<p>The iron sulfide compound FeS, present in petrochemical plants, is prone to spontaneous combustion when exposed to air, posing a significant threat to the safety of production processes in the petrochemical industry. Accurately assessing the spontaneous combustion tendency of FeS is crucial for preventing fire and explosion hazards in petrochemical storage tanks. In this study, FeS was passivated with 3%, 6%, and 9% concentrations of passivator for 2 to 8&#xa0;h, and the effects of gas-phase passivation on FeS were investigated. Thermodynamic and pore structure parameters were extracted through simultaneous thermal analysis and BET specific surface area measurement experiments. The entropy-weight TOPSIS method was then used to evaluate the passivated samples and determine the optimal passivation conditions. The results show that the entropy-weight method assigns dominant weights to the duration temperature in the room-temperature spontaneous combustion stage (ΔT₁, 10.2%) and the activation energy in the high-temperature combustion stage (E₃, 9.3%), indicating that the thermodynamic suppression effect, driven by the passivation mechanism, plays a more significant role than the changes in the microstructure. TOPSIS analysis reveals that the FeS sample passivated for 8&#xa0;h with a 9% passivator concentration has the lowest spontaneous combustion tendency (composite score of 0.743), while the highest risk is associated with the sample passivated for 2&#xa0;h with a 3% concentration (composite score of 0.341). The passivator concentration is positively correlated with inhibition efficiency, and selecting a gas-phase passivator concentration between 6% and 9%, with a passivation time of over 4&#xa0;h, ensures higher safety and superior passivation effectiveness.</p>