Effects of four carboxyl-containing additives on imitation gold electroplating Cu-Zn-Sn alloys in an HEDP system

The requirements for using noncyanide imitation gold plating as decorative electroplating are increasing; thus, continuously improving the quality of the coating of the imitation gold plating and optimizing the coating process have become the current priority. In this work, hydroxyethylidene diphosphonic acid (HEDP) was used as the main complexing agent, CuSO4·5H2O, ZnSO4·7H2O and NaSnO3·3H2O were the main salts, and NaOH and anhydrous sodium carbonate were used as the buffers to prepare the electroplating solution. Using sodium citrate (SC), sodium potassium tartrate (SS), sodium gluconate (SG), and glycerol (Gl) as four additives, the effects of the number of carboxyl groups on the properties of a Cu-Zn-Sn alloy coating were compared. The electrochemical analysis showed that Cu-Zn-Sn alloy codeposition occurred at -0.50 V vs. Hg|HgO. The scanning electron microscopy (SEM) results showed that the particle size of the coatings obtained with carboxyl-containing additives was more uniform than that obtained with the electroplating solution without additives. The X-ray fluorescence spectrometry (XRF) analysis revealed that the composition of the Cu-Zn-Sn alloy coating obtained by using SC as an additive in the electroplating solution was 89.75 wt% Cu, 9.61 wt% Zn, and 0.64 wt% Sn, and the color of the coating was golden yellow. The X-ray diffraction (XRD) pattern showed that the coating was a mixture of Cu, Cu5Zn8, CuSn, Cu6Sn5, and CuZn phases. The analysis of the electroplating solution by UV, IR and NMR spectroscopy methods indicates that the additives improve the coating by affecting the complexation reaction of metal ions. These results can provide technical and theoretical guidance for developing Cu-Zn-Sn ternary alloy Preprints (www.preprints.org) | NOT PEER-REVIEWED | Posted: 13 July 2020 doi:10.20944/preprints202007.0276.v1 © 2020 by the author(s). Distributed under a Creative Commons CC BY license. electrodeposition technology with the new cyanide-free HEDP alkaline electroplating system.

model pointed out that when an organic additive was present, the additive competed with Cu 2+ in the solution, so the concentration at the electrode reaction interface was relatively stable. A.
Dianat [23] studied the molecular dynamics of the additives PEG, bis(3-sulfopropyl)-disulfide (SPS) and chloride in copper metal deposition reactions by density functional theory and found that PEG inhibited the reaction function, while SPS and chloride played a promoting role. F.
Lallemand [24] studied whether organic additives affect the deposition of metals through the current density and concluded that additives do not affect the mechanism of metal deposition but do affect the metal deposition rate. F.L.G. Silva [25] showed that using benzotriazole and cystine as additives can change the composition of the copper-zinc alloy and improve the corrosion resistance of the coating in the citrate electroplating Cu-Zn alloy system. L.F. Ding [26] studied the effects of four nitrogen-containing additives on the HEDP electroplating system and finally found that using ammonium fluoride as an additive can make the particles of the coating more uniform and produce coatings with better brightness and gloss. So far, although many different types of electroplating systems and additives have been reported, they have only been described in terms of their effects, and their mechanisms have not been explained, which has restricted the development of using additives in imitation gold plating. In this work, the HEDP system was used. Four carboxyl-containing additives were added to the system and include sodium citrate (Na3C6H5O7, SC), sodium potassium tartrate (C4H4KNaO6, SS), sodium gluconate (C6H11NaO7, SG), and glycerol (C3H8O3,Gl), where the number of carboxylic acid groups contained in these four molecules is 3, 2, 1, and 0, respectively.
The purpose of this work is to study the effects of four additives on the coating and the reaction mechanism. Therefore, the system uses the process parameters of L.F. Ding [26] in the previous study, such as current density, plating time, pH and so on. Scanning electron microscopy (SEM), X-ray fluorescence spectrometer (XRF) and X-ray diffraction (XRD) and other methods were used to characterize the morphology, composition and crystal morphology of the alloy coatings. The mechanism of complexation in the imitation gold of the electroplating solution was explored by UV, IR and NMR, which laid a certain theoretical foundation for future research on imitation gold plating mechanisms. Beichenfangzheng Reagent Factory) were selected for study. Water was purified using a water purification system (PALL Cascada II I 30, USA). All the reagents were of analytical grade.

