1 Introduction

In recent years, researchers have gone beyond conventional alloy design concepts and proposed the production of multicomponent alloys, called high-entropy alloys (HEAs), by mixing a variety of major alloying elements in approximately equiatomic ratios. HEAs possess extraordinarily stable microstructures, as well as a wonderful potential in excellent mechanical properties and corrosion resistance. Consequently, HEAs have attracted considerable attention and possess wide application potential in industrial manufacturing fields (Ref 1,2,3,4,5).

Among the various types of HEAs, equiatomic CoCrFeMnNi HEAs, known as Cantor alloys, have a unique single-phase face-centered cubic (FCC) structure (Ref 6,7,8). To date, CoCrFeMnNi HEAs have been widely studied because of their outstanding mechanical properties (Ref 7, 9,10,11). For instance, Kim (Ref 12) investigated the deformation behavior and high-cycle fatigue (HCF) properties of a CoCrFeMnNi HEA manufactured by selective laser melting (SLM). The SLM-built HEA had an exceptional yield strength of 774.8 MPa and an elongation of 30.8%. Schuh (Ref 13) used arc melting to prepare a CoCrFeMnNi HEA that was then subjected to severe plastic deformation (SPD) under high-pressure torsion. The hardness of the drop-cast ingots was always maintained above 520 HV and reached a maximum of 910 HV. Jiang (Ref 14) manufactured a CoCrFeMnNi HEA by spark plasma sintering (SPS) and studied the mechanical properties. The SPSed CoCrFeMnNi HEA showed a uniform elongation (UE) of 9.5% and a gigapascal yield strength. The abovementioned results show that CoCrFeMnNi HEA could be used as a material for load-carrying structures.

Reinforced concrete is a common building structural material that is widely used in many industries (Ref 15,16,17,18). In the service process, the corrosion resistance of metals is critical for the durability of reinforced concrete (Ref 19,20,21,22,23). The corrosion properties of CoCrFeMnNi HEAs have been previously reported. For example, Luo (Ref 24) compared the anti-corrosion resistance of a CoCrFeMnNi HEA and a 304 stainless steel (SS) in a 0.1 M H2SO4 solution. A thinner passive film formed on the 304 SS than on the CoCrFeMnNi HEA. Xu (Ref 25) prepared a CoCrFeMnNi HEA by SLM and investigated the corrosion resistance of the HEA in 3.5 wt.% NaCl solution. The sample had a low corrosion current density (icorr) of 4.737×10− 7A·cm−2. Pathak (Ref 26) used vacuum casting to produce CoCrFeMnNi HEAs and analyzed their corrosion behavior. The HEAs had excellent anti-corrosion properties in both NaCl and NaOH solutions. Luo (Ref 27) explored the effect of carbon on the corrosion behavior of CoCrFeMnNi HEAs in a chlorinated concrete solution and found that carbon changed the doping concentration, but not the semiconductor type of passive film. Zhu (Ref 28, 29) reported a HEA with secondary passivation characteristics, and increasing iAC changed the corrosion form of CoCrFeMnNi HEA in a carbonate/bicarbonate solution. Thus, previous studies on the corrosion behavior of HEAs have mainly been conducted in acidic, neutral and weakly alkaline environments. With a high corrosion resistance, CoCrFeMnNi HEA is likely to be used in reinforced concrete structures in future.

A large number of studies have demonstrated that alternating current (AC) interference significantly aggravates metal corrosion in electrical facilities, pipeline transportation, and other industries (Ref 30,31,32,33). Given the wide application of reinforced concrete, the influence of AC interference on the corrosion behavior of reinforced concrete should be explored in depth. Few studies (Ref 33) have shown that concrete containing discrete steel fibers has an inherent corrosion resistance to stray AC interference. Increasing temperature can affect the corrosion reaction rate of alloys (Ref 34,35,36,37). It has been reported that AC interference can generate a sufficiently large quantity of heat in concrete structures that the temperature can reach 50 °C (Ref 38). However, few studies have been conducted on the corrosion behavior of CoCrFeMnNi HEAs in simulated concrete pore solution. Thus, it is necessary to study the effect of AC interference and temperature on the corrosion resistance of CoCrFeMnNi HEA for potential application to reinforced concrete.

