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Article

Ni-Co Electrodeposition Improvement Using Phenylsalicylimine Derivatives as Additives in Ethaline-Based Deep Eutectic Solvents (DES)

by
Enrique Ordaz-Romero
1,
Paola Roncagliolo-Barrera
2,*,
Ricardo Ballinas-Indili
3,
Oscar González-Antonio
1 and
Norberto Farfán
1
1
Department of Organic Chemistry, Faculty of Chemistry, Universidad Nacional Autónoma de México, Ciudad Universitaria, México City 04510, Mexico
2
Department of Metallurgical Engineering, Faculty of Chemistry, National Autonomous University of Mexico, Ciudad Universitaria, México City 04510, Mexico
3
Department of Chemical Sciences, Facultad de Estudios Superiores Cuautitlán, Universidad Nacional Autónoma de México, Cuautitlán Izcalli, Estado de México 54740, Mexico
*
Author to whom correspondence should be addressed.
Coatings 2025, 15(7), 814; https://doi.org/10.3390/coatings15070814
Submission received: 27 May 2025 / Revised: 4 July 2025 / Accepted: 8 July 2025 / Published: 11 July 2025
(This article belongs to the Special Issue Electrochemistry and Corrosion Science for Coatings)

Abstract

The development of metallic coatings as Ni-Co alloys, with particular emphasis on their homogeneity, processability, and sustainability, is of the utmost significance. To address these challenges, the utilization of phenylsalicylimines (PSIs) as additives within deep eutectic solvents (DES) was investigated, assessing their influence on the electrodeposition process of these metals at an intermediate temperature of 60 °C, while circumventing aqueous reaction conditions. The findings demonstrated that the incorporation of PSIs markedly enhances coating uniformity, resulting in an optimal cobalt content of 37% and an average thickness of 24 µm. Electrochemical evaluations revealed improvements in charge and mass transfer, thereby optimizing process efficiency. Moreover, computational studies confirmed that PSIs form stable complexes with Co (II), modulating the electrochemical characteristics of the system through the introduction of the diethylamino electron-donating group, which significantly stabilizes the coordinated forms with both components of the DES. Additionally, the coatings displayed exceptional corrosion resistance, with a rate of 0.781 µm per year, and achieved an optimal hardness of 38 N HRC, conforming to ASTM B994 standards. This research contributes to the development of electroplating bath designs for metallic coating deposition and lays the groundwork for the advancement of sophisticated technologies in functional coatings that augment corrosion resistance and mechanical properties.

1. Introduction

Nickel and cobalt superalloys are prized in the aerospace industry for their remarkable strength and stability at high temperatures, making them perfect for turbine engines and jet propulsion systems. These superalloys are capable of withstanding extreme temperatures and resisting stress and corrosion [1,2]. Electroplating is crucial for manufacturing these alloys because it enables precise control over their composition and structure, thereby enhancing their overall performance and properties. The alloy formation process presents numerous challenges, and key factors that can impact quality and efficiency. Common issues involve internal stresses within the coating caused by hydrogen release, as well as the need for precise control over the microstructure and chemical composition of nickel-cobalt superalloy coatings, which significantly influence their mechanical properties, such as hardness and fatigue resistance. A pivotal aspect in the mechanism of alloy formation is the rapid nucleation of cobalt in comparison to nickel, as this constitutes an anomalous codeposition type [3,4]. To achieve controlled thickness, uniform coatings, and defect-free surfaces, several alternatives are available for managing this deposition process, including adjustments in concentration, surfactant additives, or complexes that influence ion migration and redox reactions [5]. The utilization of complexes is constrained by their solubility in aqueous solution and temperature, thereby necessitating the exploration of methods in non-aqueous systems [2,6].
The development of environmentally friendly and cost-effective electroplating processes that meet aerospace standards introduces an additional layer of complexity. Deep eutectic solvents (DES) represent a novel, eco-friendly approach that offers an alternative to conventional electrolytes in nickel, cobalt, and chromium processes. This approach has the potential to mitigate carcinogenic risks, reduce water and energy consumption, and facilitate the evaluation of a broad spectrum of organic compounds soluble in ethaline [7,8,9].
Recent studies have centered on the role of binders as additives in metal electroplating. These additives have been shown to have a significant impact on ion migration, concentration polarization, reduction potential, and electrochemical crystallization of electrodeposited grains. These elements have been demonstrated to enhance the microstructure and Faradaic performance of deposits [10,11,12,13]. Alesary H.F. et al. showed that the signals in cyclic voltammetry changed with different amounts of nicotinic acid in DES, altering the structure parallel or perpendicular to the electrode [14]. Andrew P. Abbott’s group investigated the role of additives in nickel and cobalt electroplating processes. Their findings revealed that nicotinic acid, ethylenediamine, and boric acid exert a limited effect on the amount of nickel deposited [15]. However, these additives have been observed to influence ionic reduction by forming Ni (II) complexes, thereby affecting the electroplating process. This, in turn, alters the diffusion rate and concentration of metal ions.
The selection of appropriate ligands necessitates the consideration of the structure of complexes with the metals to be deposited. For instance, nickel (II) complexes exhibit octahedral geometry, while cobalt (II) complexes manifest tetragonal geometry. These orbitals enable the rapid formation of coordination complexes with Lewis bases through electron donation [16,17,18,19,20,21]. Schiff bases are classified as azomethines or imines with organic side chains, and they are represented by the general formula R1R2 C = NR3. The synthesis of these compounds is straightforward, enabling the formation of stable chelates with metal ions through azomethine, hydroxyl, and amine groups. This facilitates charge and mass transfer during the electroplating process of metal alloys. The Schiff base H2L and its metal chelates have been synthesized and characterized as binuclear complexes using polar organic solvents [22,23,24,25,26].
Phenylsalicylimines (PSIs) are easily synthesized compounds that can be formed using inexpensive raw materials and can form metal cation complexes through chelated rings, making them potential candidates for selective bonding with cobalt and nickel [27,28,29]. These ions have been found to form stoichiometric complexes with Co (II), Ni (II), and Cu (II) in the form [M(L)X]X and [M(L)SO4], where L is 3,3’-thiodipropionic acid bis(4-amino-5-ethylimino-2,3-dimethyl-1-phenyl-3-pyrazoline) [30,31,32,33,34,35]. Additionally, they are low-cost compounds, which favor their implementation in industry. They are a favorable alternative for sustainable use in Ni-Co electroplating.
Most existing research has focused on enhancing nickel mobility, primarily toward cobalt ions. This study explores two phenylsalicylimine compounds with modified functional groups to hinder cobalt ion mobility during nickel-cobalt alloy deposition in ethaline, a deep eutectic solvent.

