Investigating the synergistic inhibition of amine-based corrosion inhibitor and monoethylene glycol on top-of-the-line corrosion of API 5L X65

Top-of-line corrosion (TLC) is a critical integrity threat for wet natural gas pipelines, where condensed acidic electrolytes containing CO 2 and H 2 S attack the upper internal surface. Although amine-based volatile corrosion inhibitors and Monoethylene Glycol (MEG) are used for corrosion control and hydrate prevention, respectively, their interfacial interaction under TLC conditions remains poorly understood. This study addresses this knowledge by evaluating the synergistic inhibition performance of a commercial amine-based inhibitor combined with MEG on API 5L X65 steel under varying conditions, including inhibitor only (0-3000ppm), MEG only (40-90wt%), and their synergistic combination. Corrosion inhibition performance was evaluated using weight-loss and electrochemical techniques, while surface films were characterized by XRD, SEM/EDS, and FTIR to elucidate the protective mechanism. A pronounced synergistic effect was observed at the optimal concentration of 1000ppm of the amine-based inhibitor combined with 40 wt% MEG, which markedly reduced the TLC rate, as determined by Tafel polarization, to 0.052 mm/year (η tafel =94.97%), and increased the polarization resistance (R t ), obtained from EIS measurements, to 5794.3Ω.cm 2 , thereby significantly outperforming the inhibitor alone (η tafel =86.67%) and MEG alone (η tafel =56.52%) at the same concentrations. Mechanistically, surface characterization confirmed that MEG acts as a co-structuring component, establishing a hydrogen-bonding network that densifies the organic film and promotes the formation of a compact, adherent FeCO 3 -rich scale. Furthermore, the synergistic interaction between the corrosion inhibitor and MEG was confirmed by both the Absolute Synergy Model (S abs =2.035) and the Adjusted "1+1>2" Synergy Model (S adj.1+1>2 ​=1.618), demonstrating the effectiveness of the combined system for TLC mitigation in sour gas pipelines.