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Liu et al. Microstructures 2023;3:2023020 https://dx.doi.org/10.20517/microstructures.2023.02 Page 11 of 27

PITTING CORROSION
The pitting corrosion mechanism includes both initiation and propagation mechanisms. The initiation and
stabilization mechanisms of metastable pitting are still not comprehensively understood. Research on the
pitting corrosion mechanism is mostly performed on single-phase austenitic materials, and there is little
research on duplex stainless steel, which is more complicated. Current research on the pitting corrosion of
duplex stainless steels mostly focuses on the correlation between various material factors with the pitting
corrosion resistance and their mechanisms. These material factors include alloying composition and
microstructure. As duplex stainless steels are mainly applied in chloride-containing environments, the
default environment is a Cl-containing reducing environment unless specifically mentioned.

Alloying elements
Nitrogen element
Nitrogen is the element that solves the welding problems of duplex stainless steels and makes it widely
commercially available. Nitrogen slightly increases the pitting potential and protection potential of duplex
[45]
stainless steels . When the nitrogen content in 2,507 is increased from 0.15 wt.% to 0.27 wt.%, the
corrosion rate decreases by approximately 85% . However, previous studies mainly focused on corrosion
[46]
evaluation and not on the corrosion mechanism, and some topics remain debatable. Nitrogen is mainly
distributed in the austenite phase in duplex stainless steels, and the solubility of nitrogen in the ferrite phase
+
is extremely low. In austenitic stainless steel, nitrogen exists as NH in the passive film . Therefore, the
[47]
4
addition of nitrogen leads to a larger difference between the Pitting Resistance Equivalence Number (PREN)
values of the two phases. According to the point of view that the phase with the lowest PREN represents the
pitting corrosion resistance, the addition of nitrogen has little effect on the pitting corrosion resistance,
which is not consistent with the experimental results.

Molybdenum element
Molybdenum can significantly increase the pitting corrosion resistance of duplex stainless steels. When
molybdenum content is below 1.0 wt.%, molybdenum exhibits no obvious improvement in pitting
corrosion resistance . However, adding 1.5 wt.% Mo can increase the pitting potential by at least
[48]
150 mV . This is mainly attributed to two reasons. Firstly, molybdenum exists in the passive film in the
[49]
form of MoO , MoO , and MoO . These oxides render the passive film more stable . Secondly,
2-
[49]
3
2
4
molybdenum accumulates in the pits and hinders further dissolution . Additionally, Tian et al. found that
[50]
tetravalent Mo species only exist in the passive film of 2,205 duplex stainless steel, as compared to single
ferrite phase or single austenite phase . This indicates that molybdenum plays an important role in the
[51]
interaction between the austenite and ferrite phases. However, the exact interaction between the austenite
and ferrite phases is unclear. The influence mechanism of molybdenum on austenite and ferrite in different
stages of pitting corrosion still needs to be comprehensively understood.
Nickel element
Nickel changes the composition of the passive film by introducing metallic Ni, NiO, Ni(OH) , and NiCl .
[52]
2
2
[46]
This is beneficial for pitting corrosion resistance . A nickel content of 5-13 wt.% increases the pitting
potential in 1 M hydrochloric acid by 500 mV . Nickel also narrows the protection potential range .
[53]
[53]
However, the content change of nickel causes a significant change in the ratio of the two phases , and the
[54]
ratio of the two phases has a greater impact on the pitting corrosion ability. When studying the influence of
nickel, it is necessary to control the ratio of the two phases by heat treatment. Additionally, the influence of
nickel on pitting corrosion is still mainly observed using electrochemical tests, and comprehensive research
needs to be conducted from the perspective of the corrosion morphology and the corrosion rate.