# base64_Investigation on the corrosion resistance of epoxy resin coatings modified by high-entropy oxides Investigation on the corrosion resistance of epoxy resin coatings modified by high-entropy oxides > **Boxin Yan¹, Chao Wang^1,2, Yihui Liu¹** > > ¹Hubei Key Laboratory of Advanced Technology for Automotive Components & Hubei Collaborative Innovation Center for Automotive Components Technology, Wuhan University of Technology, Wuhan, Hubei, 430070, China > > ²Corresponding author’s e-mail: wchao@whut.edu.cn > > **Abstract:** High-entropy oxides, as an emerging class of ceramic materials, exhibit exceptional high-temperature stability, superior corrosion resistance, and excellent hardness and strength, rendering them promising candidates for surface protection applications. In this study, high-entropy oxide filler Y₂(Ti_0.2Zr_0.2Hf_0.2Ce_0.2V_0.2)₂O₇ was synthesized via solid-state reaction and incorporated as a nanofiller to modify epoxy resin, thereby fabricating composite coatings. The influence of varying high-entropy oxide contents on the anticorrosion performance of the composite coatings was systematically investigated, and the underlying corrosion protection mechanism of the high-entropy oxide-modified epoxy coatings was elucidated. The results demonstrate that the composite coatings incorporating high-entropy oxide exhibit outstanding anticorrosion properties, with a corrosion inhibition efficiency of 99.39% derived from polarization curve analysis. Even after immersion in 3.5wt% NaCl solution for 10 days, the corrosion inhibition efficiency remained at 95.57%. Impedance efficiency, as determined from Nyquist plots, reached 98.63%, and retained 91.75% after 10 days of immersion. # 1. Introduction Metallic materials play a crucial role in industrial development. Their corrosion not only affects the national economy and personal safety, but also poses significant environmental impacts. Among existing strategies, applying protective coatings on metal surfaces is currently one of the most widely used and effective corrosion-prevention approaches \[1\]. However, traditional organic coatings often fail to meet the demands of modern industry, and the incorporation of functional anticorrosive fillers has proven to be an effective method to enhance coating performance \[2\]. One-dimensional nanomaterials exhibit size-dependent effects \[3\] and barrier properties, which can effectively improve the performance of epoxy coatings by mitigating microcracks and voids formed during the curing process \[4\]. High-entropy oxides represent a novel class of ceramic materials \[5\], composed of multiple metallic elements that form a highly disordered structure. This unique configuration imparts exceptional properties, including superior thermal stability, outstanding corrosion resistance, and excellent hardness and mechanical strength. In this study, one-dimensional high-entropy oxide nanofillers were synthesized and incorporated into epoxy resin to develop composite coatings. The objective is to fill micro-voids generated during the coating fabrication process, thereby achieving a composite coating with enhanced hardness and corrosion resistance, and subsequently investigating its anticorrosion performance. # 2. Methods and Materials ## 2.1 Preparation of High-Entropy Oxide Y₂(Ti_0.2Zr_0.2Hf_0.2Ce_0.2V_0.2)₂O₇ The high-entropy oxide (HEO) was synthesized using the solid-state reaction method. The main preparation steps are as follows: oxide powders of the corresponding elements were weighed according to the designed chemical composition of the high-entropy oxide with a molar ratio of Y:T:Zr:Hf:Ce:V = 5:1:1:1:1:1. The powders were then mixed using a planetary ball mill for 12 hours. After milling, the resulting slurry was transferred into centrifuge tubes, centrifuged, and the lower precipitate was collected and dried in a vacuum oven for 8 hours. The dried mixture was subsequently ground and sieved through a 200-mesh sieve. The sieved powder was placed in an alumina crucible, compacted with a spatula, and sintered in a muffle furnace at 1000 °C for 4 hours. Finally, the sintered product was ground and passed through a 200-mesh sieve to obtain the high-entropy oxide filler (HEO). ## 2.2 Preparation of Composite Coatings 2g of