Determination of the Morphological Properties of Electroplated Nickel Coatings Obtained Using Impinging Jet Electrode.
Ni coatings were produced on copper substrates by using impinging electrolyte jet in Watt Bath at constant temeprature. In this study, it was studied the determination of the effects of hydrodynamic parameters of unsubmerged impinging jets on morphological properties of Ni coatings. The electroplating experiments were carried out at constant voltage conditions in order to investigate the effects of jet hydrodynamic parameters which are electrolyte jet velocity (Vj) and dimensionless nozul-to-cathode surface distance (H/d), where H is the distance between the nozul (anode) and the cathode, and d is the nozul inner diameter. It is determined that as these parameters were changed, the current density passed through cathode changed. It was recorded that the change of the current density at cathode changed the morphological properties of produced Ni coatings such as the grain size of Ni coatings, thickness and surface roughness.
According to the experimental results, the thickness of Ni coatings increased with increasing Vj and reduced with increasing (H/d) at the jet stagnation point on cathode surface. The grain size of Ni coatings grew with increasing Vj up to 4 m/s. However, it shrinked with increasing Vj up to 8 m/s and with increasing (H/d) values. The surface roughness of Ni coatings reduced with increasing Vj up to 8 m/s and increased with changing (H/d) values from 0.5 to 2.0.
Keywords: Impinging jet, Nickel electroplating, Limiting current, Grain size, Surface roughness.
Ni electroplating has attracted great attention in electroplating industries because of important advantages such as enhancing the value, utility, and lifespan of industrial equipment and components, automotive parts by protecting them from corrosion. The Ni films are also commonly used in the chemical and food processing industries to prevent iron contamination.
Electroplating is a mass transfer process in which metal ions are transferred from one anode to a cathode. In conventional parallel plates electroplating technique, if a fast deposition rate is needed, it should be applied excess voltage through the cell. Although, it can be applied excess voltage in elecroplating process, the current passed through electrochemical cell will be limited. The reason for this is that metal ions cannot be reacted faster than they can arrive at the cathode surface, which is called the limiting current density. The limit is set by the plating process parameters. In the Nerst diffusion layer near the electrode surface, the diffusion rate of metal ions are slower than the reaction rate at the cathode surface. Therefore, the nickel ions concentration becomes nearly zero on the cathode surface because of the lack of supplied nickel ions, the electrochemical reaction will be stopped.
If the applied voltage is increased beyond the limiting current, some side reactions will start, such as the hydrogen gas evolution. The side reactions reduce energy efficiency and precipitate rough and powdery deposit.
On the other hand, if the electrolyte can be impinged cathode surface by impinging electrolyte jets, that is one of the convective heat and mass transfer enhancement techniques, the thickness of Nerst diffusion layer near the cathode will be decreased. As a result, mass transfer resistance arising from diffusion of nickel ions wil be decreased and the electroplating rate of the metals on the cathode surface will be increased. Because of high speed deposition is an important criteria in industrial electroplating applications, the electroplating process with impinging jet electrode (IJE) have been taken into consideration by many researchers. The electroplating rates for IJE systems are 10-15 times higher than that of conventional parallel plates techniques [1,2]. Takeuchi et al.  have declared that the IJE both enables a fast deposition rate because of a rise in the limiting current density and provides the volume fraction control of particles by changing the jet velocity.
Besides, it is informed by many researchers that the impinging jets increase codeposition rate of particles in composite electroplating [1,2].
Impinging jet systems which is a kind of convective heat/mass transfer enhancement technique  provides both the high mass transfer rates on the workpiece and has the capability of direct plating to very specific areas of the substrate. Impinging jets have played an important role in many technological areas: Heat/mass transfer enhancement systems, electroplating of materials, corrosion studies, burning systems and etc. Impinging jet electroplating tecnique has been utilized for the plating of copper, gold and nickel [4,5]. And also, it has been used for Ag electroplating of electrical contactors, formation of different kinds of nanocomposite films, and Nickel deposition .
