Electrochemical and Theoretical Evaluation of Carbothioamide-Derived Corrosion Inhibitors for Mild Steel in Acidic Environments.

Organic inhibitors that couple strong heteroatom donor centers with tunable aromatic substituents are valuable platforms for clarifying structure-adsorption relationships in steel corrosion under acidic condition. Here, three 1H-1,2,3-triazol-4-yl carbothioamide derivatives bearing para-methoxy (MTCT), hydrogen (PTCT) or para-chloro (CTCT) substituents were synthesized and evaluated as inhibitors for mild steel in 1.0 mol L-1 HCl. Gravimetric tests at 298-338 K, electrochemical experiments, surface analysis, adsorption thermodynamics evaluation and DFT calculations were combined to relate macroscopic corrosion protection to molecular adsorption. All derivatives decreased the corrosion rate and suppressed both anodic, iron dissolution, and cathodic, hydrogen evolution, indicating mixed-type inhibition. At 1.0 mmol L-1 and 298 K, the inhibition efficiency followed CTCT > PTCT > MTCT, with CTCT reaching 94.1% by weight loss and 93.7% by EIS, with the same order maintained at higher temperatures. Increasing charge-transfer resistance and decreasing double-layer capacitance supported formation of an adsorbed protective layer, while Langmuir-type adsorption and negative Gibbs free energies indicated spontaneous adsorption with increasing chemisorptive contribution at elevated temperature. DFT-based descriptors, optimized Fe(110) adsorption geometries, and PDOS analyses showed that the superior performance of CTCT arises not simply from global electron-donating ability, but from a more favorable adsorption geometry, stronger molecule-surface interaction energy, and enhanced orbital hybridization with Fe states. These results identify triazole-carbothioamide derivatives as efficient acid corrosion inhibitors and clarify how subtle para-substitution modulates adsorption-controlled protection.