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.
Short chain carboxylic acids are often present in oil and gas reservoirs beside other corrosive compounds like carbon dioxide (CO2) and hydrogen sulfide (H2S). Amongst these short chain acids, acetic acid (CH3COOH, abbreviated HAc) appears to be the most abundant. The effect of HAc on the corrosion of mild steel in sweet systems has been the subject of a number of experimental studies during the last few years, see summary below. It appears that HAc can give a significant contribution to the overall corrosion rate, and raise the uninhibited corrosion rate to several tens of millimeters per year. The worst-case corrosion rates are obviously unacceptable in the field, and robust corrosion mitigation techniques are needed. However, the introduction of the inhibitor availability concept justifies studying worst-case corrosion rates, since the corrosion during periods of inhibitor unavailability may contribute significantly to the loss of pipe wall thickness. Furthermore, the study of worst-case corrosion rates may contribute to the understanding of the corrosion mechanisms, and the development of corrosion prediction model tools.
The present paper reports the results of a laboratory study on the effect of HAc under conditions without protective corrosion films at two temperatures, 25°C and 80°C. The tests focus on the bottom of-line corrosion in gas-condensate pipeline conditions. The objective of the work was to assess the effect of HAc on the potential (worst case) corrosiveness in such systems, and to generate data that may facilitate corrosion prediction in presence of HAc. Furthermore, the paper contains a brief summary of the literature on the effect of HAc on corrosion in oil- and gas production and transportation systems. The aim was to extract relevant information and clear up some apparent inconsistencies in the literature. For example, some reports state that HAc increases corrosion, while others state that HAc acts as an anodic inhibitor. Physio-chemical data of HAc were also reviewed. Corrosion tests examining the effect of HAc on protective corrosion films were also performed in the same pro