According to the flow types, two types of impinging jet flows can be employed: Submerged and unsubmerged impinging jets. In a submerged impinging jet system, the fluid jet emerged from a nozzle and moves through a larger and stationary region containing the same fluid. In an unsubmerged jet system, the fluid jet emerges from a nozzle and moves through an air region and then impinges on a target surface. In submerged configuration, fluid jet moves through extremely more viscous fluid than air. Therefore, it exposes more shear stress forces than the other configuration. The difference between these configurations in terms of the fluid flow pattern is illustrated in Fig.1 (a-b) in the literature .
Generally, unsubmerged IJE configuration can be used in electroplating studies, since the current density on the cathode (workpiece) higher than the other configuration . Furthermore, the fluid entrainment occured in submerged impinging jets doesn't take place in unsubmerged jet configuration because of greater shear stress in the electrolyte, which is very useful effect for the electroplating processes. Submerged systems have been widely studied in heat/mass transfer systems , while unsubmerged systems, which are of central interest to this investigation, are less commonly studied. Recently, unsubmerged IJEs have been used to electrodeposite the nanocomposite films as nickel- alumina etc .
The jet flow field has been divided into three regions by many researchers: The free jet region which is under the nozzles, the stagnation flow region near the impingement zone, and the wall jet region (Fig.1) . Depending on high speed mass transfer, IJE systems can achieve high current density and fast plating rates. In the IJE systems, there are two anode configurations: either metal jet nozul may be anode or a ring shaped metal may be located inside plastic nozul as an anode . However, nozul exit should be plastic in order not to be changed diameter of nozul exit throughout the electroplating process.
The existance of various interrelated parameters in the electroplating process, such as current density, electrolyte composition, temperature, flow hydrodynamics etc. were reported by Thiemig et all. . The objective of this paper is to determine the change of characteristics of Ni deposition depending on the jet hydrodynamic parameters as jet velocity, nozul to substrate distance.
The schematic representation of the unsubmerged impinging jet electroplating system used in the present study has been shown in Fig. 2. The jet electroplating cell with cylindrical geometry located vertically was made of PVC pipe of 25 cm in height by 20 cm in diameter with a wall thickness of 0.50 cm (Fig.3).
The upper section of the cell consists of a nozul-anode system which is threaded cylindrical PVC pipe of 2.54 cm diameter, 10 cm in length and a circular PVC plate sample holder of 10cm diameter and 3mm in height. A circular copper sample was located in a hole of 12.1 mm in diameter and 0.5 mm height at the center of the cylidrical sample holder. The nozul-anode system consists of a polypropylene pipe of 15 mm in diameter and a nickel anode with cylindrical geometry located inside of the polypropylene pipe (Fig.3). The anode in the nozzle was made of pure nickel (99.98 % Ni). During the electroplating process, since anode exposes to electrochemical corrosion, its diameter is getting larger. So, the nozzle jet velocity decreases depending on the nozzle diameter. So as to prevent the increase in nozzle diameter, the nozzle tip was made of a polypropylene material. A centrifugal pump was used to form jet flow at the nozzle exit.
The electrolyte jet was impinged on the substrate through a nozzle with an inner diameter of 9 mm.
In the bottom part of the cell, the collected electrolyte is continuously pumped into the nozzle using a centrifugal pump and recirculated to the cell with by-pass using another centrifugal pump. By-pass line consists of a tangential inlet to the cell at the bottom of the cylindrical cell. By using a tangential inlet stream to electroplating cell is ensured swirl flows at the bottom of the cell . So, swirl flows yield well mixing of electrolyte and a uniform concentration. The electrolyte temperature is remained stable at 23 oC with a serpantine heater- cooler located at the bottom of the cell which is connected to a constant temperature circulator. The electrolyte jet velocity is adjusted by using a globe valve and a variable area flowmeter. The constant voltage conditions were supplied by using Wenking PGS-95 model potentiostat.
A copper disc (99.98 mass % Cu) with a diameter of 12 mm and 1mm height was employed as a cathode (substrate). The other sides of the copper substrates were insulated electrically by coating epoxy resine. Prior to nickel electroplating, the substrates were polished with 1200 degree sandpaper under aqueous conditions and then was cleaned with acetone to remove oil films. Prepared these substrates were washed with sulphuric acid to remove metal oxide layer and then rinsed with pure water. In the electroplating experiments, pure nickel (99.98 %) metal was used as a soluble anode.
