Electrochemical and Electroless Deposition of High-Entropy Alloy Thin Films: A Review of Plating Conditions, Properties, and Applications

Abstract

High-entropy alloys (HEAs) represent a breakthrough class of materials characterized by a unique combination of properties derived from their multielement compositions. This review explores the current advancements in both electrochemical and electroless deposition techniques for synthesizing HEA thin films. This paper discusses the crucial plating conditions using aqueous or organic electrolytes and various current/potential modes that influence the formation, quality, and properties of these complex alloy coatings. Particular attention is given to their emerging applications in areas such as catalysis, protective coatings, microelectronics, and liquids’ separation. A comparison of electrochemical versus electroless methods reveals insights into the advantages and limitations of each technique for research and industrial use. This comprehensive review aims to guide further innovation in the development and application of HEA coatings.

1. Introduction

Electrodeposition and electroless deposition are among the most commonly used methods for fabricating metallic or composite coatings, porous layers, or nanostructures [1]. These solution-based techniques enable the controlled and scalable synthesis of single- or multicomponent materials under mild processing conditions, at temperatures below 100 °C and under atmospheric pressure, and without the need for complex or expensive equipment. Adjusting the process parameters and bath components allows for the composition, thickness, and morphology of the deposited layers to be modified, thus achieving structural arrangements and properties that are often unattainable by other manners [2]. Electrochemical procedures offer high deposition rates and fine control via the application of current or potential, using aqueous or non-aqueous electrolytes containing metal salts. Electroless deposition, on the other hand, enables uniform coating from aqueous solutions on complex-shaped substrates without the need for an external power source. Owing to these benefits, both techniques have been recently increasingly explored for the synthesis of polymetallic alloys, including high-entropy alloys (HEAs) [3,4] and medium-entropy alloys (MEAs) [5].

The concept of high-entropy alloys was introduced about twenty years ago [6]. It involved a novel approach to alloy design, based on combining multiple principal elements in equimolar or near-equimolar ratios, resulting in high configurational entropy (above 1.5R, where R is the gas constant), which lowers the free energy of the structure (Figure 1). This contrasts with the traditional alloy model, where one or two principal metals are combined with smaller amounts of alloying elements, and the formation of brittle intermetallic compounds is common when incorporating a wide variety of elements, whether metallic or nonmetallic.

High-entropy alloys are typically solid solutions with FCC or BCC structures, composed of at least five principal elements, each present in concentrations ranging from 5 to 35 at.%. However, second-generation HEAs may consist of at least four principal elements and form dual or complex phases in non-equimolar compositions [7]. Their high configurational entropy promotes the formation of simple phases, while sluggish diffusion slows down microstructural processes, and severe lattice distortion enhances both the chemical and mechanical properties [6,7]. The complex interactions among multiple elements give rise to the so-called “cocktail effect”, showing behavior that exceeds predictions based on the rule of mixtures. As a consequence, high-entropy alloys exhibit superior mechanical strength [8], corrosion resistance [9], magnetic properties [10], and catalytic activity [11,12] compared to conventional alloys. These advantages, together with virtually unlimited compositional flexibility, create new opportunities for the development of advanced functional materials for different applications through various synthesis routes [8,11,12,13,14,15].

While the first electrodeposited high-entropy alloys were reported in 2008 [16] and have since been explored in various multimetallic systems [3,4], the application of electroless plating has remained considerably more challenging, with the first successful attempts published in 2020 [17]. In light of these recent developments, this review aims to highlight the growing interest in the use of electrochemical methods for HEA synthesis, covering both electrodeposition and electroless deposition approaches. Particular emphasis is placed on the role of electrolyte composition and current/potential modes in tailoring the structure and morphology of HEA coatings, which ultimately determine their properties and technological potential. The latter is especially important, as thin films of HEAs can exhibit not only high-performance properties comparable to those of bulk high-entropy alloys but may even surpass them, providing a significant improvement to the surface properties of the substrate [15]. Given their ability to form multicomponent alloys as solid solutions in a single step, electrochemical deposition methods offer significant advantages and appear to be highly competitive for producing HEA thin films compared to techniques like magnetron sputtering, detonation spraying, laser cladding, or physical vapor deposition. Furthermore, low-temperature alloy deposition from metal salt solutions can avoid issues commonly associated with high-temperature processes, such as elemental segregation or unwanted phase transformations.

2. Materials and Methods

This review is based on an in-depth examination of publications related to the fabrication of high-entropy coatings using electrodeposition and electroless deposition techniques. Particular attention was given to studies published within the last three years, reflecting the most current advances in the field. The literature search was guided by keywords covering synthesis methods, coating performance, structural aspects, and functional applications. The source material included peer-reviewed journal papers and academic books. The main databases used for the search were Web of Science and Scopus, supported by the additional exploration of major scientific publishing platforms such as those of the American Chemical Society, IOPscience, Royal Society of Chemistry, Taylor and Francis, Elsevier, Springer, and Wiley. Each publication was assessed based on its scientific quality, originality, and relevance to the scope of this review. The selected studies provide the basis for the analysis and discussion presented in the subsequent sections.

3. Electrodeposition

The electrodeposition of alloys is a significantly more complex process compared to the deposition of individual metals. This complexity arises not only from the selection of electrolyte composition and pH but also from the current–potential conditions required to ensure the simultaneous reduction of metal ions [18]. Metal ions may be present in the bath as simple cations, complex ions, or oxyanions. The chemical form of the metal ions determines not only their reduction potentials but also the cathodic overpotentials of the individual components. By controlling the kinetic and thermodynamic parameters of metal ion reduction, alloy formation can proceed through different mechanisms, typically classified as normal (regular, irregular, or equilibrium) or abnormal (anomalous or induced) codeposition from aqueous baths [19]. In the case of normal deposition, more noble metals are reduced and deposited preferentially. In contrast, under abnormal conditions, less noble metals may deposit preferentially (anomalous deposition), or metals that cannot be electrodeposited individually from aqueous solutions may be reduced in the presence of other metal ions and incorporated into the alloy (induced codeposition). The mode of codeposition influences the final composition of the alloy and the relative proportion of its individual constituents (Figure 2).

Abnormal codeposition behavior appears to be the most commonly exploited phenomenon in the electrodeposition of high-entropy alloys based on iron-group metals (Fe, Ni, Co) [20,21,22,23,24,25,26,27,28,29,30,31,32,33,34,35,36,37,38,39,40,41,42,43,44,45,46,47,48,49,50,51,52,53,54,55,56,57]. Metals from this group not only tend to codeposit anomalously with other metals but they also play a key role in inducing the incorporation of elements that exist in the electrolyte as oxyanions, which are otherwise not reducible on the cathode from aqueous solutions on their own (W, V, Mo, Re) [21,22,25,26,27,28,29,33,34,35,53,54,57].

