Tuning Nanocrystalline Heterostructures for Enhanced Corrosion Resistance: A Study on Electrodeposited Ni Coatings

Abstract

Tailoring the microstructural heterogeneity of metallic coatings is a promising strategy for enhancing their corrosion resistance; however, its systematic optimization remains underexplored. Here in, we present a one-step, scalable electrodeposition strategy to fabricate Ni coatings with tunable nanocrystalline heterostructures on Cu substrates by varying the current density from 1 mA/cm² to 50 mA/cm². The coating with a current density of 10 mA/cm², featuring a heterogeneous nanograin structure of coexisting small and large grains, exhibited optimal corrosion resistance in 3.5 wt.% NaCl solution, with a low self-corrosion current density of 4.48 µA/cm². Electrochemical impedance spectroscopy (EIS) and molecular dynamics (MD) simulations revealed that the heterostructure dispersed Cl⁻ adsorption sites and promoted passivation. High-resolution transmission electron microscopy (HRTEM) revealed that as the current density increased from 10 mA/cm² to 50 mA/cm², the corrosion product transitioned from a crystalline NiOOH structure to an amorphous structure, which correlated with a reduced corrosion resistance. The heterogeneous microstructure enhances durability, offering a cost-effective and alloy-free alternative for offshore applications. These findings provide a theoretical and experimental basis for designing advanced corrosion-resistant coatings.

1. Introduction

Ni-based coatings have been widely utilized in engineering applications, particularly in offshore environments, owing to their corrosion resistance and mechanical reliability [1,2]. However, the increasing demands of modern engineering, such as extended service life, resistance to corrosion, and cost-effectiveness, present challenges that conventional Ni-based coatings may struggle to address fully. To overcome these shortcomings, an effective strategy is compositional plainification [3], a design philosophy that improves material performance by microstructural optimization alone [4], avoiding the complexity and cost of alloying elements [5]. Unlike alloyed coatings, such as high-entropy alloys, which depend on compositional diversity to enhance corrosion resistance at the cost of processing complexity and resource demands, compositional plainification uses controlled grain size refinement and structural heterogeneity to achieve superior properties [6,7,8]. Recent advances have highlighted nanostructured coatings with heterogeneous microstructures [9], which are promising alternatives. These heterostructured materials exhibit synergistic properties, including enhanced mechanical strength [10], oxidation resistance [11], and corrosion resistance [12]. These properties make them suitable candidates for next-generation coatings in harsh environments.

Electrodeposition is an advantageous technique for synthesizing nanostructured coatings. It provides a scalable, single-step process for customizing microstructural features, such as grain size and phase distribution [13,14]. Electrodeposition has been successfully applied to fabricate Ni-based coatings, including pure Ni and its binary to quinary alloys [15,16,17,18]. However, its potential for producing nanocrystalline heterostructures remains unexplored. Previous studies on electrodeposition provide a foundation for this work. Studies on electrodeposited Ni-based alloys have demonstrated that deposition parameters, such as current density, electrolyte composition, and temperature, influence grain refinement and coating properties [19,20,21,22]. Among these, our earlier investigation showed that tuning the current density is a straightforward and customizable approach to introducing heterostructures into electrodeposited coatings [16]. However, a systematic understanding of how these parameters control the formation of heterostructured coatings and their corrosion mechanisms remains limited. This gap implies the need for a comprehensive approach that integrates experimental synthesis with thorough characterization and corresponding simulations.

The approach in the current work yields Ni coatings with tunable microstructures designed to meet the rigorous demands of offshore engineering. To clarify the relationship between nanoscale architecture and corrosion resistance, the samples were characterized using a suite of experimental techniques, including X-ray diffraction (XRD), scanning electron microscopy (SEM), and electrochemical tests. The adsorption mechanisms governing the corrosion behavior were simulated using molecular dynamics (MD). The corrosion products were characterized using high-resolution transmission electron microscopy (HRTEM) to reveal the transitions in the corrosion mechanism. By linking the interplay between structural heterogeneity and corrosion resistance, this work provides actionable insights into the design of durable and cost-effective coatings. These findings offer a pathway for the development of durable coatings tailored for offshore energy and chemical engineering.

