Effect of Pt Addition on the Oxidation and Corrosion Resistance of Al_0.25CoCrFeNi High-Entropy Alloy

Abstract

The effect of minor platinum alloying on the microstructure, phase composition, oxidation and corrosion resistance of Al_0.25CoCrFeNi high-entropy alloy (HEA) has been studied. It was observed that Pt does not segregate as a separate phase, but it is incorporated into the fcc solid solution. High-temperature oxidation of the as-cast Al_0.25CoCrFeNi and Al_0.25CoCrFeNiPt_0.1 high-entropy alloys was carried out in a muffle furnace under isothermal conditions at 900 °C for 0–50 h (weighing was carried out every 10 h) in air atmosphere. The specific weight gain decreased from 0.58 mg/cm² for Al_0.25CoCrFeNi to 0.31 mg/cm² for the platinum-doped Al_0.25CoCrFeNiPt_0.1 sample. It was determined that Pt triggers the formation of an interlayer Al-rich oxide phase between the outer Cr-rich oxide layer and the interior of the alloy, significantly reducing the oxidation rate of Al_0.25CoCrFeNiPt_0.1 HEA. Electrochemical tests in 0.5 M H₂SO₄ solution demonstrated passive behavior with anodic control of the process.

1. Introduction

During the last two decades, the so-called “high-entropy alloys (HEAs)” have been one of the new promising areas in materials science. HEAs consist of five or more elements with an equiatomic or near-equiatomic ratio and are characterized by a high value of the configurational entropy of mixing [1,2]. Among different classes of HEAs, the Al_xCoCrFeNi type exhibited an optimal combination of various characteristics such as strength and ductility [3,4], hardness and wear resistance [5,6] and low-temperature resistance [7,8]. Moreover, due to the increased concentrations of aluminum, chromium and nickel compared to those of the conventional industrial alloys, these HEAs demonstrate superior resistance to high-temperature oxidation [9,10] and high resistance against corrosion in various solutions [11,12]. Evaluating the corrosion behavior of Al_xCoCrFeNi HEA is a rather urgent task since these alloys are potentially promising protective coatings for repairing machine parts operating under aggressive conditions [13,14,15].

Currently, alloying different amounts of various elements is the main route to improve the oxidation and corrosion resistance of HEAs. There are data in the literature on the negative effect of alloying Si [16,17], Mn [17], Ti [18] and V [18] and the positive effect of introducing Ti/Nb [19], Y/Hf [20] and Cu [18,21] on the oxidation resistance of Al_xCoCrFeNi base HEA. In particular, alloying by Nb the Al_0.2Co_1.5CrFeNi_1.5Ti_0.3 HEA allowed the specific weight gain to be decreased from 1.4 mg/cm² to 0.7 mg/cm² after 50 h of exposure at 900 °C; the isothermal oxidation resistance was enhanced owing to the presence of the Nb-rich layer [19]. Lu et al. [20] showed that the introduction of Y/Hf in the AlCoCrFeNi HEA practically did not affect the phase composition of the oxide film (consisting mainly of Al₂O₃) but significantly reduced the possibility of cracking it and, thus, increased the protective properties of the formed layer. Dabrowa et al. [21] determined that AlCoCrCuFeNi HEA after 50 h of exposure at 1000 °C had a specific weight gain of only 0.21 mg/cm² versus 0.72 mg/cm² for the HEA without copper addition.

Several works considered the effect of alloying Ti [22,23,24], Cu [22,25,26], Si [22,27], V [22] and Mn [22] on the behavior of Al_xCoCrFeNi base HEA in various solutions, including sulfuric acid, simulating the conditions of natural resources extraction when the medium is a mixture of SO₂/SO₃ vapors and water. At the same time, with some exceptions, data were mainly obtained on the negative effect of additionally introduced elements on the electrochemical corrosion of HEAs due to the microstructural features since this leads to the creation of a number of galvanic couples and acceleration of the corrosion processes. For example, Jiang et al. in [23] demonstrated that titanium addition had a negative effect on the corrosion resistance of the base alloy in NaCl and NaOH solutions but had a positive effect on resistance in sulfuric acid solution (provided that the amount of added titanium equaled or exceeded the aluminum concentration). Pratskova et al. in [22] showed the selective dissolution of copper (when alloying the base alloy with it) in an acidic environment; on the contrary, Li et al. in study [25] indicated a positive effect of introducing copper into the composition on the corrosion behavior of HEAs in the same environment. According to Xiang et al. [27], the doping of silicon allowed pseudo-passive behavior of AlCoCrFeNiSi_0.1 alloy in 3.5 wt.% NaCl solution to be obtained, but, in the 0.5 mol/L H₂SO₄ solution, this HEA showed active–passive behavior. Pitting occurred in the chromium-depleted phase.