2. Materials and Methods

2.1. Synthesis of Phenylsalicylimines

The reactions were carried out in an open atmosphere. The organic solvents (ethanol, methanol, and diethyl ether) were used without prior purification. The starting reagents, 2-aminophenol, salicylaldehyde, 2-amino-5-nitrophenol, and 4-(N,N-diethylamino)salicylaldehyde, were purchased from Sigma-Aldrich (Merck KGaA, Darmstadt, Germany) and used without prior purification.
(E)-2-((2-hydroxybenzylidene)amino)phenol (PSI-1). The compound was synthesized from 0.5 g (4.09 mmol) of salicylaldehyde and 0.44 g (4.09 mmol) of 2-aminophenol. The reaction was refluxed in 50 mL ethanol for 3 h using a Dean–Stark trap (Pirex®, Charleroi, PA, USA). After this time, the product precipitated as a red solid, which was washed with hexane and 89% methanol (0.79 g, 3.71 mmol) Mp 165–167 °C, IR n(max) (ATR) 3045, 2838, 2693, 1625 (C=N) 1525, 1484, 1368, 1240, 1134, 1095, 1013, 738 cm−1. 1H NMR (300 MHz, DMSO-d6) [d, ppm]: 13.82 (s, 1H, H-7), 9.76 (s, 1H, H-16), 8.97 (s, 1H, H-8), 7.62 (d, 1H, J = 8.5 Hz, H-6), 7.42–7.35 (m, 2H, H-1, H-2), 7.15–7.11 (m, 1H, H-13), 7.01–6.89 (m, 4H, H-3, H-12, H-14, H-15). 13C NMR (300 MHz, DMSO-d6) [d, ppm]: 162.1, 161.2, 151.6, 135.4, 133.2, 128.5, 120.0, 120.00, 119.2, 117.1, 117.0. HR-MS (ESI-TOF) calculated for C13H11NO2 [M+1] 214.0869, found 214.0868.
(E)-5-(diethylamino)-2-(((2-hydroxyphenyl)imino)methyl)phenol (PSI-2). The compound was synthesized from 0.5 g (2.58 mmol) 4-(diethylamino)-2-salicylaldehyde and 0.28 g (2.58 mmol) 2-aminophenol. The reaction was refluxed in 50 mL ethanol for 3 h using a Dean-Stark trap (Pirex®, Charleroi, PA, USA) After this time, the product precipitated as an orange solid, which was washed with hexane and 91% methanol (0.66 g, 2.34 mmol) Mp 153–155 °C, IR ν(max) (ATR) 3058, 2895, 26449, 1835 (C=N) 1581, 1453, 1337, 1222, 1137, 1072, 973, 736 cm−1. 1H NMR (300 MHz, DMSO-d6) [d, ppm]: 9.80 (s, 1H, H-16), 8.65 (s, 1H, H-8), 7.31–7.25 (m, 2H, H-6, H-13), 7.06–7.01 (m, 2H, H-14, H-15), 6.98–6.82 (m, 1H, H-12), 6.00 (d, J = 9 Hz, 1H, H-1), 6.00 (s, 1H, H-3), 3.36 (q, J = 6 Hz, 4H, H-18, H-20) 1.12 (t, J = 6 Hz, 6H, H-19, H-21). 13C NMR (75 MHz, DMSO-d6) [d, ppm]: 166.6, 158.8, 152.2, 150.5, 134.8, 134.5, 126.6, 120.0, 118.9, 116.7, 109.5, 104.2, 97.7, 44.3, 13.0. HR-MS (ESI-TOF) calculated for C17H21N2O2 [M+1] 285.1603, found 285.1592. The two compounds evaluated are shown in Figure 1.
To characterize the compounds, NMR spectroscopy with DMSO-d6 as the solvent was used on a Jeol GX-300 NMR spectrometer (Jeol Ltd., Tokyo, Japan). Mass spectra were obtained using a JEOL SX-102 spectrometer (Jeol Ltd., Tokyo, Japan), and for IR, a JEOL FTIR spectrometer (Jeol Ltd., Tokyo, Japan) was used.

