Effect of stabilizing and brightening agents and some operated conditions on electroplating kinetics of NiCu alloys from citrate-sulfate solutions

Cite this paper: Vietnam J. Chem., 2021, 59(1), 37-41 Article DOI: 10.1002/vjch.202000094 37 Wiley Online Library © 2021 Vietnam Academy of Science and Technology, Hanoi & Wiley-VCH GmbH Effect of stabilizing and brightening agents and some operated conditions on electroplating kinetics of NiCu alloys from citrate-sulfate solutions Uong Van Vy*, Le Ba Thang, Nguyen Thi Thanh Huong, Le Xuan Que Graduate University of Science and Technology, Vietnam Academy of Science and Technology 18 Hoang Quoc Viet, Cau Giay, Hanoi 10000, Viet Nam Institute for Tropical technology, Vietnam Academy of Science and Technology 18 Hoang Quoc Viet, Cau Giay, Hanoi 10000, Viet Nam Submitted June 6, 2020; Accepted September 16, 2020 Abstract The effect of stabilizing and brightening additives, temperature and stirring on electrodeposited kinetics of NiCu alloy has been studied by the linear polarization method. The additives almost do not affect on the electrochemical reduction rate of Cu2+ ions, however it has a much affect on the reduction rate of Ni2+ ions. The presence of boric acids, saccharin or 1,4-butynediol have increased cathode polarization, inhibited precipitation alloy and shifted discharge potential of Ni2+ about 100 to 150 mV towards more negative. The solution temperature strongly affected on the polarization curve over the full range of investigated electrode potential, the cathode current increased about 1.5 times when the temperature increases from 30 to 50 oC. The stirring did not increase the discharge current of Ni2+ ions, on the contrary, it significantly increased the discharge current of Cu2+ ions, increasing from 2 to 3 times compared to when did not stir. Keywords. Stabilizing, brightening additives, temperature, stirring, electrodeposited, kinetics, NiCu alloy. 1. INTRODUCTION NiCu alloy plating can be applied as a metal corrosion protection coating or as an electrochemical catalyst electrode.[1-4] The ability to protect against to corrosion and the electrochemical catalytic activity of the alloy plating are highly dependent on the alloy composition and its surface morphology. The alloy with chemical composition and surface morphology appropriate can be fabricated by controlling solution composition and electrolyte operation. The co-deposited kinetics of Cu2+ and Ni2+ ions to form NiCu alloys in the solution containing citrate and ammonia complex have been studied.[1-3] Cu2+Cit complex ions react by two steps and are controlled by diffusion process. Ni2+ ions react through two steps including the forming the Ni+ intermediate adsorption form. The Co-deposition occurs in the range of electrode potentials from -1.0 to -1.2 V/SCE. As a result the Ni content in the alloy increases with the cathode polarization.[4] Effects of saccharin on electroplating nanocrystalline NiCu alloys were investigated. Saccharin (0.5 g/L) was proved to suppress the reduction of Cu and act as a leveling and grain size reduction agent in NiCu alloy codeposition. From steady-state polarization and impedance analysis, it is suggested that these saccharin effects are produced by the formation of [NiSa]ad (I), which reduces the adsorption of Niad (I) and suppresses Ni–Cu dendrite growth.[5] Saccharin has reduced particle size, but low efficiency for alloy with Cu content over 90 %.[6] The crystal size of the alloy can be controlled by changing the electrolyte solution temperature. Higher temperatures can give the smoother coating, smaller crystal size, higher Ni content and better corrosion resistance.[7] However, there is also research showing that the surface roughness of the coating increases with increasing solution temperature, formation of lumps on the surface of the coating leads to column structure. Increasing the solution temperature also makes the division into Cu-rich and Ni-rich phases tend to be stronger.