3.1. Electrolytes

The electrodeposition of high-entropy alloys is carried out from aqueous or organic electrolytes (

Table 1. Types of aqueous and organic baths used for electrodeposition of HEAs.

Bath Type	Al	Bi	Cr	Co	Cu	Fe	Mn	Mo *	Ni	P	Pd	Pt	Re *	Ti	W *	V *	Zn	Ref.
Aqueous Acidic Baths
sulfate–citrate–oxyanions *																		[22,34,35]
																		[53]
chloride–sulfate– citrate–oxyanions *																		[28,45,47]
																		[21]
																		[25]
																		[26]
chloride–sulfate–citrate																		[36,41,43,48]
																		[30,31]
																		[55]
																		[55]
sulfate–citrate																		[51]
																		[24]
chloride–citrate																		[52]
chloride–oxyanions *																		[27]
chloride																		[46]
																		[32,65]
Aqueous Alkaline Baths
sulfate–citrate–oxyanions *																		[33]
																		[44]
chloride–citrate–oxyanions *																		[54]
Organic Baths
chloride–DMF–AN																		[38,42,49,50]
																		[37]
																		[59]
																		[59]
chloride–nitrate(Bi)–DMF–AN																		[16]
chloride–nitrate(Co)–DMSO–AN																		[56]
chloride–glycerin																		[40]

* Metal as oxyanion in solution.

) with relatively low concentrations of metal ions, typically in the range of 0.01–0.1 M, usually in the form of chlorides and/or sulfates [16,21,22,23,24,25,26,27,28,29,30,31,32,33,34,35,36,37,38,39,40,41,42,43,44,45,46,47,48,49,50,51,52,53,54,55,56,57,58,59,60,61,62,63,64,65,66,67,68].

Acidic aqueous solutions are most commonly used; however, both acidic and alkaline baths may contain citrate ions, added either as sodium [29,35,36,43,45,48] or ammonium [26,44] salts, and sometimes as citric acid [26,29,51], to help align the deposition potentials of the constituent metals and to suppress the precipitation of hydroxides or basic salts in alkaline environments. Metals such as tungsten, molybdenum, vanadium, or rhenium are introduced as sodium or ammonium oxyanion salts. It is worth noting that acidic aqueous chloride [32,46] or mixed chloride–sulfate–citrate baths [55] have been used to successfully deposit HEAs containing 10–20 at% Al, despite the fact that aluminum typically does not undergo reduction at the cathode in aqueous media. Aqueous solutions often contain various additives [22,25,28,29,30,32,45,46,47,52,55,67,68], such as sodium hypophosphite NaH₂PO₂ and ascorbic acid C₆H₈O₆ to prevent the oxidation of Fe²⁺ to Fe³⁺, ammonium salts and/or boric acid H₃BO₃ to act as buffers, alkali metal chlorides (NaCl, KCl) to improve electrolyte conductivity, sulfanilic acid C₆H₇NSO₃ as an anti-stress agent, formic acid HCOOH, gelatin, or saccharin C₇H₅NSO₃ to refine grain structure, reduce cracking, and improve the morphology and adhesion of the coating, while sodium lauryl sulfate C₁₂H₂₅SO₄Na is commonly used as a surfactant to prevent pitting defects.

Due to variations in bath composition and the diversity of codeposited metals, it is not possible to clearly determine the influence of individual components on the quality and properties of the resulting alloys. Nevertheless, NiFeCoMoW alloys obtained from acidic solutions under galvanostatic conditions can serve as illustrative examples. Caroll et al. [22] obtained compact, crack-free amorphous deposits of Fe38Ni6Co31Mo14W11 composition with a cauliflower-like morphology from a sulfate–citrate–oxyanion bath (pH 5, 75 mA/cm²). In contrast, Zhang et al. [28] reported cracked deposits with a nearly similar composition Fe36Ni8Co23Mo13W20 obtained from a sulfate–chloride–citrate–oxyanion bath (pH 4.5, 80 mA/cm²). Despite the use of similar metal ion concentrations, bath pH, and current densities, the observed differences in deposit morphology can be attributed to the presence of different organic additives with anti-stress properties, namely saccharin [22] and sulfanilic acid [28]. The role of saccharin is particularly noteworthy, as it is widely employed in nickel plating both as a brightener and as an effective internal stress reducer [2]. Its effectiveness appears to exceed that of sulfanilic acid, as reflected in the improved structural integrity and reduced cracking of the resulting deposits.

Haché et al. [35] investigated the effect of WO₄²⁻ and MoO₄²⁻ concentrations in a sulfate–citrate bath on the composition of NiFeCoMoW alloys. They observed competitive behavior between these two metals, with increasing oxyanion concentrations leading to a decrease in current efficiency (from 42 to 45% to 20 to 25%), an effect more pronounced in the case of molybdenum species. These coatings exhibited banded structures containing nanoglass boundaries that extended throughout the bulk and were elongated in the direction of growth [35,66]. Further studies [22] showed that increasing the pH from 5 to 7 led to only a slight increase in the Mo (from 14 to 16 at%) and W (from 11 to 12 at%) contents in the alloys, accompanied by a drastic decrease in the cobalt content (from 31 to 15 at%). The concentrations of iron (30–46 at%) and nickel (6–16 at%) were more variable, although both elements appeared to vary inversely within the coating.

Ratajczyk and Donten [33] systematically analyzed the codeposition of FeCoNiCuW alloys from alkaline sulfate–citrate–oxyanion baths under galvanostatic conditions. In an unusual approach, they used ferric instead of ferrous salt to improve the stability of iron ions against oxidation. However, this solution may have resulted in very low current efficiencies for alloy deposition, reaching only about 8%. Compact alloy with similar element concentrations Fe20Co19Ni17Cu19W24 was obtained. It was observed that the presence of copper ions in the bath catalyzed the deposition of the other metals, particularly tungsten and iron. However, the codeposition mechanism was not explained.

Ju et al. [52] investigated the role of citrate ions (added as sodium salt and citric acid) and the cationic surfactant cetyltrimethylammonium bromide CTAB on the potentiostatic electrodeposition of FeNiCoCrMn alloys from a chloride-based electrolyte (pH 3.5). Single-phase alloys with a face-centered cubic structure were obtained, but their elemental composition did not show a clear dependence on the presence of additives or the applied potential. Nevertheless, it was observed that both additives influenced the surface texture of the deposits: CTAB promoted a rough and irregular surface, while the presence of citrate resulted in smooth and uniform crystalline films.

It is important to highlight a key issue concerning the composition of alloys obtained from solutions containing oxyanions. Most authors report the composition of the resulting alloys in terms of their metal content. However, these alloys may also contain oxygen, with reported levels reaching from about 4 at% up to 22 at% in the case of NiCoCuMoW alloys [57,58]. The origin of this oxygen is not clearly explained; it may result from secondary surface oxidation, or it is likely related to the reduction mechanism of oxyanion precursors. Nevertheless, the presence of oxygen attributed to metal oxides [29,30,55] is observed in coatings containing elements such as molybdenum, tungsten, and chromium.