2.2. Electrochemical Evaluation

Choline chloride (ChCl) and ethylene glycol from Sigma-Aldrich, 99% purity (Merck KGaA, Darmstadt, Germany) were used at a (1:2) ratio. Solutions of CoSO4·7H2O and NiSO4·6H2O at concentrations of 0.5 M were used. The solutions were dried in a flask (Thermo Fisher, Waltham, MA, USA) for 24 h at 80 °C under high vacuum to minimize the water content in the solvent.
The effects of PSI compounds were evaluated in cobalt and nickel solutions without additives, with ligand concentrations of 0.1, 0.3, and 0.5 mM. For the Cyclic voltammetry (CV) a PalmSens 4 potentiostat (Houten, The Netherlands) was used Cyclic voltammetry (CV) to characterize the oxidation–reduction potentials at 60 °C. Electrochemical assays were performed using a three-electrode cell with platinum electrodes and a silver-chloride reference electrode (Radiometer Analytical, Lyon, France), featuring a 1 V overpotential and an 80 mV s−1 sweep rate.

2.3. Optical Evaluation

Experimental UV-Vis and emission spectra were obtained using a Thermo Scientific Evolution 220 spectrophotometer (Thermo Fisher, Waltham, MA, USA) and a PerkinElmer FL 6500 spectrophotometer (PerkinElmer, Waltham, MA, USA), both from 200 to 800 nm. Co (II) and Ni (II) electrolytes were optically evaluated with various PSI-1 and PSI-2 ligand concentrations. UV–Vis and fluorescence methods were used to assess phenylsalicylimine and cobalt coordination compounds. Ligand solutions, at a concentration of approximately 1 × 10−6 M, were prepared in DES (EG: ChCl, 2:1 molar ratio) with varying stoichiometric ratios of metallic ions, allowing for the acquisition of absorption and fluorescence spectra for PSI-1 and PSI-2.

2.4. Cell Parameters

Subsequently, they were evaluated through cathodic polarization under 25 rpm stirring and at varying ion concentrations in NiSO4·6H2O solution (0.5 M) and CoSO4·6H2O solution (0.25 M), both of 99% purity (JT Baker, Phillipsburg, PA, USA). The concentrations of PSI-1 and PSI-2 were set at 0.3 mM. The applied overpotential was −0.7 V from the open circuit potential (OCP) at a sweep rate of 0.01 mVs−1.
Roncagliolo et al. [36] described these dynamic conditions as optimal for the deposition of Ni-Co alloys. A typical 3-electrode cell, featuring a glassy carbon rotating disk (ECR) electrode (Pine Research Instrumentation, Grove, PA, USA) as the working electrode with an area of 1 cm2, platinum as the counter electrode, and Ag/AgCl as the reference electrode (Red Rod REF201 Radiometer Analytical, Lyon, France), was employed. The temperature was maintained at 333 ± 3 K using a heating grid (Thermo Fisher, Waltham, MA, USA).

2.5. Metallic Coating Assessment

The electrodeposition process was carried out in a Haring-Hull cell with a copper cathode and a nickel-cobalt anode. Surface preparation was performed according to ASTM B254-92, with electrodes spaced 2 cm apart [37]. A DC power supply (Vimar, México City, Mexico) was used for 60 min in combination with nickel and cobalt solutions under the same dynamic conditions, temperature, and concentration for each additive PSI-1 and PSI-2 to generate a constant current.
SEM and EDS were performed using a Jeol-JCM-6000PLUS instrument (Jeol Ltd., Tokyo, Japan) to characterize the coating morphology and punctual chemical composition. Thickness was measured using a Zygo Nexview optical profilometer (Zygo, Middlefield, CT, USA).
Corrosion resistance was evaluated in 3.5% wt. NaCl solution from Sigma Aldrich, 99% purity (Merck KGaA, Darmstadt, Germany), using a VMP-3 Potentiostat-Galvanostat (BioLogic, Knoxville, TN, USA) in a three-electrode cell, with Ni-Co coating as the working electrode, an Ag/AgCl reference electrode, and platinum as a counter electrode. The open-circuit potential (OCP) was recorded for 1800 s to achieve a steady state. Then, electrochemical impedance spectroscopy (EIS) was measured, with a sinusoidal potential of ±10 mV RMS over a frequency range of 104 Hz to 10−2 Hz. After EIS, linear polarization resistance (LPR) was carried out using ±30 mV over the previously determined open circuit potential at a sweep rate of 0.166 mVs−1.
Microhardness was measured using a Vickers Shimadzu hardness tester (Shimadzu, Kyoto, Japan) with a load of 980.7 mN for 5 s, in accordance with ASTM E92-17. Each coating was tested five times longitudinally [38].

2.6. Computational Evaluation

All-electron geometric optimizations were performed for all of the species considered using a density functional theory (DFT) approach with a B3LYP/6-31g (d, p) level of theory, using a CPCM solvation model in methanol to emulate the dielectric constant of ethylene glycol, confirming that the species were those of minimum energy by observing that they only presented positive vibrations. This same level of theory was reported for the computational study of cobalt complexes with separate phenylsalicylimine ligands [35]. All calculations were performed using Gaussian16 software (Carnegie Mellon University, Pittsburgh, PA, USA) and the GaussView visualization suite [39].

3. Results and Analysis

3.1. Synthesis and Characterization of Ligands

The synthesis of phenylsalicylimines (PSI-1 and PSI-2) was performed following methodologies described in the literature [40,41,42]. The compounds were characterized, and the data obtained by 1H NMR were compared with those of the bases currently available (Figures S1 and S2, Supplementary Information). Figure 2 shows the synthesis.
Initially, the base structure was proposed as a study model, presenting a phenylsalicylimine with no substituents present (PSI-1). Subsequently, the incorporation of a functional group that could modulate the electronic density and the functionalization of phenylsalicylimines with an electron donor group R1 = Et2N (PSI-2) was contemplated, obtaining a “Push” type structure [43,44]. This system assumed that the nitrogen in the diethylamino substituent can introduce electron density into the π-conjugated system of the ligand.

3.2. Cyclic Voltammetry Study

The electrochemical potential window with the most information for the glassy carbon electrode was determined. The cobalt and nickel solutions exhibited reduction and oxidation peaks that varied due to the quasi-reversibility of the reactions, indicating that less energy was required to reduce the ions than to oxidize them (Figure S3, Supplementary Information) [45,46]. Figure 3 shows the cyclic voltammetry (CV) curves of cobalt and nickel at concentrations of 0.5 M without additives and with PSI-1 and PSI-2 ligands, respectively.
Figure 3A,B show the CV curves recorded for 0.5 M Co (II) in DES at different concentrations of additives PSI-1 and PSI-2. In the direct CV scan direction, the reduction peak located in the potential range of −0.4 V to −0.8 V was related to the process associated with the reaction Co (II)DES + 2e− → Co(s). Meanwhile, during the anodic sweep, the oxidation peak, located in the potential range of −0.3 V to 0.1 V, was related to the dissolution of the deposited metal (from Co0 to Co2+) [45,46].
Adding PSIs to the cobalt DES solution did not show potential window changes; however, a decrease in current density at reduction peaks was observed, indicating that the rate of the electrochemical reaction, specifically the reduction of cobalt ions in this case, was slower. This suggested a change in the kinetics of the process, likely due to factors like increased resistance or slower electron transfer at the electrode surface [47,48,49,50,51].
Elsewhere, a change in the shape of the anodic sweep of the CV curves was associated with the effects of functional group changes in the PSIs. PSI-1 exhibited a potential shift from −0.2 V to 0.5 V, indicating an effective decrease in cobalt oxidation and, consequently, an increase in the process output current. For PSI-2, there was no apparent change in potential, but the oxidation current decreased. The cathodic current was suppressed, indicating slower nucleation and growth of cobalt due to interactions between cobalt and the ligand. PSI-1 and PSI-2 could be adsorbed onto the electrode surface, inhibiting Co deposition and thereby reducing the growth and nucleation of Co, which increased the nucleation rate.
In Figure 3C,D, the reduction peak was in the potential range of −0.4 V to −0.7 V, associated with the reaction Ni (II)DES + 2e− → Ni(s), and the oxidation peak was in a potential range of −0.3 V to −0.6 V. The PSIs’ action of Ni (II) increased in the diffusion-controlled phenomenon, widening the window, accompanied by an increase in current, which favored the nucleation and growth processes at the electrode–solution interface. At the oxidation peak, a current increase occurred only at a concentration of 0.5 mM, which suggested that the compound promoted the passivation of nickel deposited at the highest concentration evaluated. As the reverse scan advanced, a noteworthy decrease in current was observed, which was predictably associated with the electrolysis decomposition of the solvent [52,53,54].
Changes in the functional groups of phenylsalicylimines alter the charge and mass transfer mechanisms, modifying the reduction rate of nickel and cobalt [55]. It is presumed that the amino group increased the hydrogen bond network and improved the selective complexation capacity of cobalt in ethaline-based DES, which was reflected in the solvation and reduction of metal ions transported to the interfacial layer [56,57,58]. By contrast, the ethylamine functional group led to the formation of a secondary amine that could coordinate with cobalt and nickel ions, while also influencing the overall charge distribution. As a result, this affected charge migration, transport, and transfer processes, but a greater response in cobalt oxidation was exhibited as the current density decreased compared to nickel [59,60,61]. This preliminary analysis of the interaction between phenylsalicylimines and ions was further examined through computational calculations, as elucidated below.

3.3. UV–Vis

Speciation was observed only in the cobalt solution with PSIs, attributed to its strong interactions during cyclic voltammetry tests in ChCl:EG. The ligand concentration was tested at 0.5, 1, and 1.5 equivalents in the cobalt solution with ethaline to clarify the interaction between the electrolyte components in the baths more effectively. The measurements were taken after the sample had remained under agitation at room temperature for one hour. Figure 4 shows the absorption and emission bands of the ligands’ UV–Vis spectra.
Phenylsalicylimines exhibited characteristic absorption bands in the UV-Vis spectra. For PSI-1 (Figure 4A), two strong absorption bands were observed centered at 273 and 352 nm, while for PSI-2 (Figure 4B), these were found at 420 and 440 nm due to their π–π* transitions [62]. The formation of the expected coordination compound was confirmed by the appearance of a third band, centered at 416 nm, for PSI-1. Meanwhile, for PSI-2, only a change in the intensity of the dominant absorption band became evident as, compared to PSI-1, the absorption of the free ligand bathochromically shifted in the range of 400 to 450 nm due to the presence of the Et2N group [63,64,65]. Thus, the effect of the band corresponding to the coordination compound was overshadowed by the initial band of the free ligand.
In the absorption–emission spectra (Figure 4C,D), PSI-1 and PSI-2 emissions were observed. The ligand emission bands supported the absorption findings, showing a decrease in intensity due to fluorescent deactivation resulting from the coordination of cobalt. These findings supported the effect of these compounds as additives in electrodeposition processes, acting as coordinating structures affecting cobalt migration.

3.4. Cell Parameters

After confirming the positive impact of phenylsalicylimines as additives on electrochemical reactions under static conditions, hydrodynamic tests were conducted at 25 rpm. The rules of Abner Brenner et al. [65] were used in electrodeposition of the alloy, in which the mechanism and ionic kinetics of the nickel and cobalt species were equal for the additive at a concentration of 0.3 M [66,67]. Figure 5 shows the cobalt and nickel solutions without ligand (X) and with ligands PSI-1 and PSI-2.
The PSIs’ effects were reflected in the change in OCP, exhibiting slightly negative depolarization compared to nickel and cobalt solutions without additives. This indicated an electrochemical response resulting from the modification of the metal interface’s double layer, which reduced ion desorption. Additionally, notable differences in nickel and cobalt kinetics emerged, with cobalt’s mobility being delayed and that of nickel being favored, contrasting with cobalt’s rapid transition from activation to diffusion control in the absence of additives [23,24,25]. By adding PSI-1 and PSI-2, with activation control, an intersection was observed for both Co and Ni, matching the reduction in ionic velocities at the metal interface. When PSI-1 was added, the intersection was at −0.187 ± 0.089 V vs. Ag/AgCl with a current density of 19.74 ± 57.61 μA cm−2 and for PSI-2, the intersection was at −0.172 ± 0.051 V vs. Ag/AgCl with an exchange current density of 36.08 ± 72.47 μA cm−2. Both additives met the convergence of the deposition potentials for both components, which ensured that both ions were deposited at the same rate without changing concentration, agitation, or pH. The improvement in the reduction reaction may be attributed to the better ionic mobility of the compounds formed by the additives compared to the anionic chloride complexes in ethaline, which altered the electrochemical behavior of Co (II) and Ni (II) [27]. From the current limit, the diffusion coefficient was calculated using the Levich equation, as shown in Equation (1) [66]:
D 2 3 = 1.613   J x v 1 6 n F A C ω 1 / 2
where JX = reactant flux (mol m−2 s−1), ω = rotational speed of the disk = rpm × 2π/60 (s−1), υ = kinematic viscosity (m2 s−1), DCo = diffusion coefficient of X (m2 s−1), CCo = concentration of Co (mol m−3 or mM), n = number of electrons in the reduction or oxidation reaction of the analyte (eq mol−1), F= Faraday constant, (96 485 C/mol), and A = area of the flat electrode (cm2). The kinematic viscosity of the ethylene glycol and choline chloride mixture at 60 °C was determined to be 2.58 × 10−2 A cm s2, a value typical of a highly viscous solution. The results are shown in Table 1.
It was observed that DCo PSI-1 increased up to 1.9523 × 10−6 cm s−2, and for the case of DCo PSI-2, it was up to 1.6268 × 10−6 cm s−2, compared to the mobility of cobalt, which was 2.566 × 10−6 DCo/cm s−2. Metal transport became slower, causing a change in the diffusion coefficient considering that the electrodeposition process is not only affected by the transport of the metal ion to the electrode surface but will also change the mass transport by changing the concentration of the species near the cathode. In the case of the diffusion coefficient of nickel, no significant changes in ion mobility were observed. It can be deduced that the coordination of these additives was preferentially with cobalt.
Turning favored the selectively controlled deposition of cobalt through coordination processes and helped to maintain a stable Ni deposition rate through steric interference in the electrodeposition process of the latter, as depicted. It was inferred that the absence of the Et2N group in the first ligand increased the additive’s desorption rate, leading to less ordered electrodeposition. The morphology of the deposit generated showed the changes in the diffusive phenomenon observed during the electrodeposition of the alloy. Adatom formation and electronic exchange facilitated by the ligands were the causes of this change in the cathodic current. The ligands enhanced the discharge of cations to the adatoms, making the process more rapid and lowering the diffusion coefficient values.

3.5. Ni-Co Coating Assessments

When analyzing metallic deposits, it is essential to evaluate the coating’s morphology, which should be uniform, compact, and pore-free. Figure 6 displays secondary electron SEM images of the surface, a longitudinal cross-section for thickness observation, and the EDS spectra.
The PSI-1 coating (Figure 6B) exhibited a generalized grain morphology and surface segregation, attributed to the increased cobalt content in the deposited alloy compared to the coating without additives (Figure 6A). The PSI-2 coating (Figure 6C) exhibited a homogeneous morphology, characterized by homogeneous coating with minimal contrast changes. The thicknesses determined from profilometry were 21.66 ± 9.62 μm for the coating without additive, 23.46 ± 2.06 μm for the PSI-1 coating, and finally 24.16 ± 2.36 μm for the PSI-2 coating. In the cross-sectional images, compact films were observed in both PSI-1 and PSI-2, with better levels compared to the prominent and isolated crystals shown in the coating without additives where globular growth was observed (Table S1, Supplementary Information). The film thickness is one of the critical control factors in Ni-Co alloy coatings; the standard specifies a range of 5 to 30 μm, and it is also a fundamental parameter for comparing corrosion resistance [67,68].
The EDRx spectra (Figure 6D) showed that cobalt peaks were observed with higher intensity in the presence of PSI, both in Lα and Kα and Kβ, compared to the coating without the additive. From the punctual analysis, the percentage of cobalt in the deposits of PSI-1 was 26% and for PSI-2 it was 37.69%. Research indicates that alloy ranges with the best anti-corrosion and mechanical properties are between 20% and 37% by weight of cobalt, with the PSI-2 coating being the only one that met the composition under the ASTM B994 standard [69,70,71].
EIS and LPR were employed to evaluate the corrosion resistance and mechanical strength of the unbound coating (X) with PSI-1 and PSI-2. The results presented in Figure 7 and Table 2 demonstrate the electrochemical parameters obtained from fitting the experimental results using equivalent electrical circuits (ECC) from electrochemical impedance spectroscopy. A constant phase element (Q) was employed, as shown in Equation (2), to enhance the fit:
Z C P E = 1 Q j 2 ω α 1
where Q represents the double-layer capacitance, while the parameter α represents the non-ideal electrode-electrolyte interface (case α = 1 refers to an ideal capacitance).
When performing a comparative analysis of the results obtained, the resistance of the coating without any additive presented a lower contribution compared to that of the PSI-1 and PSI-2 coatings (Figure 7A); both exhibited an increase. Moreover, the double-layer capacitance decreased, translating into better corrosion protection of the Ni-Co coatings compared to the absence of additives on primary cobalt composition and homogeneous surface morphology. PSI-1 and PSI-2 additives produced Ni-Co coatings with higher corrosion resistance and lower capacitance, indicating improved corrosion protection characteristics [72]. With the LPR (Figure 7B) results under the ASTM G59 standard [73] and assuming activation control, the corrosion rate was calculated for each of the coatings evaluated under ASTM G102 [74], resulting in average rates of 1.698 µm yr−1 for the coating without additive versus 0.996 μm yr−1 for the PSI-1 ligand and 0.781 μm yr−1 for the PSI-2 ligand, with the latter corrosion rate meeting the standard (0.8636 μm yr−1). The same trend was obtained when measuring the Vickers hardness, for the PSI-1 and PSI-2 coatings presented increases in penetration resistance, with an average value of 342 N HV (−34.6 N HRC) for the PSI-1 coating and an average value of 382 N HV (41 N HRC) for the PSI-2 coating, which was significantly different from 31 N HRC [71]. Notably, the mechanical requirements, thickness, and corrosion resistance were achieved through the performance of an alloy that can be used in anti-corrosion coating systems, which had already been applied to metal protection systems when PSI-2 was introduced.

3.6. Theoretical-Computational Analysis

This study focuses on PSI-2 and cobalt ions as key additives influencing Ni-Co alloy formation. Understanding their structure and orientation relative to the electrode is vital to assessing how PSI-Co (II) coordination compounds affect the cathode during reduction. Two geometries are proposed for the cobalt atom coordinated with an PSI and a solvent molecule: a pseudo-tetrahedral or a quadrangular geometry. The geometry depends on the nature of the coordinating atoms, with the PSI coordinating through two O atoms (weak field) and one N atom (intermediate strength).
Therefore, a computational analysis was performed to evaluate the most stable electroactive species involved in the electrodeposition process. Matsumoto et al. [75] reported that Co (II) in ethylene glycol can adopt octahedral or square geometries. It was assumed that the medium in which the ligands and coordination compounds were applied allowed for the presence of the free cation, which was available to be coordinated by the DES and the additive. Co (II) can present two values of spin multiplicity (M), and when coordinated, M = 2.4 would determine the planar square or tetrahedral geometry, respectively (Figures S4 and S5, Supplementary Information).
The phenolic structure and its phenolate state were analyzed to assess the thermodynamics related to ethylene glycol-solvated cobalt. The coordination reactions detailed in the Supplementary Information aimed to identify potential coordination products, informed by previously studied Co (II) complexes with similar PSI ligand architectures [76].
The medium consisted of a 2:1 mixture of ethylene glycol and choline chloride, leading to a second set of coordination reactions. Here, a choline molecule displaced ethylene glycol while maintaining its coordination mode. Each complex gained a 1+ charge, resulting in eight computationally studied coordination compounds.
Thermodynamic modeling of the coordination reactions indicated that Co (II) species coordinated with the ligands DES and PSI-2. The formation of two unstable species, PSI-2b and 2f, with distorted tetrahedral geometries exceeding +20 kcal/mol, was unlikely. Given the choline concentration and the stability of the deprotonated ligand PSI-2a, a square planar complex formed with ethylene glycol. The exchange of diol for a choline molecule (PSI-2e and 2i) showed a minimal energy difference (3.4 kcal/mol), making this exchange feasible at room temperature. These two species were significantly more stable and likely to undergo electrochemical phenomena. Two square CoL complexes were formed, one with ethylene glycol and one with choline, exhibiting higher polarity and stability due to the potential difference in the medium during deposition. Figure 8 depicts electrostatic potential maps for the three most relevant species considered in the study.
Figure 8A shows the ethylene glycol molecule coordination of cobalt, and Figure 8B shows the cobalt complex with PSI-ethylene glycol ligand, with the dipole moment pointing out the diethylamino substitute and the free hydroxyl group. The positive form of diethylamino is considered, as it donates electron density, potentially anchoring the electrode to aid in the reduction of Co (II) to Co (0).
In Figure 8C, the PSI-2 complex shows a clear ligand exchange for choline. It features a negative zone in orange, a slightly positive area on the diethylamino substituent, and a more positive zone on the choline ligand. This results in a notable increase in the dipole moment, creating two viable zones for the electrothermal processes of Co (II) reduction to Co (0).
The ligand with a diethylamino substituent (PSI-2) serves two main functions. First, it creates stable species with a geometry that enhances the cobalt’s electrostatic susceptibility to the cathode and improves mobility in the medium through changes in solvation and dipole moment. Second, by coordinating with Co (II), it transfers charge density to the metal, generating two positively charged sites for electrode processes.
The ligand facilitates coordination–migration–reduction cycles, desorbing after reducing cobalt to metal and then coordinating with another Co (II) atom due to favorable thermodynamics. This process contributes to alloy formation at Ni–Co interactions. Additionally, the increased dipole moment of Co (II) in thermodynamically favored square-planar complexes enhances its diffusion toward the cathode, utilizing positively charged areas as anchors.

3.7. Mechanism of PSI Additives of Ni-Co Deposition

Despite its low concentration, the ligand had a significant impact on the electrochemical reduction behavior of the cobalt ion. In this scenario, when the ligand was adsorbed onto the metal matrix, the Co (II) ion interacted strongly with the electrode surface, thereby favoring electrochemical reduction. Twisting enabled the selective control of cobalt reduction through coordination processes, thereby maintaining a stable rate of interference deposition. It is inferred that the absence of the Et2N group in the first ligand increased the desorption rate of the additive, leading to less ordered electrodeposition. The morphology of the generated deposit showed the changes in the diffusive phenomenon observed during electrodeposition of the alloy. The formation of adatoms and the electronic exchange facilitated by the ligands were the causes of this change in the cathodic current. The PSIs enhanced the discharge of cations to the adatoms, making the process faster and decreasing the diffusion coefficient values.
In summary, three significant effects can be identified. Firstly, it was demonstrated that the phenylsalicylimine compounds evaluated interacted with both cobalt and nickel, showing better molecular interactions with the PSI-2 ligand; this was supported by computational analysis and experimental results. A coating with better distribution of the nickel-cobalt alloy was obtained, exhibiting a chemical composition, thickness, hardness, and corrosion resistance that met the minimum requirements of the standard for using Ni-Co alloy as a metallic coating.

4. Conclusions

Electrochemical analysis demonstrated that the phenylsalicylimine derivatives exerted an effect that inhibited Co (II) migration while augmenting Ni (II) mobility, thus facilitating a more uniform and controlled deposition process. Modifications to the functional group influenced redox behavior; notably, the ethylamine group exhibited the most pronounced capacity to reduce Co (II) migration and enhance Ni (II) mobility within ethaline and DES systems.
Molecular calculations revealed that the phenylsalicymine ligands with a diethylamino functional group (PSI-2) performed two primary roles. It formed stable species with a geometry that increased cobalt’s electrostatic attraction to the cathode and improved its mobility by altering solvation and dipole moment. By coordinating with Co (II), it transferred charge, creating positively charged sites that facilitated electrode reactions. The increased dipole moment of Co (II) in square-planar complexes enhanced its diffusion to the cathode, using positively charged areas as anchors, and facilitated alloy formation at Ni–Co interactions.
Utilizing the PSI-2 ligand guarantees a consistent Ni-Co coating containing 37% Co, which possesses superior anti-corrosion properties and demonstrates exceptionally high microhardness levels. This adequately satisfies the mechanical and corrosive resistance requirements established by the ASTM B571 standard.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/coatings15070814/s1: Figure S1: IR, Mass, 1H and 13C NMR spectra of PSI-1; Figure S2: IR, Mass, 1H and 13C NMR spectra of PSI-2; Figure S3: Voltamperometry of Ni and Co at different scan rates; Figure S4: Computational study. Optimized geometries calculated at the B3LYP/6-31g (d, p) level of theory, using a CPCM solvation model in methanol; Figure S5: Representation of the thermodynamics for the calculated compounds showing the stability of the 8 possible complexes, where the neutral species and those with 1+ charge would be the most abundant and favored in their formation; Table S1; Coating thickness from DOX bath.

Author Contributions

Conceptualization, E.O.-R. and P.R.-B.; methodology, P.R.-B.; software, O.G.-A.; validation, R.B.-I. and P.R.-B.; formal analysis, R.B.-I. and P.R.-B.; investigation, E.O.-R. and P.R.-B.; resources, P.R.-B. and N.F., data curation, E.O.-R., P.R.-B., O.G.-A. and R.B.-I.; writing—original draft preparation, E.O.-R., P.R.-B., O.G.-A., R.B.-I. and N.F.; writing—review and editing, E.O.-R., P.R.-B., O.G.-A. and R.B.-I.; visualization, P.R.-B.; supervision, P.R.-B. and N.F.; funding acquisition, P.R.-B. and N.F. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by Dirección General de Asuntos del Personal Académico (DGAPA), Universidad Nacional Autónoma de México, grant number UNAM-PAPIIT IA100824 awarded to Paola Roncagliolo Barrera and UNAM-PAPIIT IN 200422, awarded to Norberto Farfán. Enrique Ordaz thanks CONAHCYT for awarding him his doctoral fellowship (CVU 917328). O. González-Antonio thanks CONAHCYT for his postdoctoral fellowship (CVU 289250). R Ballinas-Indili thanks CONAHCYT for his postdoctoral fellowship (CVU 619858). Supercomputing resources from DGTIC-UNAM through the “LANCAD-UNAM-DGTIC-268” Project are sincerely appreciated.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

Data are contained within the article.

Acknowledgments

P. Roncagliolo wants to thank Itzel Reyes Chaparro (FQ UNAM) for technical assistance with SEM images, Sergio García Galán (FQ UNAM) for technical assistance with Mechanical Tests.

Conflicts of Interest

The authors declare no conflicts of interest.