Watt bath was used as a fundamental electrolyte. The chemical composition of electrolyte are listed in Table1. ACS certified reagent grade chemicals were used to prapare the electrolyte. In the literature, it is informed that the major source of the Ni ions in electrolyte is nickel sulphate . The nickel chloride, which is second source of Ni ions, is used to improve anode corrosion, to increase the conductivity of the electrolyte and to ensure the uniformity of the coating thickness distribution. In order to keep the pH value of the electrolyte at the optimum level, the boric acid was used. Moreover, good quality nickel deposits can be obtained at the optimum pH levels and temperatures.
Table: Watt Bath Composition and Electroplating Conditions.
Bath Temperature (oC)###35
In the electroplating studies using with impinging jets, the mathematical relation between Jet nozul velocity (Vj) and the limiting current density (IL) can be calculated the correlation (Eq.1) which is derived by Karakus and Chin [11,12] at the jet stagnation point. On the other hand, the sample diameter used in this research is bigger than that of considered in the correlation.
So, in this study the value of limiting current density was determined experimentally. The limiting current density values are increased with increasing jet velocities in the experimental heat/mass transfer studies using with impinging jets [2,3]. In this research, the voltage corresponding to the limiting current value is used in the electroplating studies. Furthermore, when the limiting current conditions are provided, the formation reaction of hydrogen gas is prevented mainly on the cathode surface. In addition, a common voltage value was used in this study because of determination of the effects of impinging jet hydrodynamic parameters such as jet velocity (Vj), the dimensionless nozul-target surface distance (H/d) on the properties of coated metal films on substrate. This common voltage value (4.5 V) corresponds to the limiting current values obtained at the different jet velocities used in all experiments (Fig. 4).
During electroplating process, production of hydrogen gas as a side reaction along with the main reaction takes place together on the cathode. The hydrogen gas formationtion on the cathode caused to decrease in the concentration of H+ ions in the electrolyte. Therefore pH-value of the electrolyte is increased. For this reason, prior to each electroplating experimental set, the electrolyte pH-value was set at 2.85.
Nickel films were deposited by using an unsubmerged impinging jet electroplating system. The coating process variables studied were electrolyte flow rate of 0.5-5.0 L/min and dimensionless nozul to plate distance of 0.5-2.0. There exist few studies done to evaluate the influence of impinging jet hydrodynamic parameters on the morphological properties of electrodeposited nickel films. In this research, the grain size of nickel coatings, the thickness of Ni coatings at jet stagnation point, the roughness of nickel cotaings at the jet stagnation zone was analyzed using with SEM, X- Ray and AFM analysis. The thickness of Ni films on the substrate were determined using with cross- section SEM photographs of the samples.
Results and Discussion
Enhancement of electrodeposition rate by using Impinging jet:
According to Faraday's law, electrodeposition rate depends on cathode current density (i), if current efficiency is constant. Electrodeposition process should be carried out at high current density conditions to obtain fast electroplating rate. In the beginning of the study, the relationship between jet velocity (Vj) and limiting current density (iL) was determined in terms of Vj. The relationship obtained by using the impinging jet technique was also described in Fig.2 for comparison with that of the conventional electroplating technique. The electroplating at Vj=0 m/s was conventional one where only metal ions transfer by diffusion and no convection was applied. The iL obtained at Vj=0 m/s depends on only the concentration of Ni ions. The rate of increase in iL gradually decreases as Vj increases (Fig.5).
In nickel electroplating, nickel ions are transferred to cathode surface in three ways: diffusion, ion migration and convection. The slowest one of these three steps is diffusion of nickel ions. The electroplating rate is governed by the diffusion of nickel ions. It was observed by different researchers that the application of an electrolyte jet to Ni electroplating improved the transfer of nickel ions due to the enforced supplement of nickel ions [1,2,8]. In electroplating process, there is a diffusion layer with a concentration gradient of nickel ions formed next to the cathode surfaces. If the applied current between anode and cathode exceeds the limiting current value, the concentration of nickel ions will approach nearly zero on the cathode surface [2,13]. Because, the electrochemical reaction rate of nickel ions is higher than diffusion rate of nickel ions in the diffusion layer called Nerst diffusion layer. Therefore, the electrochemical reaction will be stopped.
