Construction and Verification of Pourbaix Diagrams for CO_2 Corrosion of Mild Steel Valid up to 250°C

摘要

The Pourbaix diagrams for an Fe-CO_2-H_2O system at elevated temperatures were constructed using thermodynamic theory and data and then correlated with observed CO_2 corrosion phenomena of mild steel up to 250°C. In the range 80-150°C corrosion product layer formed and the surface analysis showed a mixture of crystal morphologies on the steel surfaces, for experiments lasting 4 days. Kinetic studies conducted at 120°C show full transformation from plate-like to oblong prismatic crystals over 30 days. No obvious crystal morphology was identified on the surfaces of samples studied at 200°C and 250°C. XRD analysis indicates that FeCO_3 and Fe_2(OH)_2CO_3 formed on the steel surface at 80-150°C. At 200-250°C, the corrosion product was exclusively Fe_3O_4. With surface pH consideration, the generated Pourbaix diagrams were validated by the experimental results. Maximum weight loss corrosion rates were observed at 120°C.