Greening Industrially Applied Toxic Cadmium Plating with γ‐Ni2Zn11 Alloy in Deep Eutectic Solvents: Promising Electroplating Efficiency and Chemical Corrosion Resistance

Cadmium is an industrially applied plating metal used in aerospace applications, but electrodeposition of cadmium is toxic, and current production method will soon be banned. Therefore, electrodeposition of alternative alloys in environmentally friendly solutions is an attractive research area. An additive‐free, nonaqueous, deep eutectic solvent (DES) containing Ni and Zn halide salts is developed in a greener environmental method. Gamma‐phase nanocrystalline Ni2Zn11 is coated onto hot‐rolled mild steel via pulse reverse current (PRC) in the developed nonaqueous solvent. The employed DES’ relatively low viscosity and sufficient conductivity enable the simultaneous reduction of Ni2+ and Zn2+ cations on microelectrode. Electrochemical quartz crystal microbalance (eQCM) analysis proves the absence of the commonly reported problem of electrode blocking during electrodeposition, revealing an efficiency of 86.07%. Appropriate PRC parameters are applied for scaled macroelectrode deposition. The plating profile exhibits 20 μm‐thick alloy deposition without cracks around 3 h. Scanning electron microscopy–electron‐dispersive spectroscopy and X‐ray diffraction analyses reveal that 15.5 wt% Ni is in the nanograin phase, and all crystal planes belong to the γ‐Ni2Zn11 phase, which is needed as an alternative to cadmium plating. Hardness and corrosion tests performed on the γ‐Ni2Zn11 coating reveal better hardness and corrosion resistance with supporting morphological evidence.

gas formation, especially for relatively electronegative metal plating.As a result, it caused an undesirable situation known as hydrogen embrittlement in the final product.Many aqueous electrolyte systems also consisted of complex multi-input mixtures of components and used different additive chemicals to stabilize adhesion and surface leveling.On top of all these negative effects, aqueous systems evaporated rapidly due to their high vapor pressures, making bath stabilization difficult in continuous production.
Deep eutectic solvents (DESs) belong to the group of nonaqueous solvents and offer relatively low vapor pressure, nonflammability, easy preparation, low cost, and a wide electrochemical window.When electrochemical coatings on DESs were examined, it was seen that DES systems known as type-III were predominantly used for Zn, [8][9][10] Ni, [11] and Zn-Ni [12] electrodeposition.Type-III DESs systems were commonly prepared with a mixture of hydrogen bond acceptors and hydrogen bond donors such as choline chloride (ChCl) and ethylene glycol (EG).Recently, Lei et al. used type-III DES consisting of a mixture of ChCl and EG with propylene glycol and boric acid additives to obtain a zinc nickel alloy containing 15-19 at wt% Ni. [4] However, it is also reported that choline and glycol derivatives cause an inhibition effect on the substrate, especially for the deposition of electronegative metals.The blocking effect was mediated by the adsorption of molecular derivatives such as glycol, mainly choline, onto the double layer of the deposited surface.Another study also mentioned ChCl as a commonly used type-III deep eutectic system component that more predominantly blocks the electrode and causes serious hydrogen evolution via single-electron transfer shown in Reaction (1). [13]H þ e À ↔RO À þ H ads (1)   In the reaction, R can come from EG and ChCl as ÀH, ÀCH 2 CH 2 OH, and ÀCH 2 CH 2 N(CH 3 ) 3þ .The blockage of EG and ChCl was investigated for two Type-III DESs systems comprising ZnCl 2 .It was found that choline-free system revealed significantly lower blockage activity and relatively higher coating efficiency. [13]Compared to Type-III, Type-IV systems were focused less, and they were first introduced to the literature by Abbott et al., [14] but no report was found on gamma-phase Zn-Ni coatings in Type-IV DESs systems.
This study applied a ChCl-free and additive-free Type-IV DES prepared by a simple mixture of zinc chloride: EG at a molar ratio 1:4 was used to investigate the formation of Zn-14 wt% Ni coating on hot-rolled mild steel (MS).The study aimed to prevent surface blocking of type-III DESs coming from ChCl and prepare an electrochemical bath with as few components as possible as additive free.The investigation started by measuring the physical properties of formulated type-IV DES, and electrochemical tests were performed with a microdisc electrode to examine the Zn and Zn-Ni deposition.Then, bulk coatings were carried out to decide the optimum conditions.The measurements were also analyzed by electrochemical quartz crystal microbalance (eQCM) to determine the actual coulombic efficiency of the system.The results showed that Zn-15.5 wt% Ni coating consisting of only nanoparticulate Ni 2 Zn 11 phase could be coated with sufficient thickness for corrosion protection in type-IV deep eutectic systems.Scanning electron microscopy and high-resolution optical material microscopy techniques were applied for morphological investigations.Solid-state phase investigation of the produced coatings was studied with X-ray diffraction (XRD).Corrosion resistance was carried out by a nondestructive technique: electrochemical impedance spectroscopy (EIS) and a destructive technique: Tafel.