Electroplating experiment
The electrolysis system used intelligent constant-current power (WJY-30V/10A) as the power source. The electrocoating device consisted of an electroplating solution, a cathode plate, and an anode plate. The electroplating solution effective volume was 100 mL. The cathode was a 304 30 mm×70 mm×1.0 mm stainless steel plate, whose single-side effective area was 12.0 cm 2 . The anode was made of Cu0.7Zn0.3 with 30 mm×70 mm×1.0 mm dimensions and had a single-side effective area of 15.0 cm 2 . During the electrodeposition process, the temperature was maintained at 298 K, and the current density and electrolysis time were 3.5 A· dm -2 and 60 s, respectively.

Characterization of the coating properties
The macroscopic morphology of the coatings was recorded by an optical camera (Canon A590 IS). The surface morphology of the coating was characterized by A-HITACHI-SU8010 scanning electron microscope (SEM).The content of the alloying elements in the coatings was analyzed using an HMST-II-MZ X-ray fluorescence spectrometer (XRF). The images and spectra were collected with a 20 kV accelerating voltage. The phase analysis of the coatings was carried out by Bruker D2 Phaser X-ray diffractioninstrument (XRD) (National Taiwan University of Science and Technology, China).

Characterization of the complexation reaction mechanisms
The electrochemical tests were conducted by a PARSTAT PMC1000 electrochemical workstation. A three-electrode system was used comprising, namely, the reference electrode (RE), which is a Hg|HgO electrode, the working electrode (WE) (Φ=10 mm, area=78.5 mm 2 ), which is made of 304 stainless steel, and the counter electrode (CE) (Φ=10 mm, area=78.5 mm 2 ), which is Nuclear magnetic resonance (NMR) was performed by an Avance ш 400M instrument (Bruker, Germany).