2. Materials and Methods

2.1. Electrodeposition Preparation

Pure Cu (≥99.9%) was selected as the substrate material. The substrates were machined into small pieces (15 mm × 15 mm × 1 mm), polished with SiC papers and diamond slurry, and etched in 10 vol. % sulfuric acid for 30 s, rinsed with ultrapure water, and dried with nitrogen gas. During this process, the surface was degreased by ultrasonic cleaning. Nanocrystalline Ni coatings were electrodeposited on pure copper using the direct current method at room temperature [16]. Each liter of the electrolytic bath solution comprised 76.1 mM NiSO₄, 466.4 mM C₆H₈O₇, 163.2 mM C₆H₅Na₃O₇, 1368.9 mM NaCl, 404.3 mM H₃BO₃, 7.3 mM C₇H₄NNaO₃S, and 0.3 mM C₁₂H₂₅SO₄Na (pH = 2.5). The electrolytic bath was prepared using ultrapure deionized water. A square platinum sheet electrode (1 cm × 1 cm) was used as the anode. The electrodeposition time was 30 min for each sample. During electrodeposition, different current densities were applied: 1 mA/cm², 5 mA/cm², 10 mA/cm², 20 mA/cm² and 50 mA/cm². Based on the differences between the pre-and post-deposition weights, the estimated thicknesses for current densities of 1 mA/cm², 5 mA/cm², 10 mA/cm², 20 mA/cm², and 50 mA/cm² were 0.3 μm, 1.9 μm, 3.7 μm, 6.2 μm, and 13.4 μm, respectively.

2.2. Material Characterization

The phase structure of the electrodeposited Ni coatings was identified using XRD (Ultima IV, Rigaku Corporation, Tokyo, Japan). Cu Kα radiation was used for the measurements. The operating voltage and current were 40 kV and 30 mA, respectively. The surface morphology was observed using field emission SEM (FE-SEM, Nova NanoSEM 450, FEI, Hillsboro, OR, USA) at an operating voltage of 15 kV. The electrochemical properties were evaluated using an electrochemical workstation (CHI-660E, Chenhua, Shanghai, China) with a standard three-electrode configuration. During the measurements, the test medium was a 3.5 wt.% NaCl solution at room temperature. The three-electrode system consisted of the prepared electrodeposited Ni coating as the working electrode, a saturated calomel electrode (SCE) as the reference electrode, and a platinum mesh electrode as the counter electrode. The exposed area was 1.0 cm². Before the electrochemical measurements, the open-circuit potential (OCP) of each sample was monitored for 10 min. After the OCP measurements, the potential stabilized for all samples, confirming a steady state for subsequent electrochemical impedance spectroscopy (EIS) and polarization tests. During the EIS test, the frequency was set within a range from 0.1 Hz to 10 MHz, the sinusoidal perturbation was 5 mV, and the spectroscopy data were obtained at 0 V. After the EIS tests, the data were fitted using the ZSimpWin 3.30d software. Potentiodynamic polarization tests were performed at a scanning rate of 5.0 mV/s to evaluate the uniform corrosion of the coating. The results were further analyzed using the Tafel extrapolation method. To understand the corrosion products, freestanding TEM samples were mechanically peeled off from the samples after polarization testing. HRTEM tests were carried out using scanning TEM (STEM, Talos F200X, Thermo Fisher Scientific, Waltham, MA, USA) at an operating voltage of 200 kV. Energy dispersive spectroscopy (EDS, X-Max, Oxford Instruments, High Wycombe, UK) was used to analyze element distribution.

