The Effect of Acetic Acid on Film Formation in Carbon Dioxide Corrosion

CO2 corrosion is one of the main concerns in upstream oil and gas particularly during material selection process. The presence of other multi-corrosive species mainly acetic acid (CH3COOH) further complicates the CO2 corrosion mechanism, due to possible interaction between iron(II) carbonate (FeCO3) and iron(II) acetate (Fe(CH3COO)2) film formation. This is important since most of the CO2 prediction modeling is affected by the protectiveness nature of FeCO3 film. Most of film formation studies in CO2 environment were done by adding Fe2+ ions to induce the formation of a dense iron carbonate film. However, this methodology is not representing the operational condition where FeCO3 film naturally formed. Thus, the objective of this research is to investigate the effect of acetic acid in CO2 environment on the corrosion rate in natural filming condition. Electrochemical corrosion experiments and surface morphology investigation were performed to study the effect of 10-400 ppm acetic acid on mild steel in CO2-saturated 3-wt% NaCl at pH 5.5, 90oC in stagnant natural filming condition. Linear Polarization Resistance of 96 hours showed that in blank test (without acetic acid), the effect of FeCO3 film formation was observed to decrease corrosion rate of 2.13 mm/yr at beginning to 0.25 mm/yr at the end of the test. Based on the morphology of film, there is small amount of FeCO3 film formed and partially covered the steel surface at the end of test. The presence of a small amount of acetic acid below 60 ppm did not change corrosion rate as compared to the blank test. Corrosion rate increases significantly in the range of 2.26-2.65 mm/yr with 60-400 ppm acetic acid and affect the FeCO3 film formation. The existence of acetate and bicarbonate ions in the solution caused a competition among them to react with Fe2+ ions, resulting delay the formation of FeCO3 film and delay stability of corrosion rate. However, there was no evidence of iron(II) acetate film since the solubility of iron(II) acetate is high. There is no variation in the thickness of FeCO3 film with more than 60 ppm acetic acid. This due to the longer time needed to reach saturation limit with the increase of acetic acid concentration. Polarization sweeps result show no difference in mechanism of corrosion. However the kinetics of corrosion varies with different concentration of acetic acid. Comparison with prediction tools showed an agreement about trend of corrosion rate at high temperature. There is no sufficient information on prediction tool regarding the effect of film formation to corrosion rate. In conclusion, the presence of acetic acid increases corrosion rate of mild steel in CO2 environment, especially above concentration of 60-ppm. Acetic acid delays FeCO3 film formation by reacting with Fe2+ ions and forms soluble iron(II) acetate. Iron carbonate film which found in both blank and acetic acid added corrosion samples was not fully protective.