Effect of Heat Treatment on the Microstructure and Corrosion Resistance of Al_0.75CoCr_1.25FeNi High-Entropy Alloys

Abstract

In this work, heat treatment of three different temperatures (600 °C, 800 °C, and 1000 °C) was applied to as-cast Al_0.75CoCr_1.25FeNi high-entropy alloys (HEAs) to investigate the influence of heat treatment on their corrosion properties. Open circuit potential (OCP) and cyclic polarization tests reveal that the 1000 °C heat-treated HEA possesses excellent corrosion resistance, as indicated by the low corrosion tendency and corrosion current density. Electrochemical impedance spectroscopy (EIS) and potentiostatic polarization analyses imply the presence of a superior passive film on the 1000 °C heat-treated HEA. X-ray photoelectron spectroscopy (XPS) analysis demonstrates that the passive film formed on the 1000 °C heat-treated HEA during potentiostatic polarization tests is most corrosion-resistant since it possesses the highest ratio of Al₂O₃/Al(OH)₃ and Cr₂O₃/Cr(OH)₃.

1. Introduction

HEAs, which are metallic alloys containing multiple (at least five) principal elements with near-equiatomic proportions, have garnered tremendous attention from researchers since being discovered in 2004 [1,2,3,4]. Owing to their extraordinary physical, chemical, and mechanical properties [5], HEAs possess promising applications in numerous industrial fields such as transportation, aerospace, defense, advanced machining, biomedical engineering, and energy [6,7,8]. The remarkable properties of HEAs can be ascribed to the existence of a random solid solution (RSS) phase in them [5].

RSS phases in HEAs include FCC, BCC, and HCP phases. HEAs with a single FCC phase commonly contain late 3d transition elements and are featured by high ductility but low strength [9]. In contrast, HEAs with a single BCC phase generally possess refractory metals and are characterized by high strength but low ductility [10]. The formation of HCP phases is typically related to rare-earth elements, raising the cost of HEAs [11]. To obtain HEAs with good strength and ductility, the idea of developing HEAs with both FCC and BCC phases has been proposed. AlCoCrFeNi HEAs are typical such HEAs. They possess excellent strength and ductility, as well as superior oxidation resistance and high-temperature properties. Therefore, they are promising structural materials for automobile and aerospace applications [12]. The mechanical properties of AlCoCrFeNi HEAs have been intensively investigated over the past decade [13,14,15]. Microstructure and mechanical performance of AlCoCrFeNi HEAs are susceptible to many factors, including element content, preparation method, and heat treatment [16].

In addition to mechanical properties, corrosion resistance is another essential property that needs to be considered when using HEAs, especially in damp and salt spray environments. The corrosion behavior of HEAs has recently evoked considerable interest in the scientific community. It was reported that the corrosion resistance of HEAs is sensitive to various metallic elements, such as Al, Cr, Co, Ni, Mo, and Ti [17,18]. Al is more active in the galvanic series than other common metallic elements, including Cr, Co, and Ni [17]. Corrosion resistance of HEAs decreases as the content of Al increases [19,20]. Cr plays a significant role in promoting the corrosion resistance of HEAs. A suitable Cr content obviously strengthens the passivation ability of HEAs. However, excessive Cr induces Cr segregation in interdendritic regions, resulting in severe pitting corrosion [17,21,22]. Co addition enhances the corrosion resistance of HEAs, and the corrosion resistance increases as Co content increases [23,24]. As an essential element in classic HEAs, Ni is very important to ensure the excellent corrosion resistance of HEAs. Ni usually exists in the passivation films of HEAs in the following three forms: Ni, NiO, and Ni(OH)₂ [18,25].

Although the influence of heat treatment on the mechanical properties of AlCoCrFeNi HEAs has been studied [26,27], its effect on the corrosion behavior of AlCoCrFeNi HEAs has rarely been examined. In this work, an as-cast Al_0.75CoCr_1.25FeNi HEA was heat treated at three different temperatures (i.e., 600 °C, 800 °C, and 1000 °C) to investigate the impact of heat treatment on its microstructure and corrosion resistance. Results presented in this study may facilitate the application of AlCoCrFeNi HEAs.

