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Liu et al. Microstructures 2023;3:2023020 https://dx.doi.org/10.20517/microstructures.2023.02 Page 21 of 27

[118]
Hydrogen charging in the pre-strained duplex stainless steel can induce dislocation multiplication .
[119]
Hydrogen can cause more obvious lattice deformation in austenite than ferrite . Further studies
interpreted this phenomenon by stating that hydrogen causes slip planarity and martensite formation in
austenite [120,121] . Larsson et al. reported that hydrogen absorption causes compressive strains without
stress . This may be due to the expansion effect after austenite accommodates more hydrogen atoms.
[122]
The solubility of hydrogen atoms in the ferrite phase is significantly lower than that in austenite, whereas
the diffusion rate in ferrite is considerably faster than that in austenite. Therefore, the ferrite phase is more
prone to cracking during hydrogen embrittlement . The remarkable role of the phase boundaries in
[123]
hydrogen-induced stress cracking was recently recognized. The binding energies of phase boundaries,
[107]
ferrite boundaries and austenite boundaries are 43.6, 26.5, and 13.5 kJ/mol, respectively . Wu et al. used
experiments, numerical simulations, and theoretical analysis to demonstrate that the phase boundaries are
not only strong hydrogen traps but also act as fast hydrogen diffusion pathways .
[107]

Sulfide stress cracking
Sulfide stress cracking is common in the petrochemical industry. The occurrence of sulfide stress cracking is
related to the dissolution of the passive film, sulfide formation, localized corrosion, and hydrogen
embrittlement . In the sulfide environment, the passivation film dissolves, and a layer of sulfide is formed
[124]
on the surface . The surface is partially acidified, and the acidification effect results in corrosion
[124]
[Figure 9C1 and 9C3] . Corrosion pits are then formed in the ferrite phase. The reacidification and the
[106]
autocatalytic effects generate hydrogen atoms. Cracks initiate in the ferrite phase [Figure 9C2]. Hydrogen
atoms generate at the corrosion product/substrate interface also preferentially diffuse to the phase
[109]
boundaries and initiate cracks [Figure 9C4] .

Zucchi reported that adding sulfide to seawater increases the cracking sensitivity of 2,205 duplex stainless
steel in neutral seawater . The main control mechanism is the hydrogen embrittlement mechanism. The
[108]
sensitivity of 2,101 duplex stainless steel after adding 10 M thiosulfate to a 20 wt.% NaCl environment is as
-3
high as 40%, but when the pH is increased to above 4.5, 2,101 is not sensitive to 10 M thiosulfate . This
[106]
-2
indicates that the sulfide stress cracking is closely related to the acidification effect. The σ phase, which
preferentially precipitates in the ferrite phase, also easily becomes the initiation point. The σ phase may
[125]
crack first and the crack spreads along the interface between the ferrite phase and the σ phase .
CONCLUSIONS
This article first summarizes the failures in the last 20 years and identifies the main corrosion-related types
leading to failures of duplex stainless steels. Then the study on the formation and degradation of passive
films is reviewed. The mechanisms by which the alloying elements and microstructure affect the pitting
corrosion are summarized. Finally, the academic progress of EAC is reviewed. The main conclusions are as
follows.

(1) Among the reported failures, pitting corrosion and chloride-induced stress corrosion cracking are the
main causes. Sulfide stress cracking, Hydrogen-induced stress cracking, MIC, selective corrosion and
crevice corrosion are other failure causes. Therefore, academic studies should focus on the causes of such
failures.

(2) The evolution of passive films after immersion in water can be roughly divided into three-stage, namely,
the nucleation, rapid growth, and stable growth stages. The rupture process of the passive film is a
continuous metal oxidation process rather than a sudden rupture. The film structure and composition