Electrochemical Deposition and Corrosion Resistance Characterization of FeCoNiCr High-Entropy Alloy Coatings

Abstract

The corrosion resistance of FeCoNiCr high-entropy alloy deposits was investigated upon being prepared by current electrodeposition. The coatings were co-deposited in an electrolyte of an aqueous ferrous, cobalt, nickel, and chromium sulfates solution. Energy dispersive spectrometry analysis demonstrated that all four elements were co-deposited successfully. At the same time, the results from SEM indicate that the surface of the coating exhibits a granular morphology, with uniform density and no presence of cracks, with sizes ranging from 500 nm to 5 μm. Furthermore, X-ray diffraction patterns enunciated that the as-deposited coatings were amorphous. The polarization curves of the FeCoNiCr high-entropy alloy coating were measured by an electrochemical workstation in 3.5 wt.% NaCl, 1 mol·L⁻¹ H₂SO₄ and 1 mol·L⁻¹ NaOH solutions. The results revealed that the coating exhibited excellent corrosion resistance. The corrosion mechanism of the FeCoNiCr high-entropy alloy coating was analyzed in different environments. Moreover, the scratch testing method was employed to determine the alloy adhesion on the substrate, with higher values obtained for the FeCoNiCr alloy.

1. Introduction

High entropy alloys (HEAs) are a new class of metallic materials that have been widely studied in the scientific community in recent years. They are based on an innovative fabrication method. Unlike traditional alloys, these multicomponent materials have no predominant components and are composed of an isoatomic mixture of five or more elements. By classical metallurgical theory, it can be inferred that at the microstructural level, these materials are comprised of numerous intermetallic compounds. In contrast, due to the high mixing entropy of HEAs, they often exhibit the formation of simple solid solution structures, single-phase crystal structures, or even amorphous structures. Single-phase crystal structures are also very common. Not only the high-entropy effect, the high-entropy alloy exhibits slow diffusion and severe lattice distortion, significantly influencing its structure and properties. Therefore, HEAs have defined a research field that has attracted considerable attention in materials science and engineering [1,2,3,4].

In recent years, HEAs have been extensively studied, and numerous alloy systems with excellent hardness, strength, annealing softening resistance, corrosion resistance, wear resistance, and magnetoelectric properties have been developed. The structures of these alloy systems make them applicable to functional materials. A distinguishing feature of high entropy alloys is their remarkable capability to maintain their properties even under elevated temperatures [5,6,7,8,9].

The common practice for preparing HEAs involves the utilization of physical methods, with melting-casting techniques such as arc and induction furnace smelting being the prevailing choice [10,11]. In addition to melting-casting, mechanical alloying and rapid solidification are alternative synthetic techniques employed to produce bulk materials [12,13].Meanwhile, magnetron sputtering and laser cladding are employed to develop HEA coatings [14,15,16,17].

FeCoNiCr-type high-entropy alloys are less prone to phase transformation or the formation of precipitates. Enhancing their corrosion resistance poses a challenging task. The current general solutions include alloying and introducing new manufacturing techniques [18]. The superior corrosion resistance of FeCoNiCr MPEAs (Multi-Principal Element Alloys) is attributed to the inclusion of the Cr (Chromium) element [19]. However, Y. Qiu et al. [20]. found that FeCoNiCr MPEAs prepared using Spark Plasma Sintering (SPS) exhibited reduced corrosion resistance due to chromium segregation in the matrix. Nevertheless, high-entropy alloys mixed in the same molar ratio exhibit better corrosion resistance [21].

Electroless plating holds great potential in promoting the application of metal nanomaterials. However, due to its reliance on chemical reducing agents and more complex bath control, electroless plating is more costly compared to similar electrodeposition reactions. Additionally, the plating rate of electroless plating is relatively slower [22].

Electrodeposition, on the other hand, is an efficient and rapid process for producing coatings [23], and it is a low-cost method for fabricating high-entropy alloy thin films. This convenient method can also be applied to deposit coatings on substrates with complex geometries. Conventional metallurgical methods for fabricating alloys with large differences in melting points can be cost-intensive, whereas electrodeposition allows for codeposition at lower temperatures through the selection of appropriate solution systems, leading to energy savings [24]. However, due to the varying degrees of attraction between different ions and charges, the preparation process becomes challenging. Therefore, it is important to choose suitable complexing agents, appropriately adjust the metal ion concentrations, and optimize electrodeposition parameters [25].

