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Liu et al. Microstructures 2023;3:2023020 https://dx.doi.org/10.20517/microstructures.2023.02 Page 17 of 27

phase ratios is not highly correlated with PREN. but it is well correlated to the two-phase micro-galvanic
[23]
corrosion depth . The authors believed that this is because PREN mainly reflects the initiation process of
pitting corrosion. This is suitable for 2,205 and 2,101 duplex stainless steels, which are not resistant to
pitting corrosion. The corrosion resistance of 2,507 duplex stainless steel is higher. Therefore, the pitting
corrosion performance mainly depends on the pitting propagation process. However, according to the
pitting corrosion model proposed by Li et al. and Frankel et al., the propagation process plays a decisive role
in materials that are not resistant to corrosion [84,85] . This opinion contradicts that of Ha et al. [21,22] . However,
the model proposed by Li et al. has not been verified in the duplex stainless steel system [84,85] . Therefore, the
mechanism of the phase ratios on the pitting corrosion requires further clarification.

Precipitates
Harmful phases precipitate in duplex stainless steels during welding and isothermal aging processes. In
metals, precipitates typically have a deleterious effect on pitting resistance . There are three opinions on
[86]
interpretations of the pitting corrosion mechanisms of secondary precipitates.

The first theory is the Cr-depletion theory [Figure 8A1], where the precipitates are enriched with chromium
or molybdenum, which forms a surrounding depleted zone. The passive film at the depleted zone is weak or
absent. Pitting corrosion occurs in these areas. This theory is applicable to secondary austenite, sigma phase
(σ phase), chi phase (χ phase), CrN, and Cr N [Figure 8A2 and A3] [87-90] . Zhang et al. showed that after
2
tempering at 700 °C for a sufficient duration, the pitting initiation site of S82441 duplex stainless steel
[91]
transformed from austenite to the Cr-depleted zone . The higher the quantity of the sigma phase, the
lower the breakdown potential. However, the pitting corrosion resistance recovers to a certain degree after
aging for a sufficient time. Once the sigma phase is fully precipitated, further aging causes tungsten and
[92]
molybdenum to diffuse into the Cr-depleted secondary austenite .

The Cr-depleted α phase formed by spinodal decomposition can also induce pitting corrosion, but it is not
[93]
as strong as the σ phase . The micro-galvanic corrosion model has been proposed to interpret this
phenomenon [Figure 8B1] [94,95] . Cathodic phases enriched in chromium surround anodic phases depleted in
[94]
chromium [Figure 8B2 and B3] . Microgalvanic corrosion induces pitting corrosion in the Cr-depleted
phase, which has been proposed recently and is appliable to the α and α’ phases.

The pitting corrosion that occurs around the G phase is still under debate. The stress concentration model
assumes that a large strain field is generated around the square precipitate, which promotes the initiation of
pitting corrosion preferentially at this location [Figure 8C1]. A high-stress field has been verified using
HAFFT [Figure 8C2 and C3] . Another hypothesis is that pitting corrosion is related to the Cr-depleted
[96]
zone around the G phase, which is caused by the growth of the G phase. Silva et al. recently suggested that
the pitting corrosion around the G phase is due to galvanic corrosion .
[94]

In summary, research on microscale precipitates and nanoscale precipitates has yielded a unified
understanding of the σ and χ phases, whereas the knowledge of nanoscale precipitates, such as the G phase,
α phase and α’ phase, is still lacking. Transmission electron microscopy (TEM) has been adapted to trace the
initiation sites and propagation tendency in austenite stainless steel and got good results on Cu-rich phases
and MnS . This technology is promising in studying the pitting corrosion related to nanoscale precipitates
[97]
in duplex stainless steel. The second tendency is from static research to dynamic research. The precipitation
growth process of secondary phases leads to the diffusion of alloying elements, which affects the pitting
corrosion resistance. Moreover, research on the surface passive films of the precipitates is still lacking.
Current research has reported that TiB nanoparticles help suppress pitting initiation, because it is easier to
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