Thermodynamics and Kinetics of Carbon Dioxide Corrosion of Mild Steel at Elevated Temperatures

CO₂ corrosion of mild steel in the oil and gas industry has been widely investigated. Nevertheless, research on high temperature CO₂ corrosion has been rarely conducted, and its mechanisms remain unclear. Therefore, it is important to complete an in-depth study of CO₂ corrosion of mild steel at high temperature. The entire scope of this Ph.D. research is to investigate and model CO₂ corrosion of mild steel over a range of 25-250¿¿C. The research is divided into four main sections: chemical thermodynamics, electrochemical thermodynamics, electrochemical kinetics, and the proposal of mechanisms of CO₂ corrosion.

In the chemical thermodynamics segment, water chemistry components of CO₂ systems were studied. In the absence of Fe2+, pH increased with temperature in both predicted and experimental results. With addition of Fe2+, pH did not change with temperature due to FeCO₃ precipitation.

Pourbaix diagrams for the Fe-CO₂-H₂O systems were constructed using thermodynamic theory and data, subsequently validated with observed CO₂ corrosion phenomena. In the range of 80-150¿¿C, FeCO₃ and Fe₂(OH)₂CO₃ formed on the steel surface, for experiments lasting 4 days. At 200-250¿¿C, the corrosion product was exclusively Fe3O4. Kinetic studies conducted at 120¿¿C show full transformation from plate-like Fe₂(OH)₂CO₃ to oblong prismatic FeCO₃ crystals over time. In relation to pressure effects, FeCO₃ is the more favored corrosion product than Fe₃O₄ at high pCO₂. With surface pH consideration, the generated Pourbaix diagrams were validated by experimental results.

The corrosion kinetic experiments at elevated temperatures were further investigated including the effects of pH and flow. It was concluded that corrosion rates did not monotonously decrease with temperatures due to formation of corrosion products. Corrosion rates at pH 4.0 were higher than those at pH 6.0, independent of temperature. The main corrosion product was FeCO₃ with Fe₃O₄ present at temperatures above 150¿¿C. No flow sensitivity was observed due to the formation of corrosion products.

Mechanisms of CO₂ corrosion at temperatures of 25-250¿¿C were proposed based on the current CO₂ corrosion model with an addition of Fe₃O₄ formation. The thermodynamics and kinetics of Fe₃O₄ formation were identified. As soon as thermodynamic conditions for Fe₃O₄ are achieved, it forms and protects the steel.