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The effect of acetic acid on the corrosion behavior of X 65 and C 1018 carbon steel in vapor-water two-phase stratified flow (Vsg: 2 m/s; Vsl: 0.1 m/s) at 2 bars total pressure, 1.54 bars CO2 partial pressure, pH 5.5, and 80°C was studied in a low pressure-high temperature multiphase flow horizontal loop using electrochemical and mass loss techniques. The liquid phase is composed of 1% NaCl aqueous solution containing different concentrations of acetic acid (500 to 5000 ppm total acetic acid), and the gas phase is composed of CO2-acetic acid-water vapor mixture. The corrosion rates, on both the top and bottom of the line, increase with increase in acetic acid concentration, which was attributed to the contribution of hydrogen ions by the free acetic acid to the cathodic reaction. Partial coverage of the metal surface by FeCO3 on both the top and bottom of the line is reported to be responsible for the observed localized corrosion. Surface analysis investigated with SEM and XRD is reported.A vapor-liquid equilibrium model was developed for the system studied. [ABSTRACT FROM AUTHOR]

• A novel experimental setup was used to maintain the water chemistry of the electrolytes with well-defined mass transfer characterization. • Ca2+ incorporated into the crystal structure of FeCO 3 and a substitutional iron-calcium carbonate solid solution was formed. • The porous structure of Fe 3 C networks provided "anchoring" sites for precipitation of FeCO 3 enriched with calcium. • The iron-calcium carbonate covered uniformly the steel surface and retarded the rate of the anodic reaction, causing lower general corrosion. • Presence of Ca2+ in the electrolyte, while bulk CaCO 3 precipitation was not expected, did not lead to any localized corrosion. The effect of Ca2+ on the corrosion product layers when the CO 2 -containing electrolyte is saturated with respect to CaCO 3 , as frequently observed in oil production systems, was studied by weight loss, electrochemical techniques, and surface characterizations. An improved experimental set up was used and all experimental parameters reported. Results show that precipitation of corrosion products in solutions with or without CaCO 3 was due to the favorable water chemistry obtained at the steel surface compared to the bulk solution. The presence of Ca2+ did not jeopardize the final protectiveness of corrosion product layer, but seemed to delay its growth. [ABSTRACT FROM AUTHOR]