Electrochemical Behavior of Carbon Steel in Carbon Dioxide-Saturated Diglycolamine Solutions

The electrochemical behavior of carbon steel in diglycolamine (DGA) solutions saturated with carbon dioxide (CO₂) under 4.5 MPa pressure was investigated using potentiodynamic polarization and impedance measurements at room temperature (RT) and at 100°C. Electrode equivalent circuits were proposed for interpreting the impedance measured at the higher temperature. Anodic and cathodic reaction mechanisms depended upon DGA concentration, temperature, and potential. At RT, the rate-determining step of the cathodic reduction changed from a diffusion process to an interfacial reaction with increasing DGA concentration. Corrosion rates were relatively low, and varied only slightly with DGA concentration. At 100°C, a tight corrosion product film was formed on the carbon steel surface in 0.1 M and 1.0 M DGA solutions. Anodic dissolution was inhibited strongly by slow ion diffusion in the film, and corrosion rates were much lower than those at RT. The tight corrosion product film did not form in the 5.0 M DGA solution, and the corrosion rate was very high. There were differences in the anodic dissolution mechanisms between the low- and high-concentration DGA solutions. At the high DGA concentration and at 100°C, a surface adsorption and an interfacial intermediate electro-chemical reaction played important roles in cathodic reduction and anodic dissolution, respectively.