By analyzing the literature data, it can be noted that currently there are no optimal HEA compositions which can be distinguished by high resistance against high-temperature oxidation and corrosion in aggressive environments. Thus, works in this direction are ongoing.

Platinum belongs to the group of noble metals (Ag, Pt, Au), which ensures its chemical inertness upon heating. In addition, platinum is located after hydrogen in the electrochemical series of metals, which initially implies its high corrosion resistance in the acidic environments. Thus, Pt is a promising metal for alloying into Al_xCoCrFeNi base alloys. In our previous study [28], we observed the positive effect of silver alloying into Al_xCoCrFeNi base HEA on its passivation interval in a sulfuric acid solution.

Therefore, the aim of this work is to study the microstructure, high-temperature (at 900 °C) oxidation resistance and corrosion resistance (in a 0.5 M H₂SO₄ solution) of Al_0.25CoCrFeNiPt_0.1 high-entropy alloy.

2. Materials and Methods

Ingots of Al_0.25CoCrFeNi (sample for comparison) and Al_0.25CoCrFeNiPt_0.1 HEAs were melted according to the procedure proposed elsewhere [18]. The ingots were produced by induction melting in a reducing atmosphere using metals (granules and powders manufactured by JSC “POLEMA”, Tula, Russia) of high purity (>99.9 wt.%).

For oxidation tests, specimens with dimensions of 10 mm × 6 mm × 6 mm were cut from the ingots and then polished. Oxidation tests were carried out in a Plavka.Pro PM-1 SmartKiln muffle furnace (Plavka.Pro, Korolev, Russia) at 900 °C with an exposure time of 0–50 h in air. The samples were weighed every 10 h on a laboratory analytical balance, model Sartorius MSE225S-000-DU (Sartorius Group, Göttingen, Germany), with an accuracy of 0.00001 g. High-temperature oxidation tests were carried out using three parallel measurements. Standard deviation of the average values amounted to no more than 3%.

The working electrodes for the electrochemical corrosion test and recording of polarization curves were used according to the method outlined elsewhere [22]. Polarization measurements were carried out in a standard three-electrode YaSE-2 electrochemical cell (OJSC “Gomel Plant of Measuring Instruments”, Gomel, Belarus) with a platinum auxiliary electrode using a P-30J potentiostat device (LLC “Elis”, Chernogolovka, Russia). The potentials were measured relative to mercury sulfate reference electrode at room temperature (25 °C) with a sweep rate of 5 mV/s. The use of a lower scan rate (for example, 1 mV/s) led to the effect of “leveling” a number of ongoing processes; instead of a sharp peak on the anodic branch of the polarization curve, the peak became flatter. In addition, small peaks became less noticeable. The low sweep rate simply did not allow time to record the ongoing electrochemical processes on the potentiostat, stretching them out over time. Thus, in this investigation, a sweep rate of 5 mV/s was chosen. It should also be noted that, according to the results of preliminary measurements, the mercury sulfate reference electrode is most suitable for the sulfuric acid solution in which the measurements took place. The use of a silver chloride reference electrode could lead to contamination of the working solution with chlorine ions which could, in turn, affect the measurement results.

The open circuit potential E_OCP was measured for 3600–4400 s in a cell filled with 0.5 M H₂SO₄ electrolyte solution without applying current. The recorded open circuit potentials were E_OCP = −0.005 (V) for the reference sample and E_OCP = +0.350 (V) for the sample doped with platinum. Corrosion parameters such as corrosion potential (E_corr) and current density (I_corr) were determined by the Tafel extrapolation method using both the cathode and anodic branches of the polarization curves. The polarization resistance (R_p) was calculated using the formula:

$$R_p=\frac{1}{2.303\left(\frac{1}{{\beta }_a}+\frac{1}{{\beta }_c}\right)I_{\mathit{corr}}}$$

(1)

where β_a and β_c are the slope of the anodic and cathodic straight lines of the Tafel equation, respectively, obtained by extrapolation.