In this paper, the effect of AC interference and temperature on the corrosion behavior of CoCrFeMnNi HEA in a simulated concrete pore solution with chloride was studied by electrochemical tests, atomic force microscopy (AFM) and immersion test. Furthermore, the synergy between AC interference and temperature was analyzed. The purpose of this work is to deepen the understanding on the corrosion behavior of the HEA in reinforced concrete environment.

2 Experimental Procedures

2.1 Sample Processing and Testing Solution

The equiatomic CoCrFeMnNi HEA was fabricated by high-purity (99.99%) alloy elements under an inert gas atmosphere in an induction suspension furnace. The melting temperature was risen to 1700 °C. Then, the as-cast ingot was cooled in the furnace for 30 minutes to room temperature, and remelted at least 4 times to ensure its homogeneous composition. The chemical composition (wt.%) of the HEA was Co 21.3, Cr 19.6, Ni 20.2, Mn 20.5 and Fe balance. Samples with a dimension of 1 cm × 1 cm × 0.3 cm were machined from the prepared HEA ingot, and sealed with ethoxyline resin, leaving 1 cm2 of the testing surface. Prior to each test, the sample surfaces were mechanically ground with SiC sandpapers from #320 to #2000, and then polished by using 0.5 μm diamond suspension. Finally, the samples were cleaned, and dried with cold air. In this work, a solution of 0.001 M Ca(OH)2+0.2 M NaOH+0.6 M KOH was selected (Ref 39, 40). To simulate the real concrete environment, 0.15 M NaCl was added into the prepared solution considering that there are a small amount of polluted chloride ions. The pH of the solution was approximately 13.65.

Figure 1 shows the microstructure of the HEA observed by scanning electron microscopy (SEM, FEI Quanta 250) after erosion by aqua regia for approximately 6 s. Obviously, the as-cast HEA consists of a single-phase FCC structure.

Fig. 1
figure 1

Microstructure of as-cast CoCrFeMnNi HEA

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2.2 Electrochemical Tests

The electrochemical test was conducted with a CHI660E electrochemical workstation via a conventional three-electrode system. The HEA sample was acted as the working electrode (WE), the saturated calomel electrode (SCE) was used as the reference electrode, and the large-area Pt sheet was served as the counter electrode (CE). The electrochemical test equipment for AC corrosion research on the HEA was consistent with our previous research (Ref 41, 42). The current density of a sine wave AC signal with a frequency of 50 Hz (power frequency in China) was applied to the sample as the root mean square value, which was measured by a high-precision AC clamp meter. Electrochemical tests were included potentiodynamic polarization curve, electrochemical impedance spectroscopy (EIS) spectra and Mott-Schottky curve. Before the test, the sample was soaked in the solution for 3 h to reach a stable state. Then, under the interference of different iAC (0, 50, 100, 200 A/m2), the potentiodynamic polarization curve test was performed at a scan rate of 1 mV·s− 1, and the potential range was − 1.0~1.5V (vs. SCE). The iAC values were in accord with the actual interference intensity that buried pipeline steels suffered. The EIS spectra and Mott-Schottky curve were measured to study the influence of AC on the damage behavior of passive film formed on the HEA surface. The experimental design was as follows: First, the passive film was grown on the surface of the HEA sample by potentiostatic polarization at a fixed potential of 0.16 V (vs. SCE) for 2 h, and then different iAC were applied to the electrode sample. After 5 min of AC interference, the AC power was removed. Subsequently, the EIS spectra and Mott-Schottky curve were successively measured. The EIS test was performed at open circuit potential from 100 kHz to 10m Hz with a 10 mV interference signal. The test frequency of the Mott-Schottky curve was 1 kHz, the step length was 50 mV, and the potential sweep range was − 1.0 V~0.1 V. All tests were performed more than 3 times to ensure data reproducibility.

2.3 AFM Analysis

In the AFM test, the preparation procedure of the passive film on the HEA surface is the same as those of EIS and Mott-Schottky curve measurements. Then, various iAC of 0, 50 and 200 A m-2 were applied to the electrode with passive film for 5 min. In this test, the solution temperature was maintained at 50 °C. Subsequently, the samples were taken out, washed with deionized water, and air-dried.

The topography morphology characteristics of passive films on the HEA treated under different conditions were observed by an atomic force microscope (PSLA XE-100E AFM) with a non-contact mode.

