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Page 4 of 27 Liu et al. Microstructures 2023;3:2023020 https://dx.doi.org/10.20517/microstructures.2023.02

PASSIVE FILM
Stainless steel is more corrosion-resistant than carbon steel in many applications because a dense film is
formed on the surface, shielding the matrix from corrosive media. The passive film of duplex stainless steels
consists of nanocrystals, hydroxides, and mixed oxides . The two phases of duplex stainless steels have a
[19]
gradient in their chemical composition, raising the question of whether or not their film-formation and
film-degradation processes differ in any way. These issues have recently been addressed in passive film
research on duplex stainless steels. This section introduces the research progress from the perspective of
film formation and film degradation.

Formation of the passive film
Formation process of passive film
Chromium and molybdenum in duplex stainless steels are enriched in the ferrite phase, whereas the content
of nickel and molybdenum in austenite is higher [Table 1] [21-23] . The composition difference would lead to
the formation difference of passive film. Overall, there is a lack of research in this area, and few studies have
been published. Some scholars believe that the final passive film is dependent on the oxidation potential and
ion solubility of the individual elements. Therefore, they extrapolated from the electrochemical behavior of
pure metals in the environment to the composition of the passive film. Yao et al. predicted the passive film
composition of 2,205 duplex stainless steel at different potentials by comparing the polarization curves of
[24]
pure iron and pure chromium . The composition of the passive film in duplex stainless steel can be readily
determined using this method, which is interesting to consider. However, this view assumes that there is no
interaction between the interphase passive films. This assumption might be debatable at the phase
boundaries.

Another approach is to observe the passive process in situ. The evolution of passive films over 600 min was
characterized using electrochemical atomic force microscopy (EC-AFM) . Oxide particles were formed
[25]
separately during the first 100 min, after which they completely covered the surface [Figure 2] . However,
[25]
the low time resolution of EC-AFM, as compared to the rapid formation of passive films, is a drawback of
this technology. Passive films form within seconds of exposure. Hence, the above study could not
adequately capture the initial process of passive film formation. In recent years, researchers have also
attempted to capture the formation process in situ at the nanometric scale using high-resolution techniques.
This has yielded favorable results for the investigation of austenitic stainless steel [26,27] . Using scanning
tunneling microscopes, the local oxidation of chromium was observed . Therefore, a local inhomogeneity
[26]
is generated once the passive film is formed. However, these studies focused on austenitic stainless steel.
Further studies on the coupling process between nitrogen element and chromium element, the formation
process at grain boundaries and phase boundaries, and the differences in passive film formation between
ferrite and austenite from on the nanometric scale are promising for unveiling the mechanism responsible
for the high corrosion resistance of duplex stainless steel. Furthermore, from a cross-sectional perspective, it
is currently unknown how the passive film grows longitudinally. There are insufficient data to determine
whether the oxidation is internal or external.

The study of the in situ passive film growth process in liquid may be another research direction because the
current research is performed under electrochemical polarization conditions or in air. The state of the
passive film in air differs from that in a real liquid environment. CrOOH in the passive film, which is one of
the main components of the passive film in air, may react with water into other substances once it is
immersed in the NaCl solution .
[28]