The formation of FeCO3 was studied as a function of NaCl solutions (1 and 3% w/v), saturated with CO2 (pH ∼ 6) at 80 °C. Individual crystal growth and properties, the formation of films, and the corrosion resistance of C1018 carbon steel were assessed. Immersion experiments were conducted from 6 to 168 h. Monitoring ex situ used the substrate weight loss method with dissolved iron concentration measurements (from bulk solution) by inductively coupled plasma-mass spectroscopy (ICP-MS). After exposure, scanning electron microscope (SEM), X-ray diffraction (XRD), and Raman spectroscopy were employed to understand FeCO3 amorphous/crystalline nature and surface composition. Complementary electrochemical measurements [open circuit potential (OCP), potentiodynamic polarization] were conducted to understand the impact of NaCl on cathodic and anodic processes. The role of NaCl was significantly more nuanced than the general corrosion rates might suggest (initially decreasing with increasing NaCl concentration). The induction time and nature of the FeCO3 formed are strongly influenced by the NaCl concentration. First, increasing NaCl concentration retarded the nucleation of FeCO3 crystals. Second, this same increase also induced crystal ripening and habit modification. Collectively, this leads to a porous, less protective layer of FeCO3.