2.3. Adsorption Simulation

MD simulations were performed using LAMMPS (version: 19 Nov. 2024, Development) [22] to study Cl⁻ adsorption on Ni surfaces by comparing the grain boundary and interior regions. A symmetric tilt {210}[001] grain boundary was constructed using Atomsk 0.13.1 [23] by rotating two [001] grains ±26.57°. Cl ions were placed above the Ni surface, with two at the grain boundary and five in the grain interior to maintain charge neutrality. For the qualitative simulation, the interactions were modeled via a hybrid potential combining the embedded atom method (EAM) for Ni–Ni interactions [24] and customized Lennard–Jones (LJ) potentials for Ni–Cl (ε = 0.0025382 eV, σ = 2.300 Å) and Cl–Cl (ε = 0.00986272 eV, σ = 3.516 Å) interactions, with long-range Coulombic forces truncated at 10.0 Å and calculated using the particle–particle–mesh (PPPM) method. The Ni substrate was fixed, and simulations were performed at 300 K for 50,000 steps, that is, 25 ps, using a Nosé–Hoover thermostat. The adsorption energies were calculated from the stabilized potential energy of Cl ions in each region, and the results were visualized using OVITO 3.11.3 [25].

3. Results

Figure 1 shows the XRD patterns of the as-prepared Ni coatings at various current densities from 1 mA/cm² to 50 mA/cm². All the samples have two face-centered cubic (FCC) structures, i.e., electrodeposited Ni and the Cu substrate. The characteristic Ni peaks correspond to the (111)_FCC, (200)_FCC, and (220)_FCC planes. Interestingly, these Ni peaks vary with the applied current density. This reflects the structural differences among the electrodeposited Ni coatings. At the lowest current density, the Ni peaks are broad and weak. This suggests the presence of fine grains in the samples. With increasing current density, the Ni peaks become relatively sharp and intense. This indicates a larger grain size in the sample. At current densities ranging from 1 mA/cm² to 20 mA/cm², the samples exhibit a preferred orientation along the (111) plane, corresponding to the plane with the lowest surface energy in FCC metals. At the largest current density of 50 mA/cm², the samples exhibit a strong (220) orientation in FCC crystal growth. This aligns with our previous work [16], which demonstrates that high current densities produce strongly oriented coatings. The shift from the close-packed (111) plane to the (220) plane increases the surface energy, contributing to reduced corrosion resistance. Cu diffraction peaks remain stable as the current density increases from 1 mA/cm² to 20 mA/cm², and the intensity decreases at a current density of 50 mA/cm², indicating a thicker Ni coating.

Figure 2 shows the surface morphologies of the electrodeposited Ni coatings obtained at different current densities from 1 mA/cm² to 50 mA/cm². The grain size increases with the current density, which is consistent with the XRD results. Notably, the grain size heterogeneity varies with the tuned current density. At the lowest current density, the grains are uniformly small. At a current density of 5 mA/cm², the grains become heterogeneous, combining small and large grains, suggesting a transition in the nucleation–growth balance. The heterogeneity peaks at a current density of 10 mA/cm², with small and large grains coexisting. At a current density of 20 mA/cm², heterogeneity remains, but larger grains dominate. At the largest current density of 50 mA/cm², the grains are overgrown, and the heterogeneity is almost lost. This corresponds to the texturing observed in the XRD results at the highest current density. This heterogeneity reflects the interplay between nucleation and growth during electrodeposition [26]. The peak heterogeneity in the sample with a current density of 10 mA/cm² arises from the balance between nucleation and growth. A sufficiently high deposition rate [27] promotes grain growth, preferentially enlarging some grains, while numerous active nucleation sites sustain smaller grains. At lower current densities, nucleation dominates growth. With further increases in the current density, the loss of heterogeneity indicates that growth gradually dominates.

