As a typical material of the insert in high-level radioactive waste (HLW) geological disposal canisters, iron-based materials will directly contact with groundwater after the failure of a metallic canister, acting as a chemical barrier to prevent HLW leaking into groundwater. In this paper, anoxic groundwater was simulated by mixing 10 mM NaCl and 2 mM NaHCO₃ purged by Ar gas (containing 0.3% CO₂) with different added ions (Ca²⁺, CO₃²⁻ and SiO₃²⁻) and operation temperatures (25, 40 and 60 °C). An electrochemical measurement, immersion tests and surface characterization were carried out to study the corrosion behavior of pure iron in the simulated groundwater. The effects of Ca²⁺ on the corrosion behavior of iron is negligible, however, Cl⁻ plays an important role in accelerating the corrosion activity with the increased concentration and temperature. With increased concentrations of CO₃²⁻ and SiO₃²⁻, the corrosion resistance of iron is largely improved, which is attributed to the formation of a uniform passivation film. The independent effects of temperature on the corrosion behavior of iron are resulted from the repeated passivation–dissolution processes in the formation of the passivation film, resulting from the synergistic effects of CO₃²⁻/SiO₃²⁻ and Cl⁻. The formation of ferric silicate is dominant in the passivation film with the addition of SiO₃²⁻, which effectively protects the iron surface from corrosion.