Effect of Heat Treatment on Corrosion of an AlCoCrFeNi_2.1 Eutectic High-Entropy Alloy in 3.5 wt% NaCl Solution

Abstract

This paper studies how heat treatments influence the corrosion of an AlCoCrFeNi_2.1 eutectic high-entropy alloy (EHEA) in a 3.5 wt% NaCl solution, by comparing the corrosion behaviors of as-cast, 600 °C heat-treated, and 1000 °C heat-treated samples using microstructure characterization, electrochemical measurements, and surface characterization. The electrochemical results show that the pitting potential rises and the passive current density and passive film resistance are almost changeless with an increasing heat treatment temperature. The enhancement in the pitting corrosion resistance results from the increased amount of the Cr-rich FCC phase and decreased amount of the B2 phase rich in the Al element, which are induced by the heat treatment. On one hand, this microstructure evolution can make the passive film have more Cr₂O₃ and less Al₂O₃, thereby enhancing its protective properties, as confirmed by the X-ray photoelectron spectroscopy analysis. On the other hand, the decreased amount of the Al-rich B2 phase can make the pitting corrosion less prone to initiate since the B2 phase can act as the pit initiation site, which is supported by the observation of corrosion morphologies, due to its higher electrochemical activity. In a summary, the heat treatment is beneficial for improving the pitting corrosion resistance of the AlCoCrFeNi_2.1 EHEA.

1. Introduction

High-entropy alloys (HEAs), as a new type of multi-component alloy, typically consist of five or more primary elements. This multi-main-element characteristic endows HEAs with high-entropy, lattice distortion, the sluggish diffusion, and the “cocktail” effects, i.e., the named four core effects [1,2]. Consequently, HEAs exhibit excellent properties, such as good mechanical properties, corrosion resistance, and wear resistance [3,4,5,6,7,8,9].

In recent years, Lu et al. [10,11] introduced eutectic structures into these alloys and successfully prepared the AlCoCrFeNi_2.1 eutectic high-entropy alloy (EHEA), bringing EHEAs into the spotlight. EHEAs are an important member of the HEA family, distinguished by a structure that consists of an orderly arrangement of solid solution phases containing more different crystal structures formed through eutectic reactions. The unique microstructure of EHEAs endows them with both high strength and ductility, so they have broad application prospects in many fields. Moreover, the emergence of EHEAs addresses shortcomings such as the poor castability of traditional HEAs, offering better casting and thermomechanical processing performance. As a result, they show great practical application potentials in metal components requiring direct casting, such as propellers, rudders, and valves [12].

Due to its unique compositional design philosophy and excellent comprehensive properties, EHEAs have become a prominent subject of research within the field of materials science. Corrosion is an important consideration for the application of EHEAs [13,14,15,16,17,18,19]. However, current research on EHEAs primarily focuses on their mechanical properties, with relatively limited studies on failure mechanisms in corrosive environments. Song et al. [13] studied the corrosion of an AlCoCrFeNi_2.1 alloy in chloride-containing sulfuric acid solutions and discovered that the B2 body-centered cubic (BCC) phase underwent pitting corrosion. Hasannaeimi et al. [14] employed the scanning vibrating electrode technique to evaluate the galvanic corrosion behavior of an AlCoCrFeNi_2.1 alloy, finding that the corrosion was initiated by the preferential dissolution of the B2 BCC phase, which acted as the anode relative to the L1₂ FCC phase in locally formed micro-galvanic cells within the microstructure. Mao et al. [16] examined the corrosion behavior of an AlCoCrFeNi_2.1 EHEA, in comparison with a duplex stainless steel in simulated oil field-produced water, finding that this EHEA showed superior resistance to corrosion. Fu et al. [17] evaluated the corrosion resistance of as-cast and annealed AlCoCrFeNi_2.1 alloys in a H₂SO₄ solution, reporting that the resistance to corrosion in the annealed state was enhanced, as compared with the as-cast state, which resulted from the increased proportion of oxides in the passive film and increased thickness of the passive film, as well as the reduction in compositional disparity between the B2 phase and the face-centered cubic (FCC) phase. Duan et al. [19] discovered that the AlCoCrFeNi_2.1 alloy, after aging treatment, contained a large amount of nanoscale L1₂ FCC and BCC phases, which exhibited improved corrosion resistance compared to the samples after the solution treatment, with no precipitates.

