Effect of Substituents on Chitosan-Derived Sustainable Corrosion Inhibitors: Experimental and Computational Studies of Inhibition and Adsorption Performance

This work involves the synthesis of two chitosan derivatives by reacting chitosan, extracted from shrimp shells in eastern Morocco, with 2-nitrobenzaldehyde via a Schiff base reaction. An amino derivative of chitosan was then produced by reducing the imine group created by sodium borohydride. We investigated the molecular weight (<i>Mw</i>), crystallinity index (<i>CrI</i>), and degree of deacetylation (<i>DDA</i>) of the isolated chitosan, among other characteristic features. Thermogravimetric analysis (TGA), X-ray diffraction (XRD), and attenuated total reflectance Fourier transform infrared spectroscopy (FTIR-ATR) were used to characterize the extracted chitosan (<i>CS</i>), 2-nitroben-chitosan Schiff derivative (<i>CS-2NI</i>), and chitosan amino derivative (<i>CS-2NA</i>). In a corrosive medium of 1 M HCl, the three ligands, <i>CS</i>, <i>CS-2NI</i>, and <i>CS-2NA</i>, were used as mild steel corrosion inhibitors. Electrochemical studies were used to analyze the surface shape and assess the inhibitory efficiency. They reveal a significant inhibitory efficiency of 92.31% for the <i>CS-2NI</i> derivative, highlighting the effectiveness of the imine group (═N-) and the nitro group (-NO₂) compared to the two amino groups (-NH₂ and -NH-) present in the <i>CS-2NA</i> derivative. To support experimental research, a computational study was carried out that combined the simulated annealing technique. The three inhibitors behave as mixed-type corrosion inhibitors. Thermodynamic analyses and adsorption studies revealed that the tested inhibitors create covalent bonds with the steel surface (chemisorption) as well as physical interactions (physisorption), in accordance with the Langmuir model. The identification of the ligands' adsorption sites on the steel surface was made more accessible by computational methods, demonstrating the relationship between the inhibitory properties and the chemical structure of these biodegradable and biocompatible biopolymers.