The microstructure and surface morphology of the samples before and after corrosion tests were examined using a JSM-7001F scanning electron microscope (SEM) (JEOL, Tokyo, Japan) equipped with an energy dispersive X-ray spectroscopy detector (EDS) (Oxford Instruments, Abingdon, UK) for quantitative chemical analysis. X-ray diffraction (XRD) was carried out on an Ultima IV diffractometer (Rigaku, Tokyo, Japan) using Cu Kα. The scanning speed was 5 degrees per minute, and the scanning step was set to 0.02 degrees.

3. Results and Discussion

3.1. Microstructure and Phase Composition

Figure 1 shows the microstructure of the as-cast HEA samples and the corresponding elemental EDS mapping. According to the results, both samples demonstrated an almost homogeneous microstructure with a uniform distribution of the constituent elements. However, a slight unevenness, apparently along the grain boundaries, could be noted.

The average chemical composition of the as-cast samples is given in

Table 1. EDS-determined average chemical composition (at. %) and the configurational entropy of mixing for as-cast HEAs.

Alloy	Al	Co	Cr	Fe	Ni	Pt	ΔSmix, J∙mole−1∙K−1
Al0.25CoCrFeNi	5.86	23.47	23.88	23.21	23.58	–	1.53R
Al0.25CoCrFeNiPt0.1	5.64	22.93	23.10	22.96	23.05	2.32	1.60R

. For alloys, the change in the configurational entropy of mixing can be determined by the formula [29]:

$$\Delta S_{\mathrm{mix}}=-R{\displaystyle \sum _{\mathrm{i}=1}^{\mathrm{n}}{X}_{\mathrm{i}}}\ln X_{\mathrm{i}}$$

(2)

where R is the universal gas constant (R = 8.314 J∙mol⁻¹∙K⁻¹), and X is the atomic fraction of the elements in the alloy. When ΔS_mix ≥ 1.5R, the alloy can be considered a HEA. Using EDS data on the average composition, the changes in the configurational entropy of mixing were calculated for both alloys. According to the results (see Table 1), both samples can be categorized as HEA.

XRD patterns of the samples are shown in Figure 2. In the as-cast state, both samples exhibited a single-phase face-centered cubic (fcc) solid solution. According to the literature [30,31], Al_xCoCrFeNi base HEAs with an aluminum content of x < 0.3 crystallize into an fcc structure, which corresponds to our results. It seems that platinum is completely dissolved in the fcc solid solution phase and does not segregate as a separate phase.

3.2. High-Temperature Oxidation

Figure 3 shows the specific weight gains of the samples during the high-temperature oxidation test. Pt had a positive effect on the oxidation resistance of Al_0.25CoCrFeNi HEA. The specific weight gain (after 50 h of oxidation duration) for the alloy doped with platinum was 0.31 mg/cm², while it was 0.58 mg/cm² for the reference sample without platinum alloying. These results can be compared, for example, with the data reported by Zhu et al. [32], where a specific weight gain of 1.25 mg/cm² after 50 h of isothermal holding at 900 °C was observed for AlCoCrFeNi HEA. Moreover, Abbaszadeh et al. [33] determined a specific weight gain of 0.42 mg/cm² after 72 h of exposure at 900 °C for Al_0.5CoCrFeNi. Liu et al. [34] demonstrated a specific weight gain of 0.6 mg/cm² after 100 h holding at 1000 °C for Al_0.5CoCrCuFeNi HEA. For Al_0.3CoCrFeNi, Mohanty et al. [35] also obtained a value of 0.6 mg/cm² after 50 h of exposure at 1100 °C. Thus, Al_0.25CoCrFeNiPt_0.1 HEA shows a rather high resistance to high-temperature oxidation.

The kinetic curves in Figure 3 suggest that the oxidation process follows the parabolic law, for which the specific weight gain (W) can be related to time by the formula:

$$W=k\sqrt{\tau };$$

(3)

or

$${\left(\frac{△m}{A}\right)}^2=k_p\tau ,$$

(4)

where Δm is weight gain (g), A is surface area (cm²), k_p is the parabolic oxidation rate constant (g²/cm⁴ s) and τ is holding time (s). The determined parabolic oxidation rate constant in comparison with the literature data is given in

Table 2. Oxidation rate parabolic constant (k_p, g²/cm⁴s) of Al_0.25CoCrFeNi and Al_0.25CoCrFeNiPt_0.1 HEAs at 900 °C compared with the literature data.