2. Materials and Methods

An as-cast Al_0.75CoCr_1.25FeNi HEA was taken as the study subject. The fabrication process of this HEA and its actual composition were given in the previous study [28]. The as-cast HEA was machined into specimens with a dimension of 10 mm × 10 mm × 2 mm. As-cast HEA samples were sealed in a quartz vacuum tube and annealed for 1 h at three different temperatures (i.e., 600 °C, 800 °C, and 1000 °C). Subsequently, as-cast and heat-treated specimens were sealed using epoxy resin, with a surface of 10 × 10 cm exposed for further testing. The exposed surfaces of specimens were then processed by mechanical grinding using sandpaper (from 600# to 2000#), polishing with 2.5 μm diamond paste, cleaning in ethanol, and drying with cold air.

The electrochemical properties of the investigated HEAs were assessed utilizing an electrochemical workstation (Gamry Reference 600+, Gamry Instruments, Inc., Philadelphia, PA, USA). A conventional three-electrode setup was adopted for these experiments. Specifically, an HEA specimen, a platinum plate with a 1 cm² surface area, and a saturated calomel electrode (SCE) were set as the working electrode, auxiliary electrode, and reference electrode, respectively. A 3.5 wt.% NaCl aqueous solution was employed as a corrosive medium, simulating a marine or saline environment. Cyclic potentiodynamic polarization tests were conducted at a scan rate of 0.333 mV/s, with a scan range of −1.0 to 1.0 V vs. SCE. Furthermore, electrochemical impedance spectroscopy (EIS) measurements were performed in the frequency range of 10⁴–10⁻² Hz under natural corrosion conditions. A sinusoidal waveform with an amplitude of 10 mV was employed as the excitation signal. All experiments were conducted at room temperature.

X-ray diffraction (XRD, XRD-6000) was employed to investigate the phase composition of HEAs at a scan rate of 6°/min. A scanning electron microscope (SEM, ZEISS Sigma 500) was used to observe the microstructure of HEAs. After immersion test in 3.5 wt.% NaCl solution, the exposed surfaces of samples were characterized by X-ray photoelectron spectroscopy (XPS, ESCALAB 250Xi T, ThermoFisher Scientific, Waltham, MA, USA). All corrosion tests were replicated three times to ensure the reproducibility of the obtained results.

3. Results and Discussion

3.1. Microstructure of Al_0.75CoCr_1.25FeNi HEA

XRD patterns of as-cast and heat-treated Al_0.75CoCr_1.25FeNi HEAs are displayed in Figure 1. Note from Figure 1 that the as-cast Al_0.75CoCr_1.25FeNi HEA consists of FCC, BCC1, and BCC2 phases. Interestingly, after heat treatment, the peak intensity of the FCC phase is significantly higher than that of the BCC1 and BCC2 phases, implying that the content of the FCC phase is higher than that of the BCC1 and BCC2 phases. This phenomenon agrees well with previous studies [29,30] on AlCoCrFeNi HEAs. Moreover, in contrast to the as-cast HEA and the HEAs heat treated at 600 °C and 1000 °C, one extra phase (σ phase) exists in the 800 °C heat-treated HEA. This is consistent with the previous report [26] that heat treatment at 650 °C to 975 °C induces the formation of the σ phase in AlCoCrFeNi HEAs. The σ phase, which is brittle and hard, possesses a tetragonal structure primarily composed of Cr and Fe [31,32].

