Synthesis, characterization, and evaluation of novel coumarin azoimine as corrosion inhibitors for mild steel in acidic environment: Integrated experimental and DFT/DFTB computational study

Abstract

The development of corrosion inhibitors with tailored molecular architectures requires combining experimental performance testing with molecular-level insights. In this study, we report the first synthesis and evaluation of a coumarin–azoimine hybrid (Cum-Azo) as a novel inhibitor for mild steel corrosion in 1 mol/L hydrochloric acid. Unlike previously reported inhibitors, Cum-Azo integrates coumarin and azoimine functionalities, offering multiple heteroatoms and conjugated systems for enhanced adsorption.The protective behavior of Cum-Azo was systematically characterized using weight-loss measurements, electrochemical methods, including potentiodynamic polarization and electrochemical impedance spectroscopy, supported by detailed surface analyses (AFM, SEM, contact angle, and UV–Vis spectroscopy). Experimental results revealed that the anticorrosion capability of Cum-Azo strongly depended on concentration, achieving a peak inhibition efficiency of 84.2 % at an optimal concentration of 2.5 × 10⁻⁴ mol/L, though this protective efficiency declined at higher temperatures and extended immersion durations. Cum-Azo exhibited mixed-type inhibition behavior, with adsorption onto the mild steel surface closely conforming to the Langmuir isotherm, reflecting a hybrid mechanism of physisorption and chemisorption interactions. Advanced quantum chemical analyses (DFT, MD, and SCC-DFTB) revealed distinct electronic structures for neutral and protonated Cum-Azo, and importantly identified the formation of strong Fe–C covalent bonds (adsorption energies −2.147 to −2.139 eV). Projected density of states analysis confirmed pronounced orbital hybridization, providing direct molecular-level evidence of robust chemisorption. This unprecedented integration of experimental and computational approaches establishes Cum-Azo as a promising next-generation inhibitor and provides new mechanistic insights to guide the rational design of molecular corrosion inhibitors.