Corrosion Inhibition Performance of a Ternary Alkyl Phosphate Ester-Based Inhibitor in Simulated Geothermal CO2 Systems on AISI 1018 Steel at Elevated Temperatures

The corrosion inhibition performance of a ternary inhibitor based on alkyl phosphate esters was investigated in environments simulating geothermal CO2 systems at 60 °C and 80 °C using AISI 1018 steel as the test material. The inhibition efficiency was evaluated using mass loss method and electrochemical techniques, including linear polarization resistance (LPR), potentiodynamic polarization, and electrochemical impedance spectroscopy (EIS). Structural and surface characterization of the samples were performed using Fourier transform infrared spectroscopy (FTIR) and intermittent contact–alternating current scanning electrochemical microscopy (IC-AC-SECM). The results demonstrated a significant reduction in the corrosion rate in the presence of the inhibitor at both investigated temperatures, accompanied by an increase in polarization resistance and a decrease in corrosion current density. Electrochemical measurements indicated that the investigated inhibitor acts as a mixed-type corrosion inhibitor. FTIR analysis supported the presence of inhibitor-related species at the steel interface, while IC-AC-SECM measurements indicated a relatively homogeneous and electrochemically protected surface under the investigated conditions. The concentration–surface-coverage relationship was close to the Langmuir-type form at 60 °C, whereas greater deviations were observed at 80 °C, together with a lower apparent adsorption parameter, suggesting temperature-dependent changes in the interfacial layer. The results demonstrate the strong corrosion-protection performance of the investigated commercial ternary inhibitor formulation under the tested conditions.