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Liu et al. Microstructures 2023;3:2023020 https://dx.doi.org/10.20517/microstructures.2023.02 Page 19 of 27

Environmentally assisted cracking (EAC) is the main failure cause in duplex stainless steel. EAC is caused
[99]
by the combined effects of materials, stress, and a corrosive environment . At the mesoscopic scale, the
suggested mechanisms include: (1) The anodic dissolution mechanism, which suggests that the nucleation
[100]
and propagation of cracks result from the preferential dissolution of atoms at the crack tip ; (2) The
adsorption-induced dislocation emission theory, where absorbed ions reduce the bonding force of the
atoms at the crack tip, which promotes dislocations nucleation. Dislocations connect the holes, thereby
inducing crack propagation ; (3) The weak bond theory, where hydrogen atoms generated by the
[101]
electrochemical reaction are adsorbed at the crack tip. Hydrogen reduces the bonding force between the
atoms at the crack tip and promotes crack propagation ; (4) Hydrogen promotes local plastic deformation
[102]
theory. Hydrogen promotes the movement of dislocations in the plastic zone. Therefore, slip/microvoids
accumulate in the local plastic deformation zone and the crack propagates ; and (5) The membrane
[103]
rupture theory, which assumes that the crack tip moves because the brittle product film at the crack tip
[104]
ruptures under the action of an external force .

This section reviews the mechanisms and the recent studies on the main type of failures [Figure 9] [105-109] .
Overall, research on the EAC mechanism in duplex stainless steels is still lacking. The current research
mainly focuses on how environmental variables affect EAC susceptibility.

Chloride-induced stress corrosion cracking
Although duplex stainless steel is more resistant to stress corrosion cracking than austenitic stainless steel, it
still suffers from chloride-induced stress corrosion cracking under certain conditions , which include
[4]
seawater and alkaline environments . The cracking mechanism of chloride-induced stress corrosion
[110]
[105]
cracking is associated with ferrite dissolution and hydrogen embrittlement . The mechanism states that in
the chloride-containing environments, chloride ions penetrate the passive film of duplex stainless steel
[Figure 9A1 and A2]. Owing to the poor stability of the passive film, pitting corrosion is preferentially
initiated in the ferrite phase. An autocatalytic effect occurs at the bottom of the pits, which hinders the
repassivation of the matrix [Figure 9A3] . Under the action of stress, cracks initiate at the bottom of the
[105]
pits. Distinct chloride-induced cracks are branched. When cracks propagate into the austenite phase, they
are arrested [Figure 9A4] . Therefore, the cracks mainly propagate along the ferrite/ferrite boundaries and
[105]
austenite/ferrite phase boundaries.

Wu et al. reported that the sensitivity of 2,205 duplex stainless steel in a simulated marine atmosphere
increases with a decrease in the pH, and the main control mechanisms are the anodic dissolution
[111]
mechanism and the hydrogen embrittlement mechanism . The susceptibility of 2,101 duplex stainless
steel increases with increasing temperature, then decreases when the temperature is above 50 °C . This is
[106]
because high temperatures intensify the anodic dissolution and weaken the hydrogen embrittlement . The
[106]
addition of 5,600 ppm of nitrate prolongs the rupture time of 2,507 duplex stainless steel in 30 wt.% MgCl
2
by nine-fold because it can reduce the adsorption sites of chloride ions .
[112]
The crack propagation resistance of ferrite is weaker than that of austenite . However, some researchers
[113]
believe that the anodic dissolution crack initiation in ferrite is not due to pitting corrosion. Wickström
suggested that the chloride-induced cracks in ferrite in water droplet evaporation experiments were due to
surface dealloying . Not all circumstances make the ferrite more prone to chloride-induced stress
[114]
corrosion cracking. For example, Örnek found that the austenite in 2,205 duplex stainless steel suffers from
chloride-induced stress corrosion cracking in the drop test .
[115]