Physical Properties of Electrolyte
Type-IV DES was prepared with a direct molar mixture of ZnCl 2 and EG in molar ratios of 1 and 4, respectively.The selected type-IV system has been the subject of more diverse investigations in recent years, such as desulphurizing diesel fuel [15] and metal oxide synthesis, [16] but has not been applied to the electrodeposition of Zn-Ni alloy.The planned work aims to achieve simultaneous deposition of zinc and nickel in Type-IV DES.The viscosity and conductivity properties of the prepared DES solution were studied at a gradually increasing temperature between 25 and 70 °C, as shown in Figure 1.
In general, the preferred temperature for feasible industrial applications is usually 50 AE 5 °C.In the range of 100 °C and above, while energy cost increases significantly, material fatigue is also accelerated by corrosion and degradation.The viscosity and conductivity values at 50 °C were measured as 65 cP and 1.7 mS cm À1 , respectively, and were found to be suitable for electrodeposition.

Cyclic Voltammetry Investigation
Different amounts of NiCl 2 salt were dissolved in the prepared 1ZnCl 2 : 4EG Type-IV DES to obtain solutions containing 0.05, 0.1, and 0.2 M NiCl 2 .CV plots for bare 1ZnCl 2 : 4EG and NiCl 2 -dissolved variations are presented in Figure 2a,b respectively.When the anodic scan of Figure 2a was examined, it was difficult to detect the initial oxidation potential due to iR artefacts, but the peak potential for the Zn 0 !Zn 2þ reaction was À0.3 V.Even with Ohmic component compensation, iR artifacts occurred due to the expected accumulation of highly electroactive species in the low-conductivity environment.When the cathodic scan in Figure 2a was examined, the primary reduction peak (shown with a triangle in Figure 2a) before the Zn 2þ !Zn 0 reduction in the cathodic region scan was found between À0.28 and À0.72 V (%400 mV difference).The observed 400 mV difference for Zn 2þ !Zn 0 reduction is relatively short compared to the studied CV of zinc in the Type-III DES.The observed reduction tendency during this period, where the initial nucleation of zinc does not begin, may be due to two reasons.The first reason can be the discussed glycol reduction.This explanation is reasonable for the observed primary reduction peak in a short range, around 400 mV, since ChCl was not used and was reported far longer for the Type-III DES systems containing ChCl. [17]he second reason may be related to the electrochemical double-layer structure (EDL) in the type-IV DES system used, as it has been reported that the Gouy-Chapman-Stern EDL model is not valid for systems composed of ionic groups such as DESs. [18]Another study reported that ionic-based DES systems prevent nucleation by forming a compact layer due to the ionic solvation of the solute formed in the EDL structure. [19]ompared to choline-containing Type-III systems (usually observed <À1 V), À0.8 V is an early potential for zinc nucleation.Therefore, whether the first zinc nucleation starts at À0.8 V or an early reduction slope occurs at À0.8 V due to downstream reactions (glycol blockage or reduction) was investigated.Accordingly, a control experiment was set up in which the CV was held at À0.8 V for 60 s in the reduction region to ensure continuous current flow.The WE was then removed for surface observation.As the size of the glass body Pt microdisc WE (%15 cm) was not fit to the SEM chamber, it was investigated under high-resolution optical microscopy.As shown in Figure 2c, the WE surface is completely covered with Zn metal even at À0.8 V.No behavior was observed for electrode-blocking species, which usually appear in uncoated regions.The white regions appeared as very smooth and shiny micrometer-sized flat Zn coatings, while the black regions appeared as nanocore Zn coatings.The early reduction of the zinc cation in a choline-free solution also supports the claim that choline is the dominant surface-blocking species.In choline-containing type-III DESs, initial metal nucleation starts at more negative potentials (<À1 V) to overcome the blocking effect.Previous studies show that electroactive zinc specie in choline chloride and ZnCl 2 containing type-III DES systems is [Zn x Cl 2xþ1 ] À (x = 1,2, or 3), [20,21] and it is later declared that tetrachloride anion in the form of ZnCl 4 2À is the dominant specie. [13,22]In the cathodic polarization of the electrode in these type-III DESs systems, before electroforming of Zn on the electrode, it is mentioned that electrode is blocked at positive potentials before Zn formation takes place by either adsorbed choline, ethylene glycol by given Reaction (1), and adsorbed hydrogen, Reaction (2), or a formed intermediate species "Z" via replacing Cl À in ZnCl 4 2À with RO À species, as shown in Reaction (3). [22]ads þ H ads !H 2 (2) Kalhor et al. studied both choline chloride containing ChCl:EG mixture type-III DES [23] and 1ZnCl 2 :4EG mixture type-III DES at the molecular level, [24] and it may help to further understand the absence of electrode blockage in the studied type-IV system.It was found that following the ZnCl 2 and EG mixture, the larger clusters of EG tetramer in the structure dissociate smaller ones, such as dimer and trimer EG structured with the formation of ZnCl 2 :EG complexes.Their findings show that EG molecules interact with ZnCl 2 via both O-H•••Cl À H-bonds and Zn ← O coordination bonds.One of the distinct bonds observed only in 1ZnCl 2 :4EG DES is C-H•••Cl À H-bonds, and this decreases the number of Z specie which is also assumed to block the electrode in type-III DES before Zn reduction at the electrode. [24]oreover, the blockage-free ease reduction of Zn possibly comes from the freer atmosphere in the structure from the dissociation of Zn─Cl bonds due to the excessive lengthening of Zn─Cl bonds and negative charge increase on chlorides. [24]dentical voltammetry experiments were repeated for the studied type-IV DES solution containing NiCl 2 at concentrations of 0.06, 0.12, and 0.2 mol dm À3 .Figure 2b shows cyclic the voltammetry responses gained for the 1ZnCl 2 : 4EG type-IV DES system containing increasing concentrations of nickel cations.In the anodic scan of Figure 2b, the 0.06 mol dm À3 NiCl 2 containing 1ZnCl 2 :4EG DES revealed lower peak current density than the systems containing 0.12 and 0.2 mol dm À3 NiCl 2 .However, the overall anodic charge was approximately equal for all three.An additional anodic peak around À0.1 V was observed for Ni 0 !Ni 2þ .In the cathodic scan of Figure 2b, the observed zinc peak is identical to that of 1ZnCl 2 : 4EG DES (Figure 2a), and no reduction peak was observed for reaction Ni 2þ !Ni 0 .Thus, from the initial CV behavior, it can be concluded that simultaneous nucleation of nickel with zinc occurs.In type-III DES systems, it has been reported that nickel reduction usually starts before À0.75 V, even at as low as 0.1 mol dm À3 concentration. [25]The morphological structure of the glassy body Pt microdisc WE under an optical microscope shown in Figure 2d, with different nucleation patterns, surface finish, and color, appeared as further evidence of simultaneous Zn-Ni coating.With zinc electrodeposition alone, dull gray has often been reported as the primary color. [8,26]

