Electrolyte Temperature Dependency of Electrodeposited Nickel in Sulfate Solution on the Hardness and Corrosion Behaviors

The hardness and corrosion resistance of nickel (Ni) deposit on a substrate could be reached by controlling electrolyte temperature during deposition. In this research, the electrodeposition of Ni at various temperatures of electrolytes was performed. Electrodeposited Ni films using an optical digital camera, X-ray diffraction (XRD), scanning electron microscope with energy-dispersive x-ray spectroscopy (SEM-EDS), microhardness test


A B S T R A C T
The hardness and corrosion resistance of nickel (Ni) deposit on a substrate could be reached by controlling electrolyte temperature during deposition. In this research, the electrodeposition of Ni at various temperatures of electrolytes was performed. Electrodeposited Ni films using an optical digital camera, X-ray diffraction (XRD), scanning electron microscope with energy-dispersive x-ray spectroscopy (SEM-EDS), microhardness test, and potentiostat were investigated. The bright deposit occurred at 25 °C; an increase in the temperature to 40 °C leads to a change of color into semi-bright. Shifting to a higher temperature would increase the deposition rate, cathodic current efficiency, grain size, and oxygen content. The X-ray reflections in the planes (111), (200), and (220) correspond to as the Ni phase with a face center cubic (FCC) crystal structure. Decreasing crystallite size and micro-strain promoted to reach high hardness. Increasing the corrosion current density implies decreasing polarization resistance. The sample at the lowest electrolyte temperature has a better hardness, and the sample formed at 25

INTRODUCTION 1
Nickel (Ni) has many advantages of physical and other properties, such as better corrosion resistance and hardness [1,2]. NiO formation on the Nickel surface could form the passive nickel layer and increase the corrosion resistance and hardness. Ni is widely used as a protective coating from corrosion attacks on automotive *Corresponding Author Institutional Email: syamsuir@unj.ac.id (S. Syamsuir) vehicle components and accessories such as logos, door handles, and rearview handles [3]. For those reasons, hardness and corrosion resistance properties are needed. Numerous methods could be conducted to protect the material from corrosion attacks, such as coating, electrodeposition, hot-dip galvanizing, chemical vapor deposition (CVD), and physical vapor deposition (PVD) [4,5]. The electrodeposition process makes Ni as protective film from corrosion attack, more promising due to being faster, cheaper, and more environmental friendly [6]. Parameters such as current density, temperature, and bath formula must be considered for better hardness and corrosion resistance.
Recently, there have been many studies on the electrodeposition of Ni to increase the hardness and corrosion resistance. Wasekar  observed Ni film was electrodeposited by reverse pulsed current, and varying saccharin found less grain size and corrosion current in 5 g/l saccharin addition.
The hardness and corrosion resistance could also be reached by controlled electrolyte temperature. Chung et al. [12] varied the temperature of the electrolyte solution 5, 10, 15, and 20 °C to produce a Ni film at the highest hardness at 5 °C. Jinlong et al. [13] also reported that Ni electrodeposition at 20, 50, and 80 °C temperature variations produced the lowest corrosion rates at 20 °C. Furthermore, in previous work, by varying the temperature of the electrolyte solution, 20, 30, and 50 °C found the highest hardness at 20 °C and the lowest corrosion rate at 50 °C [14].
As mentioned above, many researchers vary bath composition to reach high hardness and corrosion resistance. Varying temperatures have been conducted; other researchers also used a complex bath. This condition could raise the new problem of waste treatment before releasing it into nature.
For that reason, the electrodeposition of Ni in 0.5 M NiSO4 solution without any additions and variations in temperature needs further investigation. In this research, the electrodeposition of Ni with various temperatures was performed using an electrolyte solution without additive addition.

1. Process and Materials
The Ni electrodeposition process used DC power supply apparatus in 0.5 M NiSO4 solution using 10mA cm -² of current density. The electrodeposition was carried out in a 250 ml electrolyte plating bath at temperatures 10, 25, and 40 °C for 1 h. The sample was designated as T10, T25, and T40 for electrolyte temperatures 10, 25, and 40 °C, respectively.
Pure Ni was used as the anode, and copper alloy [14] was used for the cathode. Before the electrodeposition process, the cathode was polished sequentially using abrasive paper of #1000 up to #3000. Moreover, the cathode was cleaned with acetone before electrodeposition was performed.