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Figure 1. Structure of phenylsalicylimines used as additives.
Figure 1. Structure of phenylsalicylimines used as additives.
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Figure 2. Synthesis of phenylsalicylimines.
Figure 2. Synthesis of phenylsalicylimines.
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Figure 3. CV curves of (A) Co (II) + PSI-1, (B) Co (II) +PSI-2, (C) Ni (II) + PSI-1, and (D) Ni (II) + PSI-2 at T 333 K.
Figure 3. CV curves of (A) Co (II) + PSI-1, (B) Co (II) +PSI-2, (C) Ni (II) + PSI-1, and (D) Ni (II) + PSI-2 at T 333 K.
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Figure 4. UV-Vis spectra. (A) Absorption spectra of Co + PSI-1, (B) absorption spectra of Co + PSI-2, (C) absorption–emission spectra of Co at PSI-1 1 eq relative, and (D) absorption–emission spectra of Co at PSI-2 1 eq relative.
Figure 4. UV-Vis spectra. (A) Absorption spectra of Co + PSI-1, (B) absorption spectra of Co + PSI-2, (C) absorption–emission spectra of Co at PSI-1 1 eq relative, and (D) absorption–emission spectra of Co at PSI-2 1 eq relative.
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Figure 5. Potentiodynamic polarization of (A) Co and Ni solutions with 0.03 mM PSI-1 and (B) Co and Ni solutions with 0.03 mM PSI-2 at 333K and 25 rpm.
Figure 5. Potentiodynamic polarization of (A) Co and Ni solutions with 0.03 mM PSI-1 and (B) Co and Ni solutions with 0.03 mM PSI-2 at 333K and 25 rpm.
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Figure 6. Coating characterization obtained by SEM and EDS for (A) blank, (B) PSI-1, and (C) PSI-2, and (D) chemical composition summary.
Figure 6. Coating characterization obtained by SEM and EDS for (A) blank, (B) PSI-1, and (C) PSI-2, and (D) chemical composition summary.
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Figure 7. Ni-Co coating evaluation, including anti-corrosion and mechanical resistance: (A) Nyquist plot; (B) LPR and Vickers hardness of the blank (black), PSI-1 (red), and PSI-2 (green) coatings.
Figure 7. Ni-Co coating evaluation, including anti-corrosion and mechanical resistance: (A) Nyquist plot; (B) LPR and Vickers hardness of the blank (black), PSI-1 (red), and PSI-2 (green) coatings.
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Figure 8. Electrostatic potential maps for (A) the neutral complex of cobalt hexacoordinated with ethylene glycol without PSIs, (B) cobalt complex with PSI-ethylene glycol ligand, and (C) the cationic complex with a PSI-choline ligand.
Figure 8. Electrostatic potential maps for (A) the neutral complex of cobalt hexacoordinated with ethylene glycol without PSIs, (B) cobalt complex with PSI-ethylene glycol ligand, and (C) the cationic complex with a PSI-choline ligand.
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Table 1. Cobalt and nickel diffusion coefficients.
Table 1. Cobalt and nickel diffusion coefficients.
Co (II)Ni (II)
CompoundE vs Ag | Ag (I)/Vιlim/µA cm−2DCo/cms−2
10−6
E vs Ag | Ag (I)/Vιlim/µA cm−2DNi/cm s−2
10−6
X−0.10515.8492.56656−0.1062.49960.29442
PSI-1−0.13110.4711.9523−0.1122.46670.29186
PSI-2−0.1172.51191.6268−0.1142.07980.26078
Table 2. Electrochemical parameter fitting of deposits without ligand (X) and with PSI-1 and PSI-2.
Table 2. Electrochemical parameter fitting of deposits without ligand (X) and with PSI-1 and PSI-2.
CoatingRsol/Ω∙cm2Rct/Ω∙cm2Qdl/µF cm−2α1
X31.91 ± 1.1810 349 ± 1.0315.542 ± 1.890.779
PSI-136.53 ± 2.1216 632 ± 3.088.931 ± 2.040.722
PSI-231.55 ± 1.9621 805 ± 4.090.145 ± 1.310.731
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MDPI and ACS Style

Ordaz-Romero, E.; Roncagliolo-Barrera, P.; Ballinas-Indili, R.; González-Antonio, O.; Farfán, N. Ni-Co Electrodeposition Improvement Using Phenylsalicylimine Derivatives as Additives in Ethaline-Based Deep Eutectic Solvents (DES). Coatings 2025, 15, 814. https://doi.org/10.3390/coatings15070814

AMA Style

Ordaz-Romero E, Roncagliolo-Barrera P, Ballinas-Indili R, González-Antonio O, Farfán N. Ni-Co Electrodeposition Improvement Using Phenylsalicylimine Derivatives as Additives in Ethaline-Based Deep Eutectic Solvents (DES). Coatings. 2025; 15(7):814. https://doi.org/10.3390/coatings15070814

Chicago/Turabian Style

Ordaz-Romero, Enrique, Paola Roncagliolo-Barrera, Ricardo Ballinas-Indili, Oscar González-Antonio, and Norberto Farfán. 2025. "Ni-Co Electrodeposition Improvement Using Phenylsalicylimine Derivatives as Additives in Ethaline-Based Deep Eutectic Solvents (DES)" Coatings 15, no. 7: 814. https://doi.org/10.3390/coatings15070814

APA Style

Ordaz-Romero, E., Roncagliolo-Barrera, P., Ballinas-Indili, R., González-Antonio, O., & Farfán, N. (2025). Ni-Co Electrodeposition Improvement Using Phenylsalicylimine Derivatives as Additives in Ethaline-Based Deep Eutectic Solvents (DES). Coatings, 15(7), 814. https://doi.org/10.3390/coatings15070814

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