Scale-Up of Zn-Ni Deposition via Pulse Reverse Current
From the optimization experiments, 0.12 mol dm À3 NiCl 2 dissolved in 1ZnCl 2 :4EG DES shown in Figure 2b (II) was found to be the most suitable solution to obtain Zn-14-16 wt% Ni alloy.Therefore, it was used as the final medium and analyzed further.Hot-rolled MS was chosen as the substrate, which is particularly difficult to coat compared to most stainless steel groups and coldrolled MS.The main reasons for favouring hot-rolled MS over cold-rolled MS are its cost and faster supply since it does not require longer preparation stages.However, hot-rolled MS is more challenging to electrodeposit because it is known for slight distortions and less precise, scaly finishing.In initial bulk deposition trials, coatings could not be uniformly provided due to the scaly finishing surface of hot-rolled MS.To overcome this problem, hot-rolled MS was subjected to submicrometer (<400 nm) electroless nickel plating for 180 s in 1ChCl:4EG Type-III DES containing 0.7 mol dm À3 NiCl 2 .The main objective was to eliminate the unfavorable synergistic effects of the hot-rolled MS substrate.Furthermore, the oxidation tendency of iron is superior to that of nickel, resulting in a more challenging deposition in oxygen-accessible environments, and the first layer of Ni coating is advantageous.
Following the electroless nickel plating, the substrate was placed in 0.12 mol dm À3 NiCl 2 -dissolved 1ZnCl 2 :4EG DES solution with an iridium-coated counter electrode for electrodeposition.Direct current (DC) electroplating was attempted initially, but the wt% Ni content was above 25% and deviated critically in continuous trials.Therefore, pulse coating (PC) and pulse reverse coating (PRC) were applied to stabilize the outcomes.During the optimization period, PC and PRC bulk coatings were performed for different current density variations and pulse periods, as shown in Table 1.In PRC variations, the weight percentage of Ni is increased by applying a longer reverse current period or a shorter forward current period.It is an expected result because deposited Zn and Ni oxidize at the reverse current period, and potentials for Zn 0/2þ and Ni 0/2þ reactions are about À500 and À100 mV, as shown in Figure 2. In other words, the oxidation potential of Zn 0/2þ is more negative than the oxidation potential of Ni 0/2þ , and deposited Zn on the electrode will reveal a more vigorous tendency for oxidation than Ni even if a low positive current applied.In PC variations, regardless of how long T off or T f periods are applied, always the amount of Zn strongly dominates the surface phase, and this may sound opposed to the explanation given above in the PRC part.However, the key point here is the tenfold-rich zinc concentration (approximately tenfold of Ni); thus, it will easily dominate the wt% of the surface phase regardless of its more negative reduction potential than nickel.If Zn and Ni had the same concentration in the applied PC, nickel would be expected to dominate the surface phase with more weight percentage.
The optimized PRC conditions revealed better results than PC.The main difference between PRC and PC is the current direction change in the former.Due to the complexity of the explanation of the applied waveforms, there are few studies on pulse techniques in DESs systems.However, it has been reported that two fundamental characteristics are usually considered to classify pulse techniques: duty cycle (θ) and frequency (f ).Duty cycle and frequency are calculated using the following equations It is recommended that the duty cycle value should be 75% and above or at least 50% in systems with low efficiency. [17]owever, depending on the kinetics, different capacitance and double-layer behaviours will likely be encountered in electrodeposition.Therefore, it is quite difficult to precisely predict the duty cycle for general aqueous systems or systems with different ionic structures without optimization experiments.The PRC technique was applied in bulk deposition with the values determined in Table 2.
In Table 2, the applied dissolution current was only 0.5 mA cm À2 , about 16 times lower than the deposition current.Furthermore, the applied time for dissolution current was only one millisecond, which is 3000 times lower than the deposition time.When equations for duty cycle (θ) and frequency (f ) were employed for optimized conditions, duty cycle and frequency Table 1.The studied deposition parameters were used in both pulse and pulse-reverse deposition conditions.were calculated as 99.97% and 333 mHz, respectively.In aqueous solutions, relatively high frequencies (>10 Hz) in both anodic and cathodic pulse cycles stand out because when lower frequencies are applied for either anodic or cathodic pulse, the double layer does not have sufficient time for proper charging or discharging as the applied short periods between sequential pulses do not comply with the renewing rate of the ionic species. [27,28]However, in DESs, even at low frequencies, cation or anion (depending on the polarization) change can be available due to excess coions in the layer, unlike known double layer theories, such as Helmholtz double layers, as applied in aqueous systems.In DESs, when surface polarization is moderate, a single layer of counterions is placed.It is balanced by an excess of coions in the following layer.However, when surface polarization is high, a mixture of counterions (anions or cations) are formed and extends over two or more layers, which causes an excess of coions in the layer below. [29]Consequently, even at short pulses periods, at low frequencies, cation or anion availability and placement on the layer can be possible.The thickness and roughness profile of the sample is shown in Figure 3.After %3 h of coating, the coating thickness was measured as 20 μm, and the surface roughness of the coated sample was around 250 nm.
Roughness at the quarter micrometer level proves a smooth and dense coating.Although no adhesion tests were performed, the nanoscale grain distribution is mostly a strong indication of good adhesion.SEM and EDS analysis were performed to analyze the overall morphology, as shown in Figure 4.The phase weight and distribution of nickel in the Zn-Ni coating were determined.SEM analysis shows that the nanograins of Zn-Ni alloy are dense and crack free, as confirmed in the roughness test.
Zinc grains are known for their thick hexagonal form, regularly distributed from one micrometer to several micrometers.However, the presented morphology revealed fine submicrometer uniform zinc grains.EDX analysis confirms the Zn-15.5 wt% Ni alloy, and the EDX mapping inset in Figure 4 shows homogeneous Ni distribution throughout the surface.T f : applied time for a cathodic current, T r : applied time for an anodic current, I f : applied current in a cathodic period, I r : applied current for an anodic period; b) T f and T r in millisecond, I f and I r in mA cm À2 , time in minute.As discussed, Zn-14-16 wt% Ni stands out with its high corrosion resistance.Although the ratio of Zn-15.6 wt% Ni confirmed by EDX is profound evidence of the existence of γ-phase Zn-Ni, XRD analyses are very important for confirming the true phase of the solid state.The XRD analysis of the sample studied in Figure 4

