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Wan et al. Microstructures 2023;3:2023014 https://dx.doi.org/10.20517/microstructures.2022.36 Page 17 of 19

samples in Figure 10, which evolved from the tiny height fluctuation during the initial period of the
immersion, due to the intrinsic heterogeneous nanoscale particles.

In summary, the addition of Cr in Cr MnFeCoNi HEAs is indeed liable to form the Cr oxide in the passive
x
film, which has good stability in the solution and improves the corrosion resistance of the HEAs [17,45] . On the
other hand, the Cr can also precipitate from the matrix, which could accelerate corrosion by enhancing the
galvanic corrosion effect. In addition, the formed honeycomb-like corroded surfaces on the HEAs would
further deteriorate the corrosion resistance. Therefore, it is vital to optimize the matrix structure of the Cr-
containing HEAs by changing the constituent elements as well as the processing methods to improve the
corrosion resistance in future work. Generally, there are two methods to improve the corrosion resistance of
the Cr-containing HEAs. One is to decrease or even eliminate the segregation of the Cr and homogenize the
composition and the other is to optimize the structure and the composition of the passive film for high Cr-
containing HEAs and eliminate the nanocrystalline-amorphous phase boundaries.
[46]

CONCLUSION
The corrosion behavior of ultrafine-grained Cr MnFeCoNi HEAs with varying Cr contents in sulfuric acid
x
solution (0.5 M H SO ) was investigated. The Cr-containing HEAs consisted of an fcc matrix and a small
4
2
amount of (Cr, Mn) O at the grain boundaries and the (Cr, Mn)-rich phase due to the compositional
4
3
segregation. During the polarization tests, the corrosion rate first increased and then decreased with the
addition of Cr. The Cr1.5 HEA showed the lowest corrosion rate due to the effective passivation. The
honeycomb-like surface was formed after electrochemical polarization tests. The passive film was mainly
composed of (Mn, Fe) hydroxide/oxide for Cr0 and (Cr, Mn and Fe) hydroxide/oxide for the Cr-containing
HEAs. The passive film was more compact and thicker with the increasing Cr concentration. However, the
existence of the nanocrystalline-amorphous phase boundaries in the passive film could reduce its stability
by providing the diffusion channel of the species. During the static long-time immersion tests, the HEAs
showed distinct corrosion behavior from that of the polarization tests, where the corrosion rate increased
exponentially from Cr0 to Cr1.5. This unexpected phenomenon should be due to the accumulation of the
galvanic corrosion effect induced by the pitting-like corrosion accompanied by the failure of the passivation
effect.

DECLARATIONS
Authors’ contributions
Conceptualization, methodology, validation, formal analysis, investigation, data curation, visualization,
writing - original draft: Wan T
Methodology, investigation: Huang Z
Investigation: Cheng Z, Zhu M, Li Z, Fu D
Methodology, investigation: Zhu W
Conceptualization, methodology, formal analysis, resources, data curation, visualization, writing - review &
editing, supervision, project administration, funding acquisition: Ren F

Availability of data and materials
The raw/processed data required to reproduce these findings are available upon request to the
corresponding author.

Financial support and sponsorship
This work was financially supported by the National Natural Science Foundation of China (No. 52122102),
the Shenzhen Peacock Team Program (No. KQTD2016053019134356) and the Guangdong Innovative &