Corrosion inhibition of E24 carbon steel in 1.0 M HCl by 2,5-Di-tert-butylphenol derivatives: Experimental, DFT, COSMO-RS and Monte Carlo Study

Corrosion of carbon steel E24 in acidic media remains a critical industrial issue, particularly in chloride-containing environments. In this study, two newly synthesized 2,5-di-tert-butylphenol derivatives, namely 2,5-di-tert-butylphenyl-2-(4-((hydroxyimino)methyl)-2-methoxyphenoxy)acetate (OX) and bis(2,5-di-tert-butylphenyl)-2,2′-(((1,4,2,5-dioxadiazine-3,6-diyl)bis(2-methoxy-4,1-phenylene))bis(oxy)diacetate (DOX), were evaluated for the first time as corrosion inhibitors for carbon steel E24 in 1.0 M HCl solution. The compounds were characterized using ¹H NMR, ¹ ³C NMR, and FT-IR spectroscopy. Their inhibitory performance was investigated by open circuit potential (OCP), potentiodynamic polarization (PDP), and electrochemical impedance spectroscopy (EIS), complemented by adsorption isotherm analysis. The results indicated that adsorption follows the Langmuir isotherm, with standard Gibbs free energy values (ΔG°ads) of −39.123 kJ·mol −1 for OX and −38.658 kJ·mol −1 for DOX, suggesting a spontaneous adsorption process involving a mixed physisorption _ chemisorption mechanism with temperature _ dependent polarization effects. Immersion-time-dependent EIS measurements and scanning electron microscopy coupled with energy-dispersive X-ray spectroscopy (SEM–EDX) confirmed the formation of a protective adsorbed film on the steel surface. Quantum chemical calculations (DFT), electrostatic potential (ESP), Mulliken charge distribution, Fukui functions, COSMO-RS, and Monte Carlo simulations further supported the experimental findings. Electrochemical results showed high inhibition efficiencies of 83.14% for OX and 86.12% for DOX at 10 −3 M concentration. Both compounds acted as mixed-type inhibitors, significantly reducing corrosion by forming a protective surface layer. EIS results confirmed this behavior through a marked increase in charge transfer resistance and a decrease in double-layer capacitance from 441.11 µF·cm −2 (blank) to 146.31 µF·cm −2 and 144.12 µF·cm −2 for OX and DOX, respectively. SEM-EDX analysis revealed smoother and more protected surfaces with reduced chlorine and oxygen signals after inhibitor addition. Theoretical studies showed that DOX exhibits a smaller HOMO-LUMO energy gap, higher softness, greater electron delocalization, stronger nonpolar COSMO-RS surface contribution, and stronger adsorption on Fe (110), particularly in aqueous acidic media. Overall, the superior performance of DOX is attributed to its larger molecular structure, extended conjugation, multiple O/N adsorption centers, and enhanced multi-site adsorption capability.