Currently, non-aqueous solvents are preferred as the solvent of choice for metal and alloy electrodeposition. This is because of their excellent chemical and thermal stability, wide operating temperature range, wide electrochemical window, high conductivity, and their characteristic of not generating hydrogen or forming hydroxides [26,27,28]. However, non-aqueous solvents are hazardous, toxic, and flammable.

Tong et al. prepared BiFeCoNiMn, MgMnFeCoNiGd, and TmFeCoNiMn multi-component HEA coatings by electrodeposition in DMF (or dimethyl sulfoxide–DMSO) and CH₃CN. These alloys have potential applications as magnetic, photoelectric, thermoelectric, and fuel cell materials [29,30,31]. Guo et al. utilized electrodeposition to fabricate FeCoNiCr high-entropy alloy coatings and investigated the effects of current density, bath temperature, and pH on the coating’s properties [21]. To date, there is a need for more knowledge regarding the electrodeposition preparation of high-entropy alloys. Furthermore, the corrosion resistance and corrosion mechanisms of the FeCoNiCr high-entropy alloy coatings in different aqueous solutions have not been reported yet [19].

In this study, FeCoNiCr high-entropy alloy (HEA) coatings were prepared using electrochemical deposition in aqueous solutions, and their corrosion resistance and corrosion mechanisms were investigated. It was found that these coatings exhibit promising anti-corrosion properties, suggesting their potential for corrosion protection applications.

2. Materials and Methods

The electrodeposition of high-entropy alloy coatings and electrochemical studies were performed at 298 K using a CHI660E electrochemical workstation (ARTISAN, Shanghai, China). Plating uses Cu as cathode material, and the FeCoNiCr HEA coatings were deposited onto Cu substrates (12 mm × 12 mm × 1 mm) using current electrodeposition in an electrolyte based on an H₂SO₄ aqueous solvent system containing FeSO₄, CoSO₄, NiSO₄, and Cr₂(SO₄)₃. Citric acid and sodium citrate were used as complexing agents. The electrolyte’s composition are listed in

Table 1. Chemical composition of the electrolyte (g/L).

FeSO4	CoSO4	NiSO4	Cr2(SO4)3	H3BO3	KCl	C6H8O7	C6H5Na3O7
2	4	20	200	25	75	92	48

.

Electrodeposition was performed for 30 min at 25 A·dm⁻². The pH of the electrodeposition was 2.5. Prior to plating, the Cu substrates were polished with abrasive paper and etched in an H₂SO₄ (160 mL·L⁻¹) and H₂O₂ (80 mL·L⁻¹) solution, followed by rinsing with double-distilled water.

A three-electrode setup was employed in the electrochemical cell. A platinum foil (17 mm × 17 mm × 0.2 mm) was utilized as the counter electrode, while a standard saturated calomel electrode worked as the reference electrode (SCE, 0.241 V versus RHE). In this study, all potentials were measured versus the SCE.

The morphology and chemical composition of the FeCoNiCr HEA coating were examined using an HS-4800 field emission electron microscope (SEM, Hitachi, Ltd., Hitachi, Japan) equipped with an INC 250 spectrometer (EDS).

The crystal structure of the coatings was determined using X-ray diffractometry (XRD, D/MAX2500HB+/PC, Rigaku, Akishima, Tokyo, Japan) with Cu Kα radiation (λ = 0.154178 nm) at a scanning rate of 2°/min and a scanning step of 0.02°. Using a wire-cutting machine J19 (HZHF, HangZhou, China)., the alloy coatings were cut into 10 mm × 10 mm square pieces for XRD measurements. The thickness of the deposits was measured with a Filmetrics F20-UVX instrument (Filmetrics, Quezon City, Philippines).

The scratch adhesion testing method was employed to measure the adhesion of the FeCoNiCr HEA coatings electrodeposited on the Cu substrates [32]. A diamond indenter with a standard geometric shape is used to apply a constant force at a predetermined distance on the surface of the coating. The indenter has a standard Rockwell C geometry with a 120° angle and a radius of 200 μm. The determined critical load (F_NC) stands for the load at which complete removal of the film is achieved, and it is related to the actual adhesion and resistance to damage of the coating/substrate. The scratch test was conducted with a normal load force that increased from 0 to 100 N at a rate of 10 mm·min⁻¹ and a distance of 10 mm.

3. Results and Discussion

Figure 1 depicts the XRD patterns of the FeCoNiCr alloy coating. For the as-deposited layers, Cu (with a cubic structure Fm-3m) diffraction peaks present due to the deposition substrate were identified as indicated by peaks at 2θ = 50.447°, 74.123°, 89.933°. The FeCoNiCr alloy coating shows a broad diffraction peak near 2θ = 45°. This result suggests that the as-deposited alloys were amorphous.