2.4 Immersion Test

The samples used in the immersion test were identical to those of electrochemical test. Prior to measurement, the sample was polished, cleaned, and dried. A high-precision electronic balance (0.0001 mg) was used to record the weight of the HEA sample. The samples with different iAC were placed in the solution at 30 and 50 °C for 96 h. After the test, dilute nitric acid was used to completely remove the salts on the sample surface. The weight-loss method was used to calculate the average corrosion rate of the HEA samples tested under various conditions. Subsequently, the surface corrosion morphology was observed and analyzed by SEM and energy dispersive spectroscopy (EDS). The above test was repeated at least 4 times.

3 Results and Discussion

3.1 Potentiodynamic Polarization Curve

Figure 2 shows the potentiodynamic polarization curves of CoCrFeMnNi HEA at different AC current densities and temperatures of 30, 40 and 50 °C in simulated concrete pore solution. Clearly, all the curves for the HEA measured under different conditions exhibit similar shapes and obvious passive characteristics. When AC current is applied at a fixed temperature, as iAC increases, the curve gradually shifts to the right, and the passive zone narrows at high iAC. With increasing solution temperature, a transition region appears at iAC of 200A/m2, and the curve also moves rightward, which indicates that the growth of the passive film on the sample becomes hard under the interference of high AC current density (Ref 43). This result indicates that AC current increases the instability and hinders the formation of the passive film. Furthermore, the higher the solution temperature is, the more harmful its influence on the passive film is. Thus, increasing iAC and solution temperature can promote the occurrence of corrosion. At 50 °C, as iAC increases, the corrosion potential (Ecorr) gradually decreases (Fig.3), and the critical pitting potential (Ep) exhibits a negative shift, which is particularly prominent at high iAC of 100 A/m2 and 200 A/m2. Lalvani and Lin (Ref 44) proposed a mathematical model for the influence of AC interference on the corrosion potential, within which Ecorr depends upon the Tafel slope ratio (r = βa/βc) and peak voltage (Epeak). In Fig. 2(c), r is less than 1. Fu (Ref 45) found that Epeak was proportional to AC current density. Thus, Ecorr becomes more negative as iAC increases. Hence, the combined action of high iAC and solution temperature increases the corrosion tendency and the susceptibility to pitting corrosion. This suggests that at high iAC, the passive film is easier to rupture, and the corrosion of the HEA is more likely to occur.

Fig. 2
figure 2

Potentiodynamic polarization curves of CoCrFeMnNi HEA at various solution temperatures and different AC current densities in simulated concrete pore solution: (a) 30 °C ; (b) 40 °C; (c) 50 °C

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Fig. 3
figure 3

Ecorr value of CoCrFeMnNi HEA under different iAC

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The fitted passive current densities (ip) of the HEA at different solution temperatures and iAC are displayed in Fig. 4. In the absence of AC current, the lowest ip of the HEA occurs at 30 °C. At a fixed temperature, ip increases with iAC, indicating that AC current accelerates the corrosion of the HEA. Similarly, at a fixed iAC, ip increases with solution temperature, which indicates that increasing solution temperature promotes the dissolution rate of passive film and reduces the corrosion resistance of the HEA (Ref 46). In particular, ip increases significantly at high iAC and temperature, suggesting that the synergistic effect of the applied AC current and the solution temperature considerably decreases the protectiveness of surface passive film and significantly facilitates the corrosion of the HEA. Some studies (Ref 42, 47) have shown that the interference of AC current causes the instantaneous fluctuation in the electrode potential, resulting in a complex current flow through the electrode that increases the corrosion current density of pipeline steels. Moreover, the increased temperature promotes the thermal movement of substances in solution, improves the reactive ability of active ions (Cl−, etc.), and enhances the penetration ability of ions, facilitating the passage of ions through the defects of passive film to contact the metal matrix, thus accelerating the electrochemical corrosion of the HEA (Ref 38, 48, 49). Therefore, the synergistic effect between the two influencing factors remarkably reduces the corrosion resistance of CoCrFeMnNi HEA.

Fig. 4
figure 4

Ip value of CoCrFeMnNi HEA under different testing conditions

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3.2 EIS and Mott-Schottky Curves

To deeply explore the detrimental effect of AC current on the passive film grown on the surface of CoCrFeMnNi HEA in the simulated concrete pore environment, EIS spectra and Mott-Schottky curves were measured. Before testing, a passive film was formed on the HEA by potentiostatic polarization, and then interfered under various AC current densities for 5 min.