On the other hand, if an impinging jet is used in such a system, the impinging electrolyte jet supplies a great quantity of nickel ions from bulk to cathode surface by force and makes the diffusion layer extremely thin. As a result, the limiting current value obtained with IJE technique is higher than that of conventional technique in electrochemical system (Fig.5). Moreover, the impinging electrolyte jet eliminates resistance of diffusion of nickel ions, so that iL is satisfied in a range governed by the convection and the migration of nickel ions by electrical field force [2,6]. In this case, the concentration polarization is prevented on the cathode surface by using the impinging electrolyte jets.
An increase was shown at pH level of the electrolyte after each set of electroplating experiments with IJE. It is attributed that the IJE increased the rate of hydrogen gas formation reaction as well as nickel deposition rate. Accordingly, as the hydrogen gas formed, the concentration of hydrogen ions decreased in electrolyte.
The effect of Vj and (H/d) on Nickel electroplating and Ni thickness at the jet stagnation point:
The effect of jet velocity on limiting current density in Ni jet electroplating is shown in Fig.5. As the jet velocity increases, the limiting current density values increase up to a certain value . It is attributed that the transfer of nickel ions by convection in the electrolyte is faster than that of by diffusion. In the situation where the convection is dominant, as jet velocity is increased, the Ni ions transfer are increased. As a result, the electric current passing through the cathode is increased.
The relationship between the limiting current density (iL) and dimensionless nozul to plate distance (H/d) is shown as a function of (H/d) in the Ni jet electroplating in Fig.6. As the dimensionless nozul to plate distance (H/d) is increased, the limiting current density value is decreased. It is attributed that the resistance of electrolyte is increased with increasing (H/d). According to the Ohm Law (Eq.2), at the constant voltage conditions the current passing through cathode surface was decreased with increasing resistance of the electrolyte. The resistance of electrolyte is proportional to the distance, (H/d), between anode and cathode (Eq.3).
where; is the specific resistance of the electrolyte, S is the cross-sectional area of anode and cathode, and l is the distance between anode and cathode.
For impinging jet electroplating with a constant voltage, the local thickness of metal deposit is proportional to the local current density changed depending on nozul jet velocity. The maximum thickness of Ni deposit was obtained at the jet stagnation point on the substrate in accordance with results given in the literature. It is recorded that the maximum heat and mass transfer rates were obtained at the jet stagnation point on the target surface in literature with impinging jets . For the present study, the total area of the cathode with circular geometry is constant and also, the voltage applied to cathode is of constant value for each nozul flow rates. However, the limiting current density values was increased as increased nozul flow rates, though the applied voltage between anode and cathode is constant (Fig.4 and 5) .
As a result, it is determined that deposited Ni thickness at the jet stagnation point on cathode surface increased depending on the increased nozul jet flow rates. It is shown in SEM photographs at Fig.7 (a-b). At the same time, the distribution of the thicknesses of electroplated Ni films versus the different jet velocities is graphed in Fig.8 at the jet stagnation point on the cathode.
It is known that the local current density values i(r) and local mass transfer coefficient values are decreased gradually from the jet stagnation point to the radial positon on the surface and the maximum mass transfer take placed at this point. At the same time, the local mass transfer coefficient values were decreased with increasing (H/d) [3,11].
In this study, it was recorded that the Ni deposited bright area on the substrate was decreased with increasing (H/d). The porous and black nickel deposit formed around the bright areas on the substrate (Fig.9.a-b-c). This is attributed the current distribution in radial position on substrate mentioned as above, and the concentration polarization because of depletion of Ni ions and formation of hydrogen gas.
In electroplating, the formation of hydrogen gas is not desired. Since, it not only reduces energy efficiency but also yields porous and black nickel deposition . The electrical conductivity of this black nickel film is very poor. As a result of increasing (H/d), the ohmic resistance of electroplated nickel surface was increased and the current value passing through cathode surface was decreased. Accordingly, as the current value decreased, both deposited nickel amount and thickness of nickel film decreased at the jet stagnation point (Fig.10).
