Comments on the Elusive Mechanism of CO2 Corrosion

The extraction of CO2 from Flue gases (14% CO2, 70 % N2 and trace of acid gases such as SO2 and H2S) and subsequent processing for the purpose of underground storage requires proper understanding of CO2 corrosion. For example most investigators agree that CO2 corrosion of mild steel is an electrochemical process which involves the anodic dissolution of iron and cathodic evolution of hydrogen ( pH lower than 4). At a pH higher than 4, the dominant reaction, according to most investigators, is the reduction of carbonic acid.

2 H₂ CO₃ (aq)   + 2e   →      H₂ (g) + 2 HCO₃^-

It will be shown that the conversion of carbonic acid (H₂ CO₃) to bicarbonate ions (HCO₃^-) is not an electrochemical reaction but simply a chemical reaction. Similar mechanism was proposed for the formation of bi-sulfide (HS^-) from aqueous dissolved H₂S.In fact, most computer models used by the petroleum industries to predict CO2 corrosion are based on the above equation. The objective is to introduce the mechanism of CO2 corrosion in accordance with the proper chemistry of CO₂ and H₂S in aqueous environment and to comment on the effects of mass transfer, reaction kinetics and shear stress during pipe flow on CO2 corrosion.