Multi-Modal Adsorption and Synergistic Corrosion Inhibition of a Collagen–BMIM·Br Composite on Mild Steel

The widespread corrosion of mild steel in acidic environments presents a persistent and economically significant challenge across multiple industries. Compounding this issue, many conventional corrosion inhibitors carry substantial environmental and toxicological hazards, underscoring the critical need for developing high-performance, environmentally benign alternatives. This study investigates a corrosion inhibitor composite comprising collagen and 1-butyl-3-methylimidazolium bromide (BMIM·Br) for mild steel in 1.5 M HCl. Gravimetric analysis demonstrated exceptional inhibition efficiency (>95%) across a temperature range of 30–60 °C, with remarkable thermal stability evidenced by less than a 1% decrease at elevated temperatures. The adsorption process was found to be spontaneous and followed the Langmuir isotherm, with thermodynamic parameters (ΔG°ads ≈ −17 to −19 kJ/mol) indicating a mixed physisorption–chemisorption mechanism. FTIR and XRD analyses confirmed molecular-level interactions and the formation of a more amorphous composite structure. A multi-modal adsorption mechanism, supported by Density Functional Theory (DFT) insights, is proposed, elucidating the synergy through ionic bridging and the formation of a co-accreted polymeric film. These findings establish the collagen–BMIM·Br composite as a highly effective, stable, and sustainable corrosion inhibitor for demanding industrial applications in aggressive acidic environments.