The Nyquist and Bode plots of passive films on the HEA interfered with by different iAC at 30 °C are shown in Fig. 5. In Fig. 5(a), each curve presents an incomplete semicircle and the characteristic of capacitive reactance. A clear difference among the radii of capacitive reactance arcs can be seen, where the largest radius is obtained for the HEA in the absence of interference by an external AC current. The larger the arc radius, the higher the protective ability of passive film is (Ref 50,51,52,53,54). As the imposed iAC increases, the radius of capacitive reactance arc decreases. This result reveals that superimposed AC deteriorates the corrosion resistance of passive film covering the HEA surface. In the Bode plots (Fig. 5b and c), the passive film without the applied AC current has the largest phase angle, and the impedance modulus value in the low-frequency stage decreases with increasing applied iAC, further indicating that the applied AC current can reduce the anti-corrosion property of CoCrFeMnNi HEA.

Fig. 5
figure 5

EIS curves of CoCrFeMnNi HEA at different AC current densities under 30 °C: (a) Nyquist and (b, c) Bode plots

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The EIS data were fitted using an equivalent electric circuit (EEC) (Fig. 6) (Ref 55, 56). In the EEC, Rs denotes the electrolyte resistance, Qf symbolizes the passive film capacitance, Rf denotes the passive film resistance, Qdl represents the nonideal double-layer capacitance and Rct is the charge transfer resistance. The model provides information about the electrochemical reactions occurring at the matrix/solution interface of a passive system. The fitted values of the aforementioned parameters are listed in Table 1. In the absence of AC current, the HEA sample possesses the largest values of Rct and Rf. As the applied iAC increases, the two indexes values gradually decrease. A decrease in Rf implies that the applied AC increases the instability of passive film and decreases the film compactness. The similar change trend for Rct suggests that the corrosion resistance of the HEA decreases with increasing iAC. Therefore, the fitted parameter results further demonstrate that the greater the applied iAC, the more adverse the effect on the corrosion resistance of the HEA is.

Fig. 6
figure 6

Equivalent electric circuit for fitting EIS data

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Table 1 The fitted electrochemical parameters for EIS diagrams of the HEA interfered with different AC current densities at 30 °C
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As has been reported in the literature (Ref 27), the corrosion resistance of the HEA was enhanced in a chlorinated concrete solution, which may be related to the change in the average grain size after hot rolling, homogenization, cold rolling and annealing treatment. Various test solutions also affected the corrosion behavior of the HEA.

It is well known that the relationship between the applied electrode potential (E) and space charge capacitance (C) can be expressed by the Mott-Schottky equation. To further clarify the influence of the applied iAC on the anti-corrosion property of CoCrFeMnNi HEA covered with passive film in a simulated concrete pore environment.

Mott-Schottky curves were measured for the passive films interfered with by various iAC. For n/p-type semiconductors, the correlation between C and E was exhibited as follows (Ref 57,58,59,60,61):

$$\frac{1}{{C^{2} }} = \pm \frac{2}{{eN\varepsilon \varepsilon_{0} }}\left( {E - E_{FB} - \frac{kT}{e}} \right)$$
(1)

where the positive sign (+) denotes the n-type semiconductor, and the negative sign (−) represents the p-type semiconductor. ε is the dielectric constant, in this paper, ε = 12 (Ref 27,28,29). ε0 is the vacuum dielectric constant (8.854×10− 14 F∙cm-1), E is the applied electrode potential, k is the Boltzmann constant (1.38 × 10− 23 J∙K− 1), E is the electron charge (1.602 × 10− 19 C), EFB is the flat band potential, and T is the Kelvin temperature (K). N is the carrier concentration (ND is the donor density, and NA is the acceptor density), and its value was calculated according to the following formula:

$$N = \frac{2}{{em\varepsilon \varepsilon_{0} }}$$
(2)

where m denotes the slope of linear part of the curve. In addition, the thickness of passive film was calculated by utilizing the following formula (Ref 62):

$$d = \sqrt {\left[ {\frac{{2\upvarepsilon \upvarepsilon _{0} \left( {E - E_{{{\text{FB}}}} - {\text{k}}T} \right)/e}}{{eN_{D} }}} \right]}$$
(3)