Electrochemical Dynamics and Efficiency Analysis by eQCM
One of the critical parameters for assessing the feasibility of electrochemical processes is the determination of the actual coulombic efficiency.The electrochemical microbalance quartz crystal technique (eQCM) can determine the electrochemical bath performance, while various data for the electrochemical system can also be obtained.For this purpose, a microbalance quartz crystal was used as the WE in a three-electrode system in CV and chronoamperometry techniques.Quartz crystals used in the eQCM technique are usually produced by coating a thin layer of chromium or carbon (depending on the crystal type) on specially cut quartz.Then, an electroactive layer such as Au, Pt, C, Ti, and Cr is coated.A PTFE cell (in-house made), schematized in Figure 6a, was used for the electrochemical tests.
The electrical circuit model that forms the operating mechanism of the eQCM system is based on the Butterworth-van Dyke (BvD) theory, [30] schematized in Figure 6b.In essence, the theory adapts the quartz crystal's electromechanical behavior to the electrochemical system's electrical behavior using a simple resistor-inductor-capacitor circuit (RLC).The first experiment given in Figure 8 shows serial frequency change (Δfs) during CV.
The Δfs value is a convenient way to check the electrochemical reversibility of a redox reaction.The change in fs starts with a cathodic sweep (from 0 V) and proceeds through increasing or decreasing mass on the crystal surface by reduction and oxidation.The ease of oscillation and, thus, the change in fs decrease with increasing crystal mass.On completing the CV, it is expected to turn the crystal to its initial state for a reversible system. [31]However, as shown in Figure 7, Δfs was measured at 0 Hz before the CV and then shifted to À1.5 kHz at the end.
From the general trend of the permanent shift in frequency (Δfs), it can be concluded that the system is not completely reversible.Following the CV, the eQCM crystal was examined  under a high-resolution microscope to investigate any discernible reason behind the shift in Δfs.The surface images of the crystal before and after the CV are shown in Figure 7a,b respectively.After the CV, a thin layer of unoxidized deposits was detected on the crystal surface.Δfs does not reveal a direct linear quantitative relation with mass or charge deposited on the surface.Before calculating the efficiency from the relationship between charge consumed and mass gained, it is important to verify whether Zn and Ni are the only species involved in CV's oxidation and reduction reactions because the reduction efficiency can also be reduced by adsorbing large glycolate or choline molecules as discussed.If significant blocking occurs due to adsorbed molecules, the mass obtained from the experimental data should differ significantly from the theoretical equivalent weight of Zn-15,6 wt% Ni.The crystal's calibration factor (C f ) must be calculated before calculating the actual molecular weight of the adsorbed or coated species on the crystal.This calculation was made from the slope of the Δfs versus charge graph, shown in Figure 8a, with the following equation where C f is the calibration factor, F is Faraday constant 96 485 C mol À1 , EA is the electroactive area, n is the transferred electron number, and MM ZnNi is the molecular weight.C f value was calculated as 143.921 Hz cm 2 μg À1 using the experimental data presented in Figure 8 in equation.It is also widely known from technical reports of crystal manufacturers and by the Sauerbrey report that the calibration factor of AT-cut 10 MHz quartz crystal is 226 Hz cm 2 μg À1 (2.26 Â 10 8 Hz cm 2 g À1 ). [32,33]The ratio between the theoretical C f and experimental C f was found as 1.5703 (C f -theoric / C f -experimental !226/143.921= 1.5703).
The mass-charge graph's slope, shown in Figure 8b, is used with the given equation below to determine the actual molecular weight of the adsorbed species, and it was calculated as 41.585 g mol À1 .
where MM ZnNi is the molecular weight, F is the Faraday constant 96 485 C mol À1 , and n is the transferred amount of electron.
When the calculated value of 41.585 g mol À1 is multiplied with C f -theoric/C f -experimental = 1.5703 (difference between theoretical and experimental crystal correction factors), the actual  mass of coated species on quartz crystal is found to be 65.300 g mol À1 .If EDX results in Figure 4 are used, the molar mass of Zn-15.5 wt% Ni can be calculated as 64.287 g mol À1 from 0.155xMM Ni þ 0.844xMM Zn .Thus, the difference between the calculated and experimental molar mass values is around 1.5%, likely due to hydrogen evolution.It can be deduced that there is no notable effect from the side molecule(s) to cause blockage by adsorbing onto the surface during the process.However, it is essential to mention that gained results do not mean the efficiency is 98.5% because charge-mass evaluation should be made for efficiency analysis.Figure 9 shows the reduction region of mass-charge relation for the performed CV presented in Figure 7.A short data distortion can be seen at around 300 s in the oxidation region; however, the corrupted data zone will not contribute negatively because the reduction zone is subject to efficiency calculation.
As shown in Figure 9, the charge consumed for the reduction region is À16.5 mC.When the calculation was made according to the 2e À transfer system using the Faraday mass-charge equation for the species with an expected molecular weight of 64.287 g mol À1 , it was found that the mass gained for the 16.5 mC transferred charge should be 5.50 μg.
The mass gained in the reduction zone (red line in Figure 9) is 4 μg.Therefore, the efficiency in the reduction zone of the CV was calculated to be 72.7%.However, deviations in efficiency calculations from direct analysis of CV data can occur; this is not always the case but occurs in most measurements.There are two well-known stages in the electrodeposition process.The first stage involves the initial nucleation of the metal(s) by reducing their cations on the substrate metal.In the first stage, the reduction of metal cations begins when the surface energy of the microbalance quartz crystal is exceeded to accept/reduce the target ion.The total energy of the first stage varies according to the surface type and size of the quartz crystal (Pt, Ti, Au, etc.).Part of the total energy is expected to be consumed to form initial bonds between the surface and the target ion (by exceeding the surface energy) and the first period of electronucleation.In the second stage, the surface will no longer be the initial quartz metal (Au, Pt, Ti, etc.) because aimed Zn-Ni nucleation occurs.Therefore, the needed energy to form ongoing Zn-Ni nuclei will be expected to be much more efficient.When the whole CV spectrum mass-charge-time triplet presented in the small inlet figure of Figure 9 was investigated, the final unremoved residue mass of the crystal was 1 μg Ni 2 Zn 11 (red line started with 0 μg and ended 1 μg).By a simple calculation, if the density of Ni 2 Zn 11 coating is 7.848 g cm À3 , [34] then by the approximation of having a dense layer on the quartz crystal, the deposition height over a surface area of 0.205 cm 2 can be calculated as 1.6088 cm g À1 .For a residual mass of 1 μg Ni 2 Zn 11 from nonreversible CV, this height would be equal to %16 nm.The 16 nm-thick rigid deposition does not affect the fundamental dynamics of the eQCM crystal or cause any oscillatory distortion.Therefore, the eQCM crystal containing %16 nm Ni 2 Zn 11 was used for efficiency calculation by chronoamperometry, as shown in Figure 10.
Previously, 16 nm Ni 2 Zn 11 -coated quartz crystal was redeposited with Ni 2 Zn 11 for 300 s in a chronoamperometry experiment, as shown in Figure 10.As discussed, the base frequency was easily tuned as the 16 nm rigid surface did not adversely affect oscillation.The charging rate was stabilized in the first ten seconds and continued for up to 5 min.The total consumed charge was calculated from current-time integration to be À71.50 mC (black line of Figure 9).The mass gained was 20.53 μg (red line of Figure 10).Faraday's mass-charge equation reveals the total mass amount will be 23.85 μg when 71.50 mC charge is consumed by Ni 2 Zn 11 coating.As a result, the actual efficiency of  Ni 2 Zn 11 coating was found to be 86.07%(measured: 20.53 μg vs calculated: 23.85 μg).As discussed in the introduction, most of the lost charge was likely consumed through hydrogen evolution from the reduction of the ROH group.

