Balancing low-temperature toughness and S-CO2 corrosion resistance: A Cr–Ni microalloying strategy for pipeline steels

This study investigates the combined effects of Cr and Ni microalloying on the low-temperature fracture toughness and corrosion resistance of high-strength low-alloy pipeline steels exposed to H2S/CO2-saturated supercritical CO2 environments. Multiscale characterization techniques were employed to correlate microstructural features with critical crack-tip opening displacement(CTOD) and laboratory corrosion behavior. Cr addition increases hardenability and strength but promotes the formation of coarse granular bainite and martensite–austenite constituents, which act as preferential crack-initiation sites and degrade fracture toughness. The introduction of Ni refines transformation products toward acicular and bainitic ferrite, increases the fraction of high-angle grain boundaries, and partially restores crack-arrest capability, leading to an improved strength–toughness balance relative to Cr alone. Corrosion results indicate that both Cr and Ni reduce corrosion rates by promoting the formation of denser and more adherent corrosion scales enriched in Cr- and Ni-containing oxides. However, regions with elevated dislocation density may locally facilitate ionic transport and impurity segregation, partially weakening passivation. Overall, the results demonstrate that appropriately optimized Cr–Ni additions can provide a balanced enhancement of fracture toughness and corrosion resistance under the present laboratory conditions, offering insight into alloying effects relevant to steels for supercritical CO2 transport.