Research Progress on the Corrosion Resistance of High-Entropy Alloy Coatings

This paper summarizes recent research on the corrosion resistance of high-entropy alloy coatings. The study primarily explores the effects of preparation processes, alloy compositions, and process parameters on the corrosion resistance of high-entropy alloys. It also offers suggestions regarding the existing challenges and key research directions for high-corrosion-resistant high-entropy alloy coatings. The authors of this work are Jia Chuntang, Sha Minghong, and others from the School of Materials and Metallurgy at Liaoning University of Science and Technology.

The insufficient high-temperature stability of most conventional alloys can degrade their mechanical properties and corrosion resistance, thereby limiting their application in extreme and highly sensitive engineering environments. In 2004, Yeh et al. broke through traditional alloy design concepts and proposed the concept of high-entropy alloys (HEAs). Initially, high-entropy alloys were defined as a new type of alloy containing five or more major elements, with each major element accounting for between 5% and 35% (atomic fraction) of the alloy composition. Due to the influence of the high mixing entropy effect, high-entropy alloys tend to form simple solid-solution structures—such as face-centered cubic (FCC), body-centered cubic (BCC), or hexagonal close-packed (HCP)—rather than complex intermetallic compounds. The unique compositional and microstructural characteristics endow high-entropy alloys with exceptional properties, including high thermal stability, wear resistance, corrosion resistance, and excellent magnetic performance. Consequently, high-entropy alloys hold great promise as candidate materials for applications in some extreme and highly sensitive engineering environments, such as nuclear power, turbine engines, and aerospace systems.

Due to their high content of expensive metals (such as Nb, W, Cr, V, Ni, Ti, and others), high-entropy alloys may have higher costs than most conventional alloys. However, this issue can be addressed by using these alloys for surface coatings. In recent years, researchers have successfully prepared high-entropy alloy coatings through processes including laser cladding, electrospark deposition, electrochemical deposition, electron-beam evaporation, and magnetron sputtering. By employing high-entropy alloy coatings, it is possible to achieve a reasonable balance between cost and performance.

Figure 1 shows the corrosion behavior of a high-entropy alloy coating, bulk high-entropy alloy, and stainless steel in a 3.5% (mass fraction) NaCl solution. As can be seen from the figure, compared to the bulk high-entropy alloy, the high-entropy alloy coating exhibits a lower current density (I corr) and a higher corrosion potential (E corr), and its corrosion resistance is comparable to that of stainless steel. Figure 2 illustrates the pitting corrosion behavior of the high-entropy alloy coating, bulk high-entropy alloy, and stainless steel in a 3.5% (mass fraction) NaCl solution. It is evident that the high-entropy alloy coating has a lower corrosion current and a higher pitting potential (E pit). Compared to both stainless steel and the bulk high-entropy alloy, the high-entropy alloy coating demonstrates superior pitting resistance.

Figure 1: Corrosion behavior of high-entropy alloy coatings, bulk high-entropy alloys, and stainless steel in a 3.5% NaCl solution.

Figure 2: Pitting behavior of high-entropy alloy coatings, bulk high-entropy alloys, and stainless steel in a 3.5% NaCl solution.

The corrosion resistance mechanism of high-entropy alloys can be summarized in the following three points:

(1) Due to the influence of the high-entropy effect, high-entropy alloys are more likely than conventional alloys to form a single solid-solution phase or an amorphous phase. As is well known, the more uniform the phase composition, the more homogeneous the alloy’s chemical composition. The formation of a single solid solution or an amorphous phase can reduce the effects of galvanic corrosion and decrease the number of microcells, thereby enhancing corrosion resistance.

