Application of the Kinetic-Thermodynamic Model for Inhibition of Acid Corrosion of Steel by Inhibitors Containing Sulfur and Nitrogen

Corrosion inhibition characteristics of four selected S-alkylisothioronium halides on steel in sulfuric acid (H₂SO₄) media were investigated at 30°C, 40°C, 50°C, and 60°C. The number of active sites on the metal occupied by a molecule of the inhibitor (1/y) and the binding constants (K) of the inhibitor to the active sites were calculated at the different temperatures. Activation free energies, enthalpies, and entropies for the inhibition processes were determined from rate-constant data measured at 30°C, 40°C, 50° C, and 60° C at two fixed inhibitor concentrations. Data were compared and fitted to a published kinetic-thermodynamic model. Results were compared with fits obtained from application of Langmuir, Frumkin, and Flory-Huggins isotherms. Results also were correlated to the chemical structure of the inhibitors, the length of the alkyl chain of the cation, and the type of the anion.