[8] With the increasing of the temperatures, the content of the nickel decreases and that of the copper increases. When the temperature is lower than 35°C the contents of nickel and copper do not vary apparently. When the temperature is larger than Vietnam Journal of Chemistry Uong Van Vy et al © 2021 Vietnam Academy of Science and Technology, Hanoi & Wiley-VCH GmbH www.vjc.wiley-vch.de 38 40°C, there are significant increases of the copper content while apparent decreases of the nickel content.[9] There are many studies on the effect of the composition of the solution and electrolysis conditions on the properties of alloy precipitates have been published. However, their effects on the electrochemical reaction kinetics to form alloy films have not been fully studied. In this paper, we will present the results of research on the effect of buffers, glazing agents and some electrolyte conditions on the kinetics of NiCu alloy precipitation from citrate-sulfate solution. 2. MATERIALS AND METHODS The solutions in this study were prepared from analytical chemicals (AR) and twice distilled water. The original solution has a composition of 0.2 M sodium citrate and 30 g/L H3BO3, both acts as buffer and complexing agent with Cu2+ and Ni2+ ions. Based on our published work,[2] we have chosen the solution with concentration of Cu2+ and Ni2+ ions are 0.05 M and 0.5 M, respectively, to study the effect of the brighting agent and the effect of temperature, stirring speed on the cathode kinetics of alloy plating. Electrochemical polarization measurements were carried out in the electrode potential range from -0.8 to -1.2 V/SCE, with a scanning speed of 5 mV/s using the electrochemical workstation Biologic VSP300. Electrochemical cell 3 electrodes consisting of a working electrode was prepared from pure copper, the counter electrode was Pt and the reference was a saturated calomen electrode (SCE). The cylindrical working electrodes, with 1 cm2 working surface area, were welded with the copper wire conductor and holded on by epoxy resin. 3. RESULTS AND DISCUSSION 3.1. Effect of boric acid Boric acid is a pH stabilizer buffer in the Watts nickel plating solution, invented by Oliver P. Watts in 1916.[12] The acid reduced effects of hydrogen evolution on the cathode, which causes an increase in the pH at the cathode surface, so the plating solution could operate stably, long service time. In order to increase the stability of NiCu alloy plating solution, we have studied using H3BO3 as a pH stabilizing buffer, as its role in Watts solution. The effect of boric acid on the polarization curve is shown in figures 1 and 2. In the origin solution, without Cu2+ and Ni2+, the appearance of boric acid caused slightly increasing cathode current at electrode potentials more negative than -0.95 V, this is due to an increasing in the concentration of H+ in the solution, however this contribution is insignificant when Cu2+ and Ni2+ ions are added. -1.2 -1.1 -1.0 -0.9 -0.8 -5 -4 -3 -2 -1 0 -1.2 -1.1 -1.0 -0.9 -0.8 E (V/SCE) i (m A /c m 2 ) 0 30 Figure 1: Polarization curves in sodium citrate 0.2 M and H3BO3 0 and 30 g/L The effect of boric acid on the polarization curve in the solution with both Cu2+ and Ni2+ ions is shown in figure 2. It can be seen that H3BO3 has negligible influence on the cathode reaction of Cu2+ ions (an insignificant increase in cathode current) but strongly influence the electrochemical reaction of Ni2+ ions. The presence of boric acid shifts the electrochemical reaction potential of Ni2+ to the negative direction about 150 mV. This means that H3BO3 inhibits the reaction of Ni2+ ions similar to nickel plating,[10] leads to a reduction in the Ni content of alloys, if it's fabricated by apply constant potentials method, and can increase tightness and flatness of the precipitated surface compare to without boric acid. -1.2 -1.1 -1.0 -0.9 -0.8 -0.7 -30 -20 -10 0 -1.2 -1.1 -1.0 -0.9 -0.8 -0.7 i (m A /c m 2 ) E (V/SCE) 0 30 Figure 2: Polarization curves in sodium citrate 0.2 M + NiSO4 0.5 M + CuSO4 0.05 M and H3BO3 0 and 30 g/L Vietnam Journal of Chemistry Effect of stabilizing and brightening © 2021 Vietnam Academy of Science and Technology, Hanoi & Wiley-VCH GmbH www.vjc.wiley-vch.de 39 3.2. Effect of saccharin Saccharin is a synthetic sweetener with a molecular