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.
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