High-Temperature Corrosion Behavior of Q345R Steel in Molten Salt Environments

This study aimed to evaluate the long-term service performance of Q345R steel in a high-temperature nitrate molten salt environment，thereby providing corrosion data and a theoretical basis for its application in emerging high-temperature fields such as molten-salt thermal energy storage and nuclear energy.Driven by the “dual-carbon” goals and the rapid development of renewable energy technologies，more stringent requirements are imposed on the high-temperature corrosion resistance of materials.Although Q345R steel is cost-effective and widely used，the long-term corrosion mechanism of Q345R steel in nitrate molten salts，particularly the dynamic evolution and failure mechanisms of the oxide film，remained unclear.Accordingly，the corrosion behavior of Q345R steel in nitrate molten salt (40%KNO3＋60%NaNO3，mass fraction) at 400 ℃ for up to 1 200 h was systematically investigated.Corrosion kinetics were obtained using the weight-loss method，and the oxide film formation process and protective mechanism were elucidated by combining optical microscopy (OM)，X-ray diffraction (XRD)，scanning electron microscopy with energy-dispersive spectroscopy (SEM-EDS)，and first-principles density functional theory (DFT) calculations.Results showed that as the exposure time increased from 300 h to 1 200 h，the mass loss increased from 0.003 3 g/cm2 to 0.008 5 g/cm2，whereas the corrosion rate exhibited a decreasing trend，dropping from 1.097 2×10－5 g/(cm2·h) within the initial 300 h to 6.392×10－6 g/(cm2·h) at 1 200 h.XRD analysis confirmed that the corrosion product on the surface was Fe3 O4，whose diffraction intensity increased with corrosion time，while the diffraction signal from the Fe substrate weakened，indicating continuous formation and progressive surface coverage of the Fe3O4 oxide film.SEM observations revealed that the corrosion morphology evolved from uniform pitting in the early stage to an “island-like” morphology caused by localized spallation of the oxide film.Cross-sectional analysis indicated that the Fe3O4 film thickness increased from 11.43 μm at 300 h to 22.50 μm at 1 200 h，accompanied by gradual densification of the film.Although minor localized delamination was observed at later stages，the film remained well bonded to the substrate overall.EDS results showed that the oxygen content increased markedly with exposure time and reached 28.14%(mass fraction) at 600 h，further confirming the progression of surface oxidation.DFT calculations demonstrated，at the atomic scale，that a charge transfer of 0.48 e occurred across the Fe/Fe3O4 interface，which enhanced interfacial chemical bonding and thereby enhancing the adhesion and protective performance of the oxide film.In conclusion，the gradual formation and densification of the Fe3O4 oxide film were identified as the key factors responsible for the decreased corrosion rate.This work clarified the corrosion evolution of Q345R steel in nitrate molten salts and provided an important reference for the lifetime prediction and formulation of corrosion protection strategies for Q345R steel in high-temperature service environments.