The summary of the aforementioned literature shows that the study of the corrosion of EHEAs mainly focuses on the AlCoCrFeNi_2.1 alloy, and its corrosion is highly dependent on the eutectic microstructure formed during casting and the precipitates formed during heat treatment. In AlCoCrFeNi_2.1 EHEAs, the FCC/B2 layered structure poses a potential risk for corrosion, due to the micro-galvanic effect caused by the difference between the Cr-rich FCC phase and Al-rich B2 phase. The presence of the aging-induced multiphase microstructure in EHEAs could further complicate the corrosion mechanism [20]. It is well known that the microstructure of EHEAs is significantly influenced by the heat treatment, with temperature serving as a critical factor in this process [21,22]. However, only a limited number of studies have specifically examined the impact of the heat treatment temperature on the corrosion behavior of the AlCoCrFeNi_2.1 EHEA, and most of them investigated the cases of high-temperature (>1000 °C) heat treatments [17,23,24]. Considering that the low-temperature (<1000 °C) heat treatment can also influence the microstructure of the AlCoCrFeNi_2.1 EHEA, thereby affecting its corrosion resistance, further investigation is warranted to clarify the effect of low-temperature heat treatments on corrosion, in order to further understand the heat treatment–microstructure–corrosion relationship of the AlCoCrFeNi_2.1 EHEA, which could be meaningful for modulating the corrosion resistance of this EHEA by heat treatments.

Accordingly, the present study focuses on the effect of a low-temperature heat treatment (600 °C and 1000 °C) on the corrosion of an AlCoCrFeNi_2.1 EHEA using electrochemical measurements by comparing the corrosion resistance of heat-treated samples with that of as-cast samples. The evolution of the microstructure induced by the heat treatment was characterized, and the surface analysis was performed to elucidate the changes in corrosion resistance.

2. Experimental

2.1. Materials and Surface Preparation

High-purity (>99.9%) raw Al, Co, Cr, Fe and Ni materials were used to prepare the ingots (1 kg) using a vacuum induction furnace, and the ingot underwent remelting five times to ensure compositional uniformity. The chemical composition (at%) of the as-cast ingot was measured as Al 16.1, Co 16.9, Cr 16.6, Fe 16.2, and Ni 34.2 using an inductively coupled plasma emission spectrometer (Optima 8300DV, PerkinElmer, Waltham, MA, USA). The samples with a dimension of 4 mm × 4 mm × 2 mm was cut from the as-cast ingot. Then, they were heated in a furnace under an atmosphere condition at a certain temperature for 2 h, followed by water quenching. The heat treatment temperature was 600 ± 2 °C and 1000 ± 2 °C, and the corresponding samples were named as 600 °C and 1000 °C samples, respectively. Subsequently, the surfaces of the samples were ground to 5000# using sandpaper for the X-ray photoelectron spectroscopy (XPS) analysis, and then polished with 0.05 μm colloidal SiO₂ suspension for the surface morphology observation. The samples used for the electron backscatter diffraction (EBSD) analysis underwent an electro-polishing treatment after grinding. For electrochemical tests, the samples were first connected to copper wires using a conductive adhesive on the back and then embedded with epoxy resin. Finally, the exposed surface area was 0.16 cm². After the epoxy resin had cured, the surfaces were polished again to 5000#. Before each test, the polished or ground samples were cleaned with ethanol and water, then allowed to dry in air.

2.2. Microstructure Characterization

The microstructural characterization was performed for the as-cast, 600 °C, and 1000 °C samples using a scanning electron microscope (SEM, Zeiss Supra 55, Zeiss, Oberkochen, Germany), which was equipped with an EBSD probe, at an accelerating voltage of 25 kV and a beam current of 13 nA. The electron beam was scanned point-by-point with a step size of 0.8 μm. The Inverse Pole Figure (IPF), grain, and phase maps were exported using specialized software to obtain the kernel average misorientation (KAM) map. The EBSD data were processed and analyzed utilizing OIM Analysis 7.3 software.

