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The corrