Electrochemical Fabrication of Ni–Co Alloy over a Wide pH Range Using Sodium Citrate as a Complexing Agent

Abstract

In this study, nickel–cobalt (Ni–Co) coatings were fabricated via electrodeposition using a 2² central composite factorial design with two central and two axial points, totaling ten experiments. The effects of pH and current density on the coatings’ chemical composition and properties were evaluated. Coatings were characterized by microstructure, morphology, magnetic properties, and corrosion resistance. The results showed that pH significantly influenced chemical composition, while current density had no notable effect. Acidic pH produced cobalt-rich coatings (43–81 at.%), with uniform morphology, higher saturation magnetization, and lower corrosion resistance. Maximum cobalt content (81 at.%) resulted in a mixed face-centered cubic (fcc) + hexagonal close-packed (hcp) phase. Alkaline pH yielded nickel-rich coatings (89–95 at.%), featuring nodular morphology, lower magnetization, higher corrosion resistance, and, exclusively, the fcc phase. The highest polarization resistance (66.1 kΩ) occurred at pH 8.83 and 60 mA/cm², while resistance decreased with increasing cobalt content. The pH effect on deposition was linked to the formation of citrate complexes: ammonia and citrate complexes promoted nickel deposition under alkaline conditions, while stable cobalt complexes dominated in an acidic pH. These findings highlight the potential to tailor Ni–Co coatings for applications such as corrosion-resistant coatings (nickel-rich) or magnetic devices (cobalt-rich).

1. Introduction

Nickel–cobalt (Ni–Co) alloys have been widely recognized as high-performance engineering materials due to their superior properties when compared to their pure constituent metals. These alloys exhibit exceptional hardness, high resistance to wear and corrosion, desirable magnetic properties, and notable electrocatalytic activity [1,2]. Such attributes render Ni–Co alloys highly suitable for diverse technological applications, including the fabrication of sensors, electromagnetic devices, and protective coatings for environments with extreme corrosive conditions. For instance, cobalt-rich Ni–Co alloys are predominantly utilized in magnetic applications due to their enhanced saturation magnetization, while nickel-rich counterparts are extensively applied as corrosion-resistant coatings, particularly in aggressive chemical environments [3].

The electrodeposition technique has emerged as the most viable and versatile method for synthesizing Ni–Co alloys. This process offers several advantages, including simplicity, cost-effectiveness, and the ability to coat substrates with complex geometries. Moreover, electrodeposition is characterized by low energy consumption and the capability to produce coatings with tailored mechanical and chemical properties. Notably, this method enables the precise control of alloy composition through the adjustment of deposition parameters, such as electrolyte bath composition, temperature, current density, and pH. These features underscore its utility for industrial-scale production and the fabrication of advanced materials [4,5,6].

Mechanistically, the electrodeposition of Ni–Co alloys is governed by the preferential reduction of cobalt ions over nickel ions, a phenomenon termed “anomalous deposition”. This process is attributed to the differences in the standard electrode potentials of the two metals, with cobalt being the less noble element [7,8,9]. As a result, the deposited alloys typically exhibit a higher cobalt content relative to the electrolyte composition. Nevertheless, the composition of Ni–Co alloys can be meticulously tailored by optimizing the operational parameters of the electrodeposition process. For instance, increasing the pH of the electrolyte bath generally promotes the deposition of nickel, leading to the formation of nickel-rich alloys. Conversely, acidic conditions favor cobalt deposition. Additionally, higher current densities are associated with smoother deposits and enhanced nickel incorporation [10,11].

The phase behavior of Ni–Co alloys also plays a critical role in determining their properties. Nickel and cobalt can form solid solutions over a wide compositional range, allowing for the deposition of alloys with varied microstructures. Cobalt-rich alloys often exhibit a mixture of face-centered cubic (fcc) and hexagonal close-packed (hcp) phases, contributing to their superior magnetic properties. In contrast, nickel-rich alloys predominantly adopt the fcc phase, which is associated with enhanced corrosion resistance and mechanical stability. Despite the extensive research on these alloys, achieving nickel-rich Ni–Co coatings through electrodeposition remains challenging due to the anomalous behavior of the system. However, specific studies have demonstrated that nickel-rich alloys can be synthesized using low Co²⁺/Ni²⁺ ion ratios and optimized process conditions [5,12,13].

Among the critical parameters that influence the electrodeposition of Ni–Co alloys, current density and pH have been widely studied due to their significant impact on the resulting alloy composition and microstructure. Low current densities are known to favor the preferential deposition of cobalt, which leads to the formation of cobalt-rich coatings. In contrast, higher current densities enhance the reduction of nickel ions, resulting in nickel-rich alloys with more compact and smoother deposits [1,14]. Similarly, the pH of the electrolyte bath plays a crucial role in determining the elemental composition of the alloy. An increase in pH typically results in a higher nickel content within the deposited material, accompanied by a corresponding decrease in cobalt content. This pH dependency can be attributed to the formation of different metal–hydroxy complexes in the solution, which influence the deposition rates of nickel and cobalt. However, these effects are not universal and are highly contingent on the specific composition of the electrolyte bath, as well as the presence of complexing agents such as citrate, which can further modulate the deposition process [15,16,17].

Sodium citrate, a commonly employed complexing agent in electrochemical processes, has shown promise in stabilizing the electrodeposition environment and promoting uniform coating formation. Citrate ions play a dual role by forming complexes with metal ions to regulate their reduction potential and maintaining a stable pH during the deposition process. Despite its widespread application, there is a notable gap in the literature regarding its use for producing Ni–Co alloys under alkaline conditions. Existing studies have predominantly focused on acidic environments, leaving the behavior of citrate in basic electrolytes largely unexplored. This study seeks to address this knowledge gap by investigating the influence of sodium citrate on the electrodeposition of Ni–Co alloys at basic pH levels [18,19].

To elucidate the effects of process parameters on alloy composition and properties, experimental design methodologies, such as factorial design and response surface analysis, were employed. These tools enable the systematic evaluation of key variables, including pH and current density and their interactions [20].

The findings of this study hold significant implications for both scientific and industrial communities. By providing a detailed understanding of the deposition mechanisms and the role of citrate in alkaline electrolytes, this work contributes to the optimization of Ni–Co alloy electrodeposition for specific applications. The ability to tailor alloy composition and properties expands the potential applications of these materials, ranging from magnetic devices to corrosion-resistant coatings in harsh environments. This research thus represents a critical step toward advancing the field of functional coatings and enhancing the performance of Ni–Co alloys in high-demand engineering applications.

2. Materials and Methods

2.1. Preparation of the Substrate and Electrolytic Solution

The substrate used for the electrodeposition process was a 1 mm-thick copper sheet with a total surface area of 8 cm². The preparation of the copper substrate involved two stages: mechanical treatment and chemical treatment. The mechanical treatment consisted of polishing the substrate surface using SiC sandpapers with progressively finer grit sizes (400, 600, and 1200 mesh) to remove impurities and homogenize the surface. The chemical treatment involved immersing the substrate in a 10% NaOH solution to remove grease and impurities, followed by rinsing with distilled water. Subsequently, the substrate was immersed in a 1% H₂SO₄ solution to activate the surface, followed by another rinse with distilled water and drying in an oven. These steps ensured that the copper substrate was adequately prepared for the electrodeposition process.

The electrolytic solution used for the electrodeposition of the Ni–Co alloy was prepared with distilled water and the following reagents: 0.2 M nickel sulfate, 0.1 M cobalt sulfate, 0.2 M boric acid, 0.3 M ammonium sulfate, and 0.3 M sodium citrate, that were purchased from Neon Commercial Ltda. (São Paulo, Brazil). The pH of the solution was adjusted using sulfuric acid or ammonium hydroxide and measured with a QUIMIS Q400RS pH meter (Brazil).

2.2. Experimental Design

A Central Composite Rotational Design (CCRD) was employed to optimize the electrodeposition process. Two independent variables, namely pH and current density, were selected for evaluation. The experimental design consisted of 2² factorial experiments + 2² axial points + 2 central points, resulting in a total of 10 experiments. The independent variables were investigated at five levels: high (+1), low (−1), central (0), extrapolated high (+1.414), and extrapolated low (−1.414). The real and coded values corresponding to the experimental matrix are presented in

Table 1. Investigation levels of the variables studied in the 2² experimental design with two extrapolations.

Factors	Levels
	−1.414	−1	0	+1	+1.414
pH	3.17	4	6	8	8.83
Current density (mA/cm2)	31.72	40	60	80	88.28

.

2.3. Electrodeposition

The electrodeposition process was carried out using an electrolytic cell comprising a platinum electrode as the anode and a copper substrate as the cathode. Both electrodes were immersed in 100 mL of an electrolytic solution containing the metallic ions to be reduced. The current was regulated using a PGSTAT302N potentiostat/galvanostat (The Netherlands), while the temperature was maintained at a constant 30 °C using a Novatecnica NT246 water bath. The deposition time for each experiment was calculated based on Faraday’s law, with a fixed total charge of 600 C.

2.4. Chemical Composition and Surface Characterization of the Alloy

The chemical composition of the obtained alloys was analyzed using Energy-Dispersive X-ray Spectroscopy (EDX) with a SHIMADZU EDX-720 energy-dispersive X-ray fluorescence spectrometer (Japan). The chemical composition was reported as the atomic percentage of each element (Ni and Co).

Current efficiency was analyzed by Faraday’s law. For calculations, a mass-based chemical composition was considered; the equation utilized is expressed by Equation (1):

$$ECC={\displaystyle \frac{w}{\frac{E_W\times I\times t}{F}}}={\displaystyle \frac{w\times F}{I\times t}}\sum {\displaystyle \frac{c_i{n}_i}{M_i}}\times 100$$

(1)

where w is the mass of the deposit in grams, t is the deposition time in seconds, I is the total current in ampere, Ew is the equivalent weight (g/equivalent), c_i is the weight fraction of the element i, n_i is the number of electrons transferred by mol of each metal, M_i is the molecular weight of i in g/mol, and F is the Faraday constant (96.485 C/mol).

The surface morphology was analyzed using Scanning Electron Microscopy (SEM) with a Tescan microscope, model Vega3 XM (Brno—Kohoutovice, Czech Republic). The microstructural characterization was performed by X-ray Diffraction (XRD) using a Shimadzu diffractometer, model 6100 Shimadzu 6100 (Japan), operating with CuKα radiation (λ = 1.54056 A°) at 40 kV and 40 mA.

The thickness measurement was performed using the gravimetric method and subsequently confirmed through the chronoamperometric method based on Faraday’s Law.

2.5. Magnetic Analysis

Magnetic hysteresis loops were measured using a Vibrating Sample Magnetometer (VSM, Microsense EZ7 model) from Milpitas, CA, USA. The samples were precisely cut and weighed on an analytical balance with a precision of 0.00001 g to obtain magnetization values normalized to the sample mass. Magnetic measurements were conducted at room temperature under a parallel magnetic field of approximately 15,000 Oe applied to the coating surface.

2.6. Corrosion Tests

Corrosion testing commenced after stabilizing the Open Circuit Potential (OCP) for 60 min. Electrochemical Impedance Spectroscopy (EIS) was subsequently conducted to evaluate the electrode–electrolyte interface at the OCP potential. The EIS procedure involved applying an alternating current with a sinusoidal signal, beginning at high frequencies (100 kHz) and progressing to low frequencies (0.01 Hz). Nyquist plots were generated and analyzed using an equivalent electrical circuit model with the NOVA 2.1 software.

Following EIS, Potentiodynamic Polarization (PP) tests were conducted to further assess the corrosion behavior. These tests were performed over a potential range of −0.3 V to 0.6 V relative to the OCP, using a scan rate of 1 mV/s with 1 mV step increments. The PP analysis provided key electrochemical corrosion parameters, including the corrosion potential (E_Corr) and corrosion current (I_Corr). These parameters were derived from the intersection of the cathodic and anodic polarization curves using the Tafel extrapolation method and the Stern–Geary equation (Equation (2)), following methodologies described by Silva et al. [21] and Costa et al. [22].

$$I_{Corr}={\displaystyle \frac{b_a.b_c}{2.3(b_a+b_C)R_p}}$$

(2)

where b_a and b_c represent the anodic and cathodic Tafel coefficients, respectively.

3. Results and Discussion

The results presented in this study provide a comprehensive analysis of the structural, morphological, magnetic, and electrochemical properties of electrodeposited Ni–Co alloys. Key findings emphasize the influence of deposition parameters, such as pH and current density, on the alloy composition and its resulting characteristics.

Table 2. Experimental design matrix and chemical composition in at.% and current cathodic efficiency of the Ni–Co alloy.

Exp.	pH	I (mA/cm2)	Ni (at.%)	Co (at.%)	CCE (%)	Thickness (µm)
1	4 (−1)	40 (−1)	36	64	66	16
2	4 (−1)	80 (+1)	41	59	45	15
3	8 (+1)	40 (−1)	93	7	65	17
4	8 (+1)	80 (+1)	89	11	58	15
5	3.17 (−1.414)	60 (0)	19	81	52	11
6	8.83 (+1.414)	60 (0)	95	5	66	18
7	6 (0)	31.72 (−1.414)	57	43	71	18
8	6 (0)	88.28 (+1.414)	56	44	86	14
9	6 (0)	60 (0)	51	49	65	16
10	6 (0)	60 (0)	51	49	65	16

presents the experimental design matrix, encompassing 10 experiments. It includes the results of the chemical composition expressed in atomic percentage (at.%) and the cathodic current efficiency (CCE) of the Ni–Co alloy.

According to Table 2, the results of this study demonstrate that the experimental design was highly effective, as it successfully encompassed nearly the entire range of chemical compositions for the alloy under investigation. The nickel content varied from 19% to 95%, while the cobalt content ranged from 5% to 81%. This broad variation in nickel and cobalt concentrations enabled a comprehensive analysis of the effects of these elements on the alloy’s properties, providing significant findings regarding its behavior. Additionally, the cathodic current efficiency (CCE) was observed to vary between 45% and 71%, indicating the feasibility of achieving coatings with good CCE values.

It was observed that a reduction in current density, combined with an increase in pH, contributed significantly to the enhancement of the coating thickness. This behavior can be attributed to the interplay between electrochemical kinetics and the chemical environment of the deposition process. Lower current densities reduce the rate of ion reduction at the cathode, allowing for a more uniform and controlled deposition process, which results in thicker layers over time [23]. Similarly, higher pH levels create conditions that favor the formation and stabilization of nickel and cobalt hydroxides, promoting the incorporation of these species into the deposited layer [15]. These findings highlight the critical influence of deposition parameters on the structural characteristics of the Ni–Co coatings, underscoring the importance of optimizing these variables for achieving desired thickness and quality in electrodeposited layers.

3.1. Effect of Input Variables on the Chemical Composition of the Alloy Ni–Co

To assess the influence of the independent variables (pH and current density) on the chemical composition of the Ni–Co alloy, the data presented in Table 2 were subjected to statistical analysis. By considering a 95% confidence level as the criterion for satisfactory results, it was possible to establish models describing the variation in the atomic percentage composition of Ni (Equation (3)) and Co (Equation (4)), as detailed below.

Ni = 51.00 + 26.56X + 5.00X² − 0.05Y + 4.75Y² − 2.25 XY

(3)

Co = 49.00 − 26.56X − 5.00X² + 0.05Y − 4.75Y² + 2.25 XY

(4)

In this context, X represents pH, Y represents current density, and XY denotes the interaction between these two variables. An analysis of variance (ANOVA) was conducted to evaluate the significance of the developed models.

Table 3. ANOVA results for the nickel and cobalt contents in at.%.

Factors	SS	df	MS	F-Value	p-Value
pH (L)	5643.471	1	5643.471	174.4325	0.000190
pH (S)	114.346	1	114.346	3.5343	0.133286
Current density (L)	0.021	1	0.021	0.0007	0.980701
Current density (Q)	103.199	1	103.199	3.1897	0.148644
Interaction	20.250	1	20.250	0.6259	0.473130
Error	129.413	4	32.353
Total sum of squares	5945.600	9

R² = 97%.

summarizes the ANOVA results for the Ni and Co contents in atomic percentage at.%, which are complementary in their contribution to the alloy’s composition..

The ANOVA results indicate that the p-value was less than 0.05 solely for the linear pH term, signifying that only this effect is statistically significant at the 95% confidence level. In this study, the determination coefficients (R²) were 97% for both Ni and Co, demonstrating that the quadratic model was highly significant in explaining the relationship between the effects and the response variables of Ni and Co contents. According to Ahmadi and [24], an R² value greater than 80% is required for the model fit to be considered significant and representative.

The statistical models are illustrated in Figure 1 as response surface plots. These plots reveal the synergistic effect of pH, within the range of 4 to 8, and current density, within the range of 40 to 80 mA/cm², on the composition of the alloy.

The response surface presented in Figure 1a demonstrates that experiments conducted in the alkaline pH range promote an increase in nickel content in the coating, whereas a reduction in nickel content is observed in experiments carried out in the acidic pH range. In contrast, the response surface for cobalt content, shown in Figure 1b, reveals that the highest atomic percentages are achieved in experiments performed in the acidic pH range, exhibiting an inverse trend compared to nickel. Additionally, the surfaces indicate that current density did not significantly influence the variation in chemical composition, consistent with the ANOVA results presented in Table 3.

Numerous studies have investigated the electrodeposition mechanism of Ni–Co alloys, and a widely accepted mechanism is represented by Equations (5)–(8), as outlined below. These equations provide a detailed depiction of the sequential electrochemical reactions involved in the deposition process, highlighting the interplay between nickel and cobalt ions in the electrolyte solution and their subsequent reduction to form the alloy. The proposed mechanism serves as a fundamental framework for understanding the anomalous co-deposition phenomenon often observed in Ni–Co systems, where the less noble cobalt ions are preferentially deposited over nickel. This theoretical representation aligns with findings from previous research and offers a comprehensive explanation for the compositional and morphological variations reported in Ni–Co alloy coatings [16,25,26].

$$2H_{2}O+2e^{-}\leftrightarrow H_2+2O{H}^{-}$$

(5)

$$M^{2+}+O{H}^{-}\leftrightarrow MO{H}^{+}$$

(6)

$$MO{H}^{+}\leftrightarrow {M{\left(OH\right)}^{+}}_{ads}$$

(7)

$${M{\left(OH\right)}^{+}}_{ads}+2e^{-}\leftrightarrow M+{OH}^{-}$$

(8)

In this context, M represents the Ni and Co atoms, and the hydroxide formed in Equation (4) facilitates the subsequent formation of MOH⁺, which further contributes to the adsorption of MOH⁺. According to Oliveira et al. [27], in acidic media, the reduction of cobalt is favored due to the increased presence of Co(OH)₂ at the electrode/electrolyte interface. This phenomenon arises from the competitive adsorption of hydroxides at the interface, which promotes the preferential reduction of cobalt as the less noble metal. Conversely, in alkaline pH conditions, the concentration of Co(OH)₂ decreases at the electrode/electrolyte interface, while Ni(OH)₂ increases, thereby favoring the reduction of nickel and its incorporation into the coating.

This behavior is clearly evident in experiments 1, 2, 5, 9, and 10, where cobalt is present in significant quantities in the alloy composition. Among these, experiment 5 exhibits the highest cobalt content, as the solution pH is situated in the strongly acidic range (pH 3.17).

To further analyze the effects of pH on the electrodeposition process, it is crucial to consider the formation of complex species, particularly given the use of reagents such as sodium citrate and ammonium sulfate. Figure 2 provides a diagram generated using the Hydra and Medusa software, version 1, illustrating the formation of these complex species as a function of the solution pH. This visualization offers a more comprehensive understanding of the chemical equilibria influencing the deposition mechanism.

In the acidic pH range (3–5), the primary species identified are MH₂Cit⁺, MHCit, and MCit⁻, whereas in the basic pH range (7–9), MCit⁻, NH₄(cit)₂⁻, and Ni(cit)₂⁴⁻ are identified, where M represents nickel (Ni) or cobalt (Co). The variation in the composition of complex species as a function of pH demonstrates the significant influence of this variable on the deposition mechanisms and, consequently, on the final composition of the coatings. The formation constant (Kf) of these complexes plays a pivotal role in this context. According to Deepatana and Valix [28], the species Ni(HCit) and Co(HCit) exhibit the same pKf value (3.19). However, the species Co(H₂Cit)⁺ has a lower pKf compared to Ni(H₂Cit)⁺, indicating a higher formation constant (Kf) for the cobalt complex. This enhanced stability of Co(H₂Cit)⁺ results in the reduced release of free Co²⁺ ions for deposition. Consequently, the higher proportion of cobalt observed in coatings formed in the acidic pH range can be attributed to cobalt’s ability to form more stable complexes under these conditions.

In basic pH, where the coatings exhibit a higher nickel content, the species Ni(cit)₂⁴⁻ and NH₄(cit)₂⁻ predominate. According to To et al. [29], most nickel–ammonia complexes form under alkaline conditions, which also favor the species NiCit⁻ and Ni(cit)₂⁴⁻. The findings of these authors indicate that increasing the concentration of NH₄⁺ reduces the fraction of NiCit⁻ and increases the fraction of Ni(cit)₂⁴⁻, which aligns with the results presented in the diagram of Figure 2. The lower Kf value of the Ni(cit)₂⁴⁻ complex facilitates the release of free Ni²⁺ ions for deposition [30]. Furthermore, the presence of NH₄(cit)₂⁻ indirectly benefits the nickel deposition process, acting as a buffering agent that stabilizes the solution’s pH and contributes to the stability of the electrolyte bath. Thus, ammonium incorporation plays a significant role in nickel deposition.

According to Golodnitsky, Rosenberg, and Ulus [25], the presence of citrate complexes reduces the incorporation of hydroxides into the deposits, enabling the production of coatings with low internal stress. Bigos et al. [31] also highlight citrate as a buffer that stabilizes the electrolyte pH and acts as a leveling agent, improving the quality of the coatings.

Thus, the effect of pH on deposition is not solely due to changes in the hydrogen potential but also to the formation and stability of specific citrate complexes with nickel and cobalt. In alkaline pH, the formation of complexes with ammonia and citrate and their respective Kf values favor nickel deposition, whereas in an acidic pH, more stable cobalt complexes promote the deposition of this element.

3.2. Surface Morphology

The morphological structure of the coatings was observed by SEM at a magnification of 3000× and is shown in Figure 3. It can be noted that, depending on the operating conditions of the electrodeposition system, the coatings exhibit different morphologies, predominantly circular structures of varying sizes.

Analyzing Figure 3a,b, which corresponds to the coatings obtained at pH 4, it can be observed that increasing the current density from 40 mA/cm² (Exp. 1) to 80 mA/cm² (Exp. 2) reduces the crystallite size and cobalt content. When the pH decreased to 3.17, along with a current density of 60 mA/cm² (Exp. 5), a smoother and more uniform surface was obtained (Figure 3d), which also exhibited the highest cobalt content (80%). This behavior aligns with the findings of Raveendran and Hegde [9], who reported that increasing the current density from 1.0 A/dm² to 4.0 A/dm² resulted in reduced grain size in surface morphologies.

The crystallite size was determined using the Scherrer equation, a widely recognized method for estimating the size of crystalline domains from X-ray diffraction (XRD) data, as described by Xinmei et al. [32]. This method considers the broadening of diffraction peaks due to the finite size of crystallites, using the relationship between the peak’s full width at half maximum (FWHM), the diffraction angle, and the wavelength of the X-ray radiation. The crystallite sizes obtained in this study ranged from 30 to 60 nanometers, indicating a nanocrystalline structure. This range reflects the influence of the deposition parameters, such as pH and current density, on the formation of the Ni–Co alloy. The results align with findings in the literature, where similar crystallite sizes have been reported for electrodeposited Ni–Co coatings under comparable conditions, highlighting the method’s effectiveness in producing nanoscale grains.

The coatings obtained in basic media (Experiments 3, 4, and 6) displayed high nickel content—93, 89, and 95 at.%, respectively. Despite having very similar chemical compositions, Figure 3c,d,f indicates morphological variations due to changes in pH and current density. The coating obtained under the conditions of Experiment 3 (Figure 3c) exhibits a cauliflower-shaped morphology with voids that disappear with increasing current density, as shown in Figure 3d (Exp. 4). According to Ghaferi et al. [33], these voids are associated with strong hydrogen evolution during the electrodeposition process. At the highest basic pH (8.83) and a current density of 60 mA/cm² (Figure 3f), the morphology displays larger nodules and a smoother surface. The cauliflower morphology in Ni–Co alloys was also reported by Radadi and Ibrahim [34] when evaluating the influence of current density.

It is noteworthy that Figure 3e,f, which corresponds to coatings obtained under the same current density (60 mA/cm²) but at different pH levels (3.17 for Exp. 5 and 8.83 for Exp. 6), exhibits entirely distinct morphologies and chemical compositions. In acidic pH, the coating displayed a higher cobalt content (81 at.%), with a smoother and more uniform surface, whereas in basic pH, it showed a higher nickel content (95 at.%) with a nodular surface. Furthermore, the coatings obtained in basic media exhibited inverse behavior to those in acidic media, as increasing the current density resulted in larger circular structures. Kamel et al. [35] reported similar behavior when evaluating the effect of current density on the morphology of Ni–Co alloys obtained in basic media (pH 10).

The morphology of coatings with nearly equiatomic compositions (Experiments 7, 8, 9, and 10) resembles needle-like structures with increasingly elongated crystallites oriented in entirely random directions. This observation was also reported by Lupi, Dell’Era, and Pasquali [36] for coatings with Co/Ni composition ratios of 1.56 and 1.87. It is worth noting that Experiments 7, 8, 9, and 10 were conducted under the same pH conditions (6). Thus, the main difference between Experiment 7 and Experiment 8 lies in crystallite size. The coating (Exp. 7) obtained under a lower current density (40 mA/cm²) exhibited smaller crystallites, whereas the coating (Exp. 8) obtained under a higher current density (80 mA/cm²) exhibited larger crystallites.

From the SEM images, it can be observed that variations in pH and current density significantly alter the morphology of the coatings.

3.3. XRD Analysis

The structure of the coatings was analyzed using X-ray diffraction (XRD), and the obtained diffractograms are presented in Figure 4. Due to the similarity in structures and the observation that current density did not influence the structure of the coatings, a single representative sample was chosen for each set of conditions. Coatings obtained under acidic pH conditions (Experiments 1, 2, and 5) are represented by Experiment 5. For coatings obtained under basic pH conditions (Experiments 3, 4, and 6), Experiment 6 was chosen as the representative. Coatings obtained at pH 6 (Experiments 7, 8, 9, and 10) are represented by Experiment 10.

According to Figure 4, the diffractograms displayed the typical structures of the Ni–Co alloy. Several studies [15,37,38] report that Ni–Co alloys can exhibit a mixed fcc (face-centered cubic) and hcp (hexagonal close-packed) structure. The main orientations observed for the alloy with an fcc structure are (111), (200), (220), (311), and (222), while the hcp structure presents orientations (002), (100), and (101). According to Ling and Wang [39], alloys with high cobalt content tend to exhibit hcp orientations in addition to fcc, meaning that with increasing cobalt content, the structure can transition from a purely fcc structure to a mixed fcc and hcp structure.

In this study, the predominant phase was the fcc phase, which is linked to higher nickel content in the chemical composition. However, the diffractogram of the coating obtained under the conditions of Exp. 5 (Ni-80Co), representing the coatings obtained in acidic media, exhibited both the fcc and hcp phases. According to the chemical composition data presented in Table 2, the coatings obtained in acidic media exhibited high cobalt content (59, 64, and 80 at.%) corresponding to Experiments 1, 2, and 5, respectively. This behavior aligns with the findings of Sharma et al. [38], who observed that Ni–Co alloys with cobalt content above 60% exhibited broader peaks and the presence of a mixed hcp + fcc phase.

3.4. Magnetic Properties of the Alloy

The magnetic properties of the coatings were analyzed in terms of saturation magnetization (MS). The magnetization versus applied field (M-H) curves recorded at room temperature are shown in Figure 5. The hysteresis loops clearly indicate that all samples exhibit soft magnetic behavior, regardless of composition, characterized by the narrow area within the hysteresis loop [40].

The coatings obtained in alkaline pH (represented by Experiment 6) exhibited lower saturation magnetization, corresponding to their lower cobalt content. Coatings obtained in acidic pH (represented by Experiment 5) showed higher saturation magnetization values, which is consistent with their higher cobalt content. This behavior is corroborated by findings in the literature. According to Karpuz et al. [41] and Özdemir et al. [42], saturation magnetization depends on the chemical composition of the Ni–Co alloy, with higher cobalt content resulting in higher saturation magnetization. According to Karimzadeh et al. [5], the saturation magnetization of the Ni–Co alloy is dependent on cobalt content because the magnetic dipole moment of cobalt is greater than that of nickel.

According to Santos et al. [43], besides chemical composition, morphology and crystalline structure significantly impact the magnetic properties of materials. Thus, it can be stated that the saturation magnetization of Experiment 10 (Ni-49Co) is similar to that of Experiment 5 (Ni-81Co) due to the refined grain morphology, especially when compared to Experiment 6 (Ni-5Co).

According to Ergeneman et al. [44], the presence of acicular (needle-like) grains indicates a coating with harder magnetic characteristics, as the increase in cobalt content tends to promote the formation of acicular morphologies. Conversely, films with nodular morphologies generally exhibit softer magnetic responses, as the presence of rounded grains is associated with deposits richer in nickel. This difference in morphologies directly reflects the magnetic performance of the analyzed coatings.

3.5. Corrosion Resistance Evaluation

The corrosion resistance of the coatings was evaluated using Potentiodynamic Polarization (PP) and Electrochemical Impedance Spectroscopy (EIS) techniques. To ensure uniform data presentation and considering that the current density did not result in significant changes, the same approach used for XRD and magnetic properties was adopted. The obtained data are graphically presented in Figure 6 and quantitatively in

Table 4. Electrochemical parameters extracted from the potentiodynamic polarization curves.

Exp.	pH	i (mA/cm2)	ECorr (V)	icorr (µA/cm2)	ba (mV/dec)	bc (mV/dec)
5	3.17 (−1.414)	60 (0)	−0.405	2.58	169	267
6	8.83 (+1.414)	60 (0)	−0.338	0.77	100	63
10	6 (0)	60 (0)	−0.418	3.89	334	247

and

Table 5. Parameters of the equivalent circuit used to adjust the EIS data.

Exp.	Rs (Ω.cm2)	CPE1 (μMho.sN)	n1	CPE2 (μMho.sN)	n2	Rp (R1+R2) (kΩ.cm2)	X2
5	47.8	24.0	0.86	4.17	0.94	22.05	0.02
6	44.6	18.1	0.91	11.5	0.52	74.70	0.005
10	45.7	38.4	0.87	2.70	0.90	29.59	0.001

.

The potentiodynamic polarization data presented in Figure 6a and the values extracted from the curves (Ecorr and Icorr), listed in Table 4, indicate that the coating obtained under the conditions of Experiment 6 (Ni-5Co) exhibited the highest corrosion resistance, being the coating with the highest nickel content. Conversely, the coating from Experiment 10 (Ni-49Co) demonstrated the lowest corrosion resistance, with the most negative E_Corr and the highest I_Corr. It was observed that increasing cobalt content in the alloy tends to make E_Corr more negative and I_Corr higher, a behavior also identified by Bakhit [45] and You et al. [46].

It is worth noting, however, that although Experiment 5 (Ni-81Co) represents the coating with the highest cobalt content, it is slightly more resistant than Experiment 10. This is likely due to its morphology. The coating in Experiment 5 presents finer grains compared to Experiment 10, which exhibits a morphology with needle-like structures. The refinement of grains in Experiment 5 may be associated with a slight improvement in corrosion resistance. According to Silva et al. [21], coatings with fine-grained morphologies exhibit greater corrosion resistance.