Characterization
For visual appearance, the picture was captured using a digital camera. Afterwards, the captured figures were analyzed by visual appearance. The mass increment of the Ni films was found by weighing the substrate before and after electrodeposition. Moreover, a mass of the Ni films is inserted in the current efficiency and deposition rate equation. The cathodic current efficiency (CCE) and deposition rate () were calculated similarly by the methode as previously reported [15].
The X-ray diffraction (XRD) identified the phase and crystal structure of the films Ni with the Cu Kα radiation (Philips binary sweep). XRD data was collected from 30° to 80° with a step size of 0.020°.
Surface morphology and composition of Ni films were analyzed with FE-SEMFEI INSPECT F50 energy dispersive analysis of X-rays (EDAX) EDS analyzer.
The potentiodynamic polarization was carried out in an electrochemical workstation (Digi-Ivy DY 2311) at 30 °C. A three-electrode cell was used with a Pt as a counter electrode, an Ag/AgCl electrode as a reference electrode, and a 1 cm² Ni film embedded in epoxy resin as a working electrode. The potentiodynamic polarization measurement performs with a 1 mV/s scan rate in 3.5% NaCl solution.
The Tafel extrapolation methods were used to find the Icorr and the Ecorr. Afterwards, the following equation was used to calculate the corrosion rate (Cor Rate) [16]; The microhardness test was carried out with a MicroMet® 5100 series. The test was conducted with a load of 100 g for 10 s and according to the ATM E384 standard.

1. Visual Appearance
Based on Figure 1, the bright deposit occurred at 25°C; an increase in the temperature to 40°C leads to a change of colour into semi-bright. A decrease in the temperature to 10°C leads to a shift in the dullness. Usually, bright deposits result in lower current densities, and dull deposits at higher current densities [17]. Furthermore, additive addition also affected the appearance quality of deposition [18].

2. Cathodic Current Efficiency and Deposition Rate
Various sample's cathodic current efficiency results can be observed in Figure 2. This behaviour indicated that the cathodic current efficiency depends strongly on temperature. Shifting to a higher temperature would affect to increase the cathodic current efficiency. Sarac and Baykul [19] found higher temperatures lead to higher cathodic current efficiency. This condition means faster Ni 2+ ions moving in the forward direction from the anode to the cathode and promotes increasing the cathodic current efficiency as the temperature of the electrolyte solution rises. Moreover, the shift to the higher temperature affects nearly equally the deposition of Ni and hydrogen evolution [20].
Electrolyte temperature decreased when the deposition was performed, affecting the decrease of the Ni films deposition rate. A measurement result of the deposition rate of electrodeposited Ni films is shown in Figure 2. Because the deposition rate is limited by poor mass transfer conditions at low electrolyte temperatures [21]. It would influence the movement of Ni 2+ ions from the anode to the cathode becomes slower with decreasing electrolyte temperature. Kang et al. [22] found that elevating the electrolyte temperature increased the deposition rate when electrodeposition Ni using electrolyte temperature 40 until 60 °C.
An increased in deposition rate would also influence the film thickness. Basori et al. [15] found that increased electrolyte temperature promoted the film's thickness due to increased in deposition rate. More thickness in the Ni films could influence the hardness.  Figure 3 illustrates the XRD pattern of electroplated Ni film samples on Cu alloy substrates at 10 °C (T10), 25 °C (T25), and 40 °C (T40). The X-ray reflections in the planes (111), (200), and (220) are defined as the Ni phase with a face centre cubic (FCC) crystal structure. According to Donegan et al. [23], Shin et al. [24] and Zhang et al. [25] these peaks are comparable to their results.

XRD
Using the JANA2006 software, the XRD pattern of Ni film samples can be very well fitted [26]. The diffraction patterns observed and the calculated results are compared in The crystallite size values of samples T10, T25, and T40 were 22.53 nm, 23.52 nm, and 24.54 nm, respectively. Based on the intercept values, for samples T10, T25, and T40 were -5.0471, -5.0901, and -5.1326, respectively. The Williamson Hall Plot method is used to calculate the crystallite size, which is formulated as follows [29,30].
The crystallite sizes of samples T10, T25, and T40 were 27.85 nm, 36.20 nm, and 41.38 nm, respectively. This value is derived from the linear regression results of the 4.sinθ vs β.cosθ curve shown in Figure 5 (b). The microstrain values of samples T10, T25, and T40 were 0.11%,

