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Liu et al. Microstructures 2023;3:2023020 https://dx.doi.org/10.20517/microstructures.2023.02 Page 9 of 27

The influence of the applied potential on the passive film can be divided into anodic potentials and cathodic
potentials, compared with the corrosion potential. One research strategy is focused on the composition
change. At low anodic potentials, the passive film exhibits p-type semiconductor characteristics, because the
passive film is composed of Cr O , FeO, NiO, and MoO , which contain many cationic vacancies. At high
2
2
3
anodic potentials, the main components are Fe O , FeOOH, CrO , and MoO . The passive film resembles
2
3
3
3
[36]
the n-type semiconductor [Figure 4G] . Therefore, the conductivity of the passive film decreases when the
applied anodic potential increases . Another research thought focuses on the film evolution modes under
[36]
[35]
applied potentials . At low anodic potentials, the passive film grows and thickens. When the applied
potential shifts to the transpassive potential range, electron removal increases the number of the valent
species and the passive film dissolves.
The applied cathodic potential is related to the hydrogen evolution reaction. Time-of-flight secondary ion
mass spectrometry (TOF-SIMS) analyses showed that hydrogen accumulates preferentially on the grain
boundaries and phase boundaries of the duplex stainless steel . In situ AFM revealed that hydrogen
[39]
charging causes a height difference on the sample surface . Further studies showed that hydrogen-charged
[39]
specimens exhibit an increased conductivity, and that the increase in conductivity in the austenite phase is
larger than that in the ferrite phase . The composition analysis of the passive film demonstrates that
[40]
hydrogen charging promotes the presence of oxygen atoms in the form of hydroxide [Figure 4H] [39,40] .
Elastic stress and tensile stress both increase the donor and acceptor densities as determined using Mott-
Schottky measurements [Figure 4I] [41,42] . Stress increases the number of dislocations on the surface.
[42]
According to the PDM models, dislocations promote the formation of vacancies in passive films .
In summary, the study on the environmental response of passive films focuses on their semiconductor
properties, compositions, and structural response characteristics. As the thickness of the passive film is only
a few nanometers, the above parameters are mostly obtained by substituting the macroscopic data into the
existing model. Therefore, its accuracy is debatable. The specific evolution process of the influence of
environmental factors on passive films is not clear. Further research can be conducted using high-resolution
observation methods coupled with an environmental test bench.

Degradation of the passive film
It was believed that the rupture of the passive film was caused by the reaction from the trivalent chromium
to the soluble tetravalent chromium at a certain potential, which was consistent with the sudden current
increase. However, new experimental results do not support this view. Using in situ synchrotron grazing-
incidence X-ray diffraction (GIXRD), a new phenomenon was observed that the passive film would thicken,
accompanied by accelerating iron dissolution, and crystallinity decreases of the passive film when the
[43]
potential increased in the passivation zone [Figure 5A-D] . This indicates that the rupture of the passive
film is not only a valence change process but also a structural change process. As the applied potential
increases to the breakdown potential, the composition and structure change simultaneously. Chromium,
nickel and molybdenum in the passive film react and form soluble substances [Figure 5E and F] . The
[44]
passive film becomes loose, whereas the film/matrix interface becomes dense owing to the enrichment of
[44]
nickel and molybdenum . These phenomena indicate that the rupture process of the passive film is a
continuous degradation process over a wide potential range, rather than a sudden change as per the
[44]
traditional definition [Figure 5G] .
Nevertheless, there are also some factors that still need to be defined. Firstly, the difference in the
degradation process of passive film between the austenite and ferrite phases is not fully understood. The