Mild steel corrosion inhibition performance of novel diazepine derivatives: Electrochemical and morphological soundings paired with quantum chemical calculations

Abstract

The inhibitory effect of diazepine derivatives as corrosion inhibitors of mild steel (MS) in 1 M HCl, namely sulfate of 2,4-dimethyl-3H-benzo[b][1,4]diazepine-1-ium (TML1) and sulfate of 2,4, 8-trimethyl-3H-benzo[b][1,4]diazepine-1-ium (TML2) was studied by electrochemical impedance spectroscopy (EIS) and potentiodynamic polarization curves (PDP) at different concentrations (10⁻³ to 5.10⁻⁵ M). The optimal concentration of 10⁻³ M was investigated for its inhibitory efficacy at diverse temperatures (293–323 K). The analysis of Tafel plots and EIS diagrams indicated that the compounds exhibited a notable adsorption behavior, which suggests that they may serve as an effective inhibitor for the control of MS rust in acidic solutions. In accordance with the Langmuir isotherm, the adsorption of inhibitors onto the MS surface occurred. The impact of the methyl groups and the influence of cross conjugation on the electron-providing and electron-accepting capabilities of the nitrogen heteroatoms (N) and π-electrons in MS oxidization has been analyzed and discussed. This may be achieved through calculations based on DFT (Density Functional Theory) at the RB3LYP [6–311 G(D,P)] level. Scanning electron microscopy (SEM) and Raman spectroscopy demonstrated the formation of an inhibitive barrier on the MS surface, which was built on the adsorbed inhibiting molecules. Moreover, quantum-based computational chemistry has been employed to investigate the relationship between the corrosion-inhibiting efficacy and adsorption strength of the inhibitors, with a particular focus on the influence of their molecular structure.