A parametric investigation on the influence and inhibition performance on CO2 corrosion of carbon steel

The deposition of hydrate and corrosion in the pipeline remain a major challenge in the petroleum industry. Internal corrosion slows and decreases the production of oil and gas when associated with free water and reacts with organic acid. Experiments had shown that the corrosion products (ferrous and anhydrous ions) combine together to form a precipitate on iron surface that are porous and non-protective. To prevent these occurrence, Mono-Ethylene glycol (MEG) is been utilized in the pipeline as an antifreeze and anti-corrosion agent. It has been observed that the MEG need to be separated from acetic acid (HAc) and acid gases which enhances corrosion of mild steel in the oil field environment. Mono-Ethylene glycol and acetic acid also has an adverse effect of lowering the solubility of mineral salts and causing a higher risk of corrosion problem. The goal of this investigation is to determine the combined effect of acetic acid and mono-ethylene glycol on corrosion of carbon steel in CO2 saturated environment at three different temperatures of 25oC, 50oC and 80oC using a 3.5% wt. NaCl solution. (Weight loss) and (electrochemical) measurements using Linear polarization resistance (LPR), Electrochemical Impedance Spectroscope (EIS) and Potentiodynamic polarization(PDP) were employed in measuring the corrosion rate as a function of time on HAc and MEG concentrations. In addition, the efficiencies of three corrosion inhibitor based chemicals (the phosphate ester and the Oleic imidazoline salt) at different concentrations were also evaluated. Furthermore, the link between the nature of the film formed on the electrode material and the corrosion characteristics were investigated using the Scanning Electron Microscope (SEM), X-ray diffraction (XRD) and X-ray photoelectron scan (XPS). The weight loss results shows that the corrosion rate with HAc increases rapidly as the concentration of HAc was added to the solutions compared to the solution without HAC. The average corrosion rate with HAc at 25oC is about 0.72 mm/yr and increased to 1.05 mm/yr at 50oC. The highest corrosion rate was noted at 80oC with approximately 2.71 mm/yr. However, addition of 20% and 80% MEG reduced the corrosion rate at all temperatures studied. The electrochemical measurement results obtained show the corrosion rate in the presence of HAc increases as different concentrations of HAc were added to the system, and decrease with increased in exposure time as a result of protective film formed on the electrode surface. At 25oC, the average corrosion rate increases from 2.6 mm/y to 3.07 mm/yr. The applications of inhibitors lower the corrosion rate at all temperatures with average corrosion rate of 0.21 mm/yr, 0.44 mm/yr and 0.56 mm/yr at 25oC, 50oC and 80oC respectively. Similarly, the Nyquist and Bode plots of the EIS results indicate that the diameter of the depressed semi-circle decreases as different concentration of HAc and MEG were added to the solution, and the capacitive semi-circle sizes also decreases on addition of 20% and 80% MEG to the system. This decreased in corrosion rate can be attributed to the formation of iron carbonate film on the surface of the steel sample. The surface examination of the exposed samples reviews that a dense layer of iron carbonate and localized corrosion were observed in the absence of HAc at 80oC. On addition of HAc and MEG, the layers of the film became porous. High magnification of the scan results shows a small pits and localized corrosion on the electrode surface. The EDX and XPS analyses on the samples reviews that some of the elements are absence on addition of HAc to the solution