Alloy	kp	Alloy	kp
Al0.25CoCrFeNi	22 × 10−13	Al0.2Co1.5CrFeNi1.5Ti0.3Nb0.05 [19]	78.5 × 10−13
Al0.25CoCrFeNiPt0.1	5.62 × 10−13	Al0.5CoCrFeNi [33]	8.91 × 10−13
Al0.6CrFeCoNi [16]	11.8 × 10−13	FeCoNiCrAl [36]	4.9 × 10−13
Al0.2Co1.5CrFeNi1.5Ti0.3 [19]	280 × 10−13	AISI 304L [37]	23 × 10−13

. It can be noted that the oxidation rate of Al_0.25CoCrFeNiPt_0.1 alloy is comparable with the available literature data on other HEAs, and it is much lower than the oxidation rate of AISI 304L steel.

The morphology of the oxide layers was studied by SEM, and the results are shown in Figure 4. It can be noted that the resulting oxide layers were quite even, without chips, cracks or pores and consist of two types of crystals; the compositions of these are given in

Table 3. Chemical composition (determined by EDS, at. %) of the oxide film formed on the surface of HEAs samples after 50 h of high-temperature oxidation. Av—average composition; LC—lamellar crystals; RC—rhombohedral crystals.

Alloy		Al	Co	Cr	Fe	Ni	Pt	O
Al0.25CoCrFeNi	Av	8.99	4.54	23.92	4.30	4.75	–	53.50
	LC	23.86	3.20	13.88	3.15	3.11	–	52.80
	RC	0.06	5.18	31.36	4.37	5.52	–	53.51
Al0.25CoCrFeNiPt0.1	Av	14.11	4.21	19.19	4.17	3.37	0.21	54.74
	LC	22.71	4.17	11.25	4.30	3.38	0.27	53.92
	RC	1.02	3.83	33.03	3.37	2.93	0.08	55.74

. Lamellar crystals (Figure 4, on the left), in addition to oxygen, have an increased content of aluminum and chromium, and rhombohedral crystals (see Figure 4, on the right) are characterized by the presence (in addition to oxygen) of chromium, nickel, cobalt and iron. If we compare the distribution of crystals over the area of the oxide film (see Figure 4, center), it can be noted that rhombohedral crystals dominated in the sample without platinum, while the proportion of lamellar and rhombohedral crystals was almost same in the sample doped with platinum. This fact is also confirmed by chemical analysis (see Table 3), since the concentration of chromium in the average composition of the oxide film in the Al_0.25CoCrFeNi sample was noticeably higher than that in Al_0.25CoCrFeNiPt_0.1 HEA. Additionally, the concentration of platinum in the composition of the oxide film formed on the surface of Al_0.25CoCrFeNiPt_0.1 alloy did not exceed 0.3 at. %.

Figure 5 shows XRD patterns recorded from the surface of the samples after high-temperature oxidation. In addition to the fcc solid solution, diffraction patterns showed peaks from the (Al,Cr)₂O₃ solid solution based on aluminum oxide, (Cr,Al)₂O₃ solid solution based on chromium oxide, NiCr₂O₄, CoCr₂O₄ and FeCr₂O₄ oxide phases. If we compare XRD data with the data from Table 3 and Figure 4, it can be assumed that lamellar crystals belong to a solid solution based on aluminum oxide, while rhombohedral crystals belong to a solid solution based on chromium oxide and spinels. No oxide compounds with platinum were found according to the results of X-ray diffraction.

The transverse section of the samples after the oxidation test was further investigated. It can be seen that the thickness of the formed oxide film for both samples was almost same, and it was about 5–7 μm, but there were differences in the thickness of the internal oxidation layer (Figure 6). Moreover, while Cr-rich oxide phase formed on the outer layer of both samples, Al_0.25CoCrFeNiPt_0.1 was characterized by the formation of an interlayer Al-rich oxide phase between the outer Cr-rich oxide layer and the interior of the alloy. For platinum-doped Al_0.25CoCrFeNiPt_0.1 HEA, the thickness of internal oxidation did not exceed 10 μm, while it was about 20–25 μm for Al_0.25CoCrFeNi, which is 2–2.5 times higher than that of Al_0.25CoCrFeNiPt_0.1.

