Introduction of newly synthesized Schiff base molecules as efficient corrosion inhibitors for mild steel in 1 M HCl medium: an experimental, density functional theory and molecular dynamics simulation study

The purposeful incorporation of aliphatic, branched chain and substituted aromatic moieties in the molecular skeleton of organic Schiff bases, in line with corrosion inhibition performance, has been conducted. Three Schiff bases, namely, 2-((2-hydroxyethylimino)methyl)-6-methoxyphenol (L-1), 2-((1-hydroxybutan-2-ylimino)methyl)-6-methoxyphenol (L-2) and 2-(2-hydroxy-3-methoxybenzylideneamino)phenol (L-3) were synthesized and subsequently assessed for the inhibition of mild steel corrosion in 1 M HCl medium. The corrosion inhibition proficiencies of the synthesized inhibitors on the mild steel surface have been investigated by gravimetric measurements, electrochemical analysis (potentiodynamic polarization, electrochemical impedance spectroscopy), surface morphological studies (FESEM and AFM), contact angle measurement and, most importantly, theoretical studies. The results obtained from the gravimetric as well as electrochemical measurements revealed that the studied Schiff bases are mixed-type inhibitors that show maximum inhibition efficiency up to approximate to 97% at the concentration of 5 mM. In theoretical studies, atomic-level calculations give deeper insights into the corrosion inhibition mechanism and the relative performance of present inhibitors. However, it has been found that normal density functional theory (DFT) calculations are not sufficient to deal with the interactions between inhibitors and metal surfaces in complex systems. As such, in order to investigate inhibitor-metal interactions, herein, the density functional tight binding (DFTB) approach has been introduced as formulated by Hohenberg, Kohn and Sham (KS-DFT). Furthermore, for more insightful studies, e.g., growth characteristics, as well as the selection of a more stable surface of the -Fe crystal morphology studies, have also been performed using the morphology module. The morphology of the -Fe crystal in equilibrium was analyzed using the Wulff construction plot. In DFTB calculations, the trans3d Slater-Koster library set has been implemented for possible pairs of interactions between inhibitor-metal complex systems (C, N, O, H and Fe). Some salient features like equilibrium adsorption configuration and charge density difference obtained from DFTB calculations have been described in detail. Furthermore, for comparison with the real world of corrosion inhibition, molecular dynamics (MD) simulation was employed in the presence of all concerned species (inhibitor molecule, Fe surface, H2O, H3O+ and Cl-), which revealed the actual adsorption configurations of the inhibitor molecule on the desired metallic surfaces.