An interesting behavior of the FeCoNiCuZn alloy in contact with the electrolyte (sulfate–chloride–citrate) was reported by Reddy et al. [41]. After the electrodeposition was completed, the alloy was left in the bath for varying periods of time (up to 60 min). A gradual increase in copper content was observed, rising from 13 at% to 58 at%, while the concentrations of the remaining metals in the alloy progressively decreased in a similar manner. This compositional shift led to a change in the phase structure, introducing metallic copper into the structure of the high-entropy alloy. These observations highlight the necessity of the immediate removal of the coatings from the plating bath to prevent unfavorable secondary reactions.

The application of non-aqueous systems opens up new possibilities for the formation of multicomponent alloys, as they eliminate hydrogen evolution that typically occurs in aqueous solutions. This modifies plating conditions and broadens the electrochemical window, allowing for the codeposition of metals with significantly different reduction potentials [37]. Organic electrolytes are typically based on aprotic solvents such as dimethylformamide (DMF) and acetonitrile (AN) [16,37,38,42,49,50], which promote uniform coating deposition and readily dissolve metal salts added in the form of chlorides. To increase the conductivity of such electrolytes, lithium perchlorate (LiClO₄) is added. Notably, this type of electrolyte was used for the first electrodeposition of the high-entropy alloy BiFeCoNiMn [16]. Alternative electrolytes include mixtures of dimethylsulfoxide (DMSO) and acetonitrile [56], as well as a novel bath based on glycerin [40], both used as solvents for metal chlorides.

The codeposition of FeNiCoCrMn alloys has been reported using both aqueous and non-aqueous electrolytes, although direct comparison of the results is difficult due to differences in deposition modes. Nevertheless, clear differences in alloy morphology have been observed: from compact but rough surfaces obtained from an aqueous chloride–sulfate–citrate bath (pH 1.5, galvanostatic deposition) [55] to coatings consisting of globular units with inter-globular porosity deposited from a chloride–DMF–AN bath under potentiostatic conditions [38], and finally to smooth morphologies with surface crystal clusters obtained using the same type of organic bath under pulse potential deposition [49]. Notably, changes in the plating conditions preserved the same crystal lattice of the alloy (FCC) [49,55], although HEAs exhibited different proportions of particular elements Fe23Ni21Co20Cr16Mn20 [55], Fe(37–41)Ni(4–10)Co(27–29)Cr(24–27)Mn(0.1–0.6) [38], and Fe(24–31)Ni(6–8)Co(18–22)Cr(7–10)Mn(35–41) [49]. The replacement of DMF with DMSO [38,56] in the organic solvent mixture can significantly modify both the composition and morphology of the alloy, although it cannot be excluded that the type of substrate, such as aluminum [38] or copper [56], may also influence nucleation and the subsequent growth of the alloy under potentiostatic conditions (Figure 3). Notably, the composition of the alloys was found to be time-dependent (30–90 min), with improved incorporation of manganese and nickel into the FeNiCoCrMn alloy over longer deposition times [38,56].

3.2. Current/Potential Regimes

Control over the elemental and phase composition of the alloys can be achieved by adjusting current density, electrolysis voltage, or cathode potential, not only in terms of their absolute values but also the mode of their application like direct or pulsed (Figure 4).

Galvanostatic deposition (GD, or direct current deposition, DC) is the most common and simplest mode of high-entropy alloy codeposition [21,22,23,24,25,26,27,28,29,30,31,32,33,34,35,36,51]. It maintains a constant current density throughout the deposition. This causes continuous metal ion consumption at the cathode, which can lead to concentration polarization and more intense hydrogen coevolution. Consequently, the method offers limited control over nucleation and crystal growth, often resulting in compositional variations through the coating thickness, non-uniform grain sizes, and uneven current distribution across the cathode surface (edge effect). Potentiostatic deposition (PD), in turn, involves maintaining a constant electrode potential versus a reference electrode throughout the deposition process. This approach allows for more accurate control over the thermodynamic conditions of metal ion reduction, which can result in improved uniformity in both HEA composition and microstructure [16,37,38,39,40,52,53,59]. Electrodeposition at constant electrolysis voltage (CV) is a much less common method used in HEA codeposition [41]. It involves maintaining a fixed voltage between the working and counter electrodes during the process. Unlike galvanostatic deposition, this method can result in fluctuations in current density with time, potentially leading to less reproducible alloy compositions and less controlled microstructures. Pulsed electrodeposition offers advantages by modulating nucleation and growth kinetics, ensuring better metal ion replenishment at the cathode surface and suppressing unwanted side reactions. This provides a powerful tool for tailoring the microstructure, morphology, and uniformity of the resulting alloy coatings. Pulsed electrodeposition can be carried out either by switching the cathode potential between two different values of PP [42,43] or by rapidly alternating the current PC [44,45,46,47,48,49,50,54,67,68] between a set value (“on cycle”) and zero (“off cycle”), each time maintaining a fixed potential or current density for a defined duration.

Zhu et al. [44] compared two electrodeposition modes (DC, PC) for CoNiWReP alloys produced from aqueous sulfate–citrate baths with tungsten and rhenium as oxyanion-type salts and phosphorus as hypophosphite. All HEA coatings were silver-white, amorphous, and adhered well to a copper substrate. Coatings deposited by direct current showed cracks, likely due to internal stress or hydrogen embrittlement. In contrast, pulsed current improved coating quality, reducing or eliminating cracks. Moreover, pulse current plating enhanced the codeposition of cobalt and tungsten while somewhat reducing the codeposition of nickel and rhenium compared to direct current (DC: Co(28–29)Ni(24–25)W(7–8)Re(31–32)P(8), PC: Co(32–37)Ni(17–23)W(10)Re(27)P(7–9)).

Yoosefan et al. [49] investigated the effect of potential pulses (“on” duty cycle, frequency) on the codeposition of a CoCrFeMnNi alloy from an organic chloride bath. Using switchable potentials (−9 V/1 V), they obtained single-phase (FCC) HEAs with inhomogeneous morphology and surface crystal clusters. The applied potential conditions did not show a consistent trend in alloy composition changes. Nevertheless, increasing the duty cycle (from 50% to 60%) led to a reduction in crystallite size, while the effect of changing frequency (2.5 kHz or 5.0 kHz) depended on the duty cycle.