Polarization curves were measured to evaluate the corrosion resistance of the Ni coatings in a 3.5 wt.% NaCl solution (Figure 3a). The self-corrosion current density, I_corr, and self-corrosion potential, E_corr, were fitted based on the polarization curves and are listed in Figure 3b. The samples exhibit different corrosion behaviors in 3.5 wt.% NaCl solution. The self-corrosion current density is the lowest in the sample with a current density of 10 mA/cm² (4.48 µA/cm²) and the highest in the sample with a current density of 1 mA/cm² (24.61 µA/cm²). The self-corrosion potential is most positive for the samples with current densities of 5 mA/cm², 10 mA/cm², and 20 mA/cm² (−0.38 V, −0.37 V, and −0.39 V, respectively). For the sample with the largest current density, the self-corrosion current density increases to 16.47 µA/cm², whereas the self-corrosion potential becomes most negative, i.e., −0.68 V. This indicates a reduced corrosion resistance and a change in the oxide kinetics. Beyond grain heterogeneity, the observed (220) texture contributes to this degradation, as its less close-packed structure increases the surface energy and enhances Cl⁻ adsorption compared to the (111)-oriented surfaces at lower current densities.

Figure 4 shows the EIS Nyquist plots of the electrodeposited Ni coatings with the fitting curves. The semicircles in the curves show the charge transfer and mass transport processes at the interface between the Ni coating and the 3.5 wt% NaCl solution. The Nyquist plots with current densities from 5 mA/cm² to 50 mA/cm² show much larger semicircles than those with a current density of 1 mA/cm². An equivalent circuit, R₁(Q₁R₂)(Q₂R₃) [28], was selected to fit the EIS data, where R₁ is the solution resistance, Q₁ and Q₂ are constant phase elements, and R₂ and R₃ represent the charge transfer (Faraday) processes. This circuit was chosen to capture the two-time constants observed in the Nyquist plots, consistent with the electrochemical behavior of the Ni coatings in 3.5 wt.% NaCl solution.

The calculated values of the parameters are listed in

Table 1. Calculated parameters of the equivalent circuits of the EIS results.

Current Density (mA/cm2)	R1 (Ω·cm2)	Q1-Y0 (Ω−1·cm2·Sn)	Q1-n	R2 (Ω·cm2)	Q2-Y0 (Ω−1·cm2·Sn)	Q2-n	R3 (Ω·cm2)
1	15.62	0.00193	1	23.6	0.00855	0.4228	36.96
5	15.85	8.716 × 10−4	0.7893	1107	2.995 × 10−5	0.9158	1410
10	34.54	2.641 × 10−5	0.905	4586	2.983 × 10−4	0.8799	2473
20	28.54	2.866 × 10−5	0.9312	3554	4.238 × 10−4	0.8431	1773
50	25.82	2.689 × 10−4	0.9792	3295	2.713 × 10−5	0.9502	7739

. R₂ is the charge-transfer resistance at the interface between the Ni coatings and the 3.5 wt% NaCl solution, while R₃ represents a deeper charge-transfer process. The maximum R₂ is 4586 Ω·cm² in the sample with a current density of 10 mA/cm². This indicates a strong charge-transfer resistance. The minimum value is 23.6 Ω·cm² in the sample with a current density of 1 mA/cm². This suggests an enhanced ion exchange across the reactive surface. Conversely, the maximum R₃ is 7739 Ω·cm² in the sample with a current density of 50 mA/cm². This implies an additional mechanism, possibly the formation of a thicker corrosion product layer during the secondary charge transfer. However, the lower R₂ and negative self-corrosion potential of the sample indicate that this layer offers limited corrosion protection.

The corrosion products of the electrodeposited Ni coatings prepared at a current density of 10 mA/cm² were characterized using TEM after Tafel polarization. In Figure 5a, the bright-field image shows the petal-like nanostructures. In Figure 5b,c, HAADF-EDS mapping reveals the presence of Ni. As shown in Figure 5d, the HRTEM results confirm the corrosion products. The fast Fourier transform (FFT) pattern inset is obtained from the area in the red box with Z = [001]_Ni. The inset of the inverse FFT (IFFT) results shows that the d-spacing of (011)_Ni is 0.26 nm. Interestingly, Figure 5e shows NiOOH at the periphery of Ni. Compared with Ni, NiOOH exhibits a larger d-spacing of 0.59 nm for the (0001) plane [29]. NiOOH facilitates the formation of a passivation layer [30,31]. In 3.5 wt.% NaCl medium, it provides initial passivation [32], reducing coating dissolution.

