The notion of high-entropy alloys (HEAs) or more generally high-entropy materials was coined quite recently and immediately awoke enormous interest by providing pathways for the development of new materials or improvement of the existing ones. In the group of high-entropy alloys, initial efforts were directed on combinations of Al with 3d transition metals, namely, Cr, Fe, Co, Ni, and to a lesser extent Cu and Mn (Miracle and Senkov, 2016). At present, the multi-principal element alloys are referred to as HEAs or compositionally complex alloys even when their configurational entropy is not high enough to secure stability of a single solid solution. Commonly, second phases are formed, including either solid solutions or intermetallics or both, which are accompanied by strong segregation of alloying elements (Pickering and Jones, 2016). AlCoCrFeNi alloys are known to have different phase composition depending on the concentration of Al. As the concentration of Al increases, their structure transforms from face-centered cubic (FCC) through mixed FCC + body-centered cubic (BCC) to BCC (Zhang et al., 2014). Since mechanical properties, that is, ductility and strength, of a number of single-phase HEAs did not meet basic requirements for structural applications, major developments were switched toward the design and fabrication of dual phase FCC + BCC alloys. One of the promising results of this work was AlCoCrFeNi_2.1, a castable eutectic high-entropy alloy with very good combination of high fracture strength and ductility (Lu et al., 2015). This composition was further modified to Co-free AlCrFe₂Ni₂, which not only had mechanical properties outperforming as-cast HEAs and many titanium alloys at room temperature but also was less expensive. The structure of this alloy, consisting of FCC(A1), disordered BCC(A2), and ordered BBC(B2), results from spinodal decomposition of the original BCC phase upon cooling. The very attractive combination of fracture strength and ductility, characteristic of this alloy, is attributed to the coexistence of harder BCC phases and softer FCC (Dong et al., 2016). In addition to mechanical testing, many HEAs were tested for corrosion resistance in different aqueous environments, mostly in 3.5 wt.% NaCl and 0.1 M H₂SO₄ (Hsu et al., 2005; Lee et al., 2008b; Chou et al., 2010; Kao et al., 2010; Lin and Tsai, 2011; Qiu et al., 2017). The results looked very promising. It was indicated, however, that some alloying additions, such as Al, Cu, or Mo, could induce elemental segregation in HEAs, leading to galvanic coupling or local breakdown of passive films formed on alloy surface in salt or acid solutions. One way to overcome this drawback is seen in microstructure refining by an adequate design of manufacturing processes (Shi et al., 2017). Among the alloying additions, Mo deserves special attention since it is known to reduce the susceptibility of stainless steels to pitting corrosion. It is also known that Mo concentration in steels should be possibly low to avoid precipitation of brittle sigma and alpha phases. Simultaneous depletion in Mo and Cr of the neighboring grains would make them more vulnerable to corrosion attacks (Park and Lee, 2014; Shang et al., 2018; Valeriano et al., 2019). Although the pitting corrosion equivalent used for stainless steels cannot be simply transferred to HEAs, it was reported that small additions of Mo increased pitting corrosion resistance of some HEAs in chloride-containing environments (Chou et al., 2010; Rodriguez et al., 2018).

In this work, corrosion of selected HEAs was investigated in aqueous solutions of sodium chloride. The principal objectives were 1) to verify a hypothesis about different corrosion behavior of alloys with the same nominal composition and different microstructure on the basis of an AlCrFe₂Ni₂ alloy produced by different manufacturing routes, 2) to assess whether or not minor additions of Mo can positively affect the corrosion resistance of the baseline alloy, AlCrFe₂Ni₂, and 3) to evaluate the susceptibility to corrosion of the FCC and BCC phases in the equimolar alloy, AlCoCrFeNi. A duplex stainless steel EN 1.4517 with excellent corrosion resistance was used as a reference material.

