Microstructural evolution of SIMP steel under stress/lead–bismuth corrosion synergy

• The applied stress significantly enhances the oxidation of SIMP steel in LBE. • The applied stress promotes oxide nucleation on the surface of SIMP steel. • Cr/Si segregate at boundaries, forming [Cr 2 O 3 + SiO 2 ], depleting protective Cr. • Cracks form in Fe 3 O 4 at 150 MPa, spread to spinel at 200 MPa. • Stress causes oxide delamination, increasing oxygen penetration. Structural materials for lead–bismuth (LBE) cooled reactors face severe challenges from LBE corrosion-stress synergy damage. This study explores the microstructural evolution of SIMP steel under synergistic tensile stress (100, 150, 200 MPa) and oxygen-saturated (approximately 1.17 × 10 -3 wt%) LBE corrosion at 550 °C for 24 h. A stress-free sample served as a reference. Results show that stress significantly intensifies corrosion, increasing oxide layer thickness from 4.08 ± 0.77 μm (stress-free) to 8.15 ± 1.52 μm (200 MPa). Stress-induced dislocations enhance diffusion of Fe, Cr, Si, and O, particularly along martensite lath boundaries, promoting growth of the duplex oxide (Fe 3 O 4 outer, Fe-Cr spinel inner) without altering composition. Stress facilitates oxide nucleation, transitioning from island-like to continuous films. Cr and Si enrichment at lath boundaries forms [Cr 2 O 3 + SiO 2 ] complexes, depleting Cr within laths and reducing spinel protectiveness. Cracking occurs in the Fe 3 O 4 layer at 150 MPa, extending to the Fe-Cr spinel at 200 MPa, with delamination and sliding compromising oxide integrity. Cracks, localized at the [Cr 2 O 3 + SiO 2 ]/spinel interface, enhance oxygen ingress, accelerating oxidation. These findings deepen the understanding of the failure mechanism of SIMP steel under stress/LBE corrosion synergy, highlighting the need for enhanced material designs for LBE-cooled reactors.