(2) The addition of elements such as Cr, Ni, Cu, Ti, and Mo can induce the formation of a passivation film on the coating surface. In oxidizing acids like nitric acid and concentrated sulfuric acid, these elements tend to be easily oxidized, forming a dense oxide film—for example, Al. ₂ O ₃ CrO ₃ , Cr ₂ O ₃ membranes, thereby reducing the corrosion rate. In alkaline solutions, the surface of alloys containing corrosion-resistant elements readily reacts with OH⁻. - Forming insoluble hydroxides that aggregate on the alloy surface to form a dense passivation film, such as Al(OH)₃. 3、 Cu(OH) 2 The film, among other factors, effectively inhibits the polarization reaction, thereby slowing down the corrosion rate and enhancing the alloy's corrosion resistance. Moreover, water also promotes the formation of the passivation film; it is generally believed that the following reaction takes place during the passivation process on metal surfaces:

Among them, (MeOH) ad It is an intermediate product, and n represents the valence of the metal ion. If the solution contains an anion A that readily destroys the passivation film, - (such as Cl - ), it will undergo the following reaction with the passivation film:

Therefore, Cl ^- It can damage the passivation film and cause pitting corrosion. Studies have shown that the appropriate addition of Mo can form a self-repairing passivation film in combination with Cr, effectively inhibiting Cl⁻. ^- Pitting corrosion caused thereby. Furthermore, studies have shown that since Me-N bonding is chemically more inert than Me-Me bonding, the involvement of nitrogen helps enhance the corrosion resistance of high-entropy alloy coatings.

(3) Compared to bulk high-entropy alloys, high-entropy alloy coatings can achieve a more uniform microstructure. Thanks to the rapid quenching effect during the preparation process, the diffusion of elements in high-entropy alloy coatings can be more effectively suppressed, leading to a more homogeneous elemental distribution and enhanced corrosion resistance.

2.1 Laser Cladding Technology

Laser cladding technology is a rapidly developing surface treatment method characterized by a fast cooling rate (10 3 ~10 6 The K/s) feature enables the prevention of compositional segregation. This technology can be used to fabricate high-entropy alloy coatings with thicknesses ranging from approximately 1 to 5 mm—significantly thicker than thin films prepared by magnetron sputtering. Laser cladding produces metallurgical bonding between the coating and the substrate, resulting in a bond strength that is much higher than that achieved by thermal spraying techniques. Zhang et al. used laser cladding to prepare an FeCo-CrAlNi high-entropy alloy coating on the surface of 304 stainless steel. The results showed that, in a 3.5% NaCl solution, the FeCoCrAlNi high-entropy alloy coating exhibited superior corrosion resistance and pitting resistance compared to 304 stainless steel. Ye et al. employed an electrochemical workstation to investigate the corrosion behavior of laser-clad CrMnFeCoNi high-entropy alloy coatings and found that these coatings demonstrated better corrosion resistance than 304 stainless steel.

2.2 Magnetron Sputtering Technology

Magnetron sputtering is the most commonly used technique for preparing high-entropy alloy thin films. During the sputtering process, the stoichiometry of high-entropy alloy thin films can be easily controlled by adjusting the chemical composition of the target material and the process parameters. Li et al. prepared FeAlCu-CrCoMn high-entropy alloy coatings using magnetron sputtering. Electrochemical tests demonstrated that the FeAlCu-CrCoMn high-entropy alloy coating exhibits excellent corrosion resistance in 3.5% NaCl, 5% NaOH, and 10% HCl solutions. 2 SO 4 The corrosion resistance of these solutions is superior to that of 201 stainless steel. In addition, they have also prepared FeAlCoCuNiV coatings, which likewise exhibit even better corrosion resistance than 201 stainless steel. Currently, the corrosion performance of high-entropy alloy coatings fabricated by magnetron sputtering has not been extensively studied. However, given the uniformity achieved through magnetron sputtering and its satisfactory corrosion resistance, research on preparing high-entropy alloy coatings via magnetron sputtering is poised to become a hot topic.

