This study systematically investigated the inhibition mechanism of NaHCO3 for mild steel corrosion in a 0.02 M naturally aerated NaCl solution at 40 C (simulating cooling water) and the critical bicarbonate/ chloride molar ratio (RC) using visual observations, potentiodynamic polarization curves, and electrochemical impedance spectroscopy. The corrosion mechanism of an anodic passive film formed with 0.20.0.40 M NaHCO3 (RC = 10) was controlled by a combination of passivation, diffusion, and charge transfer processes. Electrochemical impedance spectroscopy results showed that, compared to 0.20. 0.24 M NaHCO3, a higher NaHCO3 concentration may affect the stability of the passive film. A higher chloride concentration of 0.08 M achieved by a higher concentration cycle in the cooling water system resulted in a lower RC value of 7.5 for NaHCO3. Both NaHCO3 and Na2CO3 showed an identical corrosion mechanism; moreover, the inhibitory effect of Na2CO3 was not dependent on its alkaline pH. Taken together, these findings show the remarkable potential of NaHCO3/Na2CO3 as a commercially available, comparatively cheap, and environmentally acceptable (i.e., no organic pollution, toxicity, or eutrophication risks after discharge) corrosion inhibitor for mild steel in cooling water systems.