Figure 2 displays the SEM image illustrating the morphology of the FeCoNiCr HEA coating deposited at 25 A·dm⁻² for 30 min. The deposits on the Cu substrate consisted of spherical particles with sizes ranging from ∼500 nm to 5 μm (Figure 2). The coating was homogenous and free of cracks.

The SEM micrograph at high magnification of the FeCoNiCr HEA coating and the elemental mapping of Fe, Co, Ni, Cr are illustrated in Figure 3 and

Table 2. The chemical compositions, molar entropies of mixing, and thickness of the FeCoNiCr HEA coating electrodeposited at 25 A·dm⁻² for 30 min.

Alloy	Fe	Co	Ni	Cr	ΔSm (J·K−1·mol−1)	Thickness (μm)
FeCoNiCr	23.84	28.41	24.69	23.06	11.50	5.20

, revealing the chemical composition of the coating and confirming the presence of Fe, Co, Ni and Cr elements in the deposits. The deposits on the Cu substrate are composed of spherical, the distribution is relatively uniform, and the surface is smooth and shiny without cracks and voids.

The molar entropies of mixing and thickness of the FeCoNiCr HEA coating deposited at 25 A·dm⁻² are also shown in Table 2. The Fe, Co, Ni, and Cr atomic percentages in the FeCoNiCr alloy were 23.84, 28.41, 24.69, and 23.06 at.%, respectively.

The usual expression for the mixed molar entropy of a solid solution is given by the following Equation (1):

$$\Delta \mathrm{S}\mathrm{m}=-\mathrm{R}{\displaystyle {\sum }_{\mathrm{i}=1}^{\mathrm{n}}\mathrm{C}\mathrm{i}}\ln \mathrm{C}\mathrm{i}$$

(1)

where Ci is the molar percentage of a component, n represents the number of elements that make up the solution, R is the gas constant (8.314 J·K⁻¹·mol⁻¹), and ${\displaystyle {\sum }_{\mathrm{i}=1}^{\mathrm{n}}\mathrm{C}\mathrm{i}}=1$. The molar entropies of mixing of the FeCoNiCr HEA coating reached 11.50 J·K⁻¹·mol⁻¹ at 25 A·dm⁻².

From the critical load, F_NC, values determined by the scratch test method, and displayed in

Table 3. The thickness and critical load of electrodeposited FeCoNiCr HEA coating at different current densities.

Alloy	DK (A·dm−2)	Thickness (μm)	FNC (N)
FeCoNiCr	15	2.23	21
	20	3.42	28
	25	5.20	35
	35	3.45	20

, it was observed that the current density (D_K) had a significant influence on the adhesion, with the maximum adhesion of 35 N being achieved at a D_K of 25A·dm⁻².

The polarization curve of the FeCoNiCr HEA coating in 3.5 wt.% NaCl solution, 1 mol·L⁻¹ H₂SO₄ and 1 mol·L⁻¹ NaOH are shown in Figure 4. The coating displayed active dissolution characteristics according to the polarization curve in Figure 4a. The corrosion current density was increased suddenly, indicating that the FeCoNiCr HEA coating was undergoing pitting corrosion in 3.5 wt.% NaCl solution. Meanwhile, the E_corr and i_corr obtained by epitaxy were −0.45 V and 1.73 × 10⁻⁶ A·cm⁻², respectively. The metal corrosion resistance is inversely related to E_corr while positively related to i_corr. This demonstrates that the metal corrosion resistance improves when the E_corr value is high. Concurrently, when the i_corr value is low, the corrosion resistance is low. The experimental results illustrated the E_corr of the FeCoNiCr HEA coating in 3.5 wt.% NaCl solution was higher than that of the CoCrFeNi high-entropy alloy coating [33] prepared by vacuum hot-pressing sintering (−1.08 V). However, the i_corr of FeCoNiCr HEA coating in 3.5 wt.% NaCl solution was lower than that of CoCrFeNi HEA coating (9.44 × 10⁻⁶ A·cm⁻²). Therefore, the FeCoNiCr HEA coating prepared by electrodeposition exhibited superior corrosion resistance.

