ABSTRACT

The polarization behavior of API 5L X65 mild steel at various pH values and CO2 partial pressures was investigated at a high flow velocity. In contrast to the commonly accepted mechanistic view of CO2 corrosion, it is shown that the direct reduction of carbonic acid is insignificant at CO2 partial pressures up to 5 bar. That suggests that carbonic acid is merely a ”reservoir” of hydrogen ions and its presence only increases the observed limiting current densities by buffering the H+ concentration at the metal surface. Furthermore, the rate of anodic iron dissolution reaction was shown to be significantly increasing in the presence of CO2, suggesting that dissolved CO2 or its associated carbonate species are directly involved in the iron dissolution reaction. The findings of the present mechanistic study showed that the increased corrosion rates of mild steel in acidic solutions in the presence of CO2, which were previously associated with carbonic acid direct reduction, are in fact due to the increased rate of iron dissolution in the presence of CO2, in combination with the increased cathodic limiting currents resulting from buffering ability of dissolved CO2 and H2CO3.

INTRODUCTION

The profound effect of dissolved CO2 on increasing the corrosion rate of pipeline steel, as compared to solutions of strong acids with the same acidity, has been known for many decades. However, findings in the last two decades have challenged the commonly accepted ideas about the underlying mechanisms of this process. This study is focused on the fundamentals of the electrochemical reactions involved in the CO2 corrosion of mild steel in an attempt to further elucidate the mechanisms of the CO2 corrosion process.

Carbon dioxide (CO2) in gaseous or dissolved state is non-corrosive. However, upon dissolution in water (reaction (1)) and a subsequent hydration reaction (reaction (2)), a more reactive chemical species, carbonic acid (H2CO3), is formed. This hydration reaction is followed by dissociation reactions (reactions (3) and (4)) to form bicarbonate (HCO3-), carbonate (CO32-), and hydrogen (H+) ions; resulting in an acidic, corrosive solution.

This content is only available via PDF.
You can access this article if you purchase or spend a download.