Solution Chemistry Effects on Corrosion of Carbon Steels in Presence of CO2 and Acetic Acid

Abstract The corrosion behavior of X65 pipeline steel in solutions containing carbon dioxide (CO2), acetic acid (CH3COOH, abbreviated HAc) and 0.1-0.3 % sodium chloride (NaCl) was investigated on rotating cylinder specimens in glass cells. The tests were carried out at 25 °C (1 bar CO2) and 80 °C (0.5 bar CO2). The initial concentration of HAc was varied between 0 and 600 ppm (0-10 mM). The ferrous ion (Fe2+) concentration was in most of the tests held far below the iron carbonate (FeCO3) saturation limit. The electrode rotation rate was 1900 RPM, corresponding to ca. 1 m/s peripheral velocity. At 25 °C the HAc inhibited general corrosion, but promoted formation of deep corrosion pits. At 80 °C the corrosion was uniform; the corrosion rates were high (several tens of mm/y) in presence of HAc. The literature on CO2 corrosion in presence of HAc was reviewed. Some studies reported increased corrosivity in presence of HAc, whereas others reported that HAc works as an inhibitor. Most of the corrosion data reported in the literature was, however, found to be consistent when the effect of temperature and type of corrosion attack observed in the present work was taken into consideration. The literature data indicated that HAc corrosion could be mitigated by means of corrosion inhibitors and pH control. The physiochemical properties of HAc was also reviewed: diffusion coefficient, partial pressure of HAc over aqueous HAc solutions, HAc solubility in oil, and formation constants of ferrous and calcium acetate complexes. The effect of complex formation was investigated and discussed.