11075 A Steady State CO2 Corrosion - A Novel Testing Approach

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Product Number: 51300-11075-SG

ISBN: 11075 2011 CP

Author: Richard Woollam, Jose R. Vera, Conchita Mendez, Raul Echegoyen and Haralampos Tsaprailis

Publication Date: 2011

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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