epoxy resin was dissolved in 8mL of acetone, followed by the addition of 40mg of high-entropy oxide filler and ultrasonic dispersion for 15min. The slurry was then heated and stirred at 50°C to remove the acetone. Subsequently, 0.6g of curing agent T-31 was added, and the mixture was stirred at a constant speed for 15min. The coating was uniformly applied onto the pretreated 6061 aluminum alloy surface using a wire-wound rod coater with a wet film thickness of 200μm. The prepared samples were cured at room temperature for 2 days, followed by heating at 60°C for 4h, yielding an epoxy resin composite coating with a high-entropy oxide incorporation of 2wt%, designated as HEO-2. Following the same procedure, composite coating samples with filler mass fractions of 1wt% and 3wt% were prepared and designated as HEO-1 and HEO-3, respectively. Additionally, a pure epoxy resin coating without any filler was fabricated and designated as EP*a*² + *b*² = *c*² . $$\left( x + a \right)^{n} = \sum_{k = 0}^{n}{\left( \frac{n}{k} \right)x^{n - k}a^{k}}$$ The morphological features of the EP, HEO-1, HEO-2, and HEO-3 samples were observed using scanning electron microscopy (SEM). The electrochemical impedance spectroscopy (EIS) and polarization curves of the coatings were measured using an electrochemical workstation. EIS measurements were conducted at open-circuit potential over a frequency range from 100,000Hz to 0.01Hz, with an AC amplitude of 10mV. Polarization curves were recorded at a scan rate of 5mV/s. The corrosion inhibition efficiency \[6\] and impedance efficiency \[7\] of the coated samples were calculated using the following equations. | | | | |-----|-----|-------| | | | （1） | | | | （2） | In Equation (1), *P* represents the corrosion inhibition efficiency, *j_i* denotes the corrosion current density of the coated sample (A/cm²), and *j₀* is the corrosion current density of the blank sample (i.e., pure epoxy resin) (A/cm²). In Equation (2), *η* represents the impedance efficiency, *R_i* denotes the impedance value of the coated sample (Ω/cm²), and *R₀* is the impedance value of the blank sample (Ω/cm²). # 3. Results and Discussion ## 3.1 Microstructural Analysis The morphologies of the EP, HEO-1, HEO-2, and HEO-3 samples were examined using scanning electron microscopy (SEM), as shown in Figure 1. Numerous large bubbles are present inside the EP sample (Figure 1a). This is attributed to the high viscosity of the pure epoxy system, which traps air during stirring, and the entrapped air is unable to escape easily. The bubble control in the HEO-1 sample (Figure 1b) is similar to that in EP, with a considerable number of bubbles observed. This is due to the insufficient filler content, which leads to limited dispersion in the resin and an inability to effectively suppress bubble formation during mixing and curing. The HEO-2 sample (Figure 1c) exhibits the fewest and smallest internal bubbles, with the most uniform and complete structure. This improvement is attributed to the optimal filler content, which moderates the viscosity of the coating, facilitates bubble escape, and suppresses bubble generation. The HEO-3 sample (Figure 1d) also demonstrates good bubble control but remains inferior to HEO-2. This is because the excessive filler addition increases slurry viscosity and reduces fluidity, hindering bubble removal and leading to partial bubble retention.

**Figure 1.** SEM images of the composite coating²:(a) EP; (b) HEO-1; (c) HEO-2; (d) HEO-3. ## 3.2 Polarization Curve Analysis The polarization curves and corresponding data are presented in Figure 2 and Table 1. As shown in Figure 2 and Table 1, compared with the pure epoxy coating, the coatings containing HEO fillers exhibit a positive shift in corrosion potential and a decrease in corrosion current density, indicating improved anticorrosion performance. Among them, the HEO-2 coating shows a more positive corrosion potential and the lowest corrosion current density, demonstrating the highest anticorrosion efficiency. Even after prolonged exposure to the corrosive medium, the HEO-2 sample maintains excellent anticorrosion performance, suggesting that 2wt% HEO filler achieves good dispersion within the coating and effectively inhibits the penetration of corrosive species.