Dehestani et al. [45] applied pulse electrodeposition at different current densities (20–100 mA/cm², 50% duty cycle) and used a slightly acidic chloride–sulfate–citrate–oxyanion bath. They observed that increasing the current density did not affect the order of element contents in the FeCoNiMoW alloys, although the individual concentrations of the metals changed in three distinct ways. These trends included the following: (i) increased contents of Fe and Co, (ii) a slight increase followed by stabilization in Ni content, and (iii) decreased contents of Mo and W. Such changes were attributed to the formation of ternary complexes between iron-group metals, tungsten or molybdenum, and citrate. Since the reduction of tungsten or molybdenum appeared as diffusion-controlled at higher current densities, and due to the difficulty of transporting these heavy ternary complexes to the cathode surface, their incorporation into the coating decreased as the current density increased. With increasing current density, the initially less compact, dome-shaped deposits transformed into a very rough and coarse surface with a characteristic cauliflower-like morphology. Simultaneously, microcracks observed in the cross-sections of FeCoNiMoW alloys were attributed to differences in the atomic radii of the constituent metals and/or hydrogen coevolution during deposition. In turn, Zhang et al. [28] deposited the same alloys from a similar bath type but using a direct current. They found that increasing the current density (20–80 A/cm²) did not affect the order of the element content in the FeCoNiMoW alloys, similar to the case of pulse currents, but the individual element contents changed in different ways: (i) a decrease in Fe content, (ii) stable Co content, (iii) an increase in Ni content, and (iv) an increase in followed by stabilization of W and Mo contents. The authors stated that iron deposition was controlled by ion diffusion and concentration polarization, which was inhibited at higher current densities. In contrast, the deposition of nickel, molybdenum, and tungsten was governed by electrochemical polarization, which was enhanced as the current density increased, leading to better incorporation of these elements into the coating. These studies show that pulsed current favors the incorporation of Fe and Co, while direct current promotes the deposition of Ni, Mo, and W in FeCoNiMoW alloys due to different rate-limiting steps in metal reduction from citrate-type baths. Notably, although the coatings were homogeneous, surface cracks appeared and their density increased with rising current density, a phenomenon attributed to hydrogen embrittlement.

Soare et al. [59] investigated the effect of deposition potential (–1.5 V to –2.7 V) on the codeposition of AlCrFeMnNi and AlCrFeMnNiCu alloys from organic chloride baths (DMF–AN). They found that more negative potentials only slightly affected the incorporation of aluminum, chromium, iron, and manganese (by about 1 at%) but significantly altered the nickel and copper contents (by about 10 at%) in variable ways. All of the deposited alloys were amorphous, exhibiting spherical surface growths and increasing cracking at more negative deposition potentials.

Nagy et al. [68] investigated the codeposition of CoFeNiZn alloys from a chloride–sulfate bath using a pulsed current (PC) mode and a horizontally positioned cathode with an unconventional pentagonal geometry (Figure 5a). This setup enabled the formation of gradient coatings due to uneven mass transport along the cathode surface. The increased concentration of zinc ions in the plating baths resulted in faster transport toward the apex of the cathode compared to the bottom area. Consequently, the zinc content in the deposit varied along the substrate length from the base to the sharp corner, leading to changes in both the composition and morphology of the alloy (Figure 5b). Interestingly, as the zinc concentration in the coating increased (from 18 to 44 at%), the cobalt content and the combined concentrations of Fe and Ni decreased linearly. Regardless of the alloy’s composition, single-phase FCC coatings were obtained.

The type of current and applied potential strongly affect the codeposition behavior of HEAs, influencing alloy composition, structure, and surface features. Different metals show varying incorporation tendencies depending on both the electrolyte composition and plating conditions. However, due to the diversity of alloy systems and bath chemistries used across studies, no general trend can be clearly defined.

3.3. Properties Relevant to Applications

3.3.1. Corrosion Resistance

High-entropy alloys often exhibit excellent corrosion resistance, which is attributed to their unique multicomponent composition and stable solid solution structures. Although their corrosion behavior depends strongly on alloy composition and post-processing conditions (

Table 2. Corrosion properties of some high-entropy alloys in 3.5% NaCl (except as indicated).

Alloy	Structure	icorr, μA/cm2	Ecorr, V	Ref.
Fe7Ni50W17Mo27Mn0.1 Fe7Ni48W17Mo27Mn0.1 (annealed 200 °C)	FCC	40	−0.40	[26]
	FCC	17	−0.16
Fe(36–47)Co(23–25)Ni(3–8)Mo(7–13)W(16–20)	–	92	−0.95	[28]
Fe18Co18Ni16Mo17W31	FCC	4.4	−0.83	[27]
Fe22Co13Ni8Mo12W22Cr21	FCC	3.5	−0.34
Fe32Co24Ni27W7Cr10	FCC	2.9	−0.65	[29]
Fe18Co19Ni21Cr42	FCC	1.4	−0.32	[30]
Fe24Co28Ni25Cr23	amorphous	1.7	−0.45	[31]
		12	−0.33 1
		1.4	−0.74 2
Fe22Co19Ni30Cr29	FCC	33	−0.51	[55]
Fe23Co20Ni21Cr15Mn20	FCC	58	−0.40
Fe23Co15Ni26Cr14Al20	FCC	18	−0.39
Fe24Co26Ni14Cr9Mn27	no data	3.3	−0.23	[56]
Fe41Co27Ni4Cr27Mn0.3	no data	1.1	−0.53	[38]
Fe37Co27Ni10Cr25Mn0.6	no data	8.5	−0.75
Al15Fe26Cr21Ni27Ti14	FCC+BCC	0.09	−0.77	[46]

¹ In 1M H₂SO₄. ² In 1M NaOH.

), HEAs are considered promising materials for use in marine environments due to the formation of stable passive layers [26,27].

Divya et al. [26] compared the corrosion properties of FeNiWMoMn alloys electrodeposited galvanostatically (10 mA/cm²) from a sulfate–citrate bath. They observed that extending the electrolysis time (30–90 min) caused a slight decrease in iron content (by 4 at%) and a corresponding increase in nickel content (by 5 at%), which resulted in only a minor increase in corrosion current density and a shift in corrosion potential. Annealing the alloy (200 °C) reduced the corrosion current density by half, which was attributed to microstructural refinement and a reduction in defect density, both contributing to the formation of a more protective passive layer.

Zhang et al. [28] analyzed FeCoNiMoW alloys (DC, chloride–sulfate–citrate bath) deposited at different current densities. Although the alloy composition varied slightly and the resulting coatings exhibited some cracking, the corrosion current densities were significantly reduced compared to the substrate (304 stainless steel) by 32–67%. In turn, Mundotiya et al. [27] investigated the effect of chromium incorporation into FeCoNiMoW alloy (DC, 40 mA/cm², chloride bath). This modification led to a significant positive shift in corrosion potential (by 0.5 V), while the corrosion current density remained nearly unchanged. Both HEAs markedly improved the corrosion resistance of the multiphase nickel–aluminum bronze substrate, enhancing it by more than 36%. On the other hand, Dong et al. [29] showed that the incorporation of tungsten (about 7 at%) into the chromium-containing FeCoNiW alloy did not significantly improve corrosion resistance, but it increased the coating’s microhardness by about 180 HV (up to 675 HV) and enhanced its wear resistance.

