Technical Note: Electrochemistry of CO₂ Corrosion of Mild Steel: Effect of CO₂ on Cathodic Currents

The common understanding of aqueous CO₂ corrosion mechanism considers carbonic acid as an electroactive species. The direct reduction of carbonic acid on a steel surface is believed to be the cause of the higher corrosion rates of mild steel, as compared to that observed in strong-acid solutions with the same pH. However, in-depth quantitative analyses based on comprehensive mechanistic models, developed in recent years, have challenged this idea. In an attempt to provide explicit experimental evidence for the significance of direct reduction of carbonic acid in CO₂ corrosion of mild steel, the charge transfer controlled cathodic currents in CO₂ saturated solutions were investigated in the present study. The experiments were conducted on three different surfaces: Type 316L stainless steel, pure iron, and API 5L X65 mild steel, in order to examine the possible effect of alloying impurities on the kinetics and the mechanism of cathodic currents. The experimental polarization curves showed that at a constant pH, the charge transfer controlled cathodic currents did not increase with increasing partial pressure of CO₂ from 0 bar to 5 bar. This confirmed that the direct carbonic acid reduction was not significant at the conditions covered in the present study, and its sole effect was to buffer the hydrogen ion concentration.