2.3 Electrical Spark Deposition Technology

Electrospark deposition is an emerging material surface treatment technology that is energy-efficient, material-saving, and environmentally friendly. It involves using high-current, short-pulse discharges to deposit electrode material onto the surface of a substrate metal. Under the action of the pulsed plasma arc, tiny amounts of electrode material melt and rapidly solidify on the substrate surface, forming a coating. Li et al. prepared an AlCoCrFeNi high-entropy alloy coating on AISI 1045 carbon steel via electrospark deposition. By comparing the corrosion behavior of the coated samples with that of AlCoCrFeNi high-entropy alloy produced by copper-mold casting in a 2.5% (mass fraction) NaCl solution, they found that the corrosion current of the coated samples was significantly lower than that of the cast AlCoCrFeNi high-entropy alloy. This is because, compared to the cast AlCoCrFeNi high-entropy alloy, the Al-CoCrFeNi high-entropy alloy coating exhibits a relatively higher content of Cr oxides and Al oxides on its surface. Moreover, the coating lacks Cr-rich interdendritic phases and secondary phase precipitates, thus avoiding galvanic corrosion.

2.4 Other Preparation Techniques

The plasma arc cladding process offers numerous advantages in the preparation of high-entropy alloy coatings, including high energy transfer efficiency, minimal thermal distortion of the part, and low dilution of the substrate material. Cheng et al. used the plasma arc cladding process to fabricate CoCrCuFeNi high-entropy alloy coatings. Experimental results showed that, in a 6 mol/L NaCl solution, the CoCrCuFeNi high-entropy alloy coating exhibited superior corrosion resistance compared to 304 stainless steel. Ge et al. employed mechanical alloying and vacuum hot pressing sintering techniques to prepare CuZrAl-TiNi high-entropy alloy coatings on a T10 substrate. Compared to the T10 substrate itself, the CuZrAlTiNi high-entropy alloy coating demonstrated significantly enhanced corrosion resistance in seawater solutions, characterized by a higher corrosion potential, a wider passive region, and the occurrence of secondary passivation. Niu et al. used electron-beam evaporation to deposit Al. x FeCoCrNiCu (x = 0.25, 0.5, 1.0) high-entropy alloy coatings were deposited onto an alloy steel substrate composed of the same alloy elements. Electrochemical test results indicate that Al _0.5 FeCoCrNiCu high-entropy alloy coating in H ₂ SO ₄ The passivation region in the NaCl aqueous solution is greater than 700 mV, and it exhibits a relatively high corrosion potential (-129 mV) and a low corrosion current density (≈2.2×10⁻⁶ A/cm²). -6 A/cm²), these results indicate that Al _0.5 The corrosion resistance of the FeCoCrNiCu coating is superior to that of the unmodified substrate.

3.1 Al

Ye et al. studied the effect of Al addition on Al. x The corrosion behavior of FeCoCrNiCuCr(x=1, 1.3, 1.5, 1.8) high-entropy alloy coatings in a 0.05 mol/L HCl solution was investigated. Electrochemical experiments revealed that the addition of Al enhanced the corrosion resistance of the coatings. Among them, the Al x FeCoNiCrTi coating exhibited superior corrosion resistance compared to 314L stainless steel, with the Al 1.8 FeCoNiCuCr coating showing the best corrosion resistance performance. Niu et al. studied the effect of Al on Alx FeCoCrNiCu(x=0.25, 0.5, 1.0) high-entropy alloy coatings in solutions with a concentration of 1 mol/L H+. ₂  SO ₄ The effect of the solution and 1 mol/L HCl solution on corrosion resistance was studied. The research indicates that at a concentration of 1 mol/L H... 2 SO 4 In the solution, when the Al content is below 0.5, the material exhibits excellent corrosion resistance and pitting resistance. However, when the Al content reaches 1.0, both corrosion resistance and pitting resistance decline, though they still remain superior to those of 304 stainless steel. In a 1 mol/L NaCl solution, Al... 1.0 The pitting resistance of FeCoCrNi-Cu is superior to that of Al. 0.5 The FeCoCrNiCu high-entropy alloy coating exhibits the worst pitting corrosion resistance among 304 stainless steels.