Ju et al. [30] reported that the FeCoNiCr alloys (DC, sulfate–citrate bath) with the highest chromium content (42 at%) and with other metals at nearly constant levels exhibited the best corrosion resistance. Chromium played a key role not only in enhancing the corrosion resistance but also in increasing the hardness of the coatings, while the friction coefficient and wear rates remained relatively unchanged across all alloy compositions. In turn, Xu et al. [31] compared the corrosion behavior of FeCoNiCr alloys in different media and found that the corrosion resistance followed the order 1 M H₂SO₄ (0.11 mm/y) < 3.5% NaCl (0.016 mm/y) < 1 M NaOH (0.013 mm/y). Rong et al. [55] further showed that the incorporation of aluminum, but not manganese, into the FeCoNiCr alloy (DC, sulfate–chloride–citrate bath) reduced corrosion current density and shifted the corrosion potential in a more positive direction. However, both elements increased the microhardness of the coating by approximately 50–60 HV. Popescu et al. [38,56] obtained FeCoNiCrMn alloys from organic baths on copper (PD, DMSO–AN) and aluminum (PD, DMF–AN and DMSO–AN). The alloys exhibited low corrosion current densities, lower than those of the substrates; however, defects in the coating films produced in the DMSO–AN bath resulted in no effective protection of the aluminum substrate.

Notably, an unusually low corrosion current density (on the order of 10⁻¹ μA/cm²) was recorded for the AlFeCrNiTi alloy deposited galvanostatically from an aqueous bath [46]. The high corrosion resistance was attributed to the combination of elements with a strong tendency to form compact passive films (mainly Al, Ti, and Cr), along with a uniform, homogeneous microstructure and minimal coating defects.

3.3.2. Catalytic Activity

High-entropy alloys have recently gained attention as promising electrocatalysts due to their unique surface chemistry [23,37,42], multielement synergies [23,37,42,53], and structural stability [22,23,24,25,51]. Their catalytic activity has been explored in various electrochemical reactions, particularly in water splitting processes such as the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) (

Table 3. Catalytic activity of some high-entropy alloys—overpotentials η for HER and OER at given current density i in acid ¹ or alkaline ² solutions.

Alloy	Structure	ηHER, mV/i, mA/cm2	ηOER, mV/i, mA/cm2	Ref.
FeCoCrNi	amorphous	–	295/10 2	[24]
NiFeCoMnSn	amorphous	4/10 2	–	[23]
FeMnCuCo	FCC+BCC	–	226/10 2	[60]
FeCoNiCuMn	FCC	200/100 2	251/100 2	[51]
FeNiCoCrMn	FCC	168/10 2	231/10 2	[42]
FeNiCoCrMn	FCC	191/10 2	–	[52]
NiFeCoMnV	FCC	7/10 2	–	[25]
FeCoNiMnW	FCC	15/10 1	512/10 1	[54]
NiFeCuCoW	FCC	–	247/10 2	[21]
NiFeCoMoW	amorphous	171/10 2	–	[22]
CuMoNiCoFe	no data	12/10 2	290/10 2	[53]
NiFeCuCoCe	FCC	–	219/10 2	[61]
PtPdNiCoMn	FCC	23/10 2	–	[37]

¹ In 0.5M H₂SO₄. ² In 1M KOH.

). Effective catalysts for such reactions require not only high activity (low charge transfer overpotential) but also good electrical conductivity and chemical stability (corrosion resistance) under reaction conditions. Additionally, a high surface area with abundant active sites is crucial to enhance catalytic performance by facilitating efficient reactant access and product release.

The largest group of high-entropy systems investigated for water splitting is represented by non-noble metal alloys, including FeCoCrNi [24], NiFeCoMnSn [23], FeMnCuCo [60], FeCoNiCuMn [51], FeNiCoCrMn [42,52], NiFeCoMnV [25], FeCoNiMnW [54], NiFeCuCoW [21], CuMoNiCoFe [53], and NiFeCuCoCe [61]. To achieve a high reaction surface area, these alloys are often deposited on porous substrates such as nickel [23,42,61] and copper [52] foams, or their surfaces are purposely structured during electrodeposition to form porous morphologies [25,51].

Park et al. [53] investigated the effect of individual elements on the catalytic activity of CuMoNiCoFe alloys by synthesizing (PD, sulfate–citrate–oxyanion bath) several dozen ternary and quaternary compositions. They found that for the hydrogen evolution reaction in alkaline solution, the overpotential decreased in the order Mo > Ni > Co > Cu, indicating that molybdenum was the most beneficial element for enhancing catalytic activity. This improvement was attributed to molybdenum’s ability to modulate the binding energies of hydrogen and hydroxyl groups, thereby accelerating water dissociation. In contrast, for the oxygen evolution reaction (OER), the overpotential decreased in the order Ni > Fe > Mo > Cu, highlighting nickel as the key contributor. No correlation was observed between HER or OER overpotentials and the mixing enthalpy of the alloys, although catalysts with the lowest ΔS_mix of 1.59R exhibited the highest bifunctional activity for both reactions involved in water splitting.

Wan et al. [42] synthesized an FeCoCrMnNi bifunctional electrocatalyst with a gradient composition (PP, organic electrolyte). The gradient profile featured a decreasing concentration of iron-group metals and a simultaneous increase in chromium and manganese content across the coating thickness. The elements’ influence on HER performance followed the order Fe > Co > Mn > Cr > Ni, while for OER performance, the order was Fe > Co > Ni > Mn > Cr. The high electrocatalytic activity of these HEAs was attributed to the self-sacrificial oxidation of chromium, which exposes additional active sites, and the presence of high-valence manganese, which promotes O–O bond formation, further enhancing catalytic performance.

In turn, Ma et al. [23] investigated the role of tin atoms incorporated into NiFeCoMnSn alloys (DC, sulfate–citrate bath). They achieved excellent HER electrocatalytic performance, which was attributed to several factors: (i) the three-dimensional porous structure of the nickel foam substrate significantly increased the active surface area, (ii) the incorporation of tin atoms (much larger than the other constituent metals) into the high-entropy alloy induced strong lattice distortion due to atomic size mismatch, which shifted the d-band center and tuned the surface electronic structure, and (iii) the combined synergistic effects and lattice distortion facilitated the adsorption and desorption of reaction intermediates, thereby accelerating the reaction kinetics.

Li et al. [24] highlighted the crucial role played by the amorphous structure in FeCoCrNi alloys for efficient water splitting catalysis. They noted that the disordered atomic arrangement of amorphous materials leads to abundant structural defects, which provide numerous active sites and facilitate ion diffusion. During the OER process, surface reconstruction promotes the formation of oxygen defects and mixed metal oxides/hydroxides, whose synergistic interactions significantly enhance catalytic performance. A large active surface area and tunable electronic structure were identified as key factors in improving OER activity.

