Correlation between corrosion inhibition efficiency in sulfuric acid medium and the molecular structures of two newly eco-friendly pyrazole derivatives on iron oxide surface

The inhibition effect of newly synthesized eco-friendly pyrazole derivatives NPDM and NPD on the corrosion of the mild steel (MS) in 0.5 M H2SO4 solution was investigated by the open circuit potential (OCP) curves, potentiodynamic polarization (PP) curves and electrochemical impedance spectroscopy (EIS) along with weight loss (WL) measurements. Both compounds were confirmed to be good anti-corrosion inhibitors and their efficiency was in the order NPD > NPDM for a concentration of 10−3 M. The adsorption of the two compounds on MS surface was studied by SEM and various isotherm models. The adsorption of NPD appeared to obey the Langmuir model while that of NPDM followed Freundlich model. Corrosion inhibition was explained based on experimental results as well as computational studies. The influence of the molecular-electronic-structure on the inhibition activity was inspected by the DFT method. The global and local reactivity, before their adsorption onto the MS surface, were calculated at the B3LYP/6-311G(d,p) theoretical level for the neutral and cationic forms. The solvation hydration energy (∆GSolv) was also considered to evaluate the protection efficiency of both products. Molecular Dynamics Simulations (MDs) were carried out to model the interaction mechanism and the adsorption behavior of {10−3 M NPD or NPDM/α-Fe2O3(111)/0.5 M H2SO4} interfaces. Results derived from experimental (OCP, WL, PP and EIS) methods were supported by the theoretical investigations.