CO2 Top-of-Line-Corrosion; Assessing the Role of Acetic Acid on General and Pitting Corrosion

Abstract

Based on a review of both literature and field data, it is apparent that the role of acetic acid (HAc) in oilfield brines is both extremely complex and somewhat controversial. Although it is commonly believed that the presence of this organic compound enhances both the general and the localized corrosion rate of carbon steel, HAc has recently been reported to also act as a weak general corrosion inhibitor in specific aqueous environments. These observations prompted a study into whether such behavior is apparent in a CO2 top-of-line corrosion (TLC) scenario, i.e., when HAc dissolves into condensed water that forms on the upper internal wall of carbon steel pipelines during wet-gas stratified flow. Four different water condensation rates/temperature TLC conditions were selected to investigate the role of HAc on both the kinetics and mechanism of carbon steel dissolution. A miniature three-electrode setup was developed to characterize the real-time TLC response through the implementation of electrochemical measurements. Surface analysis techniques (microscopy and profilometry) were also performed to complement the electrochemical results. Collective consideration of the corrosion response and condensate chemistry indicates that similar effects were observed to those reported in the literature for bulk aqueous environments, in that the introduction of HAc can result in either accentuation or a minimal/inhibitive effect on general corrosion depending upon the operating conditions. The minimal/inhibitive effects of HAc were apparent at a surface temperature of 20.5°C and water condensation rate of 0.5 mL/m2·s, as no significant increase in corrosion was observed, despite a significant reduction in condensate pH being generated by the presence of HAc. X-ray photo-electron spectroscopy analysis of the inhibited steel specimen in the presence of HAc revealed the presence of iron acetate on the steel surface, which may have been at least partially responsible for the observed inhibitive effect. Extended duration experiments over 96 h revealed that both general and localized corrosion are not significantly affected by HAc addition at low temperature, whereas the level of degradation increases at higher surface temperature over longer periods.