formula of C7H5NO3S, used in the food industry because it is 200 to 600 times sweeter than natural sugars. In nickel plating solution, saccharin is used as type I brightening agent, which flattens the surface, creates a semi-bright coating, makes the coating have tighter structure, higher hardness and good corrosion resistance,[11] it is gradually buried in the platings. -1.2 -1.1 -1.0 -0.9 -0.8 -5 -4 -3 -2 -1 0 -1.2 -1.1 -1.0 -0.9 -0.8 0 0.5 1.0 1.5 E (V/SCE) i (m A /c m 2 ) Figure 3: Polarization curves in sodium citrate 0.2 M + 30 g/L H3BO3 and 0, 0.5, 1 and 1.5 g/L saccharin The effects of saccharin on the polarization curve in the origin solution are shown in figure 3. In the presence of saccharin, the cathode current density in the more negative potential range than - 1.1 V/SCE was slightly reduced. Within the scope of the study, saccharin concentrations were from 0.5 to 1.5 g/L, the concentration effect had a very small effect on the polarization curve and it was not reduced on the cathode. -1.2 -1.1 -1.0 -0.9 -0.8 -30 -20 -10 0 -1.2 -1.1 -1.0 -0.9 -0.8 i (m A /c m 2 ) E (V/SCE) 0 0.5 1.0 1.5 Figure 4: Polarization curves in 0.2 M sodium citrate + 30 g/L H3BO3 + NiSO4 0.5 M + CuSO4 0.05 M and 0, 0.5, 1 and 1.5 g/L saccharin The effect of saccharin on the polarization curve in a solution with both Cu2+ and Ni2+ ions is shown in figure 4. It can be seen that saccharin increases polarization, a stronger effect on the reaction of Ni2+, negative potential more than -0.95 V/SCE, weaker effect on the reaction of Cu2+ ions. Due to saccharin adsorbed onto the electrode surface and formed complexes with Ni2+, it makes the reaction of metal ions occure more difficult.[5] The optimal effective saccharin concentration of 1 g/L was selected for further studies. 3.3. Effect of 1,4-Butynediol The 1,4-Butynediol is widely used as a class 2 brightening agent in nickel plating solutions, it could give the plating with a very high brightening, but also increases the brittleness of the coating.[12] The effect of 1,4-butynediol on the origin and alloy plating solutions is shown in figures 5 and 6. In origin solution the 1,4-butynediol caused increasing in cathode current density, in figure 5, the higher the 1.4-butynediol concentration, the more the cathode current increases, especially in the negative potential more negative than -1.1 V/SCE, this proved that 1,4-butynediol is reduced on cathode. -1.2 -1.1 -1.0 -0.9 -0.8 -5 -4 -3 -2 -1 0 -1.2 -1.1 -1.0 -0.9 -0.8 i (m A /c m 2 ) E (V/SCE) 0 0,5 1,0 1,5 Figure 5: Polarization curves in sodium citrate 0.2 M + 30 g/L H3BO3 + 0, 0.5, 1 and 1.5 g/L 1,4-butynediol Recognizing the presence of 1,4-butynediol in the alloy plating solution, Figure 6, did not significantly affect on the reaction of copper ions but inhibits the reaction of nickel ions, the reaction potential of nickel ions was shifted about 100 mV towards the more negative direction. The effect increases with the concentration of 1,4-butynediol and reaches its optimal at 1 g/L. Saccharin and 1,4-butynediol have been studied as glossy brightening agents in NiCu alloy plating solution, the results show that the optimal Vietnam Journal of Chemistry Uong Van Vy et al © 2021 Vietnam Academy of Science and Technology, Hanoi & Wiley-VCH GmbH www.vjc.wiley-vch.de 40 concentration for each additive was 1 g/L, similar with the concentration that was applied in nickel plating. The total effect of these two additives was that the alloy plating layer had a high brightening, especially the alloy with a Ni content above 60 %. -1.2 -1.1 -1.0 -0.9 -0.8 -30 -20 -10 0 -1.2 -1.1 -1.0 -0.9 -0.8 i (m A /c m 2 ) E (V/SCE) 0 0.5 1.0 1.5 Figure 6: Polarization curves in sodium citrate 0.2 M + 30 g/L H3BO3 + NiSO4 0.5 M + CuSO4 0.05 M and 0, 0.5, 1 and 1.5 g/L 1,4-butynediol 3.4. Effect of solution temperature Solution temperature is an important parameter in electroplating in general and NiCu alloy in particular. The change in temperature leads to changes in the mobility of ions in the solution, changes in the diffusion rate of the ions thus affecting the conductivity of the solution, the discharge rate of the ions. The effect