Figure 2 exhibits SEM images of as-cast and heat-treated Al_0.75CoCr_1.25FeNi HEAs. It can be noted from Figure 2 that eutectic structures observed in the as-cast HEA still exist after heat treatment at the three different temperatures. In addition, the morphology of eutectic structures in the as-cast and heat-treated HEAs is similar. Compared to the as-cast HEA, the interface of eutectic cells is larger, and the lamellar structures are coarser in the heat-treated HEAs. Fine spherical particles exist in the BCC1 phase of the 600 °C heat-treated HEA, as exhibited in Figure 2d. Note from Figure 2e,f that the two phases of the eutectic structure in the 800 °C heat-treated HEA are interlaced with each other, and their shapes are various, presenting ordered long strips, ordered short-range patterns, and irregular loop-shaped patterns. As the heat treatment temperature rises to 1000 °C, the heat-treated HEA possesses the coarsest grains. In addition, ordered long strips dominate the microstructure of 1000 °C heat-treated HEA. Meanwhile, the spacing between the two phases increases, and the area occupied by the FCC phase enlarges with an increased proportion, which is consistent with the XRD analysis (Figure 1). This stabilizes the lattice structure and results in a more stable microstructure.

To examine the effect of heat treatment on the element distribution in the as-cast HEA, results of EDS analysis of the as-cast and heat-treated HEAs are given in Figure 3, Figure 4, Figure 5 and Figure 6. Noticeably, the essential features of element distribution in the as-cast and heat-treated HEAs are similar. Specifically, Co and Fe exhibit a relatively uniform distribution in the as-cast and heat-treated HEAs, while Al, Cr, and Ni are enriched in certain areas. The regions rich in both Al and Ni are the BCC2 phase [33].

3.2. Effect of Heat Treatment on Corrossion Resistance of Al_0.75CoCr_1.25FeNi HEA

Figure 7 shows the variation in open-circuit potential (OCP) of the as-cast and heat-treated Al_0.75CoCr_1.25FeNi HEAs in a 3.5 wt.% NaCl solution with immersion time. It is obvious that the OCP of as-cast as well as heat-treated HEAs eventually reaches a relatively stable value as the immersion time extends. The stable OCP values of the as-cast HEA and the HEAs heat treated at 600 °C, 800 °C, and 1000 °C are approximately −0.19 V vs. SCE, −0.18 V vs. SCE, −0.17 V vs. SCE, and −0.16 V vs. SCE, respectively. The stable OCP value can be used as a measure of corrosion tendency, with a high value indicating a low corrosion tendency and a low value implying a large corrosion tendency. The as-cast HEA has the smallest OCP, while the 1000 °C heat-treated HEA exhibits the largest OCP. Consequently, the corrosion tendency of the as-cast HEA alloy is the greatest, while that of the 1000 °C heat-treated HEA is the lowest. Therefore, heat treatment lowers the corrosion resistance of the as-cast HEA.

The cyclic polarization curves of the as-cast and heat-treated Al_0.75CoCr_1.25FeNi HEAs in a 3.5 wt.% NaCl solution are given in Figure 8. The cyclic polarization curve of the 600 °C heat-treated HEA exhibits the largest hysteresis loop area, indicating that this HEA possesses the highest possibility of pitting corrosion [34]. Obvious passivation regions can be observed in all HEAs, implying the formation of Al₂O₃ and Cr₂O₃ oxide films by Al and Cr during the electrochemical corrosion tests. The polarization curves of all HEAs directly shift from the Tafel region to the passivation region without an activation–passivation transition process. This indicates the natural formation of a protective passivation film under the open circuit potential. Intermittent current fluctuations occur in the passivation region, especially in the as-cast HEA and the 600 °C heat-treated HEA, which is a sign of the metastable pitting process.

Figure 8 presents the cyclic polarization curves of the Al_0.75CoCr_1.25FeNi HEAs in a 3.5 wt.% NaCl solution.

Table 1. Corrosion potential (E_corr), corrosion current density (i_corr), corrosion rate (V), pitting potential (E_pit), and passivation potential (E_pass) of as-cast and heat-treated Al_0.75CoCr_1.25FeNi HEAs in a 3.5 wt.% NaCl solution.

