Corrosion inhibition performance of 2-(2-hydroxybenzylideneamino)phenol (L(1)), 2-(5-chloro-2-hydroxybenzylideneamino)phenol (L(2)) and 2-(2-hydroxy-5-nitrobenzylideneamino)phenol (L(3)) on the corrosion behaviour of mild steel surface in a 1 M hydrochloric acid (HCl) solution is investigated by sophisticated analytical methods like potentiodynamic polarization, electrochemical impedance spectroscopy and weight loss measurements. Polarization studies showed that all the compounds are mixed type (cathodic and anodic) inhibitors and the inhibition efficiency (η%) increased with increasing inhibitor concentration. The inhibition actions of these Schiff base molecules are discussed in view of blocking the electrode surface by means of adsorption of the inhibitor molecule obeying the Langmuir adsorption isotherm. Scanning electron microscopy (SEM) studies of the metal surfaces confirmed the existence of an adsorbed film. Density functional theory (DFT) and molecular dynamics (MD) simulation have been used to determine the relationship between molecular configuration and their inhibition efficiencies. The order of inhibition performance obtained from experimental results is successfully verified by DFT and MD simulation.

In order to evaluate the effect of the functional group present in the ligand backbone towards corrosion inhibition performances, three Schiff-base molecules namely, (E)-4-((2-(2,4-dinitrophenyl)hydrazono)methyl)pyridine (L(1)), (E)-4-(2-(pyridin-4-ylmethylene)hydrazinyl)benzonitrile (L(2)) and (E)-4-((2-(2,4-dinitrophenyl)hydrazono)methyl)phenol (L(3)) were synthesized and used as corrosion inhibitors on mild steel in 1 M HCl medium. The corrosion inhibition effectiveness of the studied inhibitors was investigated by weight loss and several sophisticated analytical tools such as potentiodynamic polarization and electrochemical impedance spectroscopy measurements. Experimentally obtained results revealed that corrosion inhibition efficiencies followed the sequence: L(3) > L(1) > L(2). Electrochemical findings showed that inhibitors impart high resistance towards charge transfer across the metal-electrolyte interface and behaved as mixed type inhibitors. Scanning electron microscopy (SEM) was also employed to examine the protective film formed on the mild steel surface. The adsorption as well as inhibition ability of the inhibitor molecules on the mild steel surface was investigated by quantum chemical calculation and molecular dynamic (MD) simulation. In quantum chemical calculations, geometry optimized structures of the Schiff-base inhibitors, electron density distribution in HOMO and LUMO and Fukui indices of each atom were employed for their possible mode of interaction with the mild steel surfaces. MD simulations revealed that all the inhibitors molecules adsorbed in parallel orientation with respect to the Fe(110) surface.

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Corrosion inhibition property due to the heteroatom present in organic Schiff bases and their stereochemical conformation is assessed. Three Schiff bases, namely 2-pyridyl-N-(2′-methylamino/methylthio/methoxy phenyl)methyleneimine are employed.