Evaluating corrosion inhibition of spiro-pyrazoline-butyrolactones on carbon steel in HCl: Experimental, computational, and COSMO-RS approaches

Abstract

In this study, we report a new class of spiranic junction heterocycles; 1,3,4-triphenyl-7-oxa-1,2-diazaspiro[4.4]non-2-en-6-one (3a), 1,3-diphenyl-4-(p-tolyl)-7-oxa-1,2-diazaspiro[4.4]non-2-en-6-one (3b), and 4-(4-methoxyphenyl)-1,3-diphenyl-7-oxa-1,2-diazaspiro[4.4]non-2-en-6-one (3c); as effective corrosion inhibitors for carbon steel (CS) in 1.0 M hydrochloric acid. These compounds were synthesized in high yields (83 %–88 %) by ultrasonic-assisted cyclization of diphenylnitrilimine dipole with α-arylidene-γ-butyrolactone and were fully characterized using IR, 1H NMR, and 13C NMR techniques. Weight loss (WL) measurements, electrochemical impedance spectroscopy (EIS), and potentiodynamic polarization (PDP) tests all indicated significant corrosion inhibition efficiencies of 84.17 %, 86.17 %, and 88.12 % at 10−3 M for 3a, 3b, and 3c, respectively. PDP studies confirmed their role as mixed-type inhibitors, while activation energy evaluations suggested both chemisorption and physisorption mechanisms. Scanning electron microscopy (SEM) imaging further demonstrated that these inhibitors effectively protect the CS surface from acid-induced damage. To complement the experimental findings, conductor-like screening model for realistic solvation (COSMO-RS) and density functional theory (DFT) calculations revealed insights into compounds’ solvation properties and a direct correlation between minor structural modifications in the spiranic heterocycles and enhanced adsorption on the metal surface, respectively. The synergy between experimental and theoretical approaches underscored the potential of these newly synthesized heterocyclic structures as efficient corrosion inhibitors, providing valuable guidance for designing effective corrosion inhibitors. © 2025 Elsevier Ltd