HEAs	As-Cast	600 °C HT	800 °C HT	1000 °C HT
Ecorr/mV vs. SCE	−217.6	−274.2	−230.7	−251.2
icorr/A∙cm−2	1.12 × 10−8	7.81 × 10−9	8.96 × 10−9	9.41 × 10−9
V/mm∙a−1	1.13 × 10−10	7.87 × 10−11	9.02 × 10−11	9.48 × 10−11
Epit/mV vs. SCE	186.7	35.3	181.2	160.8
Epass/mV vs. SCE	−135.2	−192.4	−150.2	−170.6

displays the corrosion potential (E_corr), corrosion current density (i_corr), corrosion rate (V), pitting potential (E_pit), and passivation potential (E_pass) of the as-cast and heat-treated Al_0.75CoCr_1.25FeNi HEAs in a 3.5 wt.% NaCl solution derived from the cyclic polarization curves shown in Figure 8. The lower i_corr represents a slower corrosion rate. The 600 °C heat-treated HEA has the lowest E_corr and thus the largest corrosion tendency, consistent with that indicated by OCP. The corrosion rate of the as-cast HEA is highest. Heat treatment significantly lowers i_corr, but it exhibits a rising trend with increasing heat treatment temperature. Taking both corrosion tendency and corrosion rate into account, the 800 °C and 1000 °C heat-treated HEAs exhibit relatively better corrosion resistance.

SEM images of the as-cast and heat-treated Al_0.75CoCr_1.25FeNi HEAs after cyclic polarization tests in a 3.5 wt.% NaCl solution are shown in Figure 9. Recalling Figure 2, Figure 3, Figure 4, Figure 5 and Figure 6, note from Figure 9 that corrosion pits exist in the regions rich in Al and Ni. Consequently, corrosion pits in the as-cast HEA and 600 °C heat-treated HEA are reticular, while those in the 800 °C and 1000 °C heat-treated HEAs are mainly strip-like. The higher corrosion tendency of the regions rich in Al and Ni can be attributed to two reasons. Firstly, the nonequilibrium potential of Al in the 3.5 wt.% NaCl solution is the lowest among the five principal elements in the Al_0.75CoCr_1.25FeNi HEA. Secondly, Al and Ni tend to combine with Cl⁻ in the 3.5 wt.% NaCl solution, leading to local rupture of the passive film formed on the surface of the regions rich in Al and Ni.

The metastable pitting corrosion phenomenon in the passivation region is closely related to the formation, development, and repassivation of metastable pits on the surface of HEAs. Pitting corrosion first initiates in the weak sites (such as regions rich in Al and Ni) of the passive film on the surface of HEAs. To neutralize metal cations, anions in the solution (such as Cl⁻, OH⁻) tend to accumulate at these weak sites. Meanwhile, the combination of metal cations and OH⁻ acidifies the local solution, thereby promoting the development of pitting corrosion and increasing the current density. When the dissolution of metals is inhibited by the diffusion and dilution of local anions or H⁺, repassivation occurs, leading to a decrease in current density. The alternation of activation and passivation processes within the pits results in transient currents in the passivation region [20].

Electrochemical impedance spectroscopy (EIS) was conducted to analyze the passivation behavior of as-cast and heat-treated Al_0.75CoCr_1.25FeNi HEAs in a 3.5 wt.% NaCl solution. As displayed in Figure 10a, Nyquist plots of as-cast and heat-treated HEAs exhibit incomplete single capacitive arcs. The arcs for the HEAs heat-treated at 800 °C and 1000 °C are significantly larger than those of the as-cast and 600 °C heat-treated HEAs. A larger radius of the capacitive arc indicates superior corrosion resistance [35]. Bode plots were plotted to further reveal EIS characteristics of HEAs, as exhibited in Figure 10b. The overall trend of the phase angle curves of the as-cast and heat-treated HEAs is similar, showing only one time constant. The maximum phase angles of the as-cast, 600 °C, 800 °C, and 1000 °C heat-treated HEAs are 84.3°, 80.7°, 83.4°, and 80.1°, respectively, all around 80°, indicating the good stability of the passive films. What is more, the impedance modulus of the as-cast and heat-treated HEAs reaches the maximum value in the low-frequency region and then rapidly decreases to a stable level. The maximum impedance moduli of the as-cast, 600 °C, 800 °C, and 1000 °C heat-treated HEAs are 2.04 × 10⁵ Ω·cm², 2.25 × 10⁵ Ω·cm², 5.13 × 10⁵ Ω·cm², and 4.32 × 10⁵ Ω·cm², respectively. The corrosion resistance of an alloy is positively correlated with its impedance modulus [36]. Therefore, Nyquist and Bode plots disclose that the passivation behavior of the as-cast and heat-treated HEAs in the 3.5 wt.% NaCl solution is similar. Moreover, the 800 °C and 1000 °C heat-treated HEAs exhibit better corrosion resistance, in close agreement with that indicated by OCP and cyclic polarization curves.

