Abstract: Through quenching and tempering (QT) and quenching and partitioning (Q&P) processes, this study aimed to investigate the effects of microstructural modifications on the corrosion behavior and corrosion-assisted mechanical degradation of medium Ni-bearing steel. The primary objective was the identification of strategies for the enhancement of the long-term lifespan and reliability of these alloys in neutral aqueous environments. Various electrochemical evaluations and microstructural characterizations were conducted to elucidate the relationship between heat treatment processes and corrosion behavior. The findings reveal that the conventional Q&P process formed partitioned austenite with a coarse size within the martensitic matrix, which led to an uneven distribution of Ni and high kernel average misorientation and resulted in an increased susceptibility to corrosion and corrosion-induced mechanical degradation. In addition, the corroded QT sample displayed preferential attacks around cementite clusters due to selective dissolution. By contrast, a slightly higher partitioning temperature, just above the martensite transformation start temperature, provided finely distributed austenite within bainite in the microstructure, which exhibited lower corrosion kinetics and reduced susceptibility to mechanical degradation in the corrosive environment. This study highlights the potential of microstructural optimization through the Q&P process with a high partitioning temperature as an effective technical strategy for achieving the superior durability and reliability of medium Ni-bearing steel alloys in neutral aqueous environments.