Effect of Porosity on the Corrosion Behavior of FeCoNiMnCr_x Porous High-Entropy Alloy in 3.5 Wt.% NaCl Solution

Abstract

The effects of different Cr contents on the corrosion resistance of FeCoNiMnCr_x (x = 0.5;1;1.5) porous high-entropy alloys (HEAs) in 3.5 wt.% NaCl solution on corrosion resistance was investigated. With the increase in Cr content, the total porosity and permeability of the porous HEA increased. The increase in porosity improves the interconnectivity between the pores and enhances the contact area with the corrosion solution. The pore-making mechanism is mainly a powder compaction, and Kirkendall holes are caused by different elements due to different diffusion rates. With the increase in Cr content, the i_corr increases, and the E_corr decreases in the porous HEAs of FeCoNiMnCr_x (x = 0.5;1;1.5). The corrosion resistance of FeCoNiMnCr_x (x = 0.5;1;1.5) porous HEAs decreases with the increase in the Cr element. With the increase in Cr content, the weight gain rate of FeCoNiMnCr_x porous HEA increases gradually after immersion for 168 h, and the average pore size and permeability of the sample decrease gradually. The corrosion resistance of FeCoNiMnCr_x porous HEA decreases with increasing Cr content.

1. Introduction

Seawater filters are one of the key components in the fields of seawater desalination, marine resource extraction, and environmental protection. Since they serve in extremely corrosive environments, strict requirements on the performance of filter materials are put forward. Factors such as high salinity, the presence of corrosive ions (chloride ions), large temperature variations, and the attachment of marine organisms in seawater environments pose higher challenges to the corrosion resistance, strength, toughness, and other properties of materials [1,2,3]. Conventional metals corrode faster in chemical environments such as high salinity and chloride ions. They are more susceptible to fatigue damage under cyclical extreme environmental conditions such as waves and tides in seawater, greatly shortening their service life [4,5]. In the face of the complexity and variability of the seawater environment, traditional metals may not be able to fully adapt to the high temperature, high pressure, high salinity, and various harsh environments [6,7,8]. As a new alloy design concept, porous HEAs have the potential to become an important material for solving seawater filtration problems due to their unique microstructure, excellent corrosion resistance, and thermal stability [9,10,11].

In recent years, researchers have found that the FeCoNiMnCr system’s HEA has great potential for application in corrosive environments, and these advantages make the material a broad prospect for application in seawater filtration equipment [12,13]. Sun et al. [14] investigated the effect of Cr content in 3.5 wt.% NaCl solution on CoCr_xFeMnNi HEAs (x = 0.4, 0.7, 1.0, 1.3 at.%.) effect on corrosion resistance. The experimental results show that CoCr_1.3FeMnNi-based HEAs have the best resistance to Cl⁻ corrosion with high Rp values, the highest corrosion potentials, low corrosion currents, and low CPE values. Ni et al. [15] found that CoCrFeNi HEA containing the Cr element has lower corrosion current density (i_corr) than CoFeNi HEA, and the addition of the Cr element effectively improves the corrosion resistance of HEA; Ehsan et al. [16] found the CoNiMnCr_x HEA to exhibit excellent corrosion resistance in both NaCl solution and H₂SO₄ solution, and it was observed that the HEA formed a protective passivation film during immersion, which was attributed to the fact that the Cr element in the HEA could be easily oxidized in both NaCl solution and H₂SO₄ solution, thus forming a dense passivation film on the surface of the alloy; Yan et al. [17] studied the effect of Cr content on the corrosion behavior of Al_0.3Cr_xFeCoNi (x = 1.5–2.0) HEAs. The pitting corrosion resistance of the alloy increases with increasing Cr content. This is due to forming a protective Cr₂O₃-rich surface film that exhibits excellent pitting resistance and a low corrosion rate of less than 10⁻³ mm/year. Lu et al. [18] systematically investigated the corrosion behavior of Fe₅₀ Mn₃₀ Co₁₀ Cr₁₀ and CoCrFeMnNi. The experimental results show that CoCrFeMnNi exhibited better long-term corrosion performance due to the higher and more stable charge transfer resistance values found after 14 days of immersion in the NaCl solution. The above studies show that the FeCoNiMnCr system’s HEA is a new type of material with excellent corrosion resistance. The corrosion resistance of HEA can be improved by adjusting the Cr content, so the preparation of HEA into porous HEAs may also have great potential for application in the field of seawater filtration.