**Figure 2.** Polarization curves of samples with different coatings without immersion (a) and immersed for 10 days (b). **Table 1.** Polarization curve fitting data of different coating samples. | | | | | | |----------------|--------------------|------------------------|--------|-------| | Coating sample | Immersion time/day | j /(A/cm^-2) | E/V | P (%) | | EP | 0 | 1.730×10^-7 | -1.019 | \- | | | 10 | 8.013×10^-7 | -1.021 | \- | | HEO-1 | 0 | 1.308×10^-8 | -0.757 | 92.43 | | | 10 | 1.416×10^-8 | -0.517 | 91.82 | | HEO-2 | 0 | 1.059×10^-9 | -0.191 | 99.39 | | | 10 | 7.669×10^-9 | -0.416 | 95.57 | | HEO-3 | 0 | 1.424×10^-9 | -0.294 | 99.17 | | | 10 | 1.422×10^-8 | -0.744 | 91.78 | ## 3.3 Electrochemical Impedance Analysis The electrochemical impedance data are presented in Figure 3 and Table 2. Figure 3 and Table 2 clearly illustrate the performance differences among the coatings. Among the composite coatings, HEO-2 exhibits the highest impedance efficiency, reaching 98.63% initially and remaining at 91.75% after 10 days of immersion. Compared with the other samples, HEO-2 consistently shows the best impedance values and efficiency before and after immersion, indicating that the incorporation of 2wt% HEO provides superior anticorrosion performance, effectively protecting the substrate from corrosion and extending the coating service life. The HEO-1 coating demonstrates an initial impedance efficiency of 91.26%, which decreases significantly to 40.69% after 10 days of immersion. This suggests that 1wt% HEO is insufficient to notably enhance the corrosion protection capability of the coating. Combined with the SEM observations, it can be inferred that the low filler content leads to inadequate dispersion, resulting in limited reinforcement and modification effects. The HEO-3 coating shows an initial impedance efficiency of 96.71% and retains 90.62% after 10 days of immersion, indicating that 3wt% HEO significantly improves anticorrosion performance. However, compared with HEO-2, excessive filler addition leads to diminished efficiency gains and potential material waste, suggesting that 2wt% is the optimal loading for achieving balanced performance and economic efficiency.

**Figure 3.** Nyquist plot of samples with different coatings without immersion (a) and immersed for 10 days (b). **Table 2.** Electrochemical impedance data of different coating samples. | | | | | |----------------|--------------------|----------------------|-------| | Coating sample | Immersion time/day | R/(Ω) | η(%) | | EP | 0 | 8.653×10⁵ | \- | | | 10 | 5.573×10⁵ | \- | | HEO-1 | 0 | 9.902×10⁶ | 91.26 | | | 10 | 1.459×10⁶ | 40.69 | | HEO-2 | 0 | 6.324×10⁷ | 98.63 | | | 10 | 1.049×10⁷ | 91.75 | | HEO-3 | 0 | 2.637×10⁷ | 96.71 | | | 10 | 9.226×10⁶ | 90.62 | ## 3.4 Hydrophobicity Test The water absorption data obtained by the gravimetric method are presented in Figure 4. As shown in Figure 4, the water absorption rates of the epoxy composite coatings containing HEO fillers are significantly lower than that of the pure epoxy coating, and the increase in water absorption over time is also much slower. This improvement is attributed to the incorporation of HEO fillers, which partially fill the micro-voids generated during the curing process of the epoxy matrix, resulting in a denser coating structure that effectively inhibits water penetration and enhances hydrophobic performance. Furthermore, the high hardness, excellent corrosion resistance, and outstanding chemical stability of HEO contribute to greater resistance against intrusion by corrosive molecules, thereby maintaining the structural integrity of the coating and further improving its hydrophobicity.