Experimental alloys, AlCrFe₂Ni₂ (with and without Mo) and AlCoCrFeNi (equimolar and with compositions close to its FCC and BCC constituent phases), were obtained from elemental powders (alloys with Co) or granules by arc melting under an argon atmosphere. Purity of the components was: Al 4N; Co 3N; Cr 3N5; Fe 3N; Ni 3N7; Mo 2N8. The reference materials, EN 1.4519 stainless steel, and two Co-free baseline alloys (purity of components: Al 2N5; Cr 2N8; Fe 3N; Ni 2N85) were produced by ingot metallurgy (Otto Junker GmbH). Alloy marked AlCrFe₂Ni₂ (1) was cast in air and AlCrFe₂Ni₂ (2) in argon. Chemical compositions of all investigated alloys are collected in Table 1. The EN 1.4517 duplex stainless steel (STS) had the following composition in wt.%: Fe 61.77; Cr 25.18; Ni 5.70; Cu 3.00; Mo 2.74; Si 0.85; Mn 0.53; N 0.156; V 0.032; C 0.021; P 0.002; S 0.001. Samples in the form of flat plates were ground with abrasive papers up to ISO/FEPA 1200 grit and cleaned with isopropanol. For electrochemical measurements, copper wire connections were attached to each sample using conductive silver lacquer (Leitsilber 200N, Hans Wolbring GmbH). Next, the samples were mounted in epoxy resin (Crystalline 940). Potentiodynamic and electrochemical impedance spectroscopy measurements were done in a three-electrode cell: sample as working electrode (exposed area mostly about 1 cm²), saturated calomel electrode (SCE) as reference, and graphite as auxiliary electrode (area about 50 cm²), using SP-150 potentiostat controlled by Bio-Logic EC-lab V11.25 or Nova 2.1.4 Metrohm Autolab B.V. (electrochemical impedance spectroscopy) software. Liquid medium used in this work was 3.5 wt.% NaCl (at 25°C). Long-term (72 h) or short-term (2 h) open circuit voltage measurements in 3.5 wt.% NaCl were conducted for all samples. Potentiodynamic measurements were done in a regular range between −300 and +300 mV vs. open circuit voltage (OCV) (polarization resistance) and in an extended potential range between −300 and +1,500 mV vs. OCV (cyclic potentiodynamic polarization test according to ASTM G61-86). Polarization resistance measurements were done at a potential scan rate of 30 mV/min and consisted of 20 cycles. Each cycle was preceded by monitoring the variations of OCV for 2 h (before the first cycle) and for 30 min (before cycles 2–20). Cyclic potentiodynamic polarization tests were conducted at a scan rate of 10 mV/min until potential reached +1,500 mV vs. OCV or until current density reached 5 mA/cm². Then, the reverse scan was activated and continued to the initial OCV value. As previously reported (Zhang et al., 2009), potential scan rate plays an important role in minimizing the effects of distortion of the potentiodynamic polarization curves in the analyses of Tafel slopes and corrosion current density. It is noteworthy that at the selected scan rate of 10 mV/min, no significant distortions were produced. Electrochemical impedance spectroscopy measurements were carried out in the frequency range of 10 mHz–100 kHz in a single-sine excitation mode. The potential start point was the last value recorded in the 2-h OCV measurement. Three points for each frequency were recorded, and the number of points per decade was 10 in logarithmic spacing. Each sample and each type of experiment were repeated several times to assess reproducibility of the corrosion parameters. For clarity, the averaged values are presented on diagrams. Sample surfaces or cross-sections before and after the tests were examined by means of a Nova NanoSEM 200 FEI scanning electron microscope (SEM) equipped with an energy-dispersive X-ray spectroscope for chemical analysis. Selected samples were analyzed by transmission electron microscopy (TEM) using a FEI Tecnai TF20 X-TWIN (FEG) microscope, equipped with an EDAX energy dispersive X-ray detector, at an accelerating voltage of 200 kV. Thin foils were prepared by a focused ion beam (FIB) technique, using a FEI Quanta 200 3D Dual Beam FIB/SEM system equipped with an Omniprobe micromanipulator.

It has been evidenced by many studies that mechanical properties of HEAs depend on the microstructure (Tang et al., 2013, Tang et al., 2015; Wang et al., 2018). Rapidly solidifying alloys produced by arc melting or additive manufacturing are generally nanostructured, and their nano-sized components are responsible for the dispersion strengthening effect. Subsequent processing, for example, heat treatments, may, however, lead to grain coarsening and loss of superb properties.

Very impressive differences in microstructure were visible in AlCrFe₂Ni₂ alloys produced by arc melting and ingot metallurgy. The distribution of phases in the baseline alloy, AlCrFe₂Ni₂, obtained by arc melting (Figure 1A) was practically the same as that earlier described by Dong et al. (2016), with characteristic vermicular FCC and nanostructured BCC phases, while in cast alloys with the same chemical composition, the nanostructures were absent, and grains were much coarser.