This paper uses microstructure analysis, electrochemical experiments, and immersion experiments to systematically investigate the effect of Cr content on the corrosion behavior of FeCoNiMnCr_x porous HEA in 3.5 wt.% NaCl solution. By exploring the corrosion mechanism of FeCoNiMnCr_x porous HEAs in a simulated seawater environment, more technical support and references are provided for the engineering applications of FeCoNiMnCr_x porous HEAs in seawater filtration.

2. Materials and Experimentation

2.1. Material Preparation and Characterization of the Pore Structure Parameters

As listed in

Table 1. FeCoNiMnCr_x porous HEA composition ratio (at.%).

Element	Fe	Co	Ni	Mn	Cr
Sample 1	22.22	22.22	22.22	22.22	11.11
Sample 2	20.00	20.00	20.00	20.00	20.00
Sample 3	18.18	18.18	18.18	18.18	27.27

, the spherical powders of Fe, Co, Ni, Mn, and Cr elements with particle sizes of 40~70 μm (purity > 99.6%) were selected according to the isoatomic ratio to precisely configure the alloy powders. The FeCoNiMnCr_x (x = 0.5; 1; 1.5) were identified as Sample 1, Sample 2, and Sample 3, respectively, according to the different content (atomic number) of the Cr element. Then, the configured alloy powders were placed in a ball-milling bottle, with the ball-to-feed ratio set at 4:1, and run continuously in a planetary ball mill with a rotational speed of 200 r/min for 48 h. The original powder particles may have irregular sizes, and after mechanical grinding, the powder particles become more uniform in size. This change in morphology helps the powder to fill the mold better during the cold-pressing stage and improves the densification of the powder.

The electronic balance with an accuracy of 0.01 g was used to weigh 8 g of alloy powder, which was cold pressed and shaped under a 160 Mpa hydraulic press to make a cylindrical raw embryo specimen with a diameter of 25 mm and a thickness of 10 mm. During the cold-pressing process, the voids between the powder particles were compressed, and a tighter bond was formed between the powder particles. This increase in densification facilitates the diffusion of elements and alloying in the subsequent sintering stage. The raw embryo specimens were sintered in a vacuum sintering furnace with a vacuum degree of 1.3 × 10⁻³ Pa for a total sintering time of 8 h. The sintering temperature of the final FeCoNiMnCr_x (x = 0.5; 1; 1.5) porous HEA was set at 1050 °C. The preparation process is schematically shown in Figure 1.

The average pore size was measured by using a pore tester (FBP-3III (Best Instrument Technology (Beijing) Co., Ltd, Beijing, China)), with high-purity nitrogen (99.99%) as the test gas and ethanol as the wetting solution. The permeability of the porous samples was measured by the gas permeation method with the following equation:

$$K={\displaystyle \frac{Q}{\Delta P\cdot A}}$$

(1)

where K denotes the permeability (m³⋅m⁻²⋅kPa⁻¹⋅h⁻¹), Q indicates the gas flow rate (m³⋅h⁻¹), $∆$P indicates the pressure drop (kPa), and A means the size of the test area of the specimen (m²). Its open porosity was measured using the Archimedes drainage method, and the mass of the dried sample was measured before calculating the porosity M₁. The sample was then immersed in molten paraffin under vacuum, which allowed the open pores of the specimen to be filled entirely. After removing the sample, the residual medium was removed from the surface, and the mass of the dried sample was measured again using M₂. The formula is

$$P={\displaystyle \frac{M_2-M_1}{{\rho }_1\cdot V}}$$

(2)

where P denotes the opening ratio (%), V indicates the volume of the specimen after wax sealing (cm⁻³), M₁ and M₂ indicate the mass of the specimen before and after wax sealing, respectively (g), and ${\rho }_1$ indicates the density of the wax (g∙cm⁻³).