Additionally, the data obtained from the PP technique (E_Corr and I_Corr) were very close. Thus, it is recommended to consider the results provided by the EIS technique, which is widely recognized as more reliable due to its non-destructive nature and lower susceptibility to signal interference [47].

The EIS results, represented in the Nyquist diagrams (Figure 6b), show that the coatings exhibit almost semicircular capacitive arcs, a typical feature of corrosion processes controlled by charge transfer. For quantitative analysis, the experimental data were fitted using an equivalent electrical circuit of the R(Q(R(QR))) type, illustrated in Figure 6b. This model was also employed by Silva et al. [21] to describe similar spectra, reinforcing its suitability for analogous systems.

In this circuit, the elements are assigned as follows: Rs represents the solution resistance, R1 corresponds to the resistance associated with the coating, CPE1 is the constant phase element related to the capacitive behavior of the coating, R2 refers to the charge transfer resistance, and CPE2 describes the capacitive properties of the electric double layer. The choice of the constant phase element (CPE) instead of ideal capacitors was justified by the need to more accurately represent the heterogeneity and roughness of the coating surfaces, factors that significantly influence the electrochemical response. Simulations were conducted using Nova software (Metrohm, version 2.1.4), and the adjusted parameters are detailed in Table 4.

For diagrams simulated by this equivalent circuit, the sum of R1 and R2 is often used to express the corrosion resistance of materials, as described by Li et al. [48]. Based on the Nyquist diagrams and the data presented in Table 5, it was found that the coating with the highest corrosion resistance, identified by the largest capacitive arc and highest Rp value, was Experiment 6 (95 at.% nickel). On the other hand, the coating with the lowest anticorrosive performance, characterized by the smallest capacitive arc and lowest Rp value, was Experiment 5 (81 at.% cobalt). These results align with findings by Chen et al. [2], who reported that the cobalt content in coatings should be kept low to ensure superior corrosion resistance.

This observation underscores the importance of the chemical composition of coatings in modulating their anticorrosive properties, reinforcing the validity of the experimental parameters adopted in this study.

The solution resistance (Rs) showed minimal variation (44.6 to 47.8 Ω), confirming that the tests were conducted in a solution of the same concentration and under the same electrochemical cell configuration. The Rp (R1+R2) values indicate that the coating obtained under the conditions of Experiment 6 had a polarization resistance approximately three times higher than the coating obtained under the conditions of Experiment 5. These results emphasize the influence of deposition conditions on modulating the corrosion resistance of the evaluated coatings.

Relating these results to those found for magnetic properties, it can be stated that the nickel-rich coating (Exp. 6) exhibits the best anticorrosive performance and a low saturation magnetization. Meanwhile, the cobalt-rich coating (Exp. 5) presents the highest saturation magnetization and the poorest anticorrosive performance. Thus, these results highlight the uniqueness of this work and confirm what was reported by Bakhit and Akbari [3]: Ni–Co alloys rich in nickel exhibit good protective behavior, while cobalt-rich alloys display excellent magnetic properties.

4. Conclusions

This study investigated the synthesis of Ni–Co coatings by electrodeposition over a wide pH range, using a factorial design to analyze the involved variables. Although the influence of pH on the composition of coatings is well established, the novelty of this research lies in the use of sodium citrate as a complexing agent under alkaline conditions. It was observed that alkaline environments favor the formation of nickel complexes, resulting in coatings with higher nickel content, whereas acidic pH promotes cobalt deposition, as expected. However, the behavior of citrate under basic pH and its impact on Ni–Co deposition represents a novel contribution to the literature.

XRD analysis revealed that the face-centered cubic (fcc) phase predominated in coatings with high nickel content, while a mixed phase (fcc + hcp) was observed in coatings with higher cobalt concentrations (81 at.%). The different pH conditions also influenced the morphology of the coatings. In particular, Experiment 10 exhibited a needle-like morphology, associated with lower corrosion resistance in the potentiodynamic polarization tests. However, the electrochemical impedance spectroscopy results indicated that Experiment 5, which had the highest cobalt content (81 at.%), exhibited the lowest polarization resistance, suggesting a correlation between cobalt content and corrosion properties.

In terms of saturation magnetization, it was found that nickel-rich coatings, obtained under alkaline pH, exhibited lower saturation magnetization but better anticorrosive performance. In contrast, cobalt-rich coatings displayed higher magnetization but inferior anticorrosive performance.

This study demonstrates that by controlling pH conditions and employing sodium citrate, it is possible to synthesize Ni–Co coatings with optimized properties for specific applications, such as protective coatings or magnetic devices. The research presents an innovative approach to Ni–Co alloy synthesis and opens new perspectives for the development of functional coatings. Future studies may explore other complexing agents and investigate their effects on coating properties under industrial conditions.