The Al content of HEAs affects their resistance against high-temperature oxidation. The higher the Al concentration, the higher the oxidation resistance of the alloy [9]. Therefore, for Al_0.25CoCrFeNi HEA with an aluminum content of 0.25, rather low resistance to high-temperature oxidation can be expected, since a film consisting mainly of Cr₂O₃ is formed (see Figure 6a). Such a film does not have the necessary protective properties. Introducing platinum into Al_0.25CoCrFeNi does not trigger the formation of other oxide phases on the surface, but it triggers the formation of continuous aluminum oxide beneath the Cr-rich outer oxide layer (see Figure 6b). Apparently, platinum in the composition of the solid solution affects the diffusion characteristics of the elements and slows down diffusion processes that occur in the surface layers of the metal when interacting with oxygen atoms. Furthermore, the noble nature of Pt inherently improves the oxidation resistance of the alloy and prevents in-depth oxidation. As a result, there is enough time for Al to form a continuous oxide layer beneath the outer scale layer. This, in turn, significantly decreases the internal oxidation layer (see Figure 6b).

A similar effect of introducing platinum on the formation of a denser β-NiAl coating with good adhesion to the substrate material and improved high-temperature oxidation resistance was discovered in [38,39,40,41]. It has been shown that platinum in the β-(Ni,Pt)Al coating promotes the formation of a single α-Al₂O₃ scale during oxidation and improves the self-healing ability of the Al₂O₃ film. Moreover, Baskaran et al. in [39] showed the best effect of introducing platinum on the formation of the interdiffusion zone since platinum is an element that directly affects the diffusion characteristics in relation to alloys. In addition, the oxide film obtained on the surface of the Ni40Pt10Al50 bond coat (after oxidation of the sample at 1100 °C) did not show cracking or spallation and had the best protective properties. The effect of platinum on the diffusion between the (Ni,Pt)Al coating and the superalloy matrix material was also discussed in detail in a study by Bai et al. [40].

3.3. Corrosion Resistance Test

Figure 7 shows the polarization curves of the HEAs in a 0.5 M H₂SO₄ solution. Samples in sulfuric acid solution demonstrated passive behavior with anodic process control. However, according to the results of calculating E_corr and I_corr, as well as the polarization resistance (

Table 4. Potentials and current densities of Al_0.25CoCrFeNi and Al_0.25CoCrFeNiPt_0.1 HEAs in 0.5 M H₂SO₄ solution compared with different alloys reported in the literature.

Alloy	Ecorr, V	Icorr, μA/cm2	Rp, kΩcm2	Ipas, μA/cm2	ΔE	Eb, V
Al0.25CoCrFeNi	−0.132 ± 0.003	6.67 ± 0.05	1.12 ± 0.03	417	1.262	1.195
Al0.25CoCrFeNiPt0.1	−0.018 ± 0.002	3.93 ± 0.02	6.79 ± 0.05	356	1.222	1.218
Al0.25CoCrFeNiCu [22]	−0.164 ± 0.002	607 ± 3	0.088 ±0.001	271	0.077	–
Al0.5CoCrFeNiTi0.5 [23]	−0.400	320	–	24.8	0.650	0.980
FeCoNiCrCu0.5Al0.5 [25]	−0.112	4.190	–	3000	0.550	–
AlCoCrFeNiSi0.1 [27]	−0.453	304.20	–	19.35	1.350	0.925
SS 304 [27]	−0.438	105.12	–	4.47	1.261	0.925
Al0.5CoCrFeNiCu0.25 [28]	−0.112 ± 0.002	3.52 ± 0.02	7.81 ± 0.05	21.5	0.967	–
Al0.5CoCrFeNiCu0.25Ag0.1 [28]	−0.143 ± 0.002	5.21 ± 0.05	8.47 ± 0.05	28.8	1.282	–
Al0.25CoCrFeNi [42]	−0.095	16.7	–	7.1	1.080	–
SS 304 [42]	−0.185	45.3	–	19.1	0.750	–
Co1.5CrFeNi1.5Ti0.5 [43]	−0.092	30	–	9	1.130	1.089
Co1.5CrFeNi1.5Ti0.5Mo0.1 [43]	−0.071	78	–	9	1.112	1.089
AlCoCuFeNiCr [44]	−0.075	5.09	14.450	–	0.650	–
AlCoCuFeNiCrTi [44]	−0.256	39.59	1.558	–	0.940	–
Al0.3CrFe1.5MnNi0.5 [45]	−0.194	2390	–	73.9	1.176	–

), Al_0.25CoCrFeNiPt_0.1 HEA with platinum showed a slightly higher resistance to electrochemical corrosion. In comparison with the available literature data (see Table 4), it can be noted that both investigated HEAs had high corrosion resistance in 0.5 M H₂SO₄ solution. Table 4 also shows data on the passive current density (I_pas), passivation interval (ΔE) and breakdown potential (E_b).