2.2. Electrochemical Measurement

Electrochemical testing was performed using a CS310× electrochemical workstation (Wuhan Corrtest Instruments Corp.LTD, Wuhan, China) with an exposure area of 0.2826 cm². Before each test, the specimens were ground with 800#~2000# SiC sandpaper and polished with 1 μm of diamond plaster to a mirror finish. All samples were subsequently washed in anhydrous ethanol by ultrasonication for 30 min and dried in a vacuum drying oven at 45 °C for 90 min. To ensure the accuracy of the experimental data, three parallel specimens were selected for each set of experiments. The electrochemical tests were performed using a typical three-electrode system: FeCoNiMnCr_x porous HEA as the working electrode, the saturated calomel electrode as the reference electrode, and the platinum sheet as the auxiliary electrode. The corrosion solution used for the test was 3.5 wt.% NaCl, and electrochemical tests were performed at room temperature. Cathodic reduction is required to remove surface oxides from the specimen prior to electrochemical experiments. The sample was immersed in the electrolyte for 3600 s to obtain a stable open-circuit potential (OCP) followed by electrochemical impedance spectroscopy (EIS), which was performed in the frequency range of 10⁵~0.01 Hz with a sinusoidal amplitude of 10 mV. The kinetic potential polarization curves were tested at a scan rate of 0.5 mV/s and voltages in the range of −0.5~1.5 V. After the kinetic potential mechanical polarization measurements were completed, the specimens were removed from the test solution and washed with distilled water.

2.3. Immersion Test

Since 3.5 wt.% NaCl is the average salinity of natural seawater, it is close to the concentration of dissolved salts (mainly NaCl) in seawater. This concentration gives a true picture of the corrosion behavior of materials in the marine environment. And 3.5 wt.% NaCl solution is the standard simulation solution for many corrosion tests (ASTM G31 standard). Therefore, a realistic seawater environment was simulated by immersing a sintered FeCoNiMnCr_x porous HEA at 1050 °C in 30 mL of 3.5 wt.% NaCl solution for 168 h at room temperature [19]. After washing and drying with distilled water, the mass of the raw embryo specimens before immersion corrosion was weighed with an electronic balance (accuracy 0.0001 g, model ME104E (Shanghai Huyue Ming Scientific Instruments Co., Ltd., Shanghai, China)), using distilled water in order to eliminate the Cl⁻ contained in the water from causing corrosion to the specimens in advance. The weighed specimens were suspended in 30 mL of 3.5 wt.% NaCl solution, and the immersed specimens were removed at the specified time (24 h), ultrasonically cleaned in distilled water for 40 min, and dried in a vacuum oven at 50 °C for 2 h to ensure that the residues generated by the corrosion in the water were removed. Then, the mass of the specimen after immersion corrosion was weighed with an electronic balance (accuracy 0.0001 g, model ME104E (Shanghai Huyue Ming Scientific Instruments Co., Ltd., Shanghai, China)).

2.4. Microstructure Characterization

The initial samples were radiometrically analyzed for Cu-Kα using the X-ray diffraction (XRD) model XRD-7000 (Shimadzu Corporation, Kyoto, Japan) in the range of 5–90° at a scanning speed of 10°/min. Scanning electron microscope images (SEM) and compositional analysis energy spectra (EDS) diagrams of microstructures at 20 kV electron accelerating voltage were obtained by a field emission scanning electron microscope (Quanta 650FEG, FEI Company, Hillsboro, OR, USA) equipped with an energy-dispersive X-ray spectrometer (EDS) system. The morphology of the corrosion pits in the specimens after dynamic potential polarization was characterized by the super depth of field laser confocal (S-neox 090, SENSOFAR, Wuhan Zhanhua Technology Co., Ltd, Wuhan, China).

3. Results and Discussion

3.1. Microstructure Analysis

Figure 2 shows the XRD pattern of FeCoNiMnCr_x porous HEA. As shown in Figure 2, Sample 1, Sample 2, and Sample 3 are simple face-centered cubic (FCC) structures, representing an ideal crystal structure. Because of the low rotational speed (200 r/min) and the short time (48 h), mechanical grinding does not lead to the formation of the FCC phase. With the increase in Cr content, the intensity of the diffraction peaks of the FCC phase increases significantly [20]; it shows that the FCC phase increases in strength and alloying in the alloy.

