The influence of acetate ion on the rate of corrosion of carbon steel (X65) in 3 % NaC1 brine saturated with carbon dioxide has been investigated using voltammetry at a rotating disc electrode. It is shown that the rate of corrosion can only be understood if it is recognised that the cathodic process in the steel corrosion does not distinguish between the reduction of free protons and the reduction of the undissociated proton donor, acetic acid. Hence, for any brine composition it is important to consider the concentration of acetic acid as well as the pH of the brine in explaining the rates of corrosion of the steel. This requires the ability to predict the speciation of these complex solutions.
During the recovery of oil and gas, an aqueous brine saturated with carbon dioxide is usually one component of a multiphase flow through carbon steel pipes. In many oilfields, enhanced rates of steel corrosion are thought to result from the presence in the brine of the anions of weak acids, particularly acetate ion, typically at concentrations in the range of 1 - 2 mM ( 6 0 - 120ppm). Possible mechanisms for the enhancement in the rate of corrosion of carbon steels in this situation has been discussed in several papers.
