Comprehensive Experimental and Theoretical Study on the Adsorption and Corrosion Inhibition Efficiency of Pyronin B for Mild Steel Protection in HCl Solution

Mild steel (MS) is one of the most widely used materials in industry. But, this metal corrodes easily, especially in acidic solutions. Therefore, protection of this metal is critical. The use of inhibitors is one of the most practical and economical ways of achieving this. The corrosion inhibition performance of Pyronin B (PyB) for the protection of mild steel (MS) was investigated in 1 M HCl solution using the change of open circuit potential with exposure time (Eocp-t), potentiodynamic polarization, electrochemical impedance spectroscopy (EIS) and linear polarization resistance (LPR) techniques. The surface of the MS was examined by scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDX), atomic force microscopy (AFM) and contact angle measurements. The electrochemical stability of the PyB film formed on the MS surface was investigated by cyclic voltammetry (CV). The excess surface charge of the metal in the inhibited solution was estimated with the help of EIS studies. The experimental data were supported by theoretical approaches such as density functional theory (DFT) calculations, molecular dynamics (MD) and Monte Carlo simulations. It was found that PyB forms a uniformly distributed, dense, and protective organic film on the steel surface. The PyB molecules interact with the MS surface through a combination of physical and chemical interactions, the latter being dominant. The PyB acts as a mixed-type inhibitor, with a more pronounced effect on the anodic mechanism. Its corrosion inhibition efficiency depends on its concentration, reaching 95.9% efficiency at 0.01 mM. The inhibitor acts as an inhibitor without altering the mechanism of the cathodic reaction and by altering the mechanism of the anodic reaction. The overall corrosion reaction is kinetically controlled. The surface film exhibits very high electrochemical stability under potentiodynamic conditions. Theoretical approaches supported the experimental results and indicated superior inhibitory capability of PyB. The chemical reactivity of the studied inhibitor system was explained in the light of Conceptual Density Functional Theory based calculations and electronic structure principles. The binding energy, which reflects the power of the interaction between Fe(110) surface and PyB, was found to be 3.43 eV. Binding energies calculated by both DFT and MD and MC simulations support that the adsorption of the inhibitor is chemical, as noted in the experimental section.