Organic Pyridinium Salts as Corrosion Inhibitors for Mild Steel in Acidic Wastewater: Experimental and DFT Study

Heterocyclic organic compounds, namely 1,1′-methylenebis(pyridinium) bromide (Inhibitor I) and 1,1′-ethylenebis(pyridinium) bromide (Inhibitor II), were investigated as corrosion inhibitors for mild steel in acidic wastewater (0.5 M H2SO4). The inhibition performance was evaluated using gravimetric weight-loss measurements and electrochemical techniques. The results show that increasing inhibitor concentration significantly reduces the corrosion rate and enhances the inhibition efficiency, reaching maximum values of 90.42% for Inhibitor I and 87.85% for Inhibitor II at 7.5 × 10−3 M. This improvement is associated with a notable decrease in corrosion current density, indicating adsorption of inhibitor molecules at the steel/electrolyte interface. Adsorption studies reveal that both inhibitors follow the Langmuir adsorption isotherm, suggesting a mixed physisorption–chemisorption mechanism. Density functional theory (DFT) calculations and molecular dynamics simulations provide qualitative insight into the adsorption behavior, emphasizing the contribution of heteroatoms and π-electron systems to inhibitor–metal interactions. Overall, Inhibitor I exhibits superior inhibition performance, which can be attributed to its higher molecular reactivity, lower HOMO–LUMO energy gap, and higher dipole moment. The combined experimental and theoretical results demonstrate that the investigated compounds exhibit high corrosion inhibition efficiency under the studied conditions for mild steel in acidic wastewater environments.