In CO2 corrosion, iron carbonate precipitation can significantly reduce the corrosion rate of the
underlying steel. Iron carbonate precipitates in an aqueous solution, when the concentrations of Fe2+ and
CO3= ions exceed the solubility limit Ksp or solubility product constant. Film properties determine CO2
corrosion rates and morphology of the attack (e.g. uniform vs. pitting corrosion). Many tests have been
conducted to understand the conditions under which iron carbonate layers with protective properties are
formed; the effect of various parameters in the protectiveness of such layer, etc. The vast majority of
these laboratory evaluations have been performed in closed systems with limited inventory of fluids.
Large amounts of iron are sometimes added intentionally into the electrolyte to induce carbonate
precipitation or may be released depending on the corrosion rate, volume-to-surface ratio and testing
time. In either case, the iron concentration in solution varies significantly with exposure time and may
affect the kinetics of iron carbonate precipitation and its effect on corrosion rates. Therefore the
applicability of constant volume tests to CO2 corrosion prediction at field conditions, where there is
continuous replenishment of fluids, can be very limited.
A new approach has been developed to understand the iron carbonate precipitation and dissolution at
steady state conditions in a once flow through continuous stirred tank reactor (CSTR) system. The new
testing apparatus allows obtaining mechanistic information about the film formation and film properties
through electrochemical impedance spectroscopy (EIS), linear polarization resistance (LPR) and weight
loss measurements in a rotating cylinder multi-electrode system. Total iron, carbonate ion, pH,
temperature and dissolved oxygen measurements are also made and aligned with corrosion rates data to
understand the effect of iron carbonate film formation at steady state conditions.
Keywords: CO2 Corrosion, Iron Carbonate, Continuous Stirred Tank Reactor System
