fig9

Figure 9. The mechanisms of (A₁-A₄) chloride-induced stress cracking, (B₁-B₄) hydrogen-induced stress cracking (C₁-C₄) and sulfide stress cracking. (A₁) In chloride-containing environments, chloride ions penetrate the passive film of the duplex stainless steel. (A₂) Due to the poor stability of the passivation film, pitting corrosion is preferentially initiates in the ferrite phase, and the autocatalytic effect occurs at the bottom of the pits, which hinders the repassivation of the matrix and initiates cracks under the action of stress. The crack is branched and stops when it propagates into the austenite phase. The crack propagates along the ferrite and austenite/ferrite phase boundaries. (A₃) Three-dimensional pitting corrosion on the surface of the duplex stainless steel. (Reproduced with permission^[105]. Copyright 2021, Elsevier). (A₄) The crack blunts when it reaches the austenite phase and propagates along the ferrite and phase boundaries. (Reproduced with permission^[105]. Copyright 2021, Elsevier). (B₁) Hydrogen atoms diffuse and accumulate at the phase boundaries. (B₂) Cracks initiate in the ferrite phase and ferrite/austenite phase boundaries and propagate in a zig-zag manner. Hydrogen promotes dislocation slip in austenite. (B₃) Hydrogen accumulates at the ferrite/austenite boundaries (Reproduced with permission^[107]. Copyright 2022, Elsevier). (B₄) Both cracked ferrite and austenite exhibit brittle fracture characteristics. (Reproduced with permission^[108]. Copyright 2006, Elsevier). (C₁) In sulfide environments, the passivation film dissolves and a layer of sulfide is formed on the surface. The surface is partially acidified, and the acidification effect produces corrosion. (C₂) Corrosion pits are formed in the ferrite phase. The reacidification and the autocatalytic effects generate hydrogen. Cracks initiate in the ferrite phase. Hydrogen generated at the corrosion product/substrate interface also preferentially diffuses to the phase boundaries and initiates cracks. (C₃) A sulfide corrosion product film forms on the surface of the sample, and the ferrite phase preferentially dissolves. (Reproduced with permission^[106]. Copyright 2014, Elsevier). (C₄) Secondary cracks preferentially initiate at the phase boundaries and ferrite (Reproduced with permission^[109]. Copyright 2006, Elsevier).