Figure 3 shows the 100× SEM image of FeCoNiMnCr_x porous HEA. As can be seen from Figure 3, the surface shows a large number of complex and irregular pores, and with the increase in Cr content, the structure and distribution characteristics of the pores show obvious differences. As indicated by the red circle marking in pictures, when the Cr content is 0.5, the pores present relatively small and more uniform distribution characteristics. When the Cr content increases to 1.5, the pores become larger and more irregular in shape, and the edges of the pores become rougher, and even some branching or connecting structures appear. Figure 4 shows the 1000× SEM image of FeCoNiMnCr_x porous HEA. From Figure 4, the morphology changed significantly with the increase in Cr content. With the increase in Cr content, the number and diameter of pores increased, while it could be observed that some pores were connected or isolated from each other, and some pores formed closed pores inside, thus forming a complex pore network. All three groups of samples have good sintered necks and connected porous structures. SEM images magnified 100× vs. 1000×, and the samples show a consistent pattern of a gradual increase in the number of holes.

Figure 5 shows the maximum pore diameter and average pore diameter of FeCoNiMnCr_x porous HEA versus Cr content, as shown in Figure 5. The maximum pore diameter increases from 3.0 μm to 3.75 μm, and the average pore diameter increases from 0.9042 μm to 1.7583 μm when the Cr content increases from 0.5 to 1.5. Comparing the trends of maximum pore diameter and average pore diameter, it is easy to see that with the increase in Cr content, the maximum pore diameter and average pore diameter show an increasing trend [21], which is because the Cr element is the main pore-making element in the sintering process of NiCrMoCo porous materials in the activation reaction. Adding Cr elements contributes to the formation of pores, and the specimen’s maximum and average pore size increases with increased Cr content [22].

Figure 6 shows the open porosity, opening ratio, and total porosity of FeCoNiMnCr_x porous HEA as a function of Cr content. As can be seen in Figure 6, with the increase in Cr content, the open porosity of the specimens increased from 17.54% to 21.25%. The opening ratio increased from 19.34% to 26.12%. The increase in total porosity is more minor, from 83.24% to 85.36%, due to the diffusion of Cr atoms in the vacuum sintering process [23], the transfer to other locations in the sample, and the forming of pores in the original location. Therefore, with increased Cr content, the number of Cr atoms also increases, and the pores naturally increase, ultimately leading to an increase in total porosity. As the Cr content continues to rise, the opening ratio is still rising, but the frequency of increase decreases slightly; this is because the ratio of closed pores also increases with the rise in Cr content; the increase in Cr content caused by the expansion in the volume of the material, resulting in some of the Cr atoms penetrating into the lattice of the other phases, the lattice expansion blocking some of the pores, and the opening ratio decreasing [24]. Therefore, there is a difference between total porosity and open porosity.

Figure 7 shows the SEM image of FeCoNiMnCr_x porous HEA observed at a magnification of 500× after 3 mm grinding. The surfaces of the specimens all show a complex pore structure. Overall, the pores occupy a considerable proportion of the surface, and this proportion gradually increases with increasing Cr content, an observation that is consistent with the porosity data derived from the calculations. Figure 8 shows the permeability of FeCoNiMnCr_x porous HEA as a function of Cr content; as shown in Figure 8, the permeability of the specimens increased from 2.49 m³⋅m⁻²⋅kPa⁻¹⋅h⁻¹ to 6.21 m³⋅m⁻²⋅kPa⁻¹⋅h⁻¹ with the increase in the Cr content. The trends are consistent with the trends of pore diameter, opening ratio, and total porosity with Cr content, all of which show an upward trend with the increase in Cr content. It can be seen that the increase in pore diameter and total porosity contributes to the increase in permeability [25].

