Molecular Design of Sustainable Α-Aminophosphonate Inhibitors for Al Alloy Protection in Saline Environments: Experimental Validation and Theoretical Modeling

The corrosion protection of Al (AA2024-T3) and its alloys in chloride-containing environments remains a major challenge due to the aggressive action of chloride ions, which can induce localized degradation and compromise the long-term durability of metallic structures. In the present study, the corrosion-inhibitory performance of two organic inhibitors, diethyl(1H-indazol-7-yl)amino(4-methoxyphenyl)methylphosphonate (DIMMP) and diethyl (3a,7a-dihydro-1H-indazol-7-yl)amino(furan-2-yl)methylphosphonate (DIFMP), was systematically investigated to protect AA2024-T3 in a 3.5 wt% NaCl medium. The inhibitory efficiency and adsorption behavior of the selected compounds were evaluated through a combination of electrochemical techniques, surface characterization analyses, and theoretical calculations to establish a comprehensive understanding of the corrosion protection mechanism. The experimental results demonstrated that both inhibitors significantly reduced the corrosion rate of AA2024-T3 by forming a compact and adherent protective layer on the metal surface, thereby limiting charge transfer processes and hindering the penetration of aggressive chloride ions. Potentiodynamic polarization (PDP) and electrochemical impedance spectroscopy (EIS) analyses revealed a marked increase in polarization resistance (Rp) and a substantial decrease in corrosion current density (icorr) in the presence of the inhibitors, indicating enhanced corrosion resistance and remarkable inhibition efficiencies of 95.8% and 95.5% at 10(-3) M for DIMMP and DIFMP, respectively. Surface examinations further confirmed the formation of a homogeneous protective film and the mitigation of corrosion-induced surface deterioration. Adsorption studies suggested that the inhibition process predominantly occurs through the spontaneous adsorption of inhibitor molecules onto the Al surface, leading to the establishment of an effective barrier against corrosive species. Density Functional Theory (DFT) calculations provided molecular-level insights into the adsorption mechanism, highlighting the crucial role of electron-rich heteroatoms and it-electron systems in strengthening the interaction between inhibitor molecules and the Al substrate. Theoretical findings exhibited excellent agreement with experimental observations, confirming the superior adsorption affinity and protective capability of the investigated inhibitors. The synergistic integration of experimental and computational approaches demonstrates that the studied inhibitors constitute promising and environmentally benign candidates for enhancing the corrosion resistance of Al in saline environments. These findings contribute to the rational design of high-performance corrosion inhibitors and provide valuable insights into the development of sustainable strategies for the protection of Al-based materials in aggressive chloride media.