Synthesis, Characterization and Corrosion Inhibition on 4‐Ethyl − 3‐Thiosemicarbazide Meldrum's Acid in HCl: Weight Loss, Surface Analysis, Electrochemical and DFT–Fukui Studies

In the present study, the synthesis, characterization, and corrosion inhibition properties of 4‐ethyl − 3‐thiosemicarbazide Meldrum's acid (4E3TSCMA) for mild steel in 1 M HCl solution have been investigated. The inhibitor was synthesized successfully, and its structure was confirmed by elemental analysis, ATR‐FTIR and NMR spectroscopy. The corrosion inhibition properties of 4E3TSCMA were investigated by weight loss, electrochemical impedance spectroscopy, potentiodynamic polarization, scanning electron microscopy with energy‐dispersive X‐ray spectroscopy, atomic force microscopy (AFM), and X‐ray photoelectron spectroscopy. Weight loss measurements showed that the inhibition efficiency increased with the increase in the concentration of the inhibitor and reached the highest value at 0.005 mM, which indicates that the inhibitor was adsorbed on the surface of the mild steel in a monolayer. Adsorption studies showed that the inhibitor followed the Langmuir adsorption isotherm with a high value of the correlation coefficient ( R 2 = 0.9979). The calculated Gibbs free energy of adsorption (ΔG° ads = −24.33 kJ mol −1 ) indicated that the adsorption process was a mixed adsorption process. The activation energy ( E a ) calculated was 33.31 kJ mol −1 , the activation enthalpy (ΔH ‡ ) was 30.87 kJ mol −1 , and the activation entropy (ΔS 0 ) was −210.61 J mol −1 K −1 . The positive ΔH ‡ is evidence that the corrosion process is endothermic, and the negative ΔS ‡ is evidence that the activated complex is more ordered. The electrochemical studies confirmed that 4E3TSCMA is a mixed‐type corrosion inhibitor. The SEM and AFM images showed that the surface damage and roughness of mild steel were significantly reduced in the presence of the inhibitor. The overall results showed that 4E3TSCMA has high corrosion inhibition ability in the acidic medium, based on the adsorption‐controlled mechanism, which is characterized by a compact, stable, and protective adsorbed layer on the mild steel surface. Based on the results of the Fukui function analysis, N10 and S9 were identified as the main electron‐donating reactive sites, and S9 and C12 were found to have significant dual reactivity, which suggests their potential role in inhibitor–mild steel surface interactions.