4. SEM-EDS
The electrolyte temperature significantly influences the surface morphology of the Ni film. The SEM-EDS image ( Figure 6) represents the outermost surface areas. The T10 sample shows a clear island-like structure [31]. Raising the temperature to 25 °C would change the island-like structure to a compact nodule without porosity. When the electrolyte temperature increases to 40 °C, a nodule is still seen in the surface morphology, but a little porosity is shown in the grain boundary.
Raising the temperature also influences the grain size. Compared to Figure 2, it can be seen a shift to the higher temperature on electrodeposited Ni films delivering a higher cathodic current efficiency. This behaviour means higher cathodic current efficiency produces larger grain sizes [32]. Besides current efficiency, deposition rate also contributed to grain size [33]. As seen in Figure 6, an increase in the temperature creates a larger grain size due to an increase in current efficiency and deposition rate. Lin et al. [33] reported that controlling the deposition rate leads to forming of various grain sizes. Ni and O content is seen on the EDS examination. At high temperature, less O content is visited, and in the lowest temperature, more extensive O content is seen ( Figure 6). This behaviour indicated that the oxygen content depends strongly on the temperature. Various research has been found that forming a NiO would improve corrosion resistance and hardness [34,35]. Jabbar et al. [36] found C content in the Ni films increased by an electrolyte temperature due to the exhibit of graphene 0.2 g/l in the electrolyte solution. In contrast, within Jabbar et al. [36] research, unseen O content exists on the Ni surfaces due to probably the sample transport and storage prior to EDS characterization [14].

5. Microhardness
The Microhardness test result of Ni films can be seen in Figure 7. The Ni film's microhardness increases, corresponding to the electrolyte temperature decrease [22]. This behaviour perfectly agrees with Chung et al. [12] research where microhardness increases linearly when electrolyte temperature is decreased.
Compared to the XRD result, it can be seen an increase in the electrolyte temperature leads to an increase in the micro-strain. An increase in the microstrain indicates more crystal defects that are formed and could influence the physical and mechanical properties [14,37]. Therefore, less miro-strain would increase the hardness.
The slow deposition rate at a lower temperature promoted less crystallite size [12]. Less crystallite size could result in high hardness. Bigos et al. [10] found that shifting to a larger crystallite size decreases hardness. Martínez et al. [38] also found that reducing crystallite size would increase the hardness. This statement is in alignment with the present study.
Besides crystallite size, film thickness also contributed to the influence of the hardness. Compared to Figure 2, an increase in the electrolyte temperature can increase the deposition rate, which promotes increasing the film's thickness. Therefore, more thickness of the Ni films could increase the hardness [39].

Figure 7. Microhardness test result of Ni films
The hardness of electrodeposited Ni using various bath compositions and electrolyte temperatures is summarized in Table 1.
Wasekar et al. [7] found that a saccharine in the solution significantly influences the hardness. Comparing between Figure 7 and Table 1, bath composition also contributed to high hardness besides electrolyte temperature [7]. Moreover, compared to previous work, where electrodeposited by adding H3BO3, resulted less hardness than present research [14].

6. Potentiodynamic Polarization
The potentiodynamic polarization scan results in 3.5% NaCl solution are shown in Figure 8. Shifting to high temperatures leads to producing more negative Ecorr. This phenomenon contradicts our previous work, which could be the boric acid effect in the electrolyte solution [14]. Adding more boric acid has increased the cathodic current efficiency so that it will affect the properties of the Ni film formed. Moreover, adding boric acid also could influence strain and stress in the Ni films [40]. Increased internal stress will cause slight cracks in the Ni films [41]. Hence, those cracks would influence the Ecorr of the Ni films.
Based on Figure 8, it can be seen that Ecorr has a more positive value than Gu et al. [8] results. This behaviour could have different morphological results. It is seen that Gu et al.
[8] had a smaller grain size than this research. Elias and Hedge [39] reported grain size would influence Ecorr.
Based on Table 2, sample T25 has shown less corrosion rate. Moreover, the corrosion current density increases, implying decreasing polarization resistance, which means the film will be easier to corrode [42]. Furthermore, samples T10 and T40, showing active loops, indicate uniform corrosion on the film's surface ( Figure 8) [43].
Compared to SEM result, compact morphology was observed in sample T25, whereas porous morphology is found in the other samples. Xu et al. [44] found porosity could influence corrosion resistance. Higher porosity   would increase the corrosion rate. Therefore sample T25 has a higher corrosion resistance than other samples. Elias and Hedge [39] reported nodule form of the grain size would influence corrosion resistance. Based on Figure 6, nodule forms are shown in the T25 and T40 samples. Unfortunately, porosity is seen in the T40 sample. Therefore sample T25 is resulting better corrosion resistance. All conditions mentioned above can be corroborated by sample T25 has better corrosion resistance.

CONCLUSIONS
The characteristic of Ni films at various deposition temperature was investigated. The surface morphology of the sample looks different as the change of the electrolyte temperature. Increased electrolyte temperature is related to increase in grain size, deposition rate, cathodic current efficiency, micro-strain, and crystallite size. Moreover, decreased electrolyte temperature is related to increase in oxygen content and hardness. Sample T25 has better corrosion resistance than the other samples. Furthermore, the sample with the lowest electrolyte temperature has better hardness than the other samples.

ACKNOWLEDGMENTS
This work was financially supported by the Engineering Faculty of Universitas Negeri Jakarta under grant No. 065a/5.FT/PP/V/2020.