In-depth insight into the inhibition mechanism of the modified and combined amino acids corrosion inhibitors: “intramolecular synergism” vs. “intermolecular synergism”

In this work, amino acid (L-histidine (LHD)) and 1-phenylthiourea (PT) were mixed as the combined corrosion inhibitors (LHD + PT). Meanwhile, LHD was also modified with phenyl isothiocyanate to develop a novel corrosion inhibitor (phenylcarbamothioyl)histidine (PT-HD), which has a combined structure of LHD and PT. Then LHD + PT is the “intermolecular synergism”, while PT-HD could be regarded as the “intramolecular synergism” of LHD and PT. The inhibition performances of LHD + PT and PT-HD for the corrosion of carbon steel in CO2-saturated formation water were investigated by electrochemical tests and surface analyses. Additionally, the intrinsic difference in the inhibition performance and mechanism of LHD + PT and PT-HD were in-depth revealed by theoretical calculations. The experimental results indicate that PT-HD exhibits better inhibition performance than LHD + PT (inhibition efficiency of 99.2% for 0.4 mM LHD + PT and 99.3% for 0.4 mM PT-HD), i.e., the “intramolecular synergistic inhibition effect” is stronger than the “intermolecular synergistic inhibition effect”. The theoretical calculations also indicate that PT-HD has stronger adsorption ability and more effective prevention to the invasion of aggressive species. The higher inhibition efficiency of PT-HD is associated with the fact that the LHD and PT fragments in PT-HD molecule are closely linked by chemical bond, forming a more compact adsorbed inhibitor film on steel surface.