed to have a large effect on Fe deposition behavior20).
| \[ {\rm Fe}^{2+} + \mathrm{H}_2 \mathrm{O} = {\rm FeOH}^+ + \mathrm{H}^+ \] | (1) |
| \[ \mathrm{FeOH}^+ + \mathrm{e}^- = {\rm FeOH}_{\rm ad}\] | (2) |
| \[ {\rm FeOH}_{\rm ad} + \mathrm{H}^+ + \mathrm{e}^- = {\rm Fe} + \mathrm{H}_2 \mathrm{O}\] | (3) |
Fe-Ni alloy electrodeposition also shows the anomalous type behavior, and noble Ni is suppressed by Fe deposition. Since the dissociation constant of FeOH+ is extremely small compared to that of NiOH+, FeOH+ can stay in high concentration near the cathode, and the intermediate adsorbent FeOHad exists in high concentration. It is considered that this FeOHad causes the suppression of Ni deposition9).
When the concentration of NiOH+ near the cathode in Zn-Ni deposition is compared to the FeOH+ concentration in Zn-Fe deposition, because of the difference of the dissociation constant, it is estimated that the NiOH+ concentration is lower than the concentration of FeOH+. This difference of the concentrations of FeOH+ and NiOH+ near the cathode may possibly be the reason why the change of the Ni deposition site blocking effect of Zn(OH)2 is small compared to the case of Zn-Fe deposition.
The pH for FeOH formation in the sulfuric acid bath is located in range of 6.3 to 7.09), and the pH for Zn(OH)2 formation in the sulfuric acid bath is also 6.0 to 6.514).
On the other hand, the NiOH+ concentration is very low under the pH condition of Zn(OH)2 generation. From the point of view of the confliction of blocking by Zn(OH)2 and the intermediate adsorbent, in the case of Zn-Ni deposition, adsorption sites may be considered to be smaller compared to the case of Zn-Fe. This is estimated to be the reason why the effect of the change of Zn(OH)2 block sites on Ni deposition is small in the case of Zn-Ni alloy deposition.
The Zn-Ni alloy electrodeposition behaviors with different Zn concentrations, flow rates and pH conditions were compared under high bath flow rate and high current density conditions. A comparison of the partial current densities of Zn and Ni revealed the following.
