Synergistic theoretical and electrochemical evaluation of sulfonamide-based inhibitors for mild steel corrosion in HCl.

The corrosion inhibition performance of three sulfonamide-based compounds, namely 4-methyl-N-(pyridin-2-yl)benzenesulfonamide (SAP4), N,N'-benzene-1,4-diylbis(4-methylbenzenesulfonamide) (SAP5), and N,N'-(cyclohexane-1,2-diyl)bis(4-methylbenzenesulfonamide) (SAP6), was investigated for mild steel in 1 M HCl using combined theoretical and experimental approaches. Density functional theory (DFT) calculations provided insight into the electronic properties and reactive sites of the molecules, while Monte Carlo simulations suggested favorable adsorption on the Fe (110) surface. Experimental results from potentiodynamic polarization (PDP) and electrochemical impedance spectroscopy (EIS) demonstrated high inhibition efficiencies, reaching approximately 94%, 95%, and 89% for SAP4, SAP5, and SAP6, respectively, at 1 × 10-3 M. The polarization results indicate that the inhibitors affect both anodic and cathodic reactions, suggesting a predominantly mixed-type inhibition behavior. Adsorption studies revealed that the inhibitors follow the Langmuir isotherm model, with high correlation coefficients and negative values of (-36.66 to -35.32 kJ mol-1), indicating spontaneous adsorption involving both physical and chemical interactions. The effect of temperature was evaluated over the range of 298-328 K, where the inhibitors maintained relatively high inhibition efficiencies, although a slight decrease was observed with increasing temperature. Surface analysis further confirmed the formation of a protective adsorbed film on the mild steel surface. Overall, the combined findings highlight the potential of sulfonamide-based compounds, particularly SAP5, as effective corrosion inhibitors for mild steel in acidic environments.