Ferritic/martensitic 9Cr heat-resistant steels are extensively used in high-temperature steam power systems due to their balanced creep strength and oxidation resistance. However, during start-up, shut-down, hydrotesting, and wet layup conditions, these alloys experience aqueous exposure where electrochemical corrosion and localized attack may compromise structural integrity. Although molybdenum (Mo) is a key alloying element in 9Cr steel families, its independent role in governing passive film stability and breakdown remains unclear due to strong compositional coupling in commercial grades. This study aims to isolate and quantify the intrinsic effect of Mo on passivation and localized corrosion behavior using a systematic model alloy approach. A Fe–9Cr–xMo (x = 0–18 wt.%) model alloy series was developed to maintain a constant chromium backbone while varying Mo as the primary design variable. Electrochemical behavior was evaluated in chloride-containing aqueous environments using potentiodynamic polarization, cyclic polarization, repassivation potential analysis, and critical pitting temperature measurements. Microstructural characterization was performed to correlate electrochemical responses with compositional heterogeneity and secondary phase formation. Mo content modifies passive behavior in a non-linear manner. Intermediate Mo levels enhance passive film stability, increase breakdown and repassivation potentials, and elevate critical pitting temperature, indicating improved localized corrosion resistance. In contrast, higher Mo contents promote microstructural heterogeneity and metastable pitting activity, reducing repassivation efficiency and narrowing the passive stability window. These results suggest a composition-dependent transition from Mo-enhanced passivation to heterogeneity-assisted breakdown. This work establishes a quantitative composition–passivation–breakdown relationship for Fe–9Cr–xMo alloys and identifies a mechanistic threshold in Mo content controlling localized corrosion resistance. The findings provide fundamental insight for optimizing 9Cr-based steels subjected to combined steam and aqueous exposure in energy infrastructure applications.