Chandran et al. [37] designed PtPdNiCoMn high-entropy alloys (PD, organic bath) for HER catalysis by combining noble and non-noble metals, guided by five key descriptors: hydrogen absorption energy, surface M–H bond length variance, mixing enthalpy, electronegativity difference, and cationic preference. Manganese contributed redox activity and cost reduction, while cobalt and nickel stabilized the FCC phase due to their magnetic properties. Substituting platinum and palladium with non-isoelectronic metals introduced chemical heterogeneity, maintaining catalytic activity while reducing reliance on precious metals. The high configurational entropy improved the distribution and accessibility of active sites, resulting in excellent HER activity, even in challenging media like simulated and real seawater.

High-entropy alloys have also demonstrated promising catalytic performance in the electrochemical oxidation of organic compounds such as methanol [62,63] and glycerol [64]. For instance, Fu et al. [63] synthesized a mesoporous PtPdRhRuCu alloy via soft-template-assisted electrodeposition. They used a polystyrene–poly(oxyethylene) block copolymer in a tetrahydrofuran–water system to form metal-decorated micelles, which migrated to the cathode and were reduced into a film. Subsequent removal of the template with THF yielded a mesoporous catalyst exhibiting 35-fold higher activity for methanol oxidation than commercial platinum black. This enhancement was attributed to the presence of diverse adsorption sites that facilitated intermediate adsorption and lowered the energy barriers for dehydration and CO radical removal pathways. In turn, Song et al. [64] synthesized self-supported nanosheet arrays of an FeCoNiCuP alloy, which functioned as a highly efficient and bifunctional electrocatalyst for both hydrogen evolution and glycerol oxidation in seawater. Theoretical calculations indicated that Fe sites can act as catalytic centers for glycerol oxidation to formate, while CoFeNi bridge sites can serve as active centers for the hydrogen evolution reaction.

The catalytic performance of HEAs is significantly affected by deposition conditions, including bath composition and current/potential regime, which shape their surface structure and elemental distribution. Notably, many HEA catalysts demonstrate good durability and stable activity for more than 100 h of operation [22,23,42,60,61], even in aggressive media such as acidic and alkaline electrolytes or in the presence of chloride ions (seawater) [61,64]. This confirms their potential for long-term use in harsh electrochemical environments.

3.3.3. Wettability

The wettability of electrodeposited HEA surfaces is not a common subject of study [27,32,40,50], despite its importance for both corrosion susceptibility and gas bubble detachment (hydrogen and oxygen) during electrocatalytic water splitting. Yoosefan et al. [50] investigated the effect of pulse parameters on the wettability of CoCrFeMnNi alloys deposited from an organic bath. Regardless of the duty cycle and pulse frequency (−9 V/1 V), hydrophilic surfaces were obtained with water contact angles ranging from 41° to 56°. The sample with the highest contact angle exhibited the lowest corrosion current (6.7∙10⁻² μA/cm²), while the other coatings showed approximately twice the corrosion current. In contrast, Mundotiya et al. [27] found that the incorporation of chromium into a multicomponent alloy enhanced hydrophobicity, with water contact angles of about 86° for FeCoNiMoW and 110° for FeCoNiMoWCr. This clearly translated into improved corrosion resistance of the alloy, resulting from the formation of a passive chromium oxide layer. Mohan et al. [32] obtained a hydrophilic AlCrFeCoNiCu alloy with an initial water contact angle of 63°, which was further enhanced to 125° after annealing (600 °C). Notably, the same behavior was found for a somewhat shorter deposition time [65]. This increase in hydrophobicity was attributed to the formation of air pockets on the surface, with their fraction rising from 23% to 66% [32]. The improvement in surface properties also led to better corrosion resistance, as indicated by the increased polarization resistance of the coating, which was attributed to the prevention of corrosive media penetration due to the presence of stable alumina and chromium oxides acting as protective barriers.

A separate group of applications involves HEA coatings for the purpose of water–oil mixture separation. Zhang et al. [40] synthesized a ZnFeCoNiMn alloy potentiostatically from a chloride–glycerol electrolyte on an iron foam substrate, followed by soaking in ethanol (4 h). The resulting material exhibited superhydrophobicity (water contact angle of 154°) and superoleophilicity (oil contact angles of 0° for acetone, DMF, and ethanol), also displaying the lotus effect (water droplet sliding angle of 3°). The superhydrophobicity was maintained in aqueous solutions over a wide pH range (2–14), with contact angles remaining above 150° after 10 days of exposure to air, water, and mechanical abrasion. The HEA-coated mesh showed high separation efficiency for water–oil emulsions (up to 99% for dichloromethane and soybean oil), even under simulated rough marine conditions. Combined with high corrosion resistance (on the order of 10 μA/cm²), the material can offer promising durability for oil-spill remediation applications.

3.3.4. Magnetic Properties

The magnetic properties of high-entropy alloys (

Table 4. Magnetic properties of some as-deposited high-entropy alloys.

Alloy	Structure	Coercivity, Oe	Saturation Magnetization, emu/g	Ref.
Bi19Fe21Co19Ni22Mn19	FCC	20 °C: 0	–	[15]
		−268 °C: 100	–
		20 °C: 658 1	–
Fe7Ni48W17Mo27Mn0.1	cubic	20 °C: 121	–	[26]
		20 °C: 103 2	–
Co20Cu21Fe19Ni17Zn23	FCC + BCC	20 °C: 19	20 °C: 82	[43]
AlCrFeNiTi	FCC + BCC	20 °C: 0	20 °C: 83–106	[46]
FeCoNiMoW	FCC + amorphous	20 °C: 7–34	20 °C: 42–107	[47]

¹ Alloy annealed at 600 °C. ² Alloy annealed at 200 °C.

) have also attracted attention due to their tunability through compositional design and potential applications such as non-volatile memory, magnetic sensors, and nano-transformers [16,26,43,46,47].

Yao et al. [16] reported the magnetic properties of the first electrodeposited HEA coating. The single-phase BiFeCoNiMn alloy, produced potentiostatically from an organic bath, exhibited soft magnetic and paramagnetic behavior, while annealed alloys demonstrated hard magnetic properties. This change in magnetic properties was attributed to the variation in inter-granular distance between nanocrystals; when the grains are well separated (beyond the exchange correlation length), magnetic hardening is favored, whereas increased grain density reduces the inter-granular spacing, leading to magnetic softening. Other changes in magnetic properties caused by annealing were also observed by Divya et al. [26] for FeNiWMoMn alloys. They found variable coercivity in alloys deposited for different times (30–90 min), with the hysteresis loop for the as-deposited alloy being much wider than for the annealed alloy, indicating lower coercivity and hard magnetic properties in the non-annealed state. These changes during heat treatment (200 °C) were attributed to the relaxation of internal stresses and a decrease in defect density.