2.3. Electrochemical Measurements

Electrochemical measurements were used to assess the corrosion resistance, which were performed in a typical three-electrode system using an electrochemical workstation (Gamry Reference 600P, Gamry Instruments, Warminster, PA, USA). In the three-electrode system, the sample acted as the working electrode, a platinum foil was used as the auxiliary electrode, and the reference electrode was the saturated calomel electrode (SCE) that was connected to the solution through a salt bridge. Initially, the open circuit potential (OCP) was measured for 1 h to reach a quasi-steady state. Following this, a 10 mV sinusoidal potential disturbance was applied at the OCP, with a frequency range from 100 kHz to 10 mHz, to conduct the electrochemical impedance spectroscopy (EIS) tests. The EIS parameters were obtained by fitting the EIS spectra using an equivalent circuit with Gamry Echem Analyst software (Version 6.33). The potentiodynamic polarization tests started from −300 mV vs OCP with a forward potential scan at a rate of 0.1667 mV/s until the anodic current density reached 1 mA/cm². The corrosive solution was 3.5 wt% NaCl at a temperature of 25.0 ± 0.5 °C. All electrochemical measurements were conducted in triplicate to ensure reproducibility, while the potentiodynamic polarization tests were repeated 5 to 7 times.

2.4. Surface Analyses

XPS analysis was conducted by using a Thermo VG ESCALAB250 (Waltham, MA, USA) X-ray photoelectron spectrometer to analyze the composition of the passive film formed on the as-cast, 600 °C, and 1000 °C samples after immersion in a 3.5 wt% NaCl solution for 24 h. A monochromatic Al Kα (1486.6 eV) X-ray source was employed, operating at a power of 150 W (15 kV, 10 mA). The photoelectron take-off angle was set at 90°, and the base pressure was approximately 10⁻⁸ Pa. The XPS measurements were conducted at a total energy of 50.0 eV, utilizing an energy step of 0.1 eV and achieving an energy resolution of 0.5 eV. The detailed XPS spectra of Al 2p, Co 2p, Cr 2p, Fe 2p, Ni 2p, O 1s, and C 1s were obtained, and the analysis area was 500 μm × 500 μm. All binding energies were calibrated using the standard C 1s peak at 284.6 eV, and the binding energies from the NIST XPS database were used for reference. The peak fitting of detailed XPS spectra was conducted using the XPS Peak 4.1 software. Additionally, the surface corrosion morphologies of the as-cast samples, as well as those treated at 600 °C and 1000 °C, were observed using the SEM after conducting the potentiodynamic polarization tests in a 3.5 wt% NaCl solution. The SEM observation was performed at an acceleration voltage of 20 kV and a beam current of 20 nA.

3. Results and Discussion

3.1. Microstructure Characterization

Figure 1 displays the SEM microstructure of the as-cast (Figure 1a), 600 °C-treated (Figure 1b), and 1000 °C-treated (Figure 1c) AlCoCrFeNi_2.1 EHEA samples. All the samples exhibit a typical eutectic lamellar structure characteristic, which is consistent with the literature [6,22,25,26]. The lamellar structure is comprised of two phases. The EDS mapping indicates that the dark regions are enriched in Ni and Al elements, while the bright regions are enriched in Cr, Fe, and Co elements. Meanwhile, as seen from Figure 1a, the interlayer spacing of the lamellar structure is approximately 2 μm for the as-cast sample. When the heat treatment temperature rises, the interlayer spacing widens, and the lamellar structure becomes significantly coarser, with a more pronounced interface between the two phases.

Figure 2 displays the EBSD results, including the IPF, phase, and KAM maps. Both the as-cast and heat-treated AlCoCrFeNi_2.1 EHEAs exhibit a random orientation, as shown in Figure 2a1–c1. The phase map shown in Figure 2a2–c2 reflects the distribution and proportion of different phases. It is obvious that all the studied EHEAs consist of FCC and BCC phases. Based on the literature and EDS mapping results (Figure 1), it can be confirmed that the BCC and FCC phases are, respectively, B2 and L1₂ phases [6,15,22,25,26].

