Reactions of pipeline steels in carbon dioxide solutions

This paper describes the effects of dissolved carbon dioxide on the electrochemical reactions of two pipeline steels: a low alloy steel and a 13% Cr stainless steel. The dissolved gas reacts with the iron component of these steels in such a way as to contribute to their depassivation. The mechanism proposed is that of complexing with Fe (II) within the hydroxide film to give both ferrous carbonate and a dissolved complex. The effect is negligible for the 13% Cr stainless steel, but contributes to the corrosion of the low alloy steel. The cathodic regime representing evolution of hydrogen is also affected by the presence of dissolved carbon dioxide. At low cathodic overpotentials this appears to arise from a pH buffering action. At high cathodic overpotentials there is some evidence that the carbon component of the dissolved CO 2 is reduced, probably to carbon monoxide, and this reduces the rate of hydrogen evolution by cathode poisoning. The roles of pH buffer and of electrolytic anion are discussed.