Since the passivation behavior of the as-cast and heat-treated Al_0.75CoCr_1.25FeNi HEAs is similar, the same equivalent electrical circuit (shown in Figure 11) was used to simulate the experimental results, with the values of equivalent electrical circuit elements given in

Table 2. Values of equivalent electrical circuit elements (Figure 11) for the as-cast and heat-treated Al_0.75CoCr_1.25FeNi HEAs in a 3.5 wt.% NaCl solution.

HEAs	As-Cast	600 °C HT	800 °C HT	1000 °C HT
Rs/Ω·cm2	7.16	10.76	12.57	15.96
Rp/Ω·cm2	2.39 × 105	2.48 × 105	5.98 × 105	7.30 × 105
Qp/Ω−1·cm−2∙s−n	2.76 × 10−5	1.59 × 10−5	1.06 × 10−5	1.83 × 10−4
n	0.93	0.90	0.92	0.88

. Here, n, R_s, R_p, and Q_p denote the dispersion index, solution resistance, polarization resistance, and capacitance of the passive film, respectively. The values of n of the as-cast, 600 °C, 800 °C, and 1000 °C heat-treated HEAs in the 3.5 wt.% NaCl solution are 0.93, 0.90, 0.92, and 0.88, respectively, all within the range of 0.8 to 1. This indicates that a passive film is formed on the surface of the as-cast and heat-treated HEAs, and the passive film is smooth and uniform. R_p of the as-cast, 600 °C, 800 °C, and 1000 °C heat-treated HEAs are 2.39 × 10⁵ Ω·cm², 2.48 × 10⁵ Ω·cm², 5.98 × 10⁵ Ω·cm², and 7.30 × 10⁵ Ω·cm², respectively. R_p gradually increases with the increase in heat treatment temperature, and it experiences the largest rise when the temperature increases from 600 °C to 800 °C. Therefore, the 800 °C and 1000 °C heat-treated HEAs exhibit superior corrosion resistance in the 3.5 wt.% NaCl solution, and the 1000 °C heat-treated HEA possesses a more corrosion-resistant passive film.

Potentiostatic polarization tests were performed on the as-cast and heat-treated HEAs in a 3.5 wt.% NaCl solution to explore the formation process of passive films, the results of which are displayed in Figure 12. The holding potential for potentiostatic polarization tests was 0 V vs. SCE. Figure 12a exhibits the current–time transient curves of the as-cast and heat-treated HEAs. In the initial stage (from 0 to 100 s), the current density of all HEAs decreases rapidly, indicating the nucleation and rapid growth of the passive film. Subsequently, the current density seems to stabilize at a certain level, with very slight fluctuations. After about 1200 s, the current density of the 600 °C heat-treated HEA exhibits an overall upward trend, with relatively large fluctuations, while that of other HEAs is still stable. This implies that the passive film of the 600 °C heat-treated HEA is not stable, unlike that of other HEAs. It is obvious from Figure 12a,b that the stable current density of the 1000 °C heat-treated HEA is much lower than that of other HEAs, indicating that the passive film of the 1000 °C heat-treated HEA possesses the best stability.