Corrosion Investigation
Commonly applied Tafel and EIS corrosion tests were employed for corrosion analysis.While the widely applied Tafel technique provides valuable data on corrosion by evaluating polarization curves, EIS provides more detailed information on the corrosion process from a different perspective.Since the EIS technique provides conditions similar to the natural atmosphere (with applied very low 10 mV excitation potential), relatively more accurate morphological observation can be obtained.The applied potential in the EIS technique is as low as 10 mV, contrary to the Tafel technique.The surfaces to be corroded in 3.5% NaCl solution conditions are exposed to chlorine attacks for a relatively long time (%90 min), including the stabilization time of OCP, and only moderate corrosion occurs in EIS compared to Tafel.Comparisons of corrosion damage between different surfaces in EIS tests can be more apparent than in Tafel tests because the Tafel technique accelerates the corrosion intensity with an applied electromotive force of AE250 mV (vs OCP).The examined coated surface can be severely damaged or completely lost in the Tafel technique, especially for moderate corrosion resistance surfaces.The Tafel technique provides easy morphological evaluation for corrosion-resistant pure metals such as Ti or alloys, such as stainless steel groups.In the Tafel experiment, 3.5 wt% NaCl solution was applied as corrosion medium at 25 °C for MS, Cd-plated MS, and Ni 2 Zn 11 -coated MS.The acquired Tafel voltametric scan curves in the Tafel experiment are presented in Figure 11; active-passive corrosion behaviors are observed for all three samples.
When samples are immersed into 3.5 wt% NaCl solution, Reactions given in ( 7) and ( 8) for Ni 2 Zn 11 -coated sample, Reaction given in (9) for Cd-coated sample, and Reaction given in (10) for MS substrate start to take place on the electroactive surface.
The PDP curves can be evaluated depending on the potential domains.The cathodic polarization lines (parallel to the potential axis) below À0.90V AgCl , À0.82V AgCl , and À0.65V AgCl for Ni 2 Zn 11 -coated sample, Cd-coated sample, and MS substrate, respectively, correspond mainly to the hydrogen evolution as given in Reaction (11).
Reaction ( 11) is accompanied by oxygen consumption Reactions given in ( 12) and ( 13), starting from the emerging slope of lines (these are the lines parallel to the potential axis or perpendicular to the current axis) to form the OCP point at À0.80V AgCl for Ni 2 Zn 11 -coated sample, at À0.78V AgCl for Cdcoated sample, and À0.63V AgCl for MS.
Each metal exhibited different behaviors at different potentials in the anodic parts of the polarization curves.The anodic branch of PDP is magnified, as shown in Figure 12.The MS with a black line showed no passivation zone, but the corrosion rate slowed down from À0.4 to À0.3V AgCl (slope of line against current axis increased).Above À0.3V AgCl , the MS corroded severely (the slope of the line against the current axis decreased significantly and ran almost parallel to the axis).The Ni 2 Zn 11 -coated sample entered the passivation zone at À600 mV.However, the Cd-coated sample first stepped into the semipassivation region at À530 mV and then into the passivation region at À630 mV.The observed passivation zone for Cd-and Ni 2 Zn 11 -coated samples is due to protective corrosion products, such as metal oxides and hydroxides, formed by Reaction given in ( 11) and ( 12). [35]While NiO/Ni(OH) 2 and ZnO/Zn(OH) 2 can be formed on the corrosion zone of Ni 2 Zn 11 , CdO/Cd(OH) 2 will be formed in the Cd-coated sample.
In addition to metal oxide and hydroxide corrosion products, metal chloride hydroxide can also be formed by Reaction (16).
In general, chloride salts, such as metal chloride hydroxide, are less likely to form on the surface in short periods of time, and chloride salts may be less stable compared to stable oxide corrosion products.Unstable corrosion films can form a relatively conductive path at the interface of solution and the electroactive site, resulting in accelerated corrosion.As mentioned, after the Tafel corrosion tests, there is no coating left to be considered visually/morphologically.In fact, it was observed that the base substrate was attacked by chloride and formed pitting corrosion, as shown in Figure 12.
In morphological evaluation, MS showed that the most severe corrosion revealed relatively larger regions of deep pitting corrosion.When Cd and Ni 2 Zn 11 coatings were compared, both lost their coatings.However, the Cd-coated substrate intensely interacted with chloride compared to Ni 2 Zn 11 coating, and many small pitting holes formed on its surface.The dataset obtained after the applied Tafel fitting is presented in Table 3.
When E corr values were compared, the nobler E corr was found for MS, followed by Cd and Ni 2 Zn 11 coatings.However, the corrosion rate is more dependent on the kinetic components, such as J corr , β a , and β c , rather than the thermodynamic component, E corr .The Cd-coated sample produced the lowest J corr value of 33.92 μA cm À2 , which was closely followed by the Ni 2 Zn 11 sample with 48.51 μA cm À2 .However, between two competing coatings, Ni 2 Zn 11 and Cd, the former revealed the twofold higher β c , known for its energetic effect in the double layer on surface redox processes.This finding is also another sign of the strength of imposed bridge via formed oxide films to prevent charge transfer. [36]These findings are also provided in the performed EIS data presented in the supplementary data file.Consequently, the corrosion resistance of the Ni 2 Zn 11 can be classified as similar to or slightly better than the general corrosion resistance of Cd.