Figure 6 shows the TEM results of the corrosion products of the Ni coatings at a current density of 50 mA/cm². In Figure 6a, the bright-field image shows irregular, loosely packed nanostructures, which differ from the compact clusters of the sample with a current density of 10 mA/cm². The inset selected-area electron diffraction (SAED) patterns in Figure 6a display diffuse rings. This confirms the amorphous nature [33] of the corrosion products. Porous amorphous corrosion products reduce protective efficacy by facilitating Cl⁻ penetration [34], likely due to their higher permeability and reduced barrier properties. A crystalline oxide inner layer is typically covered by an outer NiOOH/Ni(OH)₂ layer at high potentials [35]. Under high current density, elevated overpotential, or prolonged exposure, the NiO layer grows rapidly and disorderedly due to irreversible oxidation reactions [35]. This disordered growth produces loosely packed structures, increasing susceptibility to Cl⁻ adsorption and subsequent corrosion. This property, coupled with hydration, remarkably increases the surface area exposed to corrosive species, promoting localized corrosion, such as pitting, and increasing corrosion rates, making it less protective than NiOOH. Thus, these amorphous corrosion products indicate a significant corrosion severity. This transition likely results from the elevated anodic overpotential for the sample with the largest current density, which accelerates Ni oxidation and promotes the rapid growth of a disordered structure [35], while the lower overpotential for the samples with lower current densities supports the formation of ordered NiOOH. Correspondingly, the sample exhibited reduced corrosion resistance, as indicated by a high I_corr of 16.47 µA/cm² and E_corr of −0.68 V. The highest R₃, 7739 Ω·cm², was also observed in the EIS results. This can be interpreted as resistance associated with a deeper, diffusion-limited process within the amorphous corrosion product, such as NiO·nH₂O. Figure 6b,c confirms the presence of Ni through HAADF-EDS mapping. In Figure 6d, no lattice fringes are observed in the HRTEM images. Corresponding to the red boxed region in Figure 6d, the FFT inset shows diffuse and broad rings. These results confirm the non-crystalline character, i.e., the lack of long-range order.

4. Discussion

The corrosion resistance of the Ni-coated electrode, which was electrodeposited in 3.5 wt.% NaCl solution is fundamentally related to their microstructure. In this study, the microstructure was customized by tuning the applied current density during electrodeposition (from 1 to 50 mA/cm²). The prepared heterostructures remarkably enhanced corrosion resistance by leveraging extended grain diversity to mitigate corrosion. MD simulations were carried out to understand the adsorption mechanism of Cl⁻ on the Ni surface in the earliest stage of corrosion (Figure 7). MD simulations revealed stronger Cl⁻ adsorption at the grain boundaries (−4.36 eV) than at the grain interiors (−1.74 eV), offering additional insights that complement the density functional theory (DFT) calculations [36]. The deeper energy well at the boundary (2.62 eV difference) shows enhanced electrostatic and van der Waals interactions [37], indicating that the grain boundaries are preferential sites for attack before passivation layers such as NiOOH or NiO·nH₂O are formed. Thus, grain boundaries serve as initial adsorption sites before passivation layers such as NiOOH or NiO·nH₂O. Furthermore, the texture evolution from (111) to (220) at 50 mA/cm² amplifies this degradation, highlighting the critical role of the grain structure and crystal orientation in governing the corrosion behavior.

