1,4-Phenylenediamine-based Schiff bases as eco-friendly and efficient corrosion inhibitors for mild steel in HCl medium: Experimental and theoretical approaches

The application of Schiff bases as corrosion inhibitors is one of the economic, practical, and effective techniques to retard metal corrosion in aggressive environment. The continuous exploration of novel, lower-cost, more efficient, and eco-friendly corrosion inhibitors is an everlasting pursuit of excellence. In this study, two 1,4-phenylenediamine-based single Schiff base isomers, (E)-4-((pyridin-2-yl-methylene)amino)aniline (PMAA2) and (E)-4-((pyridin-3-ylmethylene)amino)aniline (PMAA3), were synthesized by one-step reaction. The corrosion inhibition mechanism and structure–activity relationship of PMAA2 and PMAA3 for mild steel in HCl solution were analyzed using electrochemical and weight loss experiments, morphology analysis, density functional theory (DFT) calculation, and molecular dynamic (MD) simulation. The results demonstrated that the corrosion inhibition efficiency of PMAA2 and PMAA3 exhibited an increasing trend with the elevation of inhibitor concentration while showing a slight decrease with the temperature rise. The optimal corrosion inhibition efficiency for 800 mg L−1 PMAA2 and PMAA3 in 1.0 mol L−1 HCl solution is 90.88 % and 91.78 % at 303 K respectively by using weight loss experiment. The electrochemical experiment results indicated that PMAA2 and PMAA3 are mixed-type corrosion inhibitors, with a preponderant control of anodic dissolution. Both the thermodynamics of adsorption and the kinetic of corrosion reaction were evaluated and discussed in detail. Their adsorption upon mild steel complies with Langmuir adsorption isotherm and is involved in phys- and chemisorption concurrently. The results from surface morphology analysis exhibited that a dense protective film on the surface of mild steel is formed through inhibitor adsorption, which results in changes in the hydrophilicity and hydrophobicity of the metal surface. The calculations of DFT and Fukui index also indicated that both the neutral and pronated species of PMAA2 and PMAA3 possess donor–acceptor interactions with the surface iron atoms through both forward and back-donations. Furthermore, MD simulations further revealed that the neutral molecules as neat-flat and parallel orientation are adsorbed strongly on the Fe (1 1 0) surface but the pronated species perform in bent fashion on the Fe (1 1 0) surface. Based on the experimental analysis and the theoretical calculation, a possible adsorption and corrosion inhibition mechanism was proposed rationally. These findings display good correlations between the experimental investigation and the theoretical computation and are of fundamental importance to understanding the adsorption and corrosion inhibition mechanisms of the inhibitors.