Steady State CO2 Corrosion—A Novel Testing Approach

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 may be released into the electrolyte depending on the corrosion rate, volume-to-surface ratio and testing time. 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 iron carbonate film formation at steady state conditions.