of solution temperature on the polarization curve in NiCu alloy plating is shown in figure 7. It can be seen that the temperature of the solution strongly affects the polarization curve throughout the measured electrode potential, the cathode current density increases by about 1.5 times when the temperature increases from 30 to 50 oC, at reaction potential of Cu2+ and reaction potentials of both ions, but the influence on each potential region is also different. At the reaction potential of Cu2+ ions, when the temperature increases, the cathode current increases and reaches the limit when the temperature rises to 45 oC, this is because the temperature strongly affects to Cu2+ ion diffusion, due to the low concentration so the effect is hight. In the reaction potential of both Ni2+ ions, the increase in temperature tends to shift the reaction potential of Ni2+ ions to more positively, about 50 mV when the temperature increases from 30 to 50 oC, which means that the temperature has an impact to thermodynamics and so electrochemical react of Ni2+ ions carry out more easily. -1.2 -1.1 -1.0 -0.9 -0.8 -30 -20 -10 0 -1.2 -1.1 -1.0 -0.9 -0.8 i (m A /c m 2 ) E (V/SCE) 30 o C 40 o C 45 o C 50 o C Figure 7: Effect of temperature on polarization curve in sodium citrate 0.2 M + 30 g/L H3BO3 + NiSO4 0.5 M + CuSO4 0.05 M + saccharin 1 g/L + 1,4-butynediol 1 g/L 3.5. Effect of stirring speed Stirring increases the diffusion of the ions, thus more strongly affecting the electrochemical reaction of ions with lower concentrations in the solution. The effect of stirring speed on the polarization curve in NiCu alloy plating solution is shown in figure 8. -1.2 -1.1 -1.0 -0.9 -0.8 -30 -20 -10 0 -1.2 -1.1 -1.0 -0.9 -0.8 i (m A /c m 2 ) E (V/SCE) 0 300 400 500 Figure 8: Effect of stirring speed on polarization curve in sodium citrate 0.2 M + 30 g/L H3BO3 + NiSO4 0.5 M + CuSO4 0.05 M + saccharin 1 g/L + 1,4-butynediol 1 g/L When stirring the solution, in the reaction region of Cu2+ ions, the electrode potential more positive than -1.05 V/SCE, the cathode current density increases sharply, from 2 to 3 times higher than that of without stirring. While stirring did not increase the rate of reaction of Ni2+ ions. As a result, the obtained plating alloy is red in color of copper. 4. CONCLUSIONS Studies on the effect of boric acid, saccharin and Vietnam Journal of Chemistry Effect of stabilizing and brightening © 2021 Vietnam Academy of Science and Technology, Hanoi & Wiley-VCH GmbH www.vjc.wiley-vch.de 41 1,4-butynediol on NiCu alloy plating kinetics have been performed. The results showed that boric acid and bright additives did not change the electrochemical reaction potential and cathode current density of the reaction of Cu2+ ions, but they inhibited the reaction of the Ni2+ ions, increasing polarization and reduction of cathode current density. Suitable concentrations for saccharin and 1,4-butynediol are 1 g/L for high-gloss plating, better effect on coatings with high Ni content. The solution temperature and stirring speed do not significantly affect the precipitation of Ni but greatly affect the precipitation of Cu. When increasing the temperature of the solution as well as increasing the speed of stirring, the cathode current density for Cu2+ ions increases significantly, especially when increasing the stir, greatly affecting the composition of the alloy plating. REFERENCES 1. E. Chassaing, K. Vu Quang. Mechanism of copper- nickel alloy electrodeposition, J. Appl. Electrochem., 1987, 17, 1267-1280. 2. Uong Van Vy, Le Xuan Que. Electrochemical deposition of NiCu alloys in citrate solutions, Vietnam J. Chem., 2017, 55(5), 585-588. 3. P. Calleja, J. Esteve, P. Cojocaru, L. Magagnin, E. Vallés, E. Gómez. Developing plating baths for the production of reflective Ni-Cu films, Electrochim. Acta, 2012, 62, 381-389. 4. Ramona Y. Ying. Electrodeposition of Copper- Nickel Alloys from Citrate Solutions on a Rotating Disk Electrode I. Experimental Results, J. Electrochem. Soc.: Electrochemical Science and Technology, 1988, 135(12), 2957-2964. 5. Xinwei Cui and Weixing Chenz. 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