Quantitative investigations on stress corrosion cracking initiation and propagation behavior of 20Cr-25Ni-Nb stainless steel in supercritical carbon dioxide at 650°C

This study quantitatively evaluates the stress corrosion cracking (SCC) behavior of 20Cr-25Ni-Nb austenitic stainless steel in 650°C supercritical CO₂ (s-CO₂) environments, supporting the development of advanced s-CO₂ Brayton cycle systems for nuclear reactors and high-performance fuel cladding. Results demonstrate that under constant loads, SCC initiation time decreases exponentially with increasing load, with rapid initiation occurring above 0.8 times the yield strength (YS) within 120 h. Under constant stress intensity factors (K), crack growth rate (CGR) also increases exponentially with increasing K, exceeding 20 MPa√m results in CGR above 1.0 × 10⁻⁷ mm/s. Microstructural analysis reveals that SCC initiation in a 650°C s-CO₂ environment results from pronounced preferentially intergranular oxidation (PIO) along with carburization-induced oxide amorphization that further disrupts passivation and facilitate the crack initiation. Crack propagation occurs via synergistic grain boundary (GB) weakening from creep deformation, stress-assisted GB oxidation in Cr-depleted zones, and carburization ahead of the crack tip. These findings establish quantitative design limits for 20Cr-25Ni-Nb components in s-CO₂ systems.