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Page 2 of 19 Wan et al. Microstructures 2023;3:2023014 https://dx.doi.org/10.20517/microstructures.2022.36

[8]
promising candidates in extreme structural applications . Among the HEAs, face-centered-cubic (fcc)
CoCrFeMnNi HEA is of particular interest due to its excellent combination of high strength-ductility-
toughness at cryogenic temperature [9-11] . For example, Gludovatz et al. reported that the CrMnFeCoNi HEA
showed remarkable fracture toughness at cryogenic temperatures down to 77 K, comparable to the
austenitic stainless steels and high Ni steels . Such HEA and its sub-family generally have high Cr content,
[10]
much above the required amount (12 at.%) to obtain “stainless” behavior on the surface for conventional
corrosion-resistant alloys, such as stainless steel and nickel-based alloys , and thus, are expected to have
[12]
superior corrosion resistance.

The mechanism of Cr to enhance the corrosion resistance of alloys is through the formation of a dense Cr
[15]
oxide-based passive film [13,14] , which serves as an effective barrier on an alloy across a wide range of pH .
There have been numerous reports on the corrosion behaviors of CoCrFeNi-based HEAs in recent few
years [16-22] . Ye et al. found that the CrMnFeCoNi HEA coating showed nobler corrosion potential (E ) and
corr
lower corrosion current (I ) derived from the potentiodynamic polarization tests than 304 stainless steel
corr
(304SS) in a 0.5 M H SO solution . The AlCoCuFeNiCr HEA immersed in sulfuric acid solution showed
[21]
4
2
better corrosion resistance than the Cr-free HEA, attributing to the formation of the compact Cr O passive
2
3
film on the surface . However, the corrosion resistance of CrMnFeCoNi HEA was inferior when compared
[17]
with 304 L stainless steel in a 0.1 M H SO solution, because the passive film on the CrMnFeCoNi HEA was
4
2
very unstable due to the low content of Cr and the extensive formation of metal hydroxide in the passive
[22]
film . Similar phenomena were also observed in several Cr-containing HEA systems, such as
AlCoCrFeNi [18,19] and FeCoNiCrCu HEAs . For these HEAs, the distribution of Cr element in the matrix is
[20]
x
relatively inhomogeneous and the Cr-depleted phase as anode would become a sensitive site to induce the
formation of pitting. As a result, the general corrosion rate would be accelerated by the galvanic corrosion
effect. Even with the same composition system of CoCrFeMnNi HEA, distinct corrosion behavior was
observed [21-24] . Wang et al. found that the oxide film on HEA was duplex, comprising a Cr/Mn inner oxide
layer and a Cr/Fe/Co outer oxide/hydroxide layer . For now, the effect of Cr content on the corrosion
[25]
mechanism in the HEAs has not been unraveled in detail. The matrix structure, composition, and the
competing effect of the passive film and the pitting on the corrosion behavior can be revealed by exploring
the Cr-containing HEAs with varying Cr concentrations.
In this work, the corrosion behavior of the Cr MnFeCoNi (x = 0, 0.6, 1, and 1.5, respectively) HEAs with
x
varying Cr concentrations in a 0.5 M H SO solution was investigated. The matrix microstructure was
2
4
characterized using X-ray diffraction (XRD), scanning electron microscopy (SEM) and transmission
electron microscopy (TEM) in detail. Moreover, the corrosion behavior of the Cr MnFeCoNi HEAs was
x
examined using the potentiodynamic polarization tests and the electrochemical impendence spectroscope
(EIS), along with the static immersion tests. Then, the corroded surface was analyzed using SEM, TEM,
atomic force microscope (AFM) and X-ray photoelectron spectroscopy (XPS). Our main objective is to give
an in-depth understanding of the effect of Cr on the corrosion behavior of the Cr MnFeCoNi HEAs in
x
sulfuric solution.

EXPERIMENTAL PROCEDURE
Alloy fabrication
The Cr MnFeCoNi (x = 0, 0.6, 1, and 1.5) HEAs (hereafter denoted as Cr0, Cr0.6, Cr1, Cr1.5, respectively)
x
were prepared using commercially pure (99.9 wt.%) elemental powder of Cr, Mn, Fe, Co and Ni as the
starting material. The powder mixture was put into a stainless steel vial with a ball-to-powder mass ratio of
5:1 and subjected to high-energy ball milling for 9 h using a SPEX 8000D mill at ambient temperature in an
argon glovebox. The ball-milled powders were consolidated by spark plasma sintering (SPS-211Lx,