Mechanistic insights into carbon steel corrosion inhibition by benzodiazepine derivatives in hydrochloric acid

The protection of carbon steel from aggressive acidic environments remains a critical challenge in industrial and defence-related applications. In this study, two newly synthesised benzodiazepine derivatives, namely 9-ethyl-2,3,4,9-tetrahydrobenzo[b]cyclopenta[e][1,4]diazepin-10(1H)-one (SR3) and 1-ethyl-4-phenyl-1H-benzo[b][1,4]diazepin-2(3H)-one (SR4), were investigated as corrosion inhibitors for carbon steel in 1 M HCl solution. Electrochemical measurements, including potentiodynamic polarisation and electrochemical impedance spectroscopy, were complemented by comprehensive surface and spectroscopic analyses (SEM, EDS, AFM, XRD, contact angle, and UV-Vis). Both compounds exhibited high inhibition efficiencies, reaching 85.1% for SR3 and 92.8% for SR4, with adsorption behaviour following the Langmuir isotherm model. Polarisation results indicated a mixed-type inhibition mechanism, while surface analyses confirmed the formation of a compact and adherent protective film on the steel surface. Density functional theory and molecular dynamics simulations provided molecular-level insights into the adsorption configurations and interaction strength of the inhibitors on the Fe(110) surface, in good agreement with the experimental observations. Overall, these findings demonstrate the effectiveness of benzodiazepine-based frameworks as promising molecular inhibitors for carbon steel corrosion in acidic media.