**Figure 4.** Water absorption of different coating samples after soaking for different time. ## 3.5 Anticorrosion Mechanism Analysis The anticorrosion mechanism of the epoxy composite coating is illustrated in Figure 5. The corrosion protection primarily arises from the physical barrier effect provided by the anticorrosive fillers. Pure epoxy resin has high viscosity, which facilitates air entrapment during mixing with the curing agent, leading to the formation of micropore. In addition, solvent evaporation during epoxy curing also contributes to micropore formation. As shown in Figure 5(a), when corrosive species penetrate the coating, they can reach the metal substrate through these micropore, initiating corrosion reactions upon contact. Therefore, pure epoxy resin exhibits relatively poor corrosion resistance in electrochemical tests. Figure 5(b) presents the schematic diagram of the corrosion protection mechanism after incorporating HEO fillers. As an anticorrosive filler, HEO possesses high hardness, strong corrosion resistance, and excellent chemical stability owing to its lattice distortion effect, sluggish diffusion effect, and high-entropy effect. The appropriate addition of HEO not only suppresses bubble formation and fills the voids in the epoxy matrix, but also acts as a physical barrier, creating a “tortuous path” or “maze effect” that significantly delays the permeation of corrosive molecules toward the substrate.

**Figure 5.** Schematic diagram of corrosion resistance mechanism of composite coatings (a) pure epoxy resin coating (b) epoxy resin composite coating with HEO. # 4. Conclusion Electrochemical measurements demonstrated that the HEO-2 coating exhibits excellent corrosion resistance. Before immersion, the corrosion protection efficiency reached 99.39%. After immersion in 3.5wt% NaCl solution for 10 days, the coating retained a high protection efficiency of 95.57%. The impedance efficiency obtained from the Nyquist plots was 98.63%, and remained at 91.75% after 10 days of immersion. Hydrophobicity tests further confirmed that the HEO-2 epoxy composite coating possesses outstanding water-repellent properties. The initial water absorption rate was 2.37%, and only slightly increased to 2.72% after 10 days of immersion in 3.5wt% NaCl solution, indicating that the HEO-2 composite coating has a denser microstructure, resulting in improved hydrophobicity and enhanced corrosion protection. As a high-entropy oxide, Y₂(Ti_0.2Zr_0.2Hf_0.2Ce_0.2V_0.2)₂O₇ exhibits excellent structural stability, high hardness, and superior corrosion resistance. Its incorporation into the epoxy matrix significantly enhances the anticorrosion performance of the composite coating. The combination of inorganic fillers with organic coatings represents an important research direction for advanced protective coatings, offering broad application prospects and warranting further investigation. **References** 1. McMahon Matthew E, Santucci Raymond J. Jr, Glover Carol F, Kannan Balaji, Walsh Zachery R, Scully John R (2019) A Review of Modern Assessment Methods for Metal and Metal-Oxide Based Primers for Substrate Corrosion Protection. Frontiers in Materials.,6:190. 2. Sepideh P, Ebrahim G, Alimorad R, Mohammad R V (2018) Corrosion protection properties of novel epoxy nanocomposite coatings containing silane functionalized graphene quantum dot. Journal of Alloys and Compounds.,731:1112-1118. 3. H Liu, A Tang, W Xu, et al. (2025) Effect of carbon-based filler dimensions on the anti-corrosion performance for epoxy composite coating. Inorganic Chemistry Communications., 180(1): 114913. 4. Y Li, S Liu, F Feng, et al. (2024) Preparation and Characterization of Graphene Oxide/Carbon Nanotube/Polyaniline Composite and Conductive and Anticorrosive Properties of Its Waterborne Epoxy Composite Coatings. Polymers., 16(18): 264. 5. Almishal S.S.I, Furst M, Tan Y, et al. (2025) Thermodynamics-inspired high-entropy oxide synthesis. Nat Commun.16: 8211. 6. J Chen, X Li, et al. (2024) Influence of corrosion inhibitors on aging mechanism of epoxy resin coatings for copper 62 alloy in simulated marine environment. Corrosion Reviews., 43(4):457-467. 7. Gata Joseph A. (2023) Methodology for development of smart epoxy coatings incorporated with Ethylenediamine-N, N'-disuccinic ac-id (EDDS) layered double hydroxides (LDHs) for corrosion protection of XC38 carbon steel. Materials. INSA de Lyon.