Adsorption Mechanism of Eco-Friendly Corrosion Inhibitors for Exceptional Corrosion Protection of Carbon Steel: Electrochemical and First-Principles DFT Evaluations

In the present work, we represent two thiazolidinediones, namely (Z)-5-(4-methoxybenzylidene) thiazolidine-2,4-dione (MeOTZD) and (Z)-5-(4-methylbenzylidene) thiazolidine-2,4-dione (MeTZD), as corrosion inhibitors for carbon steel (CS) in 1.0 M HCl solution. Techniques for gravimetric methods, electrochemical measurements, and morphological characterization were used to conduct experimental evaluations. Additionally, calculations based on the fundamental principles of Density Functional Theory (DFT) were employed to simulate inhibitor–iron interactions. Experimental results indicated that investigated inhibitors can significantly enhance the corrosion resistance of CS, reaching a performance of 95% and 87% at 5 × 10⁻³ mol/L of MeOTZ and MeTZD, respectively. According to gravimetric and electrochemical experiments, inhibitor molecules obstruct corrosion reactions by adhering to the CS surface, which follows the Langmuir isotherm model. On the other hand, the morphological analysis showed a well-distinguished difference between unprotected and protected CS surfaces as a result of the inhibitors’ addition to HCl. Projected density of states and interaction energies obtained from first-principles DFT simulations indicate that the studied molecules form covalent bonds with iron atoms through charge transfer.