• Conventional Q&P processing with coarse partitioned austenite increases corrosion susceptibility and mechanical degradation in medium Ni-bearing steel.
• QT samples show preferential corrosion around cementite clusters due to selective dissolution.
• A slightly higher partitioning temperature (just above Ms) yields finely distributed austenite within bainite, improving corrosion resistance and mechanical stability.
• Microstructural optimization via Q&P at high partitioning temperature is a promising strategy for enhancing durability and reliability in neutral aqueous environments.