Conclusion
In the literature, type-III DESs systems dominate the electrodeposition studies, especially the DESs formulations prepared by ChCl and EG with also other chemical additives.In the performed study, the prepared type-IV DES solution revealed low viscosity and high electrochemical conductivity at a feasible industrial temperature range between 40 and 55 °C, similar to mentioned type-III systems.However, no additives were used for applied type-IV DES formulation; a simple mixture of ZnCl 2 and EG was prepared.The DES formulation used was utilized for the first time in this study for gamma-phase zinc-nickel electrodeposition.It revealed superior properties compared to the commonly used type-III systems.CV was applied to confirm the simultaneous reduction of zinc and nickel cations, and morphological data was supported.Optimized pulse-reverse electrodeposition parameters resulted in nanorough Zn-Ni coatings with a thickness of about 20 μm on the MS substrate.Further investigations through CV and electrochemical microbalance quartz crystal techniques revealed that the reported problem of choline blockage of the cathode in type-III DES systems was eliminated in the studied type-IV system.
SEM-EDX examination of the surface showed Zn-15.5 wt% Ni alloy, and as aimed, XRD analysis revealed that each crystal plane belongs to γ-Ni 2 Zn 11 .The prevalently accepted BvD theory employed in the new generation eQCM analyzer proves the absence of electrode blockage and Coulombic deposition efficiency.The coating efficiency of the deposition was 86%.Cdcoated samples (according to the ASTM B766 -86/2015) were obtained for comparison in the corrosion analysis section.In the Tafel corrosion analysis, the cathodic curves of MS, Cd, and Ni 2 Zn 11 samples included hydrogen and oxygen formation reactions.In the anodic branch, Cd and Ni 2 Zn 11 surfaces portrayed resistance to corrosion by forming oxide and hydroxide structures in the electroactive zone.The morphological damages observed after potentiodynamic corrosion tests confirmed that the most corroded sample was MS, followed by Cd and Ni 2 Zn 11 coating, respectively.This study has shown that an alternative coating can be possible in additive-free DES without frequently reported cathode blockage.Employed type-IV DES contains ecofriendly, not restricted chemicals, and its properties, such as nonflammability, nontoxicity, low vapor pressure, and low cost, make it an evaluable candidate in place of toxicityproven, already restricted cadmium plating systems.A few general trends can be written if the performed study is translated to other metals investigated in various DESs. 1) Type IV DESs can be further examined for electrodeposition of other employed electronegative metals in engineering applications.2) DESs are inherently nonaqueous, but bath components should be chosen cautiously to sustain high efficiency in electrodeposition.
3) Eliminating electrode-blocking components in DESs systems can also eliminate the requirement for various chemical additives for stable, highly efficient plating baths.