The effect of Vj on grain size:
The grain size of nickel deposit was identified by using X-ray diffraction analysis. Depending on increasing Vj values, X-ray diffraction analysis results of nickel deposits were shown in Fig.11. Using impinging jet, the deposited nickel matrix in Watt Bath exhibited a face centered cubic lattice with (111), (200), (220), (311) and (222) orientations. In the same graph, the peaks for copper are shown as well. According to the XRD analysis results in Fig.11, as the value of Vj was increased from 1 to 4m/s, the intensity of nickel peaks were increased. On the other hand, as the value of Vj was changed from 4 to 8m/s, the intensity of nickel peaks were decreased. The value of IL is increased with increasing Vj as mentioned above (Fig.5). Accordingly, it would have been expected an increase in peak intensity with increasing Vj for nickel deposit.
It is interpreted that, the amount of nickel deposit increased with changing the value of Vj from 1 to 4m/s, and the amount of nickel de posit decreased with increasing the value of Vj from 4m/s to 8. This situation was explained with dimensionless limiting current density, N, in the literature . Notation of N can be expressed as the ratio of ohmic resistance to mass transfer resistance in electroplating. The high values of N corresponds to the circumstances where the electrodeposition reaction is limited ohmic resistance. In this situation, the ohmic resistance will become predominant. In the electroplating by using impinging jet, the value of N for a given bath may be changed by varying the electrolyte jet velocities at nozzle. In the jet electroplating studies, it is observed that the jet flow covered the surface of substrate throughly at low jet speeds such as 1m/s. However, in the situation where Vj is 4m/s, the convective mass transfer rate of nickel is greater than that of 1m/s.
As a resul of this, nickel electroplating rate increased in the situation where Vj is 4m/s. Contrarily, in the situation where Vj is 8m/s, the impinging jet flow couldn't cover whole surface of the sample and impinged only a small part of the sample surface, and spattered from the sample surface to outside. In this situation, active cathode area is decreased. Therefore, the resistance of the cathode surface increased. Accordingly, as the resistance of the sample surface increased, the current passing through cathode decreased at the constant voltage. As a result of this, the deposited nickel amount on surface was decreased [2,11]. Consequently, the grain size of Ni grew with increasing Vj up to 4 m/s. However, it shrinked with increasing Vj up to 8 m/s.
The effect of (H/d) on grain size:
Depending on increasing (H/d) values, X-ray diffraction analysis results of nickel deposited by using impinging jet were observed in Fig.12. According to the XRD graphs, the nickel matrix exhibited with (111), (200), (220), (311) and (222) orientations. It is specified a decrease in the intensities of nickel peaks with increasing (H/d) in Fig.12. Correspondingly, this implies a decrease in the amount of deposited nickel on the cathode as (H/d) increases. This situation also is attributed that the porous and black nickel formation on cathode yielded the increase of the ohmic resistivity of cathode surface (Fig.9.a-b-c). As a result of increasing (H/d), the ohmic resistance of electroplated nickel surface was increased and the current value passing through cathode surface was decreased. Besides, this situation can be attributed the increase of the distance (H/d) between cathode and anode.
As a result of this, the electrical resistance of electrolyte will increase and the current passing through cathode will decrease. It is recorded that the heat-mass transfer rate on the target surface decreased with increasing (H/d) as other studies on impinging jets in the heat-mass transfer showed in the literature . Consequently, the grain size of Ni shrinked with increasing (H/d) values.
The mean grain size of electrodeposited nickel films were determined from the diffraction line broadening using the Debye-Sherrer methods (Eq.4) [15,16].
where is wavelength of X-Ray (0.1541 nm), D is the full width at half-maximum peak intensity in radian (FWHM), is the diffraction angle (peak position), and D is particle diameter size in nanometers.
In the literature, it is expressed that the grain size of Ni coatings can be changed by some parameters, such as current density (i), bath temperature (T) and the concentration (Ci) of coatings additives in the coating bath [1,8,17]. Because in this study no additives were used in the bath and the bath temperature was hold at 351oC, the variations of the grain size of nickel coatings depends on the current density on the cathode alone. As the experimental results of this study, the current density on the cathode increased with increasing Vjet. Correspondingly, the grain size of nickel coatings increased with increasing jet velocity (Fig.13).
On the other hand, since the current density on the cathode surface decreased with increasing (H/d), the grain size of nickel coatings decreased with increasing (H/d) (Fig.14). Several researchers have reported that the grain size of nickel deposits increased with increasing current density [10,17,18]. They attributed it to a decrease in the concentration of Ni ions near the cathode surface and the formation of hydrogen gas at cathode-electrolyte interface. Conversely, few researchers have asserted that the grain size of nickel deposit decreases with increasing current density as a result of higher overpotential which increases nucleation rate [10,19].
