The rate of corrosion of X65 carbon steel in oilfield brines containing carbon dioxide (CO_2) is increased by the presence of acetate (or unions of other weak acids) in the brine. It is confirmed that the increase in corrosion rate may be correlated with the equilibrium concentration of acetic acid (HOAc) in the brine and this may be calculated provided the full brine composition is known. It is, however, essential to take into account the ionic strength of the brine, the bicarbonate present, the partial pressure of CO_2, and the temperature, as well as the acetate concentration. Moreover, it is shown that the role of acetic acid is as a reactant in the cathodic processes contributing to corrosion; acetic acid, a proton donor, as well as free proton, reduces to give hydrogen gas. Calculation of the relative rates of all possible cathodic reactions at the corrosion potential leads to the conclusion that acetate-enhanced corrosion will be a potential hazard when the equilibrium concentration of acetic acid in the brine is >1 mM (60 ppm). This has been confirmed experimentally.
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