According to the study of the phase structure, micro-morphology, and pore structure of the specimen during the sintering process, the process of pore formation in the preparation of the porous HEAs of FeCoNiMnCr_x by activated reactive sintering is divided into two stages. Interstitial pores generated after powder pressing: the effect of pressing pressure on the interstitial pores is essentially realized through the plastic deformation of metal powder particles in the pressing process and the impact on the interstitial gap of the press blanks [26]. With Fe, Co, Ni, Mn, and Cr metal powder in the press molding process, there are tiny pores in the specimen due to the metal Cr being a challenging phase; the amount of plastic deformation in the pressing process is small, so the higher the Cr content of the interstitial holes, the higher the porosity. Kirkendall pores formed by elemental-biased diffusion: Differences in the intrinsic diffusion coefficients can cause the Kirkendall effect as the sintering temperature is gradually increased during the preparation of the material [27,28]. This is a phenomenon in which atoms of a substance are exchanged with vacancies due to differences in the intrinsic diffusion coefficients of different elements, and the vacancies are continuously aggregated to form pores. The Arrhenius equation [29] is able to calculate the diffusion rate of each element when sintering is carried out as an activation reaction, and from this, it is possible to summarize the dominant elements that lead to the formation of pores.

$$D={{\displaystyle D}}_0\exp \left({\displaystyle \frac{-Q}{RT}}\right)$$

(3)

where D is the diffusion coefficient of the substance at a temperature of T, D₀ is the diffusion coefficient of the substance at the reference temperature, Q is the activation energy per mole of atoms (J/mol), T is the thermodynamic temperature (K), R is the gas constant, and the value is 8.314 (J/(mol·K)).

Due to the inconsistent intrinsic diffusion coefficients of Fe, Co, Ni, Mn, and Cr, a strong Kirkendall effect can be induced. At activation reaction sintering temperatures below 1050 °C, the diffusion rates of the elements can be calculated based on the Arrhenius equation described above. Since the activation reaction sintered at a temperature of 1050 °C, this corresponds to a T of 1323K.D₀ which is 8.52 × 10⁻⁴ m²·s⁻¹, and Q is 292.1 kJ·mol⁻¹. The diffusion rate of Cr atoms in Ni atoms is 2.49 × 10⁻¹⁵ m²·s⁻¹, that of Cr atoms in Fe atoms is 3.25 × 10⁻¹⁵ m²·s⁻¹, and that of Cr atoms in Co atoms is 8.07 × 10⁻¹⁶ m²·s⁻¹, which is obtained by taking into account the Arrhenius equation. In 1050 °C, Cr atoms in the Fe atom diffusion rate are the largest in the whole process of the diffusion of Cr to Fe in the diffusion of the main. As Cr and Fe atoms have the same crystal structure, the difference between their atomic radii is less than 15%, and for the same cycle of similar elements, the element Cr can diffuse solid solution into the Fe cell, resulting in its lattice distortion and lattice constants, due to the study of the Cr element in the content of the larger; after sintering, through the Kirkendall effect, the elemental particles diffuse into each other, the boundary at the particle bonding surface is blurred, and a sintered neck is formed.

3.2. Electrochemical Corrosion Behavior and Analysis of Corrosion Shape

Figure 9 shows the potentiodynamic polarization curves of FeCoNiMnCr_x porous HEA. From Figure 9, it can be seen that the FeCoNiMnCr_x porous HEA reaches directly from the activation stage to the over-passivation stage, bypassing the typical activation–passivation transition zone, which indicates the spontaneous passivation property of the alloy [30]. The electrochemical corrosion parameters, including the corrosion current i_corr and E_corr voltage, were derived from the fitting of Tafel curves. The corresponding parameters are shown in

Table 2. Polarization curve fitting results for FeCoNiMnCr_x porous HEAs.