The FeCoNiCr HEA coating exhibited an activation-passivation phenomenon according to the polarization curve in Figure 4b. Meanwhile, the E_corr and i_corr obtained by epitaxy were −0.33 V and 1.25 × 10⁻⁵ A·cm⁻², respectively. Experimental results have shown that the i_corr of FeCoNiCr HEA coating in a 1 mol·L⁻¹ H₂SO₄ solution is lower than that of FeCoNiCr HEA coating in a 0.5 mol·L⁻¹ H₂SO₄ solution [19]. Moreover, the icorr of the FeCoNiCr HEA coating in 1 mol·L⁻¹ H₂SO₄ solution was lower than that of the AlCoCrFeNiCu HEA coating (2.50 × 10⁻⁵ A·cm⁻²) [34] and 304 stainless steel (2.14 × 10⁻⁵ A·cm⁻²) [35]. Therefore, the FeCoNiCr HEA coating exhibited excellent corrosion resistance and a strong passivation tendency.

According to the polarization curve in Figure 4c, the FeCoNiCr HEA coating exhibited active dissolution characteristics. The E_corr and i_corr obtained by epitaxy were -0.74 V and 1.38 × 10⁻⁶ A·cm⁻², respectively. Furthermore, the i_corr of FeCoNiCr HEA coating in 1 mol·L⁻¹ NaOH solution was lower than that of Al0.3CoCrFeNi HEA (5.20 × 10⁻⁶ A·cm⁻²) [36], Zr₅₅Al₁₀Cu₃₀Ni₅ amorphous alloy (0.40 × 10⁻⁵ A·cm⁻²) [37], and AZ91D magnesium alloy (1.02 × 10⁻⁵ A·cm⁻²) [38].

In order to further demonstrate the corrosion resistance of FeCoNiCr high-entropy alloy coatings, the average corrosion rate (r_corr) can be calculated using the parameter i_corr obtained from the polarization curve, based on Faraday’s Law [39]. The Equation (2) calculation is as follows:

$$\mathrm{Corrosion} \mathrm{Rate}\left(\mathrm{m}\mathrm{m}/\mathrm{year}\right)=3.27\times 10^{-3}\times \frac{{\mathrm{i}}_{\mathrm{corr}}}{\rho }\times \mathrm{E}\mathrm{W}$$

(2)

where ρ is the density of the alloy (in g/cm³), i_corr (in µA/cm²) is the corrosion current density, and EW (the equivalent weight of the alloy) can be expressed as Equation (3):

$$\mathrm{EW}={\left({\displaystyle \sum \frac{{\mathrm{n}}_{\mathrm{i}}{\mathrm{f}}_{\mathrm{i}}}{{\mathrm{W}}_{\mathrm{i}}}}\right)}^{-1}$$

(3)

where n_i is the valence of the ith element of the alloy, f_i is the mass fraction of the ith element in the alloy, and W_i is the atomic weight of the ith element in the alloy.

The average corrosion rate r_corr of the FeCoNiCr HEA coating in 3.5 wt.% NaCl solution, 1 mol·L⁻¹ H₂SO₄ and 1 mol·L⁻¹ NaOH can be calculated by formulae (2), (3); they were 1.57 × 10⁻², 1.14 × 10⁻¹ and 1.26 × 10⁻² mm·year⁻¹, respectively.

The post-corrosion morphology of the FeCoNiCr HEA coating in 3.5 wt.% NaCl solution, 1 mol·L⁻¹ H₂SO₄ and 1 mol·L⁻¹ NaOH were show in Figure 5. The polarization curve results were consistent with the coating morphology after corrosion. Corrosion pits were observed on the surface of the FeCoNiCr HEA coating after corrosion in 3.5 wt.% NaCl solution, as illustrated in Figure 5a. According to the theory of oxide films [40], Cl⁻ has a high permeability, allowing it to penetrate and destroy the oxide film on the FeCoNiCr HEA coating surface. Cl⁻ is enriched in the area where the FeCoNiCr HEA coating surface is connected to the electrolyte. In addition, Cl⁻ readily forms complexes with the metal elements in the high-entropy alloy coating, destroying the oxide film on the coating surface. According to adsorption theory [41], the corrosion mechanism of the FeCoNiCr HEA coating in 3.5 wt.% NaCl solution can be expressed as follows: (1) M + Cl⁻ → (MCl⁻), (2) (MCl⁻) + H₂O → (MOH) + HCl + e⁻, (3) (MOH) + H₂O → (MOH)₂ + H⁺ + e⁻, and (4) M → Mn⁺ + ne⁻. When the FeCoNiCr HEA coating was exposed to the NaCl solution, the metal element (M) in the coating dissolved into the solution under the action of Cl⁻, causing pitting corrosion on the coating surface. Therefore, pitting corrosion induced by the co-action of oxygen depolarization and Cl⁻ was the failure mode of the FeCoNiCr HEA coating in 3.5 wt.% NaCl solution.

