Effect of bicarbonate on corrosion of carbon steel in CO2 saturated brines

The effect of bicarbonate concentration (HCO3−) on bare steel surface corrosion was investigated experimentally in ambient CO2-saturated solutions. In the presence of CO2, pH was adjusted by adding NaHCO3 solution or powder. Corrosion rate decreased with pH increasing at the range of pH 4–5, was little changed at pH 5–6 and interestingly increased at pH 6–8. Thermodynamic calculations of CO2 aqueous speciation showed that only bicarbonate concentration increased as pH increased from 4 to 8 while the other corrosion-active species including proton and carbonic acid either decreased or changed little, respectively. Thus we have demonstrated that bicarbonate is an active corrosion species and is important to corrosion at pH 6–8. Our earlier mechanistic corrosion model (Han et al., 2011a) was modified to incorporate bicarbonate-induced corrosion and demonstrated good agreement with experimental observations.