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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
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 corrosion mechanism of X65 carbon steel was evaluated in water environment by increasing CO2 partial pressure from 1 bar at 24 °C to 110 bar at 40 °C, reaching conditions that include water saturated with supercritical CO2.
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The NORSOK M-506 is an empirical model based on experiments undertaken in a single phase water flow loop. The data is from experiments with low content of iron ions in the water phase, and the model is thus regarded to give a fair representation of the maximum corrosion rate in a CO2 corrosion controlled system.
The literature has been reviewed with respect to information gained in the recent 20 years on CO2 corrosion of materials used in the oil and gas industry. The paper discusses the effect of materials related, medium-related and interface-related parameters on general (uniform) and localized corrosion. Part II