3.2 Ti

Qiu et al. studied the effect of Ti on Al. 2 CrFeNiCoCuTi x (x=0, 0.5, 1.0, 1.5, 2.0) The influence of high-entropy alloy coatings. Compared to Q235 steel, Al 2 CrFeNiCoCuTi x The self-corrosion current density of the high-entropy alloy coating decreased by 1 to 2 orders of magnitude, and the self-corrosion potential became more positive. As the Ti content increased, the Al2CrFeCoCuNiTix high-entropy alloy coating in 0.5 mol/L HNO₃... ₃ The corrosion resistance in the solution is enhanced. Shi Hai et al. prepared Ni. _1.5 Co _1.5 FeCrTi x High-entropy alloy coatings—research indicates that as the Ti content increases, Ni... _1.5 Co _1.5 FeCrT ix High-entropy alloy coating in 0.5 mol/L HNO₃ 3 The corrosion resistance in the solution is enhanced because of Ni. _1.5 Co _1.5 FeCrT ix The surface of the high-entropy alloy coating in HNO ₃ A dense passivation film readily forms in the solution.

3.3 Ni

Qiu et al. studied the effect of Ni content on Al. ₂ CrFeCoCuTiNi x The corrosion behavior of high-entropy alloy coatings (with x = 0, 0.5, 1.0, 1.5, 2.0) in 1 mol/L NaOH solution and 3.5% NaCl solution was investigated. The experiments showed that as the Ni content increased, the Al... ₂ CrFeCoCuTiNi x The corrosion resistance of high-entropy alloys first increases and then decreases, with Al playing a role in this trend. ₂ CrFeCoCuTiNi _1.0 It exhibits the best corrosion resistance. The reason can be attributed to the following: While Ni possesses excellent corrosion resistance, its atomic radius is relatively small. When the Ni content is high, the alloy’s lattice distortion becomes severe, thereby affecting the alloy’s microstructure and subsequently compromising its corrosion resistance. Wu et al. studied FeCoCrAlCuNi. x The corrosion behavior of high-entropy alloy coatings (x = 0.5, 1.0, 1.5) in a 3.5% NaCl solution shows that, as Ni is added, the corrosion resistance also exhibits a trend of first increasing and then decreasing. Among these, the Fe-CoCrAlCuNi alloy... _1.0 Has the best corrosion resistance.

3.4 MB

Li Dongliang et al. studied the effect of Mo content on FeCrNiMnMo. x B _0.5 The effect of high-entropy alloy coatings with x = 0, 0.4, 0.8, and 1.0 on microstructure and properties was investigated. The study found that in a saturated salt cement slurry solution, FeCrNiMn-Mo... _0.4 B _0.5 It exhibits the best corrosion resistance, owing to the formation of a passive film by Mo and Cr, which inhibits Cl. ^- Erosion occurs. As the Mo content further increases, Mo tends to enrich at grain boundaries, leading to an uneven coating composition and a decline in corrosion resistance.

3.5 Other elements

Cai et al. studied the effect of Cu on FeCoCrNiCu. x Regarding the impact of coating corrosion resistance, studies have shown that the addition of Cu reduces the passivation capability of the cladding layer, thereby deteriorating the alloy's corrosion resistance. This phenomenon can be attributed to the segregation of Cu at grain boundaries upon Cu addition, forming Cu-rich phases that induce galvanic corrosion and consequently lower the corrosion resistance of the cladding layer. Cheng et al. investigated the effect of Nb on the corrosion resistance of high-entropy alloy coatings and found that the impedance of Nb-containing coatings was 14 times and 1.6 times higher than that of 304 stainless steel and the Nb-free coating, respectively. This indicates that the addition of Nb enhances the corrosion resistance of the coating. Qiu et al. studied the influence of Co content on Al... ₂ CrFeCo x The effect of CuNiTi high-entropy alloy coatings on corrosion resistance was studied. The research found that as the Co content increases, Al... ₂ CrFeCoxCuNiTi high-entropy alloy coating in HCl and H ₂ SO ₄ The corrosion resistance in the solution is enhanced. This is because the incorporation of Co leads to the formation of a dense passivation film on the alloy surface. Zhang et al. prepared FeCrNiCoB via laser cladding. x Coating. When 0.5 < x < 1.0, the corrosion resistance of the coating improves with increasing boron content. As x approaches 1.25, the borides transition from the orthorhombic crystal system (Cr, Fe). ₂ B transforms into a tetragonal (Fe, Cr) phase. ₂ B, this will reduce the coating’s corrosion resistance, but it still exhibits better corrosion resistance than ASTM 304L stainless steel.