Sample	icorr (A/cm2)	Ecorr (V)
Sample 1	6.98 × 10−5 ± 0.3458 × 10−5	−0.3329 ± 0.0208
Sample 2	1.148 × 10−4 ± 0.0286 × 10−4	−0.3745 ± 0.0118
Sample 3	1427 × 10−4 ± 0.0912 × 10−4	−0.4091 ± 0.0177

. i_corr potential is a characteristic or property of a metal surface that reflects the ease (or tendency) of losing electrons in the presence of an electrolyte. In contrast, corrosion current density is related to the flow of electrons during corrosion and is a true reflection of the corrosion behavior of a material. Therefore, a lower i_corr value means the material is more corrosion resistant, and a higher E_corr value means less of a tendency to corrode [31]. From Table 2, it can be seen that the corrosion behavior of the specimens shows a significant trend. First, i_corr increased with increasing Cr content from 6.98 × 10⁻⁵ A/cm² in Sample 1 to 1.427 × 10⁻⁴ A/cm² in Sample 3; it is shown that the increase in Cr content did not effectively inhibit the increase in i_corr, which is related to the interaction of other elements in the alloy with Cr, failing to form a desirable corrosion-resistant protective layer. Secondly, E_corr decreases gradually with increasing Cr content, from −0.3329 V in Sample 1 to −0.4091 V in Sample 3. A decrease in E_corr usually means that the alloy is more susceptible to corrosion reactions in the same environment, and the alloy’s corrosion resistance is diminished. It can be concluded from Table 2 that Sample 1 possesses the lowest i_corr as well as the highest E_corr. This implies that the increase in Cr content did not significantly enhance the corrosion resistance of the alloy.

Figure 10 shows the impedance profile of the porous HEA FeCoNiMnCr_x. Nyquist Figure 10a shows that each curve is characterized by an unfinished curve, indicating that the corrosion on the specimen surface is controlled by the charge transfer process of the interfacial electrochemical reaction [32]. The capacitive arc illustrated in Figure 10 is an important feature in electrochemical impedance spectroscopy, and the radius of the capacitive arc reflects the magnitude of the charge transfer resistance, which is a crucial parameter to measure the ease of charge transfer at the electrode/electrolyte interface [33]. A larger radius indicates that the interface has higher charge transfer resistance and more protective passivation film. Hence, the larger arc diameter indicates better corrosion resistance of HEA, Sample 1 > Sample 2 > Sample 3, consistent with the results of potentiodynamic polarization curves analysis. The inserted equivalent circuit (EEC) in Figure 10a is used to fit the impedance data [34]. In the equivalent circuit, Rs is connected in series with Rp, and CPE1 is connected in parallel with Rp, where Rs denotes the solution resistance, Rp denotes the polarization resistance, and CPE1 is a constant phase element. Since the inhomogeneity and roughness of the specimen surface can lead to an undesirable capacitive response of the measurements, the constant phase element CPE1 is introduced. CPE is a constant phase angle element instead of the ideal capacitance, which can be used to study the impedance behavior in the system more accurately.

Table 3. Impedance curve fitting results for FeCoNiMnCr_x porous HEAs.

Sample	Rs (Ω∙cm2)	Rp (Ω∙cm2)	Y0	n
Sample 1	14.18	25.19	3 × 10−4	0.70838
Sample 2	15.42	18.74	6 × 10−4	0.68634
Sample 3	13.5	13.47	1.6 × 10−3	0.60528

shows the results of fitting the electrochemical impedance of the FeCoNiMnCr_x porous HEA with n < 1 for each sample, which suggests that a passivation film of defective nature is formed on the surface of the alloy [35]. This is because the formation of pores leads to the optimization of the electrolyte environment, while Cl⁻ has a strong adsorption capacity and is able to invade the interior of the pores, promoting the occurrence of electrochemical reactions and destroying the integrity of the passivation film. And the inner wall of the hole is usually not smooth. These characteristics will lead to vulnerable areas and defects in the passivation film. Smaller pores have better dimensional uniformity and more uniform distribution than larger pores. The interconnectivity of larger pores increases significantly as the porosity increases. For samples with lower porosity, it is unlikely that most of the pores will trap a significant volume of solution, which results in a sample that is relatively resistant to corrosion. With the increase in Cr content, the total porosity of FeCoNiMnCr_x porous HEA increases, and the corrosion resistance decreases.

