New Heterocyclic Compound as Carbon Steel Corrosion Inhibitor in 1 M H₂SO₄, High Efficiency at Low Concentration: Experimental and Theoretical Studies

A new aromatic Schiff base with azo linkage has been synthesized and characterized by FT-IR, 1H-NMR, and 13C-NMR spectroscopy. The new compound 2-(((5-mercapto-1,3,4-thiadiazol-2-yl)imino)methyl)-4-(p-tolyldiazenyl)phenol (5, denoted as AT) was tested as a carbon steel corrosion inhibitor in 1 M H2SO4. The presence of AT in 0.04 mM concentration at 303 K achieved excellent inhibition efficiency values, 96.6 and 97.4% by potentiodynamic polarization and weight loss measurements, respectively. The adsorption process of AT on carbon steel surface was found to obey Langmuir adsorption isotherm with the highest Kads value 476,190 M−1 at 313 K, and ΔG values −25.53, −26.49, −25.97, and −25.78) kJ mol−1 over the studied range of temperatures 303–333 K, indicating the spontaneous formation of stable protection film through a strong adsorption process. Density function theory (DFT) studies were employed for further investigations about the nature of the interaction between the molecules of AT (both of its tautomers) and metal surface. SEM and AFM analysis were used to confirm the inhibition by comparing the morphology of the corroded surface with the inhibited one.