Mott–Schottky analysis was conducted to evaluate the semiconductive characteristics of passive films formed on the as-cast and heat-treated HEAs. Based on the Mott–Schottky theory [37,38], the relation between C_sc and potential E is given as

$$\frac{1}{C_{\mathrm{s}\mathrm{c}}^2}=\frac{2}{\epsilon {\epsilon }_{0}e{N}_{\mathrm{D}}}\left(E-E_{\mathrm{F}\mathrm{B}}-\frac{kT}{e}\right) \mathrm{for}\mathrm{n}\text{-}\mathrm{type}\mathrm{semiconductor}$$

(1)

$$\frac{1}{C_{\mathrm{s}\mathrm{c}}^2}=-\frac{2}{\epsilon {\epsilon }_0{eN}_{\mathrm{A}}}\left(E-E_{\mathrm{F}\mathrm{B}}-\frac{kT}{e}\right) \mathrm{for}\mathrm{p}\text{-}\mathrm{type}\mathrm{semiconductor}$$

(2)

where $\epsilon$, ${\epsilon }_0$, $e$, $E_{\mathrm{F}\mathrm{B}}$, $k$, $T$, $N_{\mathrm{D}}$, and $N_{\mathrm{A}}$ denote the dielectric constant of the passive film (taken as 15.6 [39]), vacuum permittivity constant elementary charge (8.85 × 10⁻¹⁴ F·cm⁻¹ [40]), elementary charge (1.602 × 10⁻¹⁹ C), flat band potential, Boltzmann constant, absolute temperature, donor density, and acceptor density, respectively. A positive slope signifies n-type behavior, while a negative slope represents p-type behavior [37]. The Mott–Schottky curves of the passive films formed on the as-cast and heat-treated HEAs are shown in Figure 12c. Noticeably, the Mott–Schottky curves exhibit a positive slope in the potential range of −1.0 V to −0.8 V, while the slope shifts to a negative value as the potential increases from −0.8 V to −0.6 V. This phenomenon means that the passive films of the as-cast and heat-treated HEAs exhibit both n-type and p-type semiconductive behavior, implying that the passive films are bilayer-structured [37].

Based on Equations (1) and (2), $N_{\mathrm{D}}$ and $N_{\mathrm{A}}$ can be calculated, the results of which are given in Figure 12d and

Table 3. N_A and N_D of as-cast and heat-treated Al_0.75CoCr_1.25FeNi HEAs in a 3.5 wt.% NaCl solution.

HEAs	As-Cast	600 °C HT	800 °C HT	1000 °C HT
ND	1.46 × 1020	1.52 × 1020	1.29 × 1020	9.93 × 1019
NA	1.36 × 1020	1.46 × 1020	1.03 × 1020	8.65 × 1019

. $N_{\mathrm{D}}$ and $N_{\mathrm{A}}$ are measures of defect density in passive films, with a higher $N_{\mathrm{D}}$ and $N_{\mathrm{A}}$ signifying a higher defect density and thus a more unstable film structure [41]. Therefore, passive films with a higher $N_{\mathrm{D}}$ and $N_{\mathrm{A}}$ are more vulnerable to ion accumulation and penetration. The $N_{\mathrm{D}}$ and $N_{\mathrm{A}}$ of the as-cast and 600 °C heat-treated HEAs are high, and they drop rapidly as the heat treatment temperature increases from 600 °C to 800 °C and further to 1000 °C. The 1000 °C heat-treated HEAs possess the lowest $N_{\mathrm{D}}$ and $N_{\mathrm{A}}$ and thus the best corrosion resistance, consistent with that implied by OCP, cyclic polarization curves, and EIS analysis.