From the Bode Figure 10 in Figure 10b, it can be seen that Sample 1 has the largest impedance modulus |Z| in the low-frequency region, meaning it has the best corrosion resistance. The phase angle–frequency in Figure 10b also shows that Sample 1 has the largest phase angle. By comparing the phase angles of all the samples in the range of 10²–10⁴ Hz, it is found that as the frequency decreases, the phase angles of Sample 2 and Sample 3 are always lower than that of Sample 1, and the widths of the samples are also lower than that of Sample 1. The decrease in phase angle indicates the continuous accumulation of the number of charges on the surface of the alloys, which suggests that the dissolution rate of the passivation films of Sample 2 and Sample 3 is higher than that of Sample 1, which reduces their corrosion resistance. Upon analyzing the Bode plot in Figure 10b, a time constant exists for the AC impedance, which is consistent with the result that the specimen used in the Nyquist plot has only one capacitive arc.

As shown in Figure 11, SEM images of FeCoNiMnCr_x porous HEA after immersion testing and polarization in 3.5 wt.% NaCl solution reveals the critical effect of Cr content on the corrosion behavior of the alloy. SEM observations clearly show that the degree of corrosion experienced by FeCoNiMnCr_x porous HEA is significantly aggravated with the gradual elevation of Cr content. Specifically, the metal matrix and its pores are extensively covered with a thick continuous layer of corrosion products around the interior, which are dense and continuous, indicating that the corrosion process is deep and extensive [36,37]. The severity of the corrosion was further confirmed by the presence of significant pitting on the metal surface, accompanied by the formation of loose corrosion products. In contrast, the Sample 1 specimen produced the least amount of corrosion products under the same conditions, with a relatively clean surface and minimal signs of corrosion, which is strong evidence that the Sample 1 specimen has the best corrosion resistance. This observation is consistent with the results of previous electrochemical testing, which together validate the superior corrosion resistance of the Sample 1 specimen.

Figure 12 shows the three-dimensional morphology of FeCoNiMnCr_x porous HEA after the polarization test; from Figure 12, it can be seen that the corrosion pits are randomly distributed on the surface of the specimen, and the corrosion area and depth of the corrosion pits on the surface of the specimen show an increasing tendency with the increase in Cr content. The maximum depths of localized corrosion pits formed on the surfaces of Sample 1, Sample 2, and Sample 3 are −161.10 μm, −239.89 μm, and −258.55 μm, respectively, and the depths of the corrosion pits directly reflect the severity of localized corrosion in the samples [38]. The corrosion area and depth of the corrosion pits of the Sample 1 specimen are shallow compared to other specimens. This proves that the corrosion resistance of the Sample 1 specimen is relatively better. This result further confirms that an increase in Cr content leads to a decrease in the corrosion resistance of FeCoNiMnCr_x porous HEA.

The above data show that with the gradual increase in Cr content, the alloys’ porosity and average pore diameter show an upward trend. More significantly, the increase in porosity directly promotes the interconnectivity between larger pores, forming a more complex pore network. In contrast, the porosity and average pore size of the Sample 1 specimen were at the lowest level among the three specimens. This characteristic made it difficult to retain a large amount of 3.5 wt.% NaCl solution, thus enhancing its corrosion resistance to some extent [39]. Furthermore, in the Sample 2 and Sample 3 specimens, due to the increase in porosity and pore diameter in small pores, the corrosion solution becomes acidic or oxygen deficient, which leads to an autocatalytic process of accelerated localized corrosion. The corrosion resistance of FeCoNiMnCr_x porous HEA is related to the formation of a passivation film on the surface of the alloy. The surface quality of FeCoNiMnCr_x porous HEA, the amount of surface residue, and microstructural inhomogeneity are the key factors affecting the formation and quality of the passivation film. However, the preparation of FeCoNiMnCr_x porous HEA usually consists of complex thermomechanical processes. By observing the microstructure of FeCoNiMnCr_x porous HEA, it can be seen that many holes show irregular shapes with the increase in porosity. And the inner walls of the pores are usually not smooth. These features will lead to vulnerable sites and defects in the passivation film. Small pores have better size uniformity and more uniform distribution than large pores. Further analysis revealed that when the porosity increased, the interconnectivity between the pores was significantly improved, and more connected channels were formed, which provided free-flowing paths for the corrosion solution and significantly increased the effective contact area between the alloy and the corrosion solution [40]. Therefore, as the porosity increases, the degree of corrosion of the alloy increases accordingly. Comparing the porosity of the specimens with the degree of corrosion of the specimens also explains the decrease in the corrosion resistance of the FeCoNiMnCr_x porous HEA with increased Cr content.