Table 1. Phase composition ratio (%) obtained from Figure 2a2–c2.

Samples	BCC (B2)	FCC (L12)
as-cast	42	58
600 °C	38	62
1000 °C	33	67

lists the volume fractions of the FCC (L1₂) and BCC (B2) phases. With increases in the heat treatment temperature, the volume fraction of the L1₂ FCC phase increases while that of the B2 BCC phase decreases. Furthermore, it is observed that the distribution of the L1₂ FCC phase becomes more uniform. The KAM maps shown in Figure 2a3–c3 are used to characterize the geometric dislocation density within the crystal material, where brighter colors indicate a higher dislocation density. The KAM maps indicate that the B2 phase displays a higher dislocation density, which suggests increased electrochemical activity in that area [27,28]. The heat treatment at 1000 °C can decrease the area of the region with a brighter color, which could be beneficial for the corrosion resistance.

3.2. Corrosion Behavior

3.2.1. Potentiodynamic Polarization Tests

Figure 3 presents the potentiodynamic polarization curves of as-cast, 600 °C-treated, and 1000 °C-treated AlCoCrFeNi_2.1 samples in a 3.5% NaCl solution. All three samples show spontaneous passive behavior, suggesting the spontaneous formation of a passive film on the sample surface under the free-corrosion condition [29,30]. The two heat-treated samples have a broader passivation range, while the as-cast sample shows the lowest pitting potential. With increases in the heat treatment temperature, the pitting potential rises. The sample after the heat treatment at 1000 °C exhibits the largest passivation range and the highest pitting potential, implying the best corrosion resistance.

To quantitatively analyze the influence of the heat treatment on the corrosion behavior, the electrochemical corrosion parameters were extracted from the potentiodynamic polarization curves, as shown in

Table 2. Electrochemical parameters derived from the potentiodynamic polarization curves presented in Figure 3.

Samples	Ecorr (mVSCE)	Epit (mVSCE)	Epit − Ecorr (mVSCE)	Icorr (μA/cm2)	−βc (mV/dec)	ipass at −0.1 VSCE (μA/cm2)
As-cast	−242 ± 7	135 ± 15	377 ± 47	0.28 ± 0.01	174 ± 5	0.34 ± 0.03
600 °C	−236 ± 9	171 ± 21	407 ± 42	0.29 ± 0.02	172 ± 8	0.35 ± 0.02
1000 °C	−228 ± 6	210 ± 27	438 ± 53	0.28 ± 0.04	180 ± 4	0.33 ± 0.02

, including the pitting potential (E_pit), corrosion potential (E_corr), and the passive current density (i_pass), as well as the corrosion current density (I_corr) and cathodic Tafel slope (β) obtained by cathodic Tafel fitting. As seen in Table 2, the heat treatment has a minor effect on E_corr, I_corr, β, and i_pass, but significantly alters E_pit and the range of the passivation potential region (E_pit-E_corr). As the heat treatment temperature increases, the E_pit gradually increases. Specifically, the E_pit rises from 135 ± 15 mV_SCE (as-cast sample) to the maximum value of 210 ± 27 mV_SCE (1000 °C sample), indicating that the low-temperature heat treatment can enhance the pitting corrosion resistance of the AlCoCrFeNi_2.1 alloy. The phenomenon that the pitting resistance of AlCoCrFeNi_2.1 EHEAs in chloride-containing solutions improves with increases in the heat treatment temperature has also been reported in the literature [23,24]. This demonstrates that the heat treatment is beneficial for enhancing the pitting resistance AlCoCrFeNi_2.1 EHEAs.

3.2.2. EIS Tests

Figure 4 presents the EIS spectra of the as-cast and heat-treated AlCoCrFeNi_2.1 alloys in a 3.5 wt.% NaCl solution. As seen in Figure 4a, for all the three samples, the Nyquist plots exhibit a distinct capacitive arc with relatively high impedance values, indicating the formation of a dense protective passive film on the sample surface [29]. From the Bode plot shown in Figure 4b, it can be observed that all the samples possess a large phase angle (close to 90°) in a wide frequency range, and the impedance modulus reaches as high as 10⁶ Ω cm² at the lowest frequency of 0.01 Hz, further confirming the formation of a protective passive film on the sample surface [29].