Pavithra et al. [43] investigated Co20Cu21Fe19Ni17Zn23 alloys electrodeposited using pulse plating from an aqueous solution. The dual-phase (FCC + BCC) alloy with nanocrystalline grains (5–20 nm) exhibited high saturation magnetization (82 emu/g) and relatively low coercivity (19 Oe), indicating soft magnetic behavior. The relatively high saturation magnetization of these HEA thin films was primarily attributed to their composition of ferromagnetic elements (Fe, Co, Ni), even with the addition of diamagnetic elements such as Cu and Zn, as well as their two-phase crystalline structure. In contrast, a single-phase crystalline structure, due to its symmetry, can potentially reduce saturation magnetization by neutralizing some atomic magnetic moments. Additionally, soft magnetic properties are generally enhanced in nanocrystalline materials compared to their microcrystalline counterparts.

In turn, Dehestani et al. [47] investigated the effect of pulsed current parameters (duty cycle, frequency) on the properties of FeCoNiMoW alloys produced from a sulfate–citrate bath. They observed that increasing the frequency from 10 to 100 Hz and decreasing the duty cycle from 80% to 10% resulted in higher Mo and W content and a greater amorphous phase in the HEA deposits. Additionally, as frequency increased and duty cycle decreased, the surface of the coatings became more compact. All coatings exhibited soft magnetic properties. The highest magnetic saturation (107 emu/g) was observed at a frequency of 10 Hz and a duty cycle of 10%, with the alloy showing the highest iron and cobalt content (Fe33Co30Ni15Mo13W9), while the lowest coercivity (7 Oe) was obtained at a frequency of 100 Hz and a duty cycle of 40%, with the alloy showing the lowest cobalt content (Fe29Co25Ni22Mo14W9).

3.3.5. Mechano-Structural Properties

Some electrodeposited high-entropy alloys have been characterized to evaluate their mechanical properties and structural performance [32,46,65,66]. Compared to conventional nanocrystalline materials, HEAs show enhanced stability as the number of constituent elements increases, due to their high configurational entropy and complex atomic interactions. Mohan et al. [32,65] investigated the microstructural behavior of electroplated Al-CoFeMnNiCu alloys after heat treatment at 600 °C. They observed a transformation from a globular morphology, with a refinement of grains, an increase in compactness, and a subsequent enhancement of coating hardness. Other studies [16,59] confirmed the transformation of amorphous as-deposited coatings into solid solution crystalline structures upon annealing.

Haché et al. [66] systematically analyzed the mechanical and structural properties of nanocrystalline NiFeCoW, amorphous NiFeCoMo, and nanoglass NiFeCoMoW alloys. Their findings showed that grain size stability improved with the addition of elements, with the following order of enhancement: Ni < NiCo < NiFeCo < NiFeCoW < NiFeCoMo ~ NiFeCoMoW. The five-element alloy did not result in further improvements compared to the four-element precursor due to the limitations of the compositional flexibility of molybdenum and tungsten during electroplating. They also observed a significant improvement in thermal stability in the electrodeposited HEAs, as indicated by a doubling of the activation energy for grain growth in NiFeCoW and NiFeCoMoW (both about 3 eV) compared to nanocrystalline Ni (1.3 eV). Additionally, these HEAs exhibited a 60% increase in hardness after annealing at about 500 °C, reaching maximum hardness (8.4 GPa). This hardening trend followed an inverse Hall–Petch relationship, where grain boundary relaxation and nanoglass structural breakdown played a dominant role in the inverse regime.

Nagy et al. [68] demonstrated the influence of zinc content on the mechanical properties of electrodeposited CoFeNiZn alloys with FCC structure. They reported that for Zn concentrations in a range of 25–44 at%, the coatings exhibited a hardness of approximately 4.5 GPa and an elastic modulus of around 130 GPa. At lower zinc contents (down to 18 at%), both parameters decreased. These values were significantly lower than those reported for other electrodeposited CoFeNiZn coatings produced in a similar bath, which was attributed to the presence of not only FCC but also the BCC and amorphous phases reported in the earlier study [67].

3.4. Data-Driven Design of HEAs

Computational calculations and machine learning [69] are increasingly used to predict the composition and properties of HEAs, significantly accelerating their design by processing large datasets and enabling targeted optimization. Key advances include the use of high-quality databases, domain-specific descriptors, and techniques like text mining, transfer learning, and inverse design to improve prediction accuracy and guide experiments. Computational simulations are still relatively rarely used in the context of the electrochemical deposition of HEA coatings, but they are worth mentioning as a promising tool [20,38,39].

Sopittakamol et al. [20] used machine learning to analyze Ni–Co anomalous codeposition as a foundational model for the synthesis of multicomponent alloys. Based on data from the literature, they found that higher concentrations of cobalt ions and the presence of agitation favor an increased Co-to-Ni ratio in the deposits, while higher current density reduces it. Interactions between bath composition, temperature, and current density determine whether the process is activation- or mass-transport-controlled, with agitation enhancing cobalt deposition at high current densities by reducing concentration gradients near the electrode.

Other reports [38,39] present computational simulations aimed at predicting the structure and properties of corrosion-resistant multicomponent alloys with potential applications in marine engineering. Serban et al. [39] investigated the effect of metal concentration on phase evolution in the AlCrCuFeNi system by analyzing the redistribution of solid solutions during solidification. Through optimization calculations based on selected elemental concentrations and boundary conditions, they proposed an alloy composition of Al10Cr35Cu15Fe12Ni28. The simulated phase diagram for this composition revealed predominant BCC and FCC phases and indicated enhanced thermal stability. These predictions were experimentally validated by electrodepositing the alloy from a chloride–organic electrolyte at a constant potential of −3 V, resulting in only a 2% average deviation between nominal and actual compositions. The deposited alloy exhibited a uniform structure and demonstrated improved corrosion resistance in artificial seawater compared to pure copper. In turn, Popescu et al. [38] conducted similar studies on the CoCrFeMnNi alloy. They analyzed the domains of solid solution formation by varying criteria (e.g., Allen electronegavity, atomic radius difference, density, intermetallic formation enthalpy to mixing enthalpy ratios) and evaluated the corrosion resistance of the coating deposited on an aluminum substrate. However, the substrate itself exhibited better corrosion resistance than the deposited coating.

