| Summary: | Previously we have proposed a model for the acetate-enhanced corrosion of carbon steel in oilfield brines based on acetic acid (C2H4O2, HOAc) as the dominant reactant in the cathodic reactions leading to corrosion and suggested that corrosion rates may be predicted from a knowledge of the equilibrium composition of the brine; concentrations of HOAc > 1 mM should flag a warning to possible acetate-enhanced corrosion. In this paper, we show that acetate ions in the brine also contribute to enhanced rates of corrosion through a second, although less-important, mechanism-the catalysis of the hydration of carbon dioxide (COj to carbonic acid (H2CO3). We also consider further the role of bicarbonate. Bicarbonate increases the pH and therefore decreases the equilibrium concentration of HOAc in the brine, and, indeed, it is the presence of bicarbonate in brines that prevents acetate-enhanced corrosion from being a major hazard in a large number of oilfield pipelines. It is also the concentration of bicarbonate in the brine that controls the structure of the corrosion film on the carbon steel surface.
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