To quantitatively evaluate the effect of heat treatment on the EIS responses, the widely used equivalent circuit, which is shown in the inset of Figure 4a, was employed to fit the EIS spectra using the Gamry Echem Analyst software (Version 6.33) [31,32]. In this equivalent circuit, R_s represents the solution resistance, while Q_f and R_f denote the capacitance and resistance of the passive film, respectively. Due to the non-uniformity that develops on the sample surface during corrosion, a constant phase element (CPE) was used to replace the ideal capacitive element to improve the fitting quality. The impedance of the CPE can be expressed as follows [33]:

$$Z_{\mathrm{C}\mathrm{P}\mathrm{E}}={\displaystyle \frac{1}{Q{\left(jw\right)}^n}}$$

(1)

where Q represents the CPE modulus, ω is the angular frequency, and n is the CPE exponent. Typically, the value of n is less than 1, and when n equals 1, it corresponds to an ideal capacitor. The EIS parameters obtained by fitting the EIS spectra using the equivalent circuit are presented in

Table 3. Electrochemical parameters obtained by fitting the EIS spectra using an equivalent circuit.

Samples	Rs (Ω cm2)	Qf (μF cm−2 S−n)	Rf (kΩ cm2)	n	χ2
As-cast	3.2 ± 0.2	36.9 ± 5.2	540 ± 14	0.91 ± 0.01	5 ± 3 × 10−4
600 °C	2.6 ± 0.2	34.7 ± 1.5	520 ± 9	0.92 ± 0.01	8 ± 2 × 10−4
1000 °C	3.2 ± 0.1	39.1 ± 4.9	510 ± 23	0.94 ± 0.01	3 ± 2 × 10−4

. It can be observed that passive film resistance R_f reaches as high as 10⁵ Ω cm² for all the three samples, indicating that the passive film has good protectiveness [29]. Combined with the spontaneous passive behavior exhibited in the potentiodynamic polarization curves (Figure 3), it is sure that both the as-cast and heat-treated AlCoCrFeNi_2.1 alloys in the 3.5 wt% NaCl solution are passive materials. Typically, the polarization resistance (R_p) is one of the indicators used for evaluating the general corrosion resistance of passive materials [29]. According to the definition of R_p (which is the Faradaic impedance of the equivalent circuit at ω = 0), R_p should equal to R_f in the present cases. As seen in Table 3, considering the errors, it seems that the heat treatment has few effects on the R_p, i.e., the general corrosion resistance. This agrees well with the phenomenon that the i_pass is almost independent of the heat treatment, as shown in Table 2, since the i_pass is inversely proportional to R_p [32].

3.3. Corrosion Morphology Observation

To further clarify the impact of the heat treatment on pitting corrosion resistance of the AlCoCrFeNi_2.1 alloy, the surface corrosion morphologies of as-cast, 600 °C, and 1000 °C samples after conducting the potentiodynamic polarization tests in the 3.5 wt% NaCl solution was characterized and the EDS mapping was performed correspondingly, as displayed in Figure 5. It is clear that corrosion pits are visible on all the three sample surfaces, with the largest and smallest corrosion pits observed on the surfaces of as-cast and 1000 °C samples, respectively. In other words, as the heat treatment temperature increases, the size of corrosion pits decreases. Figure 6 shows a magnified observation of the area near the corrosion pits of the 1000 °C sample, combined with the EDS mapping analysis. It can be observed that the corrosion occurring in the B2 phase is more severe than that in the FCC phase, which could result in the formation of corrosion pits to a certain extent. According to Hasannaeimi [14], during the corrosion process, a potential difference exists between the FCC phase containing more corrosion-resistant Cr element and the (Ni, Al)-rich B2 phase. In this situation, the B2 phase exhibits higher activity, acting as the local anode with a higher corrosion rate and protecting the cathodic FCC phase. As a result, the B2 phase in the AlCoCrFeNi_2.1 EHEA can initiate the occurrence of pitting by preferential corrosion. As the heat treatment temperature increases, the amount of B2 phase decreases, thus leading to less severe pitting corrosion. This aligns with the electrochemical results.

