The synergism effect of urea and potassium iodide (KI) has been studied in two synthesized inhibitors, Inhibitor A (imidazole-free) and Inhibitor B (imidazole-containing) on the corrosion inhibition of A516 steel. The experiment was carried out in CO2�saturated 3 wt NaCl solutions through an electrochemical test and surface analysis. The electrochemical measurement showed the corrosion inhibition efficiency (IE) for Inhibitors A gives 69�90, while 65�88 for Inhibitors B. The surface morphology of steel after 7 days immersion in both synthesized inhibitors shows a reduction in corrosion attack as viewed through a Scanning Electron Microscope (SEM) and Energy-Dispersive X-ray (EDX). Analysis through X-ray Photoelectron Spectrometer (XPS) revealed the coordination of nitrogen elements on the steel surface indicating the interaction of significant functional groups. Both inhibitors performed competitive adsorption which characterized an antagonistic reaction. The inhibition mechanism was influenced by the molecular structure of inhibitors. The heterocyclic ring of imidazole with the possible anchoring site favors the ionic interaction. Functional groups of unshared electron pairs on nitrogen atoms have contributed to the coordination complexes roles as a ligand. Besides, the involvement of co-counter ions also influenced corrosion inhibition. The occurrences in bulk-interface-surface prior to the adsorption were discussed. © 2019 Elsevier B.V.