As illustrated in Figure 5b, there was no corrosion on the grain surface of the FeCoNiCr HEA coating. Corrosion was only limited to the grain boundary and cracks and took the form of relatively uniform corrosion pits. Therefore, the corrosion mechanism of the FeCoNiCr HEA coating in a 1 mol·L⁻¹ H₂SO₄ solution was uniform corrosion. The electrochemical corrosion process of the coating in 1 mol·L⁻¹ H₂SO₄ solution can be expressed as follows: 2H⁺ + 2e⁻ → H₂ and M → Mⁿ⁺ + ne⁻. Therefore, the FeCoNiCr HEA coating exhibited a passivation phenomenon in the H₂SO₄ solution, forming some stable compounds, such as MSO₄ and M₂O₃ [42], on the surface. These compounds had a dense structure, hence inhibiting the corrosion of the coating. Therefore, uniform corrosion caused by hydrogen depolarization was the failure mode of the FeCoNiCr HEA coating in a 1 mol·L⁻¹ H₂SO₄ solution.

Figure 5c depicts that the FeCoNiCr HEA coating surface after corrosion was relatively flat, with only a few corrosion pits evenly distributed. Therefore, the corrosion mechanism of the FeCoNiCr HEA coating in a 1 mol·L⁻¹ NaOH solution was uniform corrosion. The electrochemical corrosion process of the FeCoNiCr HEA coating in a 1 mol·L⁻¹ NaOH solution can be expressed as follows: 2M + O₂ + 2H₂O → 2M²⁺ + 4OH⁻ and M²⁺ + 4OH⁻ → MO₂²⁺ + 2H₂O. The metal element (M) on the surface of the FeCoNiCr HEA coating reacts with OH⁻ in a 1 mol·L⁻¹ NaOH solution to form NaMO₂ or Na₂MO₂ [43]. Therefore, the uniform corrosion caused by the oxygen absorption reaction was the failure mode of the FeCoNiCr HEA coating in 1 mol·L⁻¹ NaOH solution.

4. Conclusions

FeCoNiCr HEA coatings were prepared by electrochemical deposition at 25 A·dm⁻² in an aqueous ferrous, cobalt, nickel, and chromium sulfates solution. The EDS results illustrated that the four elements, Fe, Co, Ni, and Cr, were successfully deposited. Meanwhile, the results from SEM indicate that the surface of the coating exhibits a granular morphology, with uniform density and no presence of cracks, with sizes ranging from 500 nm to 5 μm. The XRD patterns demonstrated that the as-deposited coatings were amorphous. Furthermore, the scratch testing method was employed to determine the alloy adhesion on the substrate, yielding higher values of 35 N.

The E_corr, i_corr and r_corr of the FeCoNiCr HEA coating in 3.5 wt.% NaCl solution were −0.45 V, 1.73 × 10⁻⁶ A·cm⁻² and 1.57 × 10⁻² mm·year⁻¹, respectively. As a result, the corrosion mechanism was pitting caused by the co-action of oxygen depolarization and Cl⁻. The FeCoNiCr HEA coatings demonstrated superior corrosion resistance, outperforming some amorphous and five-element high-entropy alloys because of their higher E_corr and lower i_corr.

Moreover, the E_corr, i_corr and r_corr of the FeCoNiCr HEA coating in 1 mol·L⁻¹ H₂SO₄ solutions were −0.33 V,1.25 × 10⁻⁵ A·cm⁻² and 1.14 × 10⁻¹ mm·year⁻¹, respectively. Therefore, the corrosion mechanism was uniform corrosion caused by hydrogen depolarization.

Finally, the E_corr, i_corr and r_corr of FeCoNiCr HEA coating in 1 mol·L⁻¹ NaOH solution were −0.74 V, 1.38 × 10⁻⁶ A·cm⁻² and 1.26 × 10⁻² mm·year⁻¹, respectively. As a result, uniform corrosion caused by the oxygen absorption reaction was the corrosion mechanism. The i_corr was low in 1 mol·L⁻¹ H₂SO₄ acidic solution and 1 mol·L⁻¹ NaOH alkaline solution, implying improved corrosion resistance.

Consequently, the FeCoNiCr HEA coating prepared in this study has excellent corrosion resistance, which provides a theoretical basis and practical basis for its application in engineering practice. These results demonstrate that electrodeposition is a promising method for synthesizing FeCoNiCr HEAs.