3.3. Analysis of Immersion Corrosion Test Results

Figure 13 visualizes the corrosion weight gain of the FeCoNiMnCr_x porous HEA after 168 h of immersion in 3.5 wt.% NaCl solution. After this series of immersion experiments, it is observed that the corrosion weight gain of the alloy shows a clear trend: the lowest corrosion weight gain is observed for Sample 1, followed by Sample 2, and finally, Sample 3. Specifically, Sample 1 demonstrated minimal corrosion weight gain, which strongly suggests that Sample 1 has superior corrosion resistance under the same test conditions.

Table 4. Changes in average pore size before and after corrosion of FeCoNiMnCr_x porous HEA (μm).

Sample	Before	After	Change Rate (%)
Sample 1	0.9042	0.5329	−41.06
Sample 2	1.6249	0.8626	−46.91
Sample 3	1.7583	0.8770	−50.12

shows the changes in the average pore size before and after the corrosion of FeCoNiMnCr_x porous HEAs, and the changes in the pore structure of all samples are significant. With the increase in corrosion time, the generation of corrosion products may accumulate within the pores and form blockages, leading to a decrease in the effective porosity, thus decreasing the average pore size [41]. The average pore size of Sample 1 has the lowest rate of change compared to Sample 2 and Sample 3. Therefore, the corrosion resistance of Sample 1 is better than that of Sample 2 and Sample 3.

Table 5. Changes in permeability of FeCoNiMnCr_x porous HEA before and after corrosion (m³⋅m⁻²⋅kPa⁻¹⋅h⁻¹).

Sample	Before	After	Change Rate (%)
Sample 1	2.46	1.85	−24.67
Sample 2	5.11	3.79	−25.83
Sample 3	6.08	4.17	−34.15

shows the change in permeability of FeCoNiMnCr_x porous HEAs before and after corrosion. It is observed that the permeability of all the samples decreases with the increase in corrosion time, and the rate of change before and after corrosion is around 25%. The permeability of Sample 1 has the lowest rate of change compared to the rate of change in Sample 2 and Sample 3, which again indicates that Sample 1 has the best corrosion resistance. In general, with the increase in Cr content, the corrosion products increased, the average pore size and permeability decreased, and the immersion results were basically consistent with the polarization curves.

4. Conclusions

In this paper, the effect of different Cr contents on the corrosion behavior of FeCoNiMnCr_x porous HEA in 3.5 wt.% NaCl solution is investigated, and the following conclusions are drawn based on the experimental results and the above discussion:

(1) FeCoNiMnCr_x (x = 0.5, 1.0, 1.5) porous HEA is a single FCC solid solution phase without any second phase. The maximum pore size, average pore size, total porosity, and permeability of the porous HEAs increase with the increase in Cr content. The pore-making mechanism mainly involves the interstitial pores after powder pressing, and the Kirkendall pores are caused by different elements in the diffusion process due to different diffusion rates.

(2) From the polarization curves of the FeCoNiMnCr_x porous HEA, pore formation creates a better corrosion environment, and the continued accumulation in the amount of charge on the surface of the alloy leads to an accelerated rate of dissolution of the passivation film. By analyzing the SEM images of the FeCoNiMnCr_x porous HEA, the maximum depth of the corrosion pits was found to increase gradually as the porosity increased, the interconnectivity between the holes improved significantly, and the corrosive solution contact area increased damage to passivation film, leading to more severe corrosion.

(3) The weight gain rate of FeCoNiMnCr_x porous HEA immersed in 3.5 wt.% NaCl solution for 168 h was calculated, and the weight gain rate of the specimen Sample 1 < Sample 2 < Sample 3. Because the specimen’s average pore size and permeability decreased due to the increase in corrosion products, the FeCoNiMnCr_x porous HEA decreased with the increase in Cr content. Corrosion resistance decreased.