The so called Hitec molten salt (40%NaNO₂-7%NaNO₃-53%KNO₃, by mass fraction), as an efficient heat transfer and thermal storage medium, plays a crucial role in the flexibility retrofitting of thermal power plants. The dynamic compatibility between this medium and the storage tank structural material, Q345R carbon steel, constitutes a key scientific issue determining the long-term reliability of the system. Addressing the current lack of corrosion data and mechanistic understanding under dynamic conditions, herein, the influence of flow velocity (0-2 m/s) of the salt on the corrosion kinetics and underlying mechanisms of Q345R carbon steel in Hitec molten salt at 400 ℃ for 1000 h was assessed. Quantitative results demonstrate that the average annual corrosion rate of the steel increases significantly with the increasing flow velocity, namely from 18.20 μm/a for static state increases to 20.64 μm/a and 21.82 μm/a for 1 and 2 m/s, corresponding to increment of 13.5% and 19.93%, respectively. Then a quantitative correlation between the corrosion rate and the flow velocity of salt was established for this material system. Microstructural characterization revealed that although the phase composition of the corrosion products was consistent across different flow velocities (namely Fe₂O₃, Fe₃O₄, FeCr₂O₄), however, their formation and evolution mechanisms differ fundamentally. Dynamic flow not only mechanically compromises the integrity of the surface oxide scale, leading to protective function loss and accelerated spallation, but also significantly enhances the inward migration of corrosive species (e.g., oxygen) and the selective dissolution of the key alloying element Cr, synergistically exacerbating the corrosion process. From the perspectives of kinetics and micro-mechanisms, a comprehensive failure model for Q345R carbon steel was also proposed as follows: the initial protective oxide formation, intermediate flow-dominated oxide scale degradation competing with internal oxidation, and late-stage inward migration and depletion-dominated internal corrosion development. In sum, the findings may provide good reference for the design and safety assessment of highly reliable molten salt storage tanks.