An investigation on the inhibitory action of benzazole derivatives as a consequence of sulfur atom induction

•The inhibitory behavior of three benzazole based molecules was studied.•Weight loss and EIS technique revealed a marked difference between three molecules’ potential inhibitory actions, although having the same adsorption isotherm.•AFM and contact angle measurements showed a decrease in roughness and hydrophillicity as a result of introducing sulfur which was related to better adsorption of the molecule and formation of more integrated inhibitor film.•The most constant results was obtained by HF method with 6-31G (d,p) basis set in water medium. The inhibitory action of three benzazole based molecules namely 2-methyl benzimidazole (2-MBI), 2-methyl benzothiazole (2-MBT) and 2-mercapto benzthiazole (2-SHBT) in 1M HCl solution was studied by gravimetric analysis and electrochemical impedance spectroscopy (EIS). Results showed that the inhibitor adsorption on the iron surface was according to Langmuir adsorption isotherm for 2-MBI and 2-MBT and Flory Huggins Isotherm for 2-SHBT. Surface roughness obtained by Atomic Forced Microscopy (AFM) revealed that a good inhibitor decreases the surface roughness significantly which can be related to the formation of more integrated molecular film of inhibitor on steel surface. Based on contact angle (CA) measurements as the efficiency of the inhibitor molecules improve the hydrophobicity increases. These three molecules were chosen to see the effect of introducing sulfur atom into the structure the main effect of which would be on electronic parameters. To better understand this effect, the quantum chemical descriptors including: EHOMO, ELUMO, energy gap (ΔE), dipole moment (μ), hardness (η), softness (σ), electronegativity index (χ), fraction of electrons transferred (ΔN), that are most relevant to the potential action of a molecule as corrosion inhibitor, have been calculated in water and vacuum. Electronic parameters of these three inhibitors have been studied using DFT/B3LYP, and HF methods with 6-31G (d,p) basis set.