CO₂ / H₂S Corrosion

Carbon dioxide (CO₂) corrosion, commonly referred to as sweet corrosion, is driven by CO₂ and involves the formation of carbonic acid upon contact with water. It is frequently encountered in oil and gas production and transmission systems and can occur simultaneously with the presence of hydrogen sulfide (H₂S). Individually, these acid gases, when dissolved in water, can initiate corrosion reactions on metal surfaces, leading to material degradation over time. However, the combined effect of CO₂/H₂S corrosion is far more complex and still poses significant risks to pipeline and equipment integrity, potentially leading to leaks, unplanned shutdowns, safety incidents, environmental impact, and costly repairs. Therefore, effective corrosion management strategies including proper corrosion modelling followed by material selection, chemical inhibition etc. are essential to maintaining safe and efficient oil and gas operations.

General Information

CO₂ corrosion in oil and gas production and transmission systems has been extensively studied over the last decades. The effects of various key parameters, such as temperature, pH, CO2 partial pressure and the presence of H₂S etc., have also been examined and, in many cases, these findings are used to assess and predict system corrosivity. The following chapter provides a general overview of CO₂/H₂S corrosion, the impact of these parameters and corrosion modelling approach.

Mechanism

Anhydrous CO₂ (dry gas) is considered 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 / H₂S 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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^{1 - A constant amount of 100 ppm of bicarbonate was added, as specified in the source paper}
^{2 - A constant fluid velocity of 20 ft/s (6.1 m/s) was used, as specified in the source paper}
^{3 - With no oil protection, as specified in the source paper}