4. Electroless Deposition

Electroless deposition is a chemical method for forming metallic coatings without the use of an external electrical current, enabling uniform coatings even on complex-shaped surfaces. This technique relies on controlled redox reactions between metal ions and reducing agents in solution [70] via a displacement reaction with more active metal (aluminum) [2]. The choice of reducing agent (e.g., sodium hypophosphite, dimethyl borane, sodium borohydride, glucose) must be appropriate for the specific metal (typically nickel, cobalt, copper, noble metals) being deposited, as the metal itself acts as a catalyst for the oxidation of the reducer, ensuring the continuity of the deposition process [70]. As such, the process is inherently autocatalytic, with the growing metal layer sustaining the reaction once initiated. Electroless deposition allows for the production of single-metal coatings, as well as binary, ternary, and even quaternary alloys, often containing small amounts of nonmetals such as phosphorus or boron originating from the reducing agent. The successful deposition of multicomponent alloys requires that at least one of the metal components exhibits catalytic activity toward the oxidation of the reducing agent, which is essential for sustaining the autocatalytic process.

Although autocatalytically deposited multicomponent alloys have already been discussed in the literature [70,71], these were not considered high-entropy alloys because they typically did not meet the fundamental criteria of HEAs, such as containing at least five principal elements in near-equimolar ratios and achieving high configurational entropy. Instead, many of these coatings were designed for specific functional purposes (e.g., corrosion protection, hydrogen evolution catalyst) without regard to entropy-based alloy design principles. In fact, only a very limited number of studies [17,72,73,74] have successfully demonstrated the electroless deposition of true high- or medium-entropy alloy coatings (

Table 5. Electroless deposited high-entropy coatings.

Alloy	Deposition Conditions	Structure	Properties	Ref.
CuNiFeCrMo-GO 1	chloride–hypophosphite bath; pH 1–2; 80–85 °C, 1 h	FCC + BCC	hardness, wear	[17]
NiCoWZrP 1	chloride–citrate–hypophosphite bath; pH 8–9; 85 °C, 1 h	FCC	corrosion, adhesion, wear	[72]
CuNiCo 2	sulfate–complexing baths; room temperature; thermal diffusion 800 °C, 2–10 h	FCC	corrosion	[73]
CuCoNiFe 2	sulfate–complexing baths; room temperature; substrate etching in NaOH; thermal diffusion 800/1000 °C, 8 h	two-phased	strength	[74]

¹ Autocatalytic deposition. ² Al-induced displacement deposition.

).

Premlata et al.’s paper [17] appears to be the first study reporting on an electrolessly deposited high-entropy alloy matrix reinforced with graphene oxide, specifically CuNiFeCrMo–GO. The alloy was synthesized from an acid chloride–hypophosphite bath, with lactic acid serving as a stabilizing agent. The HEA matrix exhibited a cell-like morphology with particles growing laterally toward the smooth substrate surface. The resulting nanocomposite coating formed a solid solution comprising a mixture of FCC and BCC phases. Although only metallic elements were identified in the alloy, no detailed compositional ratios were provided. The deposited coating showed a significant enhancement of hardness (325 HV) exceeding that of the mild steel substrate by approximately 2.5 times and a 27% reduction in wear rate.

Unni and Jothi [73] investigated the formation of a medium-entropy Ni69Co19W2Zr0.8P9 alloy using an alkaline chloride–citrate–hypophosphite bath, supplemented with lactic acid as a stabilizer and a zwitterionic surfactant to enhance adhesion. The resulting nanograined coating, with an average crystallite size of 0.4 nm, demonstrated improved corrosion resistance compared to mild steel and a NiZrP coating, although it was inferior to that of conventional NiP alloys. Nevertheless, the quaternary alloy exhibited superior adhesion and wear resistance relative to its binary and ternary counterparts.

Dai et al. [74] developed a method for forming high-entropy alloys on aluminum substrates using immersion plating. Equimolar layers of copper, nickel, and cobalt were sequentially deposited, followed by heat treatment to induce thermal diffusion of the elements. This process transformed the initially multilayered structure, composed of distinct phases (HCP-Co, FCC-Cu, FCC-Ni), into a single-phase (FCC) ternary alloy with a homogeneous elemental distribution across both the surface and cross-section. The resulting alloy exhibited superior corrosion resistance compared to single-metal coatings. The method was further adapted to obtain a CuNiCoFe alloy [75]. Initially, metal layers of Ni, Cu, Co, and Fe were sequentially deposited via an Al-induced process onto a printed microlattice substrate. Following the removal of the substrate, the resulting multilayer coating was annealed to promote alloying. This thermal treatment yielded alloy tubes with a two-phase structure consisting of a CuCoNiFe-rich primary phase and a Cu–Ni-rich secondary phase. Ultimately, a CuCoNiFe high-entropy alloy microlattice ultralight material (0.4 g/cm³) with a greater plasticity than nickel-based material was successfully fabricated.

5. Conclusions

The electrodeposition of high-entropy alloys offers a versatile, cost-effective approach for producing advanced coatings with superior mechanical, chemical, and catalytic properties. The ability to control composition, structure, and morphology through adjustments in electrolytes and deposition modes allows for the customization of coatings for a variety of substrates and complex geometries. Furthermore, the scalability and low-temperature process conditions make electrodeposition an attractive method for industrial applications in sectors such as energy, aerospace, chemical catalysis, corrosion protection, etc.

However, several challenges persist, limiting the widespread application of electrodeposited HEAs (Figure 6). Achieving uniform alloy composition, especially in equimolar ratios, remains difficult, and the formation of thick, compact coatings with uniform properties over the entire surface is often challenging. Additionally, the use of various electrolytes tailored for different metal combinations in HEAs complicates the process of comparing the individual effects of factors such as plating conditions on alloy properties and composition. This, combined with the lack of systematic studies on the relationships between plating parameters, coating properties, and alloy composition, creates uncertainty and limits the reproducibility of results.

Despite these constraints, the field holds considerable potential for overcoming the current limitations. Advances in deposition strategies, new electrolyte formulations (including unexplored deep eutectic solvents [75] and ionic liquids [76]), and the integration of electrochemical additive manufacturing techniques [77] could provide solutions for more consistent, reproducible results. The use of artificial intelligence and high-throughput experimentation offers promising opportunities for optimizing plating conditions and predicting alloy behavior. In particular, machine learning-based computational design methods can help uncover the mechanisms and implications of anomalous metal deposition in multi-principal element systems [20], which is crucial for the rational design of high-entropy alloy compositions and properties [69]. With continued research and the development of standardized procedures, electrodeposited HEAs could replace conventional coatings in critical industrial applications, paving the way for the next generation of materials with unprecedented performance.

The electroless plating (autocatalytic and displacement) of high-entropy alloys remains a relatively new and challenging area, with limited data available on achieving uniform composition and thickness. A key challenge in this process is the need for suitable reducing agents that can effectively reduce various types of metal ions simultaneously, ensuring the proper deposition of all alloy components with deposition time. A technical difficulty may arise from the necessity to initiate the reduction process, which requires catalytic activation or the use of a galvanic couple. Additionally, maintaining the chemical stability of multicomponent plating baths over time is essential, as the spontaneous degradation or precipitation of individual metal species in bulk electrolytes at elevated temperatures (near 90 °C) can significantly impact deposition efficiency and reproducibility.