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.
At CORROSION 2002 -we reported the rates of corrosion of carbon steel (X65) in simulated brines containing 3 % NaCl and various concentrations of sodium acetate (NaOAc), each saturated with carbon dioxide1. It was demonstrated that the rate of corrosion could be understood in terms of the concentration of undissociated acetic acid formed when the acetate containing brines were acidified by contact with carbon dioxide. In another paper2, the speciation of the solutions was calculated and it was confirmed that the chemistry and electrochemistry (at both Pt and C steel) of the acetate/CO2 solutions is identical to an acetate buffer of the same pH prepared by simple addition of acetic acid to aqueous sodium acetate. This paper now reports the extension of these studies to oilfield brines. Inevitably, such brines have a much more complex composition. They have different ionic strengths and contain an assortment of additional inorganic ions as well as organic contaminants. The former, particularly bicarbonate, may be expected to influence the pH of the solutions and hence the concentration of acetic acid while the latter could adsorb on the steel surface and thereby also influence the rate of corrosion. The studies have employed brines from oilfields in Britain and Central America.
In general, the speciation of the medium can influence the corrosion of a metal through either the composition of the film formed on the surface of the corroding metal or through the thermodynamics and kinetics of the anodic and/or cathodic reactions leading to corrosion.
In the case of carbon steel in a brine, a dominant factor resulting from the presence of acetate and carbon dioxide is the formation of acetic acid. This leads to a change in the main cathodic reaction leading to corrosion from H+ + e- ½H2 (1) or H2O + e- ½H2 + OH- (2) to HOAc + e- ½H2 + OAc- (3)
Reaction (3) occurs by the mechanism where reaction (4) is followed by reaction (1). HOAc H+ + OAc- (4)
Reaction (4) is a rapid reaction and hence the hydrogen evolution reaction on a metal surface does not distinguish free protons and undissociated acetic acid as reactants3-7. Moreover, in many of the brines containing acetate and carbon dioxide, the concentration of acetic aci