Environmentally Sustainable Approach of Corrosion Inhibition of Mild Steel in 1 N HCl and 1 N H₂SO₄ via Antihistamine Loratadine (LT) and Its Amine Derivatives: Computational and Experimental Analysis

The efficacy of pharmaceuticals in mitigating corrosion on metallic substrates has led to the development of a new class of inhibitors that are economically efficient and offer significant environmental benefits. In the present study, for the anticorrosion action of loratadine (LT), 4-(8-chloro-5,6-dihydro-11H-benzo [5,6] cyclohepta[1,2-<i>b</i>] pyridin-11-ylidene)-1-piperidinecarboxylic acid ethyl ester and its amine derivatives (LT1, LT2, and LT3), a theoretical study using DFT was conducted to elucidate the molecular interactions and complex formation mechanisms between these inhibitors and mild steel. The Δ<i>E</i> values for the studied inhibitors-2.17 eV (LT), 3.904 eV (LT1), 3.906 eV (LT2), and 3.85 eV (LT3)-indicate that the LT inhibitor shows greater reactivity compared to the other LT amine derivatives, particularly in terms of electron donation to the metal substrate. After that, the corrosion inhibitory efficacy of parent molecule LT was examined on steel substrate in a medium 1 N hydrochloric and 1 N sulfuric acid and was found to have an efficiency of 98.52 and 80.58%, respectively, (308 K for 100 ppm concentration) deliberated through the electrochemical techniques (PDP and EIS) and gravimetric technique. The reduction in Cdl values from 684.06 to 43.15 μF/cm² in 1 N HCl and from 693.41 to 83.91 μF/cm² in 1 N H₂SO₄ indicates an adsorption process in which inhibitor molecules displace water adsorbed on the metallic substrate, creating a barrier that prevents the metallic substrate from corrosive damage. The surface adsorption aligned with the Langmuir adsorption isotherm with Gibbs-free energy -51 kJ/mol in 1 N HCl and -49.23 kJ/mol in 1 N H₂SO₄. The AFM analysis with an average roughness of 16.29 nm in HCl and 49.23 nm in H₂SO₄ validated LT to form a barrier on mild steel, opposing corrosion. The decelerative effect of LT inferred from theoretical and experimental data comply, making them credible corrosion inhibitors.