Speciation and electrochemistry of brines containing acetate ion and carbon dioxide

The corrosion of carbon steel, oilfield pipelines is accelerated when the anion of a weak acid (usually considered to be acetate) is present in the carbon dioxide saturated brines that usually accompany the production of the oil. This paper reports the voltammetry of such media at both platinum and carbon steel (X65) rotating disc electrodes in order to demonstrate that the solution composition as well as the surface films determine the rate of corrosion. It is shown that the cathode reactions at Pt and in the corrosion process at steel are the parallel reduction of free proton and the strongest proton donor in the solution. In these solutions, the strongest proton donor is acetic acid and at the usual pH of the brines, the concentration of acetic acid is substantially higher than that of the free proton. As a result, the concentration of the proton donor, the undissociated weak acid, rather than the pH, is a critical factor determining the rate of corrosion of the steel. A quantitative interpretation of the voltammetry and the corrosion behaviour requires a knowledge of the speciation within these complex aqueous solutions and in this paper, the speciation is calculated using equilibrium constants from the literature and a computer package (phreeqc 2.2).