Mannich-bases of chitosan as a structurally tunable polysaccharide for interfacial protection in acidic media: Experimental, theoretical, and surface investigations.

This work examines the potential of four sustainably engineered Mannich-bases of chitosan (MbCs) namely 2-[(piperidin-1-yl)methyl]chitosan (PMC), 2-[(4-methoxypiperidin-1-yl)methyl]chitosan (MPMC), 2-[(4-hydroxypiperidin-1-yl)methyl]chitosan (HPMC), and 2-[(4-carboxypiperidin-1-yl)methyl]chitosan (CPMC) towards mild steel (MS) corrosion in 0.5 M H2SO4 ambient. The FTIR, 1H NMR and UV-Vis spectroscopy confirmed the successful synthesis of MbCs. Potentiodynamic polarization (PDP) and Electrochemical impedance spectroscopy (EIS) findings facilitated the validation of their protective behaviour, remarking MPMC as the most potent inhibitor with 95.5% inhibition efficiency (E%). The PDP findings identified the inhibitors to be mixed-type. The order of inhibition efficiency attained was MPMC (95.5%) > HPMC (93.4%) > PMC (91.4%) > CPMC (90.3%). The marked inhibitory impact can be attributed to the heteroatoms incorporated into modified chitosan, which enhance electron density, hydrophobicity, and film compactness, thereby promoting the devisement of an inhibitory layer over the MS surface. The associated adsorption mechanism was found to be interpreted most proficiently via the Langmuir adsorption isotherm. Surface analyses via SEM-EDS and AFM remarked a substantial reduction in surface degradation and roughness in the inhibitor ambient. Further, theoretical findings including molecular orbital analysis, electrostatic potential mapping, Fukui function analysis, and density functional tight-binding (DFTB) calculations were employed to examine the relevance amidst E% and the molecular structure of an inhibitor. All the findings exhibit a strong concordance validating the protective abilities.