The results of several electrochemical tests carried out in this work indicate that there is a noticeable influence of grain size and distribution of phases on the exposed surface on the corrosion behavior. In the OCV measurements, the trends were similar for the three AlCrFe₂Ni₂ samples, that is, the potential increased with time, but the values recorded for the arc-melted material were less negative than those recorded for both cast ones. Numerous spikes on the OCV curves generally reflect local phenomena (pitting), such as damage of a protective layer by ions in the solution (Kabir and Mahmud, 2011). Polarization resistance describes the ability to resist the electron transfer from one phase to another. Values increasing over time suggest improvement of the corrosion resistance. The average values of polarization resistance over 20 consecutive cycles were higher for the arc-melted alloy than those for the cast ones. Significant scatter of the experimental points in consecutive cycles indicates that protective layers formed on alloy surfaces were not stable enough. Tafel polarization curves were very similar in shape, and again some of the derived characteristic parameters, such as corrosion current, pitting potential, and corrosion rate, listed in Table 2, were more advantageous for the arc-melted sample. The differences in corrosion behavior can be attributed to different relative areas of individual phases and density of grain/phase boundaries on the surface exposed to the solution, especially when some phases are more reactive than others, as is evidenced by depletion of the BCC phase in aluminum and nickel.

Chloride ions are known to interact with the native oxide films, leading to their breakdown, particularly in the areas where these films are thinner, for example, over intermetallic precipitates in Al-based alloys (Shao et al., 2003; Yasakau et al., 2006; Shi et al., 2017). Failure of the oxide barrier triggers chemical reactions of active metals, such as aluminum, with water, accompanied by the evolution of hydrogen: 2 Al + 6 H 2 O = 2 Al 3 + + 6 OH − + 3 H 2 ↑ .

Susceptibility of the exposed grains to pitting corrosion is also dependent on their crystallographic orientation ‐ crystallographic planes with higher atomic density, higher coordination number, and lower surface energy are expected to be naturally more resistant (Shahryari et al., 2009). As it has been illustrated by numerous examples (Ralston et al., 2010), the reduced grain size has positive effects on corrosion rate only in cases when an oxide/passive film can form on the surface of metallic materials. Otherwise, dense grain boundaries are likely to increase surface reactivity and the corrosion rate. Also, segregation of impurities may contribute to the role of grain refinement in corrosion behavior (Miyamoto et al., 2008). Intergranular corrosion is, namely, often induced by impurity segregation and precipitation at grain boundaries. It has been reported that dilution of the segregated impurities at grain boundaries accounts for the beneficial effect of grain size reduction on corrosion resistance (Palumbo et al., 1990; Wang et al., 1993; Palumbo and Erb, 1999).

In the alloys investigated in this work, major impurities due to processing in air or argon were Al oxide or nitride particles. These might contribute to abrupt fluctuations of potential or current in electrochemical tests. There was no evidence, however, that the alloy processed in air was much more susceptible to pitting than the one processed in argon [AlCrFe₂Ni₂ (1) and AlCrFe₂Ni₂ (2), respectively]. Another issue is that all metals used for arc melting had better purity than those used for casting, and thus increased concentration of impurities in the intercrystalline regions of the coarse-grained microstructure might lead to worse corrosion resistance.

As mentioned earlier, the Mo additions contributed to some refinement of alloy microstructure. In sodium chloride solution, for the three Mo-containing alloys, the values of measured electrochemical parameters were less advantageous or comparable with the baseline arc-melted alloy. In the OCV measurements, the most striking difference was disappearance of potential spikes at the higher contents of Mo in the alloy, which suggested that local phenomena became less pronounced. In the Tafel plots, the curves shifted toward higher potentials with increasing content of Mo, and fluctuations of current were practically absent for AlCrFe₂Ni₂Mo_0.15. The values of charge transfer resistance (R _ct) derived from the Nyquist plot increased with the Mo content and were in a range typical of alloys with a similar composition and microstructure (Lu et al., 2015). The values of solution resistance (R _s) were similar for all the alloys and consistent with the earlier reported data (Kumar et al., 2017). The double layer capacitance (Q _dl) was approximately the same as reported by other researchers (Pan et al., 1998; Kumar et al., 2017). In comparison with other HEAs, such as Al _x CrFe_1.5MnNi_0.5 (Lee et al., 2008b), FeCoNiCrCu _x (Hsu et al., 2005) or Co_1.5CrFeNi_1.5Ti_0.5Mo _x (Lee et al., 2008a), tested in NaCl solutions, all Co-free alloys investigated in this work had much better corrosion resistance.

