Abstract
CO2 corrosion has been recognized as a major problem in internal pipeline corrosion. In the presence of water, CO2 forms carbonic acid, a weak acid which partially dissociates as a function of pH and the solution temperature. According to many studies, the presence of CO2 and therefore, carbonic acid enhances the corrosion rate of mild steel by accelerating the cathodic reaction. The exact mechanism of carbonic acid reduction at the metal surface is still being debated. When the reduction of the adsorbed carbonic acid molecule occurs at the metal surface, the mechanism is called "direct reduction", originally proposed by deWaard and Milliams in 1975. An alternative explanation has carbonic acid providing additional hydrogen ions via its dissociation while the dominant cathodic reaction is reduction of hydrogen ions; this mechanism is referred to as a "buffering effect". In the present study, electrochemical techniques such as linear polarization resistance (LPR), potentiodynamic sweeps and electrochemical impedance spectroscopy (EIS) were used in order resolve this dilemma, i.e. to investigate the exact mechanism of the cathodic reaction in the presence of carbonic acid. It was found that carbonic acid affects only the limiting cathodic current, but has no effect on the charge transfer current. The charge transfer current is found to respond only to a change in pH, indicating hydrogen ion reduction as the main cathodic reaction. The buffering effect is therefore considered to be dominant; the direct reduction of carbonic acid appears to be insignificant compared to the reduction of hydrogen ions in the range of conditions covered by this study.
