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Wan et al. Microstructures 2023;3:2023014 https://dx.doi.org/10.20517/microstructures.2022.36 Page 3 of 19

Fujidempa Kogyo. Co., Ltd, Japan) at 950 °C for 5 min under a pressure of 45 MPa in vacuum condition
(~ 6 Pa).

The sintered cylindrical HEAs were cut into discs using electrical discharge machining for subsequent
characterization and tests. For electrochemical tests, the samples were cut into discs with a size of
Φ = 10 mm × 3 mm, and sealed in epoxy with one surface exposed. The exposed surface was first
mechanically ground using SiC paper down to 2400 grit, followed by vibratory polishing using a 0.06 μm
alumina suspension. The polished samples were then cleaned with deionized water and alcohol and dried in
air. For static immersion tests, both surfaces of the samples of the discs were ground and polished following
the same procedure as those used for the electrochemical tests.

Microstructure characterization of the as-sintered HEAs
The phase of the as-sintered Cr MnFeCoNi (x = 0, 0.6, 1, and 1.5) HEAs was identified using XRD patterns
x
recorded on a Rigaku Smartlab-9KW diffractometer with Cu-Kα radiation (45 kV, 200 mA). The data were
collected with 2θ in the range of 30° to 80° with a step size of 0.02° at a scanning speed of 10°/min. The
surface morphology and composition were examined using SEM (TESCAN MIRA 3, Czech Republic)
coupled with energy-dispersive X-ray spectroscopy (EDS). Electron backscatter diffraction (EBSD) was
performed to characterize the grain structure of the HEAs. TEM (FEI Tecnai F30, USA) and high-angle
annular dark-field scanning TEM (HAADF-STEM) were used to characterize the microstructure of the as-
sintered HEAs. The TEM samples were prepared using site-specific focused ion beam (FIB; FEI Helios
TM
Nano Lab 600i) lift-out technique. Besides, platinum cap layers with a thickness of ~1 μm were deposited
before milling the trench to protect the surface.

Electrochemical corrosion and immersion tests
Electrochemical measurements were performed in a 0.5 M H SO solution at ambient temperature using a
2
4
Princeton Applied Research Versa Studio (PARSTAT 4000A, Ametek Scientific Instruments, USA). A
standard three-electrode cell system was performed during all the measurements using a platinum sheet as
the counter electrode, a saturated calomel electron (SCE) as the reference electrode and the Cr MnFeCoNi
x
HEAs as the working electrode. Before EIS and potentiodynamic polarization tests, the specimens were
subjected to the potentiostatic polarization at -0.6 V vs. SCE for 5 min to remove the surface oxidation, and
then the open-circuit potential (OCP) was measured for 2 h in order to obtain a steady or quasi-steady
potential value. EIS tests were performed at OCP with the sinusoidal potential amplitude of 10 mV in the
frequency range from 100 kHz to 10 mHz. The data derived from EIS tests were fitted and analyzed using
the Zsimpwin software. Then, the potentiodynamic polarization tests were conducted with the scanning
range from -0.25 V vs. OCP to 1.3 V vs. SCE at a scan rate of 1 mV/s. Each electrochemical test was
performed three times to confirm the reproducibility of the results.

The static immersion tests were performed in 0.5 M H SO at ambient temperature for 15 days. Three
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4
parallel experiments were performed for each HEA. The average corrosion rate by mass loss C in mm/y
w
was calculated using the following Eq. (1):

where k is the constant of 8.76 × 10 , ∆W is the mass loss (g) after the immersion test, A is the surface area
4
(cm ), t is the immersion time (h), and ρ is the density (g/cm ) of the sample, measured by Archimedes
2
3
method.