Experimental Section
The DES solvent was used as a coating medium, prepared by a simple mixture of one molar EG [C 2 H 4 (OH) 2 ] (Aldrich, 99%) and four molar zinc chloride [ZnCl 2 ] (Aldrich, ≥98%), then stirred at 60 °C until a homogeneous, colorless liquid was formed.0.12 M nickel chloride, [NiCl 2 •6H 2 O] (Aldrich, ≥98%), was added as nickel cation source to obtain the final electrodeposition medium.All the chemicals were used as received.The viscosity and conductivity of the solution were measured by Anton Paar MCR 301 Rheometer and Thermo Scientific Orion 5-Star Bench-top Multi-parameter Meter, respectively.Cyclic voltammetry (CV) and corrosion studies were carried out with Gamry 1010 E potentiostat.A glass body Pt 1.5 mm-diameter microdisc electrode was used in voltammetry studies.For reference electrode, HCl-treated Ag/AgCl wire encapsulated in a glass tube containing 0.1M AgCl in 1ChCl: 4EG DES solution, with a glass frit connecting to the outside bulk electrolyte, was used.The counter electrode was ruthenium/iridium-coated Ti. eQCM measurements were carried out with eQCM 10 M unit (10 MHz AT-cut quartz crystals placed in a house-made cell) synchronized to Gamry potentiostat.Hot-rolled MS and copper substrates obtained from an aerospace company used in the bulk deposition were sent to a company for Cd plating according to the ASTM B766-86(2015) and used in corrosion comparison tests.Substrates were treated with 500-800-2000 grade polishing pads before plating.Pulse reverse current (PRC) for bulk deposition was carried with Power Pulse pe86CB-20-5-25-S/ GD (plating electronic GmbH) device.The roughness and thickness of the sample were measured with a Mitutoyo SJ-310 Surftest device.The morphology and chemical composition of the deposited films were analyzed by a scanning electron microscope (HITACHI SU3500, SEM) equipped with electron-dispersive spectroscopy (EDS, Oxford AZtech) for elemental analysis.Energy-dispersive X-ray (EDX, Oxford Inca) was used to analyze the crystal planes of the produced samples.XRD analysis was carried out using RIGAKU Miniflex 600 equipped with Cu Kα (λ = 1.788965Å) radiation operated at 40 kV and 30 mA in 0,02°step size at 10°-90°.Both Tafel and EIS corrosion studies were performed in 3.5 wt% NaCl solution at room temperature and coated samples served as a working electrode (WE).Ag/AgCl (3.5 M KCl) electrode was used as the reference electrode.25 cm 2 ruthenium-/iridium-coated Ti was used as a counter electrode.Before applying the corrosion tests, stable open-circuit potential (OCP) was observed.For Tafel corrosion tests, a potentiodynamic voltammetric scan at 0.166 mV s À1 was performed by scanning the samples at 0.166 mV s À1 for AE250 mV of OCP.A Gamry Faraday cage was used

