Electrochemical Modeling and Surface Characterization of 2-Mercaptobenzimidazole and Propargyl–Mercaptobenzimidazole as Corrosion Inhibitors for API 5L X70 Steel in CO2-Saturated NaCl Solution

2-mercaptobenzimidazole (2-MBI) and propargyl-functionalized MBI (propargyl-MBI) as corrosion inhibitors for API 5L X70 steel in CO2-saturated NaCl solution was evaluated using potentiodynamic polarization, electrochemical impedance spectroscopy (EIS), gravimetric weight loss, scanning electron microscopy coupled with energy-dispersive X-ray spectroscopy (SEM-EDS), and atomic force microscopy (AFM). Electrochemical tests showed that both inhibitors reduced corrosion, with propargyl-MBI exhibiting the best inhibition. propargyl-MBI achieving efficiencies of 96.1% by EIS and 97.4% by polarization at 10 ppm. After 96 h, corrosion rates decreased from 2.23 mm·year−1 (blank) to 0.40 and 0.14 mm·year−1 for 2-MBI and propargyl-MBI, respectively. SEM/EDS and AFM confirmed improved surface protection, with roughness decreasing from 142 nm (blank) to 76.8 nm (2-MBI) and 51.1 nm (propargyl-MBI). Adsorption followed the Langmuir model (R2 = 0.99), with ΔG°ads values of −25.1 and −34.25 kJ·mol−1 for 2-MBI and propargyl-MBI, respectively, indicating stronger adsorption after functionalization. Dispersion-corrected density functional theory calculations were used to elucidate the adsorption modes as well as the main interactions between the 2-MBI, in thiol and thione tautomers, and propargyl-MBI. The binding energy as well as the charge transfer were increased in the case of propargyl-MBI.