It should be noted that passivation of the alloys under study occurred at higher passive current density than for some compositions presented in the literature (see Table 4). Thus, the passivation process did not begin immediately, which may indicate the initial appearance of microgalvanic cells. However, the obtained values for the studied samples showed their high corrosion resistance to general corrosion.

Breakdown potential is defined as the surface potential at which the surface’s passive film breaks down. The more positively its value is shifted, the more corrosion-resistant the sample is. The obtained data for both HEAs showed better values compared to alloys, data for which are given in the literature (see Table 4).

The surface of Al_0.25CoCrFeNi and Al_0.25CoCrFeNiPt_0.1 HEAs practically indicated no corrosion pits, pores or pitting after electrochemical corrosion (Figure 8). However, it can be clearly seen that the Al_0.25CoCrFeNiPt_0.1 sample was subjected to a stronger effect of sulfuric acid, possibly due to a somewhat uneven distribution of platinum in the solid solution (see Figure 1). Areas with a higher concentration of platinum (see point B in Figure 8b and

Table 5. EDS chemical composition (at. %) recorded from the surface of the samples after testing in 0.5 M H₂SO₄ solution. Av—average composition; points A and B—see Figure 8b.

Alloy		Al	Co	Cr	Fe	Ni	Pt	O	S
Al0.25CoCrFeNi	Av	5.66	21.88	23.17	21.54	22.03	–	5.64	0.08
Al0.25CoCrFeNiPt0.1	Av	4.10	19.91	21.45	19.56	19.50	5.60	8.94	0.94
	A	2.40	19.68	24.64	18.11	19.36	2.62	12.08	1.11
	B	4.53	20.45	20.72	20.56	20.64	6.91	5.77	0.42

) hardly corroded, while areas with a lower content of platinum (see point A in Figure 8b and Table 5) underwent partial dissolution. If we compare the average compositions of the sample doped with platinum before and after electrochemical tests (see Table 1 and Table 5), it can be noted that all elements, except platinum, passed into the sulfuric acid solution; therefore, platinum concentration in the sample after testing was almost two times higher than that of the as-cast sample.

It is possible that the manifested negative effect of the uneven platinum distribution in the as-cast microstructure can be avoided by subjecting the sample to homogenizing annealing, which will be studied in our future work. However, due to the fact that platinum is highly incorporated into the solid solution of Al_0.25CoCrFeNi and does not segregate as second phases, it is a promising element for doping Al_0.25CoCrFeNi HEA.

4. Conclusions

The effect of alloying Pt on the microstructure, high-temperature oxidation resistance (at 900 °C) and corrosion resistance (in a 0.5 M H₂SO₄ solution) of Al_0.25CoCrFeNi high-entropy alloy was investigated. It was found out that Pt can be incorporated into the fcc solid solution phase of Al_0.25CoCrFeNi up to 2.3 at. % without segregation and phase separation. At the same time, it affects diffusion processes during high-temperature oxidation owing to its chemical inertness and consequently decreases the oxidation rate of Al_0.25CoCrFeNiPt_0.1 alloy at 900 °C compared to that of Al_0.25CoCrFeNi HEA. Apparently, the noble nature of Pt inherently improves the oxidation resistance of the alloy and prevents in-depth oxidation so that it provides enough time for Al to form a continuous oxide layer beneath the outer scale layer and protect Al_0.25CoCrFeNiPt_0.1 HEA from further oxidation. On the other hand, Al_0.25CoCrFeNiPt_0.1 also exhibited higher corrosion resistance compared to Al_0.25CoCrFeNi, which may be attributed to the synergistic effects of chromium and the chemically inert platinum in the alloy composition. In particular, the corrosion current density of Al_0.25CoCrFeNiPt_0.1 was 3.93 ± 0.02 μA/cm² versus the obtained value of 6.67 ± 0.05 μA/cm² for Al_0.25CoCrFeNi HEA.