The influence of acetate ion on the rate of corrosion of carbon steel (X65) in oilfield brines also containing CO2 has been investigated using voltammetry at rotating disc electrodes. It is demonstrated that the speciation of the brine is a major factor and it is shown that the rate of corrosion of the steel is largely determined by the concentration of undissociated acetic acid in the brine. This arises because the proton donor, as well as the free proton, is a reactant in the cathodic reaction leading to corrosion and the concentration of acetic acid is usually significantly higher than that of the free proton in the brines. In laboratory simulated brines containing only NaCl and NaOAc saturated with CO2, the pH and the concentration of acetic acid is a function only of the acetate concentration and hence there is a direct
relationship between the corrosion rate and the acetate content of the simulated brine. In real oilfield brines, the situation is more complex. Other species in the brine, particularly HCO3 - , can influence the pH and, hence, the concentration of acetic acid; predicting the corrosion rate then requires more extensive information about the composition. The role of carbonic acid, bicarbonate and carbon dioxide as potential proton donors is also discussed and it is concluded that with > 1 mM OAc - , the current density for acetic acid reduction will dominate the current density for the reduction of all other species.
Keywords: Carbon dioxide, acetate, carbon steel corrosion, voltammetry, oilfield brines.