3.4. XPS Analysis

The composition of the passive film is one of the important factors affecting the both the general and pitting corrosion of metallic materials. To explain the differences in the corrosion resistance of the three EHEAs, XPS was employed to characterize the passive film composition. Figure 7, Figure 8 and Figure 9 present the fitting results of the detailed Cr 2p, Fe 2p, Ni 2p, Co 2p, Al 2p, and O1s XPS spectra for the as-cast, 600 °C, and 1000 °C samples, respectively. Regardless of the type of samples, the passive film composition consists of nearly identical types of substances, including the metallic elements (Al, Cr, Fe, Co, and Ni), metallic oxides (Al₂O₃, CoO, Cr₂O₃, FeO, Fe₂O₃, and NiO), and metallic hydroxides (Al(OH)₃, Fe(OH)₃, Co(OH)₃, Cr(OH)₃, and Ni(OH)₂). In addition, the O1s spectrum can be separated into two peaks, O²⁻ and OH⁻, and the hydroxides are the main substances in the passive films of the three samples.

To further compare the effect of the heat treatment on the passive film, the atomic percent of each substance in the passive film was obtained and is compared in Figure 10, based on the atomic percent of metallic elements obtained from the XPS analysis and the XPS fitting results shown in Figure 7, Figure 8 and Figure 9. It is found that for both the heat-treated and as-cast samples, the passive film is primarily composed of Cr and Al oxides, while the contents of the Co, Fe, and Ni elements are relatively low. As the heat treatment temperature rises, the content of Cr₂O₃ contained in the passive film also rises, reaching the highest content at 1000 °C. This should result from the higher content of the Cr-rich FCC phase in the 1000 °C sample. It is generally believed that the Cr oxides enhance the compactness and stability of the passive film, providing protective effects [34]. In other words, Cr₂O₃ improves the passive film protectiveness; therefore, the higher pitting corrosion resistance of the EHEA heat-treated at 1000 °C could be ascribed to the higher Cr₂O₃ content in the passive film. In contrast, the as-cast sample has the lowest content of Cr₂O₃ and a relatively higher content of Al₂O₃. This could correspond to the higher content of the Al-rich B2 phase in the as-cast state. According to the literature [15,35], Al₂O₃ can increase the porosity of the passive film, making it relatively loose and easier to be broken for the pitting initiation. The increased susceptibility of the as-cast EHEA to pitting corrosion may be related to the adverse effects of Al₂O₃ on the protectiveness of the passive film. Obviously, the XPS analysis supports the results of electrochemical tests and corrosion morphology observation, demonstrating that the heat treatment at 600 °C and 1000 °C is beneficial for improving the pitting corrosion resistance of the AlCoCrFeNi_2.1 EHEAs.

4. Conclusions

This paper studied the corrosion resistance of heat-treated AlCoCrFeNi_2.1 alloys in a 3.5 wt% NaCl solution, in comparison with the as-cast counterpart, by microstructure characterization, electrochemical tests, and surface analyses. The conclusions are mainly as follows:

• The microstructure of the as-cast AlCoCrFeNi_2.1 EHEA changes after the heat treatment at 600 °C and 1000 °C. After the heat treatment, the proportion of the FCC phase enriched with Cr and Fe elements increases, while that of the BCC phase rich in Ni and Al elements decreases.
• Both the as-cast and heat-treated AlCoCrFeNi_2.1 EHEAs exhibit spontaneous passive behavior in a 3.5 wt% NaCl solution. The pitting potential of the AlCoCrFeNi_2.1 EHEA rises with the heat treatment temperature, while the passive current density and passive film resistance are almost independent of the heat treatment. These results demonstrate that corrosion resistance of the as-cast AlCoCrFeNi_2.1 in a 3.5 wt% NaCl solution is improved by the heat treatment.
• The enhanced resistance to pitting corrosion results from the decrease in the content of the Al-rich B2 phase, which can act as the pit initiation site by preferential corrosion (evidenced by corrosion morphology observation) due to the higher electrochemical activity than the Cr-rich FCC phase, and is also detrimental for the protectiveness of the passive film (evidenced by XPS analysis).