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51318-11232- Mechanism of CO2 Corrosion of Mild Steel: a New Narrative

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The increased corrosion rates of mild steel in acidic solutions in the presence of CO2, previously associated with carbonic acid direct reduction, are in fact due to the increased rate of iron dissolution in the presence of CO2

Product Number: 51318-11232-SG
Author: Aria Kahyarian, Bruce Brown, and Srdjan Nesic
Publication Date: 2018
Industry: Science of Corrosion
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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.

Key words: Metallic corrosion, acidic corrosion, mild steel, carbon dioxide, mechanism, iron dissolution

 

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.

Key words: Metallic corrosion, acidic corrosion, mild steel, carbon dioxide, mechanism, iron dissolution

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