Qiu et al. investigated the effect of scanning speed on the corrosion resistance of laser-clad AlCrFe-CuCo high-entropy alloys. The experiments showed that as the scanning speed increased, the alloy’s corrosion resistance initially improved and then declined. This phenomenon can be attributed to the rapid heating and cooling induced by the laser beam, which resulted in a finer and more uniform microstructure of the coating, reduced compositional segregation, and enhanced corrosion resistance. However, when the scanning speed became excessively high, convective currents intensified, leading to a rougher coating surface and a deterioration in corrosion resistance.

Shon et al. investigated the effects of energy input and the number of overlay layers on the corrosion behavior of laser-clad CoCrFeNi coatings. The study showed that combining higher energy input with a double-layer cladding can reduce dilution of the substrate by the coating, thereby preventing the formation of localized galvanic cells and demonstrating excellent corrosion resistance in a 3.5% NaCl solution.

Hsueh et al. investigated the effect of substrate bias on the corrosion resistance of (AlCrSiTiZr)N high-entropy alloy coatings prepared by DC reactive magnetron sputtering. The study showed that a substrate bias of -100 V can effectively enhance the corrosion resistance of the amorphous (Al‐CrSiTiZr)N thin films, owing to the densification and compressive stress induced by the substrate bias.

Shi Yanyan and colleagues studied the effect of different substrate temperatures on the corrosion resistance of FeNiCoCrMn high-entropy alloy thin films prepared by magnetron sputtering. The research showed that as the substrate temperature increased, the film thickness gradually decreased, and the corrosion resistance deteriorated. Among them, the film deposited at 100℃ exhibited the best corrosion resistance.

Over the past 14 years, research on high-entropy alloys has opened up a vast and largely unexplored field of multi-component alloys. Due to their outstanding performance, high-entropy alloys hold great potential for use in a variety of engineering environments. This article summarizes the research progress and corrosion-resistant mechanisms of high-entropy alloy coatings from three perspectives: preparation processes, alloying elements, and process parameters. To further deepen our understanding of the corrosion resistance of high-entropy alloys and to promote their application in actual industrial production, the following research recommendations are proposed for the future:

(1) Currently, the research results on passivation films mainly provide microscopic characterization but fail to explain the fundamental reasons why high-entropy alloys exhibit superior corrosion resistance. Therefore, it is necessary to conduct high-resolution analysis of the passivation films in high-entropy alloys and carry out in-depth studies on their corrosion-resistant mechanisms.

(2) Research on the microstructure of high-entropy alloys is still at the “trial-and-error” stage, which not only leads to low efficiency but also increases research costs. Therefore, launching a materials genome project—using first-principles calculations and molecular dynamics simulations to perform ab initio design of cluster structures, as well as performing both equilibrium and non-equilibrium thermodynamic calculations on high-entropy alloy coatings—to predict phase formation and transformations represents one of the key research directions for the future.

(3) A stable process regime for preparing high-quality high-entropy alloy coatings has not yet been established. Therefore, developing coating preparation processes that yield uniform microstructures with reproducibility and guiding significance will become one of the primary research directions for the application of corrosion-resistant high-entropy alloy coatings.