Figure 2 .
Figure 2. a) CV curves of 1ZnCl 2 : 4EG DES.b) CV curves of I): 0.06 mol dm À3 , II): 0.12 mol dm À3 , and III): 0.2 mol dm À3 NiCl 2 containing 1ZnCl 2 : 4EG DES.The used WE and reference electrode were made of Pt metal with 0.0176 cm 2 and 20 cm 2 surface area, respectively.The reference electrode was Ag/AgCl prepared in DES.The operation temperature and scan rate of CV were 50 °C, and 20 mV s À1 , respectively.c) The optical image for held WE at À0.8 V for 60 s. in CV (a).d) The optical image of held WE at À0.8 V for 60 s in CV (b)/II.

Figure 3 .
Figure 3.The thickness and roughness profiles of the Zn-Ni-coated MS in 0.12 mol dm À3 NiCl 2 -dissolved 1ZnCl 2 : 4EG DES solution.The coating period is 200 min at 50 °C.

Figure 4 .
Figure 4. SEM and EDX analysis of the coated sample (presented in Figure3) is shown (1% oxygen is removed from the EDX data).
is given in Figure 5a.All peaks obtained belong to Ni 2 Zn 11 crystal planes (COD1523926 database), and the sample's reference intensity ratio (RIR) was 97.6%.Therefore, the XRD results indicate the presence of merely Ni 2 Zn 11 phase with well-matched lattice planes, each corresponding to the γ-phase Zn-Ni.The hardness examination is given in Figure 5b, and the hardness values of hot-rolled MS and Cd-coated samples were measured as 140 and 51 Hυ, respectively.Both values are significantly lower than the measured 250 Hυ hardness of the Ni 2 Zn 11coated specimen.

Figure 5 .
Figure 5. a) XRD analysis of PRC-coated sample (given in Figure 4/II).All detected crystal planes are characteristic of the Ni 2 Zn 11 phase.b) The hardness measurement of 20 μm Ni 2 Zn 11 -coated sample.

Figure 6 .
Figure 6.a) Half section of the PTFE cell manufactured in-house.b) Schematic of the eQCM crystal declared in BvD theory to describe the electromechanical behavior of the quartz crystal.

Figure 8 .
Figure 8. Graphs are based on the BvD theory.a) The slope of change in frequency change versus charge for calculation of the experimental calibration factor, and b) the change in mass versus charge to calculate the actual mass of coated species.

Figure 7 .
Figure 7. Serial frequency change of quartz crystal WE (0.205 cm 2 electroactive area) during the CV scan between À1.3 and 0.5 V at 20 mV s À1 scan rate in 0.12 mol dm À3 NiCl 2 -dissolved 1ZnCl 2 : 4EG DES is shown.The experiment was conducted at 50 °C with the same setup in Figure 6a.A 25 cm 2 Ir/Rucoated Ti electrode and a silver wire pseudoelectrode were used as counter and reference electrodes, respectively.

Figure 9 .
Figure 9.The charge-mass-time relationship for the reduction region of the CV presented in Figure 7 (after 183 s, oxidation starts, and the crystal mass decreases) is shown.The smaller inset in the main figure shows the entire CV spectrum.

Figure 10 .
Figure 10.Precoated (%16 nm-thick Ni 2 Zn 11 ) eQCM crystal from the irreversible CV experiment presented in Figure 9 was used in the chronoamperometry experiment.

Figure 11 .
Figure 11.Potentiodynamic polarization curves of MS, Cd-coated MS, and Ni 2 Zn 11 -coated MS in 3.5 wt% NaCl solution at room temperature.The WEs are MS, Cd-coated MS, and Ni 2 Zn 11 -coated MS; the reference electrode is Ag/AgCl (3.5 mol dm À3 KCl), and the counter electrode is ruthenium/iridium-coated titanium.

Figure 12 .
Figure 12.After Tafel corrosion tests, corroded surface images were taken for Ni 2 Zn 11 -coated sample, Cd-coated sample, and MS base substrate with a high resolution microscope.

Table 2 .
Bulk deposition conditions for applied PRC technique.

Table 3 .
Corrosion properties were obtained from figure after fitting the polarization curves for the Tafel corrosion tests.Metal/Alloy *E corr AE error % [mV] J corr AEerror % [μA cm À2 ]