Figure 7 shows the Mott-Schottky curves of passive films on the HEA surface under interference by different iAC at 30 °C. All the curves show n-p type semiconductor characteristics. The calculated ND and NA values are shown in Fig. 8. The passive film without AC interference has the lowest values of ND and NA, and the two carrier densities rapidly increase with iAC. This indicates that increasing iAC leads to an increase in the number of point defects within the passive film. The higher the number of point defects, the less compact the passive film is, that is, the diffusion resistance of charges, ions and molecules decreases (Ref 63). This may be attributed to the fact that the migration rate of ions (such as Cl− and OH−) increases with iAC (Ref 47, 64), which enables more OH − and Cl − to react with oxygen vacancies to generate cationic vacancies, deteriorating the structural stability of passive film (Ref 29). Moreover, the increase in the number of H atoms with increasing iAC generates more defects within the film (Ref 29, 42). This implies that AC interference makes the passive film more likely to rupture. Consequently, as iAC increases, the protective ability of passive film is reduced, and the resistance to pitting corrosion of the HEA decreases. Figure 9 shows the variation trend in the thickness of passive film on the HEA under various iAC. The imposed AC current significantly thins the thickness of passive film.

Fig. 7
figure 7

Mott-Schottky plots of passive films on CoCrFeMnNi HEA interfered at different AC current densities under 30 °C

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Fig. 8
figure 8

Charge carrier densities of ND and NA within the passive films on CoCrFeMnNi HEA interfered at different AC current densities under 30 °C

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Fig. 9
figure 9

Thickness of passive films on CoCrFeMnNi HEA applied with different AC current densities

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3.3 Morphology Analysis of Passive Film

To further evaluate the relationship between iAC and passive films formed on the HEA, the topographical characteristics of films interfered with by different iAC at 50 °C were analyzed by AFM measurement. As shown in Fig. 10(a), the passive film formed on the sample without AC interference is uniform, dense and flat, with small height fluctuation. As the applied iAC increases, the passive film becomes uneven. The AFM line-scanning image presented in Fig. 10a4 shows that the height fluctuation range is within 5 nm in the absence of AC interference, increases up to 15 nm under a low iAC of 50 A/m2, and has a large amplitude of nearly 35 nm under a high iAC of 200 A/m2. Additionally, the number of defects within the passive film increases remarkably under an applied iAC of 200 A/m2. The magnified morphology in region A (green box) (Fig. 10c1-c3) and the height data of Line2 in Fig. 10(c5) demonstrate the existence of holes. The presence of holes indicates that AC current damages the integrity and compactness of passive film. These holes increase the number of channels through which harmful ions can infiltrate the substrate interface.

Fig. 10
figure 10figure 10

Morphologies of passive films on the surface of HEA under various AC current densities at 50 °C: (a1-a4) 0 A/m2; (b1-b5)50 A/m2; (c1-c5)200 A/m2

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Furthermore, Fig. 10(b1) and (c1) displays that the scratches on the surface gradually become apparent, suggesting that AC interference results in a reduction in the film thickness. Figure 9 also verifies this. The corresponding data in Fig. 11 exhibit that the passive film on the HEA surface becomes rougher with increasing iAC. The heterogeneous morphology characteristics of passive film may be conducive to the aggregation of harmful ions, leading to a decrease in the stability and protective property of passive film, and accelerating the corrosion of the HEA. Therefore, higher iAC significantly impairs its corrosion resistance.

Fig. 11
figure 11

Roughness of passive films on HEA surface under different iAC at 50 °C

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3.4 Immersion Test

Figure 12(a) exhibits the corrosion rate of CoCrFeMnNi HEA after immersion for 96 h under various iAC and solution temperatures. The corrosion rate of the HEA clearly varies for different conditions. The corrosion rate is the lowest in the absence of AC at 30 °C, and increases by approximately 1.5 times at 50 °C, showing that increasing solution temperature accelerates the corrosion of the HEA. In addition, at 50 °C, the corrosion rate gradually increases with iAC, suggesting that the imposition of AC promotes the occurrence of corrosion. Under an applied iAC of 200 A/m2, the corrosion rate is approximately 4.7 times that of the HEA without AC interference at 30 °C. This significant increase in the corrosion rate implies that a synergistic effect between AC current and temperature considerably facilitates the corrosion of the HEA. In Fig. 12(b), the pit depth exhibits a similar increasing tendency, which also demonstrates the accelerated effect the two factors have on the corrosion degree of the HEA.