3.Results and discussion
3.1Effect of additives on the morphology of the coating An optical camera was used to record the appearance of the plating tablet after electroplating.
As shown in Fig.1, when there is no additive (BR) in the electroplating solution, the color of the plating tablet is uneven and is dark yellow and partially red. When the concentration of the additive SC was increased from 10 to 15 g/L, the color of the coating changed from dark yellow to golden yellow. When the SC concentration was 22 g/L, the color of the coating was uniform and golden yellow at this time. When the SC concentration is greater than 30 g/L, the coating is golden yellow, and part of it becomes increasingly black. When the concentration of the solution exceeds 40 g/L, a white precipitate will form after the solution is left for 24 hours, and the stability of the solution will deteriorate.
When the amount of SS is 5 ~ 15 g/L, the color of the coating is uneven and light yellow. When the concentration is 20 g/L, the color of the coating is uniform and light yellow, and the stability of the solution is better at this time. When the amount of SS is 30, 35, or 40 g/L, the color of the coating is light yellow and partially blackened. When the SS concentration in the solution reaches 40 g/L and above, a white precipitate will appear after the solution is left for 24 hours, and the stability of the plating solution is poor.
When the amount of SG is 17.5~45 g/L, the color of the coating is uneven and is light yellow and partially red. When the amount of SG is less than 20 g/L, the stability of the plating solution is poor, white precipitates will form, and a wall hanging phenomenon will occur after long-term storage. When the amount of SG exceeds 40 g/L, a red precipitate will form in the solution.
Therefore, the content of SG has a greater impact on the stability of the solution.
When the amount of Gl increased from 5 to 13 mL/L, the color of the coating gradually changed from dark yellow to yellow, and the gloss of the coating was relatively poor. When the amount of Gl is 15~20 mL/L, the color of the coating is even bronze. When the amount of Gl is more than 25 mL/L, the color of the coating is obviously black. When the amount of Gl was 30 mL/L, a yellow precipitate appeared after 24 hours of standing.  (Fig 2b), and the particle size is relatively uniform. After adding 20 g/L SS to the BR, the particle size is approximately 0.1-0.5 m, the particles are uniform, there are more fine particles (Fig 2c). When 30 g/L SG is added, the particle sizes on the surface of the coating are approximately 0.1-0.3 m, but there are obvious cracks in the coating. (Fig 2d). When 20 mL/L Gl was added to the BR, the size of the spherical particles on the surface of the coating was approximately 0.2-0.5 m, but local cracks appeared obviously (Fig   2e). Compared with that of the BR, the size of the crystal particles in the coating is reduced after adding the four additives, so additives with different numbers of carboxyl groups all have the effect of grain refinement.
3.3 Effect of the additives on the copper, zinc, and tin content in the coating XRF was used to analyze the composition of the coating obtained in the presence of the four additives ( Table 1). The coating alloy contained mainly copper, the zinc content was less than the copper content, and the tin content was trace. In the BR solution, the content of the copper, zinc, and tin components in the coating was 90.12 wt%, 9.46 wt%, and 0.42 wt%, respectively. At this time, the copper content in the coating is relatively high, which can explain why the abovementioned color is dark yellow but locally reddish. With the addition of the additives, the copper content in the coating alloys has been reduced. When the content ratio of copper, zinc and tin in the coating alloy changes, the color of the obtained coating is different. The comparison shows that when 22 g /L SC is added to the BR, the Cu content is reduced to 89.75 wt%, the Zn content is 9.61 wt%, and the Sn content is 0.64 wt%. At this time, this copper, zinc and tin ratio is the best ratio. The plating tablet appears to be uniform golden yellow. When 20 g/L SS was added to the BR, the copper, zinc and tin contents were adjusted slightly, but the plating tablet was a uniform light yellow color. Compare the four diffraction peaks with additives to the diffraction peaks of the BR, the 2θ position of the diffraction peak observed when 22 g/L SC was added was almost unchanged, but the height of the diffraction peak slightly changed. This shows that using SC as an additive does not affect the material composition of the coating alloy. When 20 g/L SS was added to the BR solution, the 2θ position of the diffraction peak did not change. When the additive is SG, the height of the diffraction peak at the 2θ position at 42.124° is significantly increased, and those diffraction peaks are corresponding to CuSn, Cu6Sn5, CuZn, Cu5Zn8, and Cu. The diffraction peak at the 2θ position of 50.888° decreases, and this peak represents CuSn and Cu. The analysis shows that the Zn content in the coating alloy is relatively high at this time, which is consistent with the previous XRF test results. When 20 mL/L Gl was added, CuO (JCPDS 44-0706) diffraction peaks appeared at 41.492°, 93.576°, and 95.028°. This shows that the additives will affect the composition content of the coating, and using Gl as an additive will also affect the phase composition of the coating alloy.

