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Wan et al. Microstructures 2023;3:2023014 https://dx.doi.org/10.20517/microstructures.2022.36 Page 13 of 19

[35]
non-homogeneity and the capacity dispersion in the system , such as an uneven corroded surface. The Q
p
impendence Z is given as follows,
Q

Where Y is the proportionality factor, j is the imaginary unit, ω is the phase frequency, and n is the phase
p
shift, reflecting the degree of the dispersion for an ideal capacitance. For n = 1, Z represents a pure
p
Q
capacitance with C = Y ; for n = 0.5, it presents a Warburg resistance; and for n = 0, it presents a pure
p
p
p
p
resistance with R = Y . As a result, n increases from Cr0 to Cr1, implying that the passive film becomes
-1
p
p
p
more compact on the surface. While from Cr1 to Cr1.5, n decreases slightly, which is ascribed to the
p
discontinuous passive film containing the weak site in the deep micropore, as shown in Figure 6. Moreover,
as the value of n is in the range of 0.74-0.84, which is close to 1, the value of Y can be approximately
p
p
regarded as capacitance C of the passive film. Considering the Helmholtz model [21,36] , the value of Y is
p
p
determined by the equation as follows,
where d is the thickness of the passive film, S is the exposed surface area, ε is the permittivity of the vacuum,
0
and ε is the dielectric constant of the surface which is mainly determined by the composition of the passive
film. Therefore, the decrease of the value of Y from Cr0 to Cr1 and then the increase from Cr 1 to Cr1.5 can
p
be affected by two factors for all samples: the change of the composition and the thickness of the passive
film. Meanwhile, as Cr0.6 and Cr1 have a similar surface composition as analyzed by XPS [Figure 7], the
larger Y of Cr1 is mainly induced by the thicker passive film than Cr0.6. Notably, the decreased Y for Cr1.5
p
p
should be mainly attributed to the change of the composition rather than the reduced thickness of the
passive film. In terms of the resistance R , which reflects the corrosion resistance area of the passive film and
p
the electron migration rate at the sample’s surface/electrolyte interface, the smaller value of R indicates the
p
lower corrosion resistance. Based on this, the corrosion resistance for the four groups of HEAs first
decreases and then increases with the increasing concentration of Cr, in line with the results of the
polarization tests.
Long-time immersion
Figure 10 shows the surface morphology and average corrosion rate of Cr MnFeCoNi HEAs after
x
immersion in a 0.5 M H SO solution for 15 days at room temperature. The distribution of the micropores
4
2
on the surface has no difference from that after polarization tests, but the micropores are much larger and
the surfaces become looser. The formation of a more honeycomb-like corrosion surface, especially for
Cr1.5, suggests that the HEAs suffered from more severe corrosion during the immersion of the 15 days,
due to the absence of the long-term stable passive film. Likewise, the average corrosion rate [Figure 10E]
increases exponentially with the Cr concentration. The Cr1.5 HEA shows the highest corrosion rate, 6 times
higher than that of Cr1, which seems in contradiction with the results of the electrochemical tests. The
significant change in the corrosion behavior during the long-time immersion between the four groups of
HEAs could be attributed to the transformed corrosion mechanism. The dominant galvanic corrosion
behavior has a direct correlation with the Cr concentration, which would be discussed in detail later.

DISCUSSION
Honeycomb-like morphology after polarization tests
Interestingly, all samples show the honeycomb-like morphology after polarization tests, as shown in