The passive films grown on the surface of as-cast and heat-treated HEAs by potentiostatic polarization in a 3.5 wt.% NaCl solution for 1 h were further investigated by XPS analysis. Results of XPS analysis are given in Figure 13, Figure 14 and Figure 15. Note from Figure 13 that the spectra of Al 2p3/2 for as-cast and heat-treated HEAs exhibit three peaks, i.e., metallic Al, Al₂O₃, and Al(OH)₃ [42]. The spectra of Cr 2p3/2 for as-cast and heat-treated HEAs also possess three peaks, namely, metallic Cr, Cr₂O₃, and Cr(OH)₃ [43], as displayed in Figure 14. Three peaks (O²⁻, OH⁻, and H₂O) are observed in the spectra of O 1s for as-cast and heat-treated HEAs (Figure 15) as well. Although heat treatment does not affect the composition of peaks, it exerts some influence on the intensity of peaks. The content of Al₂O₃ and Cr₂O₃ can be used to evaluate the quality of the passivation film. The higher the ratios of Al₂O₃/Al(OH)₃ and Cr₂O₃/Cr(OH)₃, the denser the passivation film, and the better its protective effect on the HEA. The as-cast HEA has the lowest ratios, with Al₂O₃/Al(OH)₃ and Cr₂O₃/Cr(OH)₃ ratios of 27.32/41.67 and 26.02/48.58, respectively. Heat-treated HEAs exhibit higher Al₂O₃/Al(OH)₃ and Cr₂O₃/Cr(OH)₃ ratios than the as-cast HEA, and the ratios increase with increasing heat treatment temperature. The 1000 °C heat-treated high-entropy alloy possesses the highest Al₂O₃/Al(OH)₃ and Cr₂O₃/Cr(OH)₃ ratios of 45.03/28.23 and 47.41/35.20, respectively. Consequently, the passive film of the 1000 °C heat-treated HEA is denser and more stable than that of other HEAs.

3.3. Corrosion Mechanism

A schematic diagram showing the corrosion mechanism of as-cast and heat-treated Al_0.75CoCr_1.25FeNi HEAs in a 3.5 wt.% NaCl solution is presented in Figure 16. In the 3.5 wt.% NaCl solution, the chemical composition of the passive film is related to the substrate. The content of Al₂O₃ in the passive film on the surface of Al and Ni-rich phases is relatively higher. However, Wang et al. [44] found that in a 3.5 wt.% NaCl solution, Cl⁻ is adsorbed on the surface of the passive film. Under the synergistic effect of Cl⁻ and H₂O, the Al₂O₃ gradually transforms into soluble AlCl₃. The reaction process is as follows:

Al₂O₃ + 3H₂O → Al₂O₃·3H₂O(ad) → 2Al(OH)₃

(3)

2Al(OH)₃ + Cl⁻ → Al(OH)₂Cl + OH⁻

(4)

Al(OH)₂Cl + Cl⁻ → Al(OH)Cl₂ + OH⁻

(5)

Al(OH)Cl₂ + Cl⁻ → AlCl₃ + OH⁻

(6)

This would lead to the deterioration of the protective ability of the passive film.

4. Conclusions

In this work, an as-cast Al_0.75CoCr_1.25FeNi HEA was heat treated at three different temperatures (600 °C, 800 °C, and 1000 °C) to investigate the influence of heat treatment on its corrosion resistance. The 1000 °C heat-treated HEA possesses the largest OCP and thus the lowest corrosion tendency in the 3.5 wt.% NaCl solution. Cyclic polarization tests reveal that the 800 °C and 1000 °C heat-treated HEAs exhibit relatively better corrosion resistance, as indicated by the large corrosion potential and low corrosion current density. EIS analysis demonstrates that the 1000 °C heat-treated HEA has the largest impedance modulus and polarization resistance as well as a large maximum phase angle, implying the presence of an excellent passive film. This result is supported by potentiostatic polarization tests, which show that the passive film on the 1000 °C heat-treated HEA possesses the lowest current density, donor density, and acceptor density. The passive films of as-cast and heat-treated HEAs exhibit both n-type and p-type semiconductive behavior, implying that they are bilayer-structured. The passive film of the 1000 °C heat-treated HEA exhibits the best corrosion resistance because it is most stable according to the potentiostatic polarization tests, as indicated by the largest ratio of Al₂O₃/Al(OH)₃ and Cr₂O₃/Cr(OH)₃.