Mo together with Cr in steels is very effective in preventing pitting corrosion in chloride-containing media. The exact mechanisms involved are not fully understood. It is believed that Cr and Mo oxides form barriers for the penetration of chloride ions. A number of researchers claimed that Mo in the passive layer on steels was present mostly in a form of adsorbed anions (MoO₄ ²⁻), constituting a barrier against the electrochemical attack (Pourbaix, 1974; Kodama and Ambrose, 1977; Ogura and Ohama, 1984; Lu and Clayton, 1985; Brooks et al., 1986; Clayton and Lu, 1986). Mo was reported to lower the chloride ion content in the passive film on steel due to its ability to form soluble stable Mo–oxo–chloro complexes as well as insoluble chlorides and oxide chlorides on the surface (Wegrelius et al., 1999). In the case of HEAs containing aluminum in high proportions, the effect of Mo is by far less strong.

The increasing concentration of aluminum in HEAs generally leads to increased volume fraction of a BCC phase and enhancement of pitting corrosion, as illustrated by the behavior of a series of alloys, Al_xCrFe_1.5MnNi_0.5 (Lee et al., 2008a; Shi et al., 2017). The BCC phase with a low concentration of Cr and high concentration of Al produced porous nonprotective oxide layers in contact with a sodium chloride solution.

The detrimental role of the Al-rich component of the investigated alloys was clearly visible in electrochemical measurements conducted in 3.5 wt.% NaCl. The relative amount of the BCC phase in the equimolar quinary alloy was about 75 vol.%, while only about 40 vol.% in the Co-free baseline alloy and consequently the values of corrosion parameters were significantly worse: corrosion potential was much lower (−0.30 vs. −0.25 V), and corrosion current density was higher (0.116 vs. 0.049 μA/cm²). The alloys denoted as BCC and FCC with compositions close to the major phases in the equimolar alloy presented very different chemical properties. The BCC alloy with the Al/Cr atomic ratio of about 7/1 had the lowest OCV, corrosion potential, and pitting potential values among all alloys investigated in this work. Compared with AlCrFe₂Ni₂, its corrosion current density and corrosion rate were roughly three times higher (0.141 μA/cm² and 13.6 × 10^–4 mmpy, respectively). On the contrary, the alloy denoted as FCC with the reversed proportions of Al and Cr, Al/Cr atomic ratio of about 1/10, had very good corrosion resistance, comparable with the STS reference in terms of corrosion and pitting potential, corrosion current density, passivation range, and corrosion rate. It also showed exceptionally high polarization resistance and charge transfer resistance. These observations are consistent with the results previously published by other authors regarding the effects of Al, Mo, and Cr on corrosion resistance of some HEAs in aqueous environments containing chloride or sulfate species (Shi et al., 2017).

Electrochemical measurements in 3.5 wt% NaCl allowed formulation of the following conclusions regarding corrosion behavior of the investigated high‐entropy alloys:

Microstructure of the AlCrFe₂Ni₂ alloys determined by the manufacturing and processing conditions influenced their chemical behavior to some extent. Major role was attributed to the grain size, distribution of phases on the surface as well as amount and distribution of impurities. Nanostructures formed by spinodal decomposition contributed to the enhancement of corrosion resistance.

The raw data supporting the conclusions of this article will be made available by the authors, without undue reservation.

EG and UH provided general concept of this study, support, and supervision of experiments. MM-K, JC, and MJ were involved in planning and execution of corrosion tests, establishing the system of data collection and archiving. SG was involved in planning and execution of the arc-melting procedure, and characterization of the Co-free alloys. All authors contributed to characterization of samples at different stages of manufacturing and testing. JC wrote the first draft of the manuscript devoted to corrosion testing of the Co-free alloys. MM-K wrote the first draft on quinary alloys with Co. MM-K, JC, MJ, and EG were involved in the discussion and interpretation of experimental results as well as editing of the final version of the paper.

Investigations were conducted within a NADEA project (M-ERA.NET 2017) with a financial support from the National Science Centre Poland (UMO-2017/26/Z/ST8/01238). The measurements of quinary alloys were financed by the Polish Ministry of Science and Education—Subsidy No. 16.16.160.557.

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.