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Liu et al. Microstructures 2023;3:2023020 https://dx.doi.org/10.20517/microstructures.2023.02 Page 13 of 27

Adding 0.25 wt.% Nb to the duplex stainless steel increases the pitting potential by more than 100 mV.
[66]
Niobium forms the Z phase that surrounds the anodic inclusions . Furthermore, the Z phase has a low
[66]
mismatch with the matrix, and there is no gap between the matrix and Z phases . Therefore, niobium
addition decreases the number of pitting nucleation sites.

It has been reported that tantalum additions can hinder the precipitation of harmful phases, which
improves the pitting corrosion resistance [67,68] . Further studies showed that tantalum forms nitrides that coat
the CaS and (Al, Ca) oxides. When CaS and (Al, Ca) oxides dissolve, stable nitrides containing tantalum
prevents further propagation of pitting corrosion .
[69]
Other elements
Aluminum was found to have a negative effect on pitting corrosion. The negative effect is not obvious when
contents are below 1 wt.%, but it is obvious when the contents are above 1.5 wt.% . This is because the
[70]
Al O film formed by aluminum is porous. Studies have reported that Al O forms an Al O layer on the
3
3
2
2
2
3
surface only when the content is above 4-6 wt.%.
The addition of 0.2 wt.% silver is detrimental to pitting corrosion resistance. Silver increases the fraction of
[71]
secondary austenite . Pits preferentially initiate at secondary austenite. Additionally, the solubility of silver
in steel is very low, and pitting corrosion is preferentially initiated at the interface between the Ag-
containing precipitates and the matrix.
Sulfur addition of 0.001 to 0.053 wt.% increases the pitting nucleation sites by approximately 3.5 times .
[72]
Manganese sulfides increase significantly, which deteriorates the pitting corrosion resistance. The pitting
potential decreases by approximately 700 mV in 3 M NaCl .
[72]

Rare earth metals (REMs) exist in duplex stainless steels in the form of precipitates. The addition of REMs
[73]
can refine polygonal Mn inclusions into uniform-shaped REM oxides . These REMs oxides are ball-
shaped at the interface and act as cathodes during corrosion . Pitting corrosion occurs at the matrix near
[74]
[74]
the oxide/matrix interface, rather than at the oxide . The frequency of metastable pitting also decreases .
[74]
However, Kim et al. did not clarify why pitting corrosion initiates in the matrix near the oxide/matrix
interface instead of in the matrix away from the REM oxides .
[74]
Adding 0.01-0.2 wt.% tin was also found to improve corrosion resistance, and its effect was better when it
was compounded with copper , but the specific mechanism is still unclear.
[75]
Ruthenium addition of approximately 0.28 wt.% can increase the pitting corrosion potential in sulfuric acid
by 100 mV . The passive current decreases dramatically with the addition of ruthenium. This indicates that
[76]
ruthenium can hinder anodic dissolution. However, in the only report on this topic, the conclusion was
mainly drawn using macroscopic electrochemistry and weight loss, and the partition and influence
mechanism of ruthenium between the two phases is yet to be understood.

In summary, the mechanism of alloying elements on the pitting corrosion of duplex stainless steels can be
roughly divided into six mechanisms. In the stabilization mechanism [Figure 6A] , elements accumulate in
[49]
the passive film and at the bottom of pits, which stabilize the passive film and hinder further dissolution in
the corrosion pits. This mechanism has been verified for molybdenum in the solid solution state . In the
[49]
ineffective mechanism [Figure 6B] [49,59] , the element does not stabilize or deteriorate the passive film. When
pitting corrosion propagates, this kind of element is also corroded. This mechanism applies to manganese