Fig.12
figure 12

Corrosion rate (a) and pit depth (b) of CoCrFeMnNi HEA immersed for 96h under different conditions

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The surface corrosion morphologies of the HEA under various iAC at different temperatures are shown in Fig. 13. Without AC application (Fig.13a-d), the corrosion degree of the HEA at 30 °C is very slight, and a few small pits are sporadically distributed on the sample surface. When the solution temperature rises to 50 °C, the number and size of pinhole pits increase. The difference in the corrosion degree is attributed to the increase in the solution temperature. The amount of pits gradually increases with iAC, which indicates that the applied AC facilitates the initiation of pitting corrosion. Moreover, an increase in pit size is exhibited. In particular, as shown in Fig. 13(h) and (j), when iAC is increased to 100 and 200 A/m2, different size pits dissolve and merge into large corrosion pits. This change in the corrosion morphology implies that increasing iAC greatly enhances the corrosion rate of the HEA, and promotes the susceptibility to pit corrosion. Combining the change rules for the corrosion rate, it can be concluded that the combined effect of AC and solution temperature significantly increases the corrosion rate of CoCrFeMnNi HEA in the simulated concrete pore environment. This law may result from the damaging influence the two factors have on the passive film grown on the HEA surface. According to the abovementioned analysis, increasing iAC and solution temperature can increase the driving force of ionic migration, and promote the migration rate of harmful Cl−: the enhanced adsorption of Cl− ions on the surface of passive film, significantly damages the compactness of the film, and increases the pitting sensitivity of the HEA. Additionally, a micro-galvanic corrosion cell can form between the bare substrate and the damaged film that accelerates anodic dissolution of the matrix (Ref 65, 66), and facilitates the initiation of pits (Ref 67,68,69).

Fig.13
figure 13

Corrosion morphologies of CoCrFeMnNi HEA immersed for 96 h at different conditions: (a, b) 0 A/m2 at 30°C; (c, d) 0 A/m2 at 50 °C; (e, f) 50 A/m2 at 50°C; (g, h) 100 A/m2 at 50 °C; (i, j) 200 A/m2 at 50 °C

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Figure 14 exhibits the surface corrosion morphologies and EDS results for HEA samples under different applied iAC at 50 °C. Without AC interference, point 1 represents the FCC matrix, and point 2 corresponds to the interior of pit. A comparison of the corresponding data shows that the contents of Mn and Cr elements in the interior of pit increase, whereas the contents of Fe, Co, and Ni elements exhibit a decreasing trend (which is particularly marked for Co and Ni). Under the interference of different iAC of 50 and 200 A/m2, the variation trend in the elemental contents inside the pits is analogous to that obtained without AC application. Hence, the formation of pits may be attributed to the selective dissolution of Fe, Co and Ni. This dissolution behavior weakens the compactness and integrity of passive film, and reduces the protectiveness of the film formed on the HEA surface to some extent.

Fig. 14
figure 14

EDS point-scanning results of CoCrFeMnNi HEA at different AC current densities under 50 °C: (a) 0 A/m2; (b) 50 A/m2; (c) 200 A/m2

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Figure 15 shows the EDS mapping result for the HEA sample interfered with by an iAC of 200 A/m2 at 50 °C. An obvious reduction in the contents of Fe, Co, and Ni elements distributed inside the pit is observed. This change further confirms that the occurrence of pits is attributed to the selective dissolution of Fe, Co and Ni. This phenomenon may be caused by the inhomogeneous distribution of elements in the local region of the matrix.

Fig. 15
figure 15

EDS mapping images of CoCrFeMnNi HEA interfered under 200 A/m2 at 50 °C

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4 Conclusions

The effect of AC current and solution temperature on the corrosion behavior of CoCrFeMnNi HEA in simulated concrete pore environment was investigated. The main conclusions are as follows:

  1. (1)

    The imposed AC current increases the amount of defects within passive film, damages the integrity and compactness of the film, thins its thickness, and hinders the film formation.

  2. (2)

    AC interference and solution temperature generate a synergistic effect, which remarkably reduces the stability and protective property of passive film formed on the HEA, and significantly aggravates the occurrence of pitting corrosion.

  3. (3)

    Pits formed on the HEA are due to the selective dissolution of Fe, Co and Ni elements.