CO2 Corrosion

Carbon dioxide corrosion, also known as sweet corrosion, is driven by CO2 and involves the formation of carbonic acid when contact with water. This acid attacks metal surfaces, leading to material degradation over time. Sweet corrosion is commonly found in gas-condensate and oil production systems as well as in produced water handling areas. It poses a serious threat to the integrity of pipelines and production equipment in oil and gas fields, potentially leading to leaks, production shutdowns, environmental damage, and costly repairs. Effective corrosion management strategies such as material selection and the use of inhibition, are essential to maintaining safe and efficient oil and gas operations

General Information

CO₂ corrosion, particularly in oil and gas production / transmission systems, has been extensively studied over the last few decades. The effects of various process parameters, such as temperature, pH, CO₂ partial pressure, and others, have been well examined and, in many cases, incorporated into relevant CO₂ corrosion prediction models. The following chapter provides a general overview of CO₂ corrosion and the impact of these parameters.

Mechanism

Anhydrous CO₂ (dry gas) is considered a non-corrosive. However, in the presence of water, it dissolves and forms weak carbonic acid, which leads to a reduction in pH and subsequent corrosion of carbon steel. The fundamental mechanism of CO₂ corrosion is relatively well understood and has been described by many researchers, including the seminal work of De Waard and Milliams, Crolet, Dugstad and others. ¹ ² ³ ⁴

Simplified anodic and cathodic reactions describing principles of CO₂ corrosion is presented below:

\(\ce{Fe -> Fe^2+ + 2e-}\) Reaction 1

\(\ce{H2CO3 + e- -> HCO3- + 2H+}\) Reaction 2

\(\ce{H+ + H+ -> H2}\) Reaction 3

Which can be presented in summary reaction:

\(\ce{Fe + 2H2CO3 -> Fe^2+ + 2HCO3- + H2}\) Reaction 4

During the progression of CO₂ corrosion, the bicarbonate ion concentration in the solution increases, which in turn raises the pH level. Once this concentration exceeds the saturation equilibrium, iron carbonate precipitates, as illustrated schematically in Reaction 5.

\(\ce{Fe + 2HCO3- -> FeCO3 + H2O + CO2}\) Reaction 5

Depending on the impact of various process parameters such as temperature, flow rate, and the CO₂/H₂S ratio, the FeCO₃ layer will exhibit varying properties, including density and porosity. However, regardless of these variations, the presence of an FeCO₃ deposit on the metal surface consistently leads to a reduction in the corrosion rate, which may be more or less significant depending on the specific conditions.

CO2 Corrosion is governed by a combination of several factors like chemical species and concentration, temperature, materials and/or flow regimes.

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References

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