The effects of Vj and (H/d) on surface roughness of Ni coatings:
The surface roughnesses of electrodeposited Ni films as a function of Vj and (H/d) observed by AFM and SEM. These are shown in Fig.15 (a-b-c). As a results of AFM investigations, the surface roughnesses of nickel coatings were decreased with the increased Vj up to 8 m/s. However, it is increased with changing (H/d) values from 0.5 to 2.0. According to the scanning electron microscopy (SEM) surface photographs (5.00 KX) of the samples in Fig.15 (a-b-c), it was shown that the surface roughnesses of the samples decreased with increasing Vj. It also can be seen from Fig.16 and Fig.17 that the surface roughness of nickel coatings obtained from AFM analysis was strongly affected by Vj and (H/d) depending on the current density. It is recorded by Wei et all.  that a high current density will increase the electroplating overpotential, that is suitable for formation of fine grains. Therefore, the surface roughness of Ni coatings decreases as the current density increases with increasing V.
On the other hand, if the current density is continually increasing, the concentration of Ni ions near the cathode will be consumed. So, Ni depletion regions expanding towards bulk solution will impede supplyment of Ni ions to cathode surface. And then, it will cause an increase in the surface roughness of Ni coatings .
An impinging electrolyte jet system was used in Ni electroplating at constant voltage in this study. This study was focused to determine the effects of the jet hydrodynamic parameters (such as Vj and H/d) of an unsubmerged impinging jet on the the morphologic properties of electroplated Ni coatings. The following results were obtained:
At constant voltage conditions, the impinging electrolyte jet increased the limiting current density 11 times at Vj is 4m/s and 14 times at 8m/s, compared to conventional parallel plate electroplating technique.
The thickness of Ni film deposited was controlled by the jet hydrodynamic parameters such as Vj and (H/d). As the increase of Vj from 1m/s to 8 at a constant (H/d), the thickness of Ni deposit at the jet stagnation point increased by two times. Depending on the increase of (H/d) by 4 times at a constant Vj, the thickness of Ni deposit at the jet stagnation point was decreased by 5 times.
The increase of jet velocity is an important parameter in terms of the increase of electroplating rate. However, when high speed jet flow was used, the electrolyte spreaded around and it couldn't cover whole surface of the cathode. On the other hand, the current passing through the cathode was reduced depending upon increase of ohmic resistance of the cathode surface when higher (H/d) values were used. As a result of these, it was more advantages to study at about 4 m/s and at lower (H/d) values of about 0.5.
The current density passing on the cathode can be controlled by jet hydrodynamics parameters such as Vj and (H/d). The morphological properties of Ni films strongly depend on current density on cathode surface. Therefore, the roughness and the grain size of Ni films produced by using jet electroplating technique were changed as Vj and (H/d) were changed.
The increase of the current density on the cathode surface caused an increase of electroplating overpotential. Accordingly, the surface roughnesses of nickel coatings were reduced with increasing Vj and increased with increasing (H/d) in the studied range of Vj and (H/d) values. Generally, it is known that the hydrogen gas, which is formed simultaneously nickel electroplating on cathode surface, can cause roughness on the electrode. On the other hand, the produced H2 gas could be swept away easily, as the electrolyte jet velocity was increased. Accordingly, as the Vj was increased, the roughness of cathode surface was decreased.
Since the value of the ohmic resistance of cathode surface, which restricts electroplating reaction, was decreased, the grain size of the nickel coatings was increased with increasing jet velocity up to 4m/s. However, the value of the ohmic resistance was increased with increasing Vj from 4m/s to 8. Therefore, it caused to a decrease in the Ni grain size.
As (H/d) was increased, the ohmic resistance of cathode surface was increased. Therefore, the active cathode surface decreased. As a result of these reasons, the decrease of the current passing through cathode caused to shrinkage of the Ni grain size.
The financial support of this research was provided by Ataturk University Research Foundation (Ataturk University BAP 2011/140).
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|Author:||Arzutug, Mehmet Emin|
|Publication:||Journal of the Chemical Society of Pakistan|
|Date:||Apr 30, 2015|
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