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Page 12 of 27 Liu et al. Microstructures 2023;3:2023020 https://dx.doi.org/10.20517/microstructures.2023.02

Tungsten element
Kim et al. reported that adding 4 wt.% tungsten increases the pitting potential and critical pitting
[52]
temperature of the duplex stainless steel. It could also promote the repassivation process . Tungsten exists
M
as WO and W in passive films. However, Torres et al. reported that adding 2.1 wt.% tungsten to the
3
duplex stainless steel decreases the critical pitting temperature when aged at 920 °C for 60 s . This change
[55]
correlates with the fact that tungsten favors the precipitation of the chi phase. The opposite results that were
obtained for tungsten are not only because the heat treatment varied but also because the steel used in the
research was not a single variable. That is, industrial steels were used as the test samples [52,55] , in which the
contents of molybdenum, silicon and other elements vary, which also influences the pitting corrosion
resistance and may obscure the influence of tungsten. Another study that controlled element tungsten more
accurately showed that tungsten is slightly beneficial in resisting pitting corrosion when in the solid solution
state . However, tungsten is detrimental when detrimental phases containing tungsten precipitate.
[56]
Copper element
Copper exists in duplex stainless steels in the form of the solid solution state or epsilon-Cu, the latter of
which exists when the sample is aged at 700-800 °C [57,58] . The addition of copper in the solid solution state
has few beneficial or even negative effects . Although copper is more stable than iron, the corrosion
[59]
product of copper dissolves into complex ions containing Cu , which have not been fully understood until
[57]
now. Epsilon-Cu has been reported to be more active than the ferrite and austenite phases , which means
[60]
that it dissolves preferentially and forms nucleation sites for pitting corrosion. However, the electrochemical
test is the main test method on this topic which cannot reflect the corrosion mechanisms microscopically.
Additionally, in situ observation of the pitting corrosion process caused by epsilon-Cu is still lacking. The
change in the pitting nucleation and pitting propagation stages after the addition of solid-solution copper is
also vague.

Manganese element
Manganese exists in passive films in the form of oxidation states II and IIII . Jang reported that adding
[61]
0.8 wt.% manganese to CD4MCU cast duplex stainless steel negatively impacts the pitting corrosion
resistance, but the pitting corrosion resistance recovers by adding 2 wt.% manganese . Jang attributed these
[62]
[62]
changes to the proportion of austenite/ferrite . Mass loss tests showed that increasing the manganese
content from 1.7 to 3.3 wt.% significantly decreases the pitting corrosion resistance . However, the pitting
[49]
potential slightly increased when adding 8.03 wt.% in the solid solution state to duplex stainless steel . Feng
[63]
[64]
et al. reported that manganese is detrimental when aging at 800 °C for forming precipitants . Therefore, it
can be concluded that manganese increases the number of pit nucleation sites when it exists in the form of
precipitants (inclusions or secondary phases). However, the current research shows that manganese-
containing inclusions exhibit different dissolution modes, and the specific mechanisms of these different
dissolution modes still need clarification. Additionally, existing studies have shown that solid-solution-state
manganese does not seem to have a significant effect on the pitting corrosion of duplex stainless steels.

Titanium, niobium and tantalum
Titanium is a strong nitride-forming element which means TiN would form upon Ti is added to duplex
stainless steels. Firstly, titanium could form TiN. Secondly, titanium in the solid solution state forms oxides.
Zhang et al. has reported adding 0.01 wt.% titanium increased the pitting potential . However, when
[65]
0.15 wt.% titanium was added, the pitting potential decreased. The authors attributed this change to the size
of TiN. However, no direct evidence was provided to support this.