In the sample with the smallest current density, the high density of nanoscale boundaries amplifies this weakness, as reflected in its electrochemical properties. In such nanocrystalline materials, triple junctions, i.e., points where three-grain boundaries intersect, exacerbate this effect below the critical grain size. Naturally, these junctions increase the defect density and provide pathways for corrosive species, such as Cl⁻, due to the segregation of impurities or alloying elements at the triple junction [38]. Additionally, the triple-junction diffusion coefficient for Ni (at d = 70 nm) exceeds that of the grain boundaries (3.5 × 10⁻⁹ m²/s vs. 2.8 × 10⁻²⁶ m²/s at 298 K) [39]. This enhances atomic mobility and potentially affects passive film formation during corrosion. The triple-junction volume fraction V_tj is estimated as follows [40],

$$V_{tj}=V_{ic}-V_{gb}$$

(1)

where V_ic and V_gb are the intercrystalline and grain boundary volume fractions, respectively [40],

$$V_{ic}=1-{\left({\displaystyle \frac{d-\delta }{d}}\right)}^3$$

(2)

$$V_{gb}={\displaystyle \frac{\left[3\delta {(d-\delta )}^2\right]}{d^3}}$$

(3)

where d is the grain size and where δ is the grain boundary thickness. This indicates a significant triple-junction network in the nanocrystalline material that can enhance atomic mobility compared to coarser structures [39]. In contrast, the prepared heterogeneous Ni coatings render this susceptibility a favorable mechanism. Compared to uniform nanograins, the mixture of small and large nanograins disperses Cl⁻ adsorption and mitigates localized corrosion, which may promote the formation of a NiOOH protective layer.

Synergy spans multiple scales in different heterogeneous materials, such as Fe–Cr–Ni alloys [41], AA 7075 Al alloys [42], and CoCrFeMnNi high-entropy alloys [43]. As corrosion progresses beyond initiation, nanoscale or ultrafine regions accelerate passivation kinetics, while microscale or coarse regions enhance durability and reduce oxidation and pitting susceptibility across diverse material systems and conditions [41,42,43]. The role of structural heterogeneity in corrosion resistance can be understood using a mechanical analogy. Similar to the way smaller nanograins impede dislocation motion to enhance strength, they also promote passivation. Larger grains provide structural stability during corrosion, similar to their role in maintaining ductility in heterogeneous materials [9]. This strategy exemplifies the potential of compositional plainification to achieve superior performance, such as corrosion resistance, through structural tuning rather than compositional complexity [3]. This approach optimizes material performance via tailored interface engineering with simpler compositions and pure elements. Traditional alloying, as seen in steel, superalloys, and high-entropy alloys strengthened with elements like Cr [44], often compromises manufacturing sustainability and complicates recycling [45]. As demonstrated with pure Ni coatings, compositional plainification enables stable nanoscale interfaces to enhance corrosion resistance without alloying. The heterogeneous structure can also enhance the mechanical properties, such as hardness, wear resistance, and fatigue resistance [9]. This work highlights the role of structural heterogeneity in corrosion properties, supporting a sustainable approach to high-performance coating design.

5. Conclusions

This study introduces a one-step, scalable electrodeposition approach to fabricate Ni coatings with tunable nanocrystalline heterostructures, and their corrosion resistance can be optimized by varying the current density from 1 to 50 mA/cm². The experimental results demonstrate that the heterostructure formed at 10 mA/cm², characterized by coexisting small and large grains, exhibits superior corrosion resistance in a 3.5 wt.% NaCl solution, with a self-corrosion current density as low as 4.48 µA/cm², outperforming other conditions. Molecular dynamics simulations revealed that stronger Cl⁻ adsorption at grain boundaries than at grain interiors initiates corrosion, whereas the heterostructure mitigates this effect by dispersing the adsorption sites. High-resolution transmission electron microscopy further revealed that as the current density increased from 10 mA/cm² to 50 mA/cm², the corrosion product shifted from a crystalline NiOOH structure to an amorphous structure, reducing the passivation efficacy. This structural optimization strategy avoids the complexity of alloying, leverages the heterogeneous microstructure of pure Ni to enhance performance, and offers a cost-effective and manufacturable coating option for offshore energy and chemical engineering. By integrating electrochemical testing, microstructural characterization, and simulations, this work elucidates the multiscale synergistic mechanisms underpinning enhanced corrosion resistance, providing clear guidance for designing high-performance metallic coatings.