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Page 22 of 27 Liu et al. Microstructures 2023;3:2023020 https://dx.doi.org/10.20517/microstructures.2023.02

change simultaneously during the breakdown process.

(3) The film structure and film thickness of ferrite and austenite are very similar, whereas the chemical
composition differs. Environment variables hardly change the double-layer structure of the passive film, but
they change the overall thickness, oxide ratio, and defect concentration.

(4) The influence mechanisms of the alloying elements on pitting corrosion are summarized as the
stabilization, ineffective, soluble precipitates, soluble inclusions, insoluble inclusions, and wrapping
mechanisms. When the state of existence of the same alloying element changes, the influence mechanism
varies.

(5) In the chloride-containing environment, ferrite is more prone to pitting corrosion than austenite.
However, reversion of the pitting corrosion resistance occurs when a sufficiently large deformation is
applied to duplex stainless steel. This is attributed to the greater number of defects generated in austenite.

(6) Three theories can be used to interpret the pitting corrosion mechanism of precipitants, namely (1) the
Cr-depletion theory suggests that Cr-depleted zones surrounding precipitates cause pitting corrosion; (2)
the microgalvanic theory proposes that the microgalvanic effect between Cr-enriched phase and Cr-
depleted phase causes pitting corrosion; and (3) the high-stress field theory suggests that the high-stress
field around the precipitates causes pitting corrosion.

(7) In chloride-induced stress corrosion cracking, chloride-induced cracks always initiate at corrosion pits
and blunt upon contact with the austenite phase. In hydrogen-induced stress cracking, phase boundaries are
not only strong hydrogen traps but also fast hydrogen diffusion pathways. The occurrence of sulfide stress
cracking is closely related to the acidification effect of sulfide.

PROSPECTS
Owing to its high corrosion resistance, low economic cost, and good mechanical properties, duplex stainless
steel is an ideal material for constructing future industrial societies. However, research on the corrosion of
duplex stainless steel is still lacking. This review proposes important scientific issues and promising research
directions in recent years.

(1) Studies on passive film formation are still lacking. The difference between the two phases at the micro-
nanoscale and the longitudinal growth process of the passivation film from the cross-sectional perspective is
still unclear. The effect of the cathodic potential on the passive film is still unclear. Therefore, it is necessary
to use in situ scanning tunneling microscopy, high-resolution transmission microscopy, and theoretical
model calculations to further explore this. Researchers need to pay more attention to the diffusion of
elements along the phase boundaries.

(2) Studies on the nucleation and repassivation process of metastable pitting and the initiation process of
submerged pitting corrosion are still lacking. Therefore, it is necessary to use in situ scanning tunneling
microscopy and neutron scattering technology to study this. It is also necessary to perform quasi-in situ
experiments, such as SKPFM and GIXRD, to understand these phenomena.

(3) The detailed mechanism of alloying elements to improve pitting corrosion still needs to be clarified
because the alloying elements may play different roles in different stages of pitting corrosion. The coupling