Effects of the additives on the phase composition of the coating
Since XRF cannot directly test the oxygen content, the oxygen content in the coating is not discussed in Table 1. Therefore, use Energy Dispersive Spectrometer (EDS) to analyze the oxygen content in different coatings. The analysis showed that no oxygen could be detected in the coatings obtained from BR and additives containing SC, SS and SG. However, 0.57% oxygen was detected in the coating containing Gl additives. This is consistent with XRD analysis results. Cu-containing compounds at the cathode. The cathode deposition peak C at -0.50 V vs. Hg|HgO represents the co-deposition precipitation peak of Cu-Zn-Sn. The potential D at -0.05 V vs.
Hg|HgO is mainly the dissolution peak of the Cu anode. There is also an anode dissolution peak E at 1.0 V vs. Hg|HgO, which is the oxidation peak of water [32][33][34][35][36][37]. With the increase in the SC content, the intensities of the deposition peaks A, B, and C all increased gradually, indicating that increasing the amount of this additive is beneficial to metal deposition. The intensity of peak D increases, and peak E disappears gradually. This shows that the increase in the SC concentration in the plating solution is favorable to the dissolution of the anode, showing that the increase in the SC content is favorable to the plating experiments. Fig 4b shows the CV curve obtained when SS was added to the solution. As the SS content increased, the cathode deposition peak A gradually disappeared, the intensity of peak B increased first and then decreased, and the intensity of peak C gradually increased. This shows that a certain amount of SS is favorable to metal deposition. With the increase in the SS content, the potential at the anode dissolution peak D is basically unchanged, the peak value gradually increases, and peak E gradually disappears, indicating that the increase in the content of the additives is favorable to anode dissolution. A comprehensive analysis of the deposition peaks A, B, and C theoretically found that when the SS content pf 20 g/L was most favorable for metal deposition. When the SG content was increased from 17.5 g/L to 30 g/L, the deposition peaks A and B disappeared, and the intensity of peak C gradually increased. This shows that theoretically, compared with that in the BR, the Cu and Sn content in the coating is less and the Zn content, which is higher. This is consistent with the conclusions of XRF and XRD. The intensity and potential of dissolution peak D do not change, peak E disappears, and a new dissolution peak F appears at 0.45 V vs. Hg|HgO. This shows that the additive SG will also affect the dissolution of the anode. When the SG content continued to increase, the co-deposition peak of Cu-Zn-Sn shifted to the position of peak G of -0.02 V vs. Hg|HgO, indicating that the electrode reaction changed at this time. However, the potential of the dissolved peak D shifted to -0.25 V vs. Hg|HgO, the peak was almost unchanged, and peak E gradually disappeared. As the content of Gl increased, the intensity of peak A gradually decreased until peak A disappeared, peak B disappeared, and the intensity of peak C gradually increased. When the concentration of Gl is 20 mL/L, the height of cathode peak A is lower than the height of the corresponding cathode peak of the BR, peak B disappears, and the intensity of peak C is substantially unchanged. This theoretically shows that the Cu content in the coating is less than that in the BR at this time, which is consistent with the abovementioned conclusions of XRF and XRD. In addition, a hybrid peak H appears at a potential of -0.8 V vs. Hg|HgO, and the intensity of the peak gradually increases with an increase in the Gl content and reaches a maximum when the Gl content is 30 mL/L. Combined with the electroplating experiments, it was found that the increase in the Gl content will cause the surface of the coating to become black, so peak H may be a black oxide formed by deposition, which is not favorable to the co-deposition of the Cu-Zn-Sn ternary alloy. This is consistent with the abovementioned XRD analysis results. In the process of increasing the content of Gl, the intensity of peak D is basically unchanged, and peak E gradually disappears, indicating that Gl does not affect the dissolution of the anode.
In summary, the additives with more carboxyl groups will have a more significant positive effect on the electrodeposition of Cu-Zn-Sn alloys, and the content of each additive will also affect electrodeposition. Gl does not contain carboxyl groups. Increasing the content of Gl is not favorable to the co-deposition of the Cu-Zn-Sn alloy. When the content of SG is more than 30 g/L, the deposition potential of the cathode shifts, indicating that the electrode reaction changes. When the content of SS is more than 20 g/L, the intensity of the cathode deposition peak will decrease slightly. The SC additives contain the most carboxyl groups, which has the most favorable effect on the electrodeposition of Cu-Zn-Sn alloys. Combined with previous research, it was found that the content of additives not only affects deposition but also affects the stability of the plating solution. It can be concluded that the optimal contents of the four additives SC, SS, SG, Gl are 22 g/L, 20 g/L, 30 g/L, and 20 mL/L, respectively. The reason for the redshift is not only the difference in the number of carboxyl groups in the four additives but also the number of hydroxyl groups in the four additives. Both carboxyl and hydroxyl groups are auxochromes, causing the wavelength of the maximum absorption peak to move in the direction of the long wave, causing a redshift. Among the additives, the total number of the two groups is the largest in the SG molecule, so the redshift is most obvious. However, comparing the UV spectra of the four additives, the position of the maximum absorption peak changes, and it is concluded that the UV absorption peak is mainly affected by the different additives. It is concluded that the maximum absorption peaks will be redshifted when additives are added to the plating solution. A proper redshift is beneficial to the formation of metal complex ions and the deposition of Cu-Zn-Sn ternary alloys.

Effect of the additives on the FTIR spectrum
The analysis in Fig 5 shows that there is an absorption peak at 3370 cm -1 in the spectrum, and it is inferred that this peaks is a stretching vibration absorption peak attributed to the free and associated hydroxyl groups. The absorption peak at 3370 cm -1 appears because HEDP, NaOH, and the four additive molecules in the electroplating solution all contain hydroxyl groups. The absorption peak at 2450 cm -1 is the stretching vibration peak attributed to the C-H bond in the methyl (-CH3) group in HEDP. When the additive is SG, the peak value of infrared absorption peak is the largest. When the additive is SS, the peak value are second. The absorption peaks at

Effect of the additives on the NMR spectrum
To investigate the reaction mechanism of the additives used in this experiment, a deuterated reagent was used in place of deionized water to prepare the electroplating solution, and the 1 H nuclear magnetic resonance spectrum was obtained. See Fig 7. The chemical shifts of the test absorption peaks are summarized in Table 2. It can be seen from the data that the peak 1 # with a chemical shift of 4.74 ppm is the absorption peak corresponding to H in the deuterated reagent [38]. HEDP has three kinds of H atoms with different chemical environments: H in HO-P-, H in the HO-C-group, and H in the -CH3 group. The chemical shifts of 3.70 ppm and 3.68 ppm of peak 2 # are formed by H in the HO-C-group. The absorption peak 4 # at a chemical shift of 1.40 ppm was generated by H in HO-P-. The absorption peak 3 # at chemical shifts of 1.28, 1.24, 1.20 ppm is a triplet formed by H in the -CH3 group, and the splitting is caused by the two Ps on the same carbon. It can be known from Table 2 that when the additives of 22 g/L SC, 20 g/L SS, and 30 g/L SG were added, the peak chemical shift and peak shape do not change, and only the peak height changes. This is because the additives will affect the complexation reaction of the metal ions, meaning that the peak-producing groups cannot rotate freely, and the height of the absorption peak changes. The peak height can reflect the reaction intensity of the metal ions and complexing agent to a certain extent. When the additive is 20 mL/L Gl, the chemical shift is 2.47 ppm, and peak 3 # appears at 2.34 ppm. Peak 3 # is a peak attributed to the two symmetric -CH2-groups in the molecular structure. The analysis shows that the four additives with different numbers of carboxyl groups will all react with metal ions.

Reaction mechanism of electroplating
This work mainly studies the electroplating process of electroplating coating alloy in HEDP system. The main salts are CuSO4· 5H2O, ZnSO4· 7H2O and Na2SnO3· 3H2O, the complexing agent is HEDP, and the buffering agents are sodium hydroxide and anhydrous sodium carbonate. The molecular formula of the main complexing agent HEDP was C(CH3)(OH)(PO3H2)2. The molecular structure was characterized to contain two phosphonic acid groups and five hydroxyl groups, often The electrodeposition of CuL2 6is completed by a two-step discharge process: and CuL 2-+ 2e -→ Cu + L 4-

Conclusion
In this work, the effect of the number of carboxyl groups on the coatings co-deposited with different phases was investigated. It was concluded by an SEM analysis that with the addition of carboxyl-containing additives, the particle size of the alloy particles was reduced, and the more carboxyl-containing additives added, the better the effect. The XRF results show that when the electroplating solution contains the SC additive with the highest carboxyl number content, the obtained coating contains 89.75 wt% Cu, 9.61 wt% Zn, and 0.64 wt% Sn. At this time, the content ratio of Cu/Zn/Sn in the coating is optimized, and its appearance is golden yellow. The XRD analysis showed that the coating was a mixture of Cu, Cu5Zn8, CuSn, Cu6Sn5, and CuZn phases.
The electrochemical analysis showed that metal co-deposition occurs at -0.50 V vs. Hg|HgO, and using a certain amount of carboxyl groups as additives can promote metal co-deposition. UV, IR and NMR spectroscopy methods were used to study the solution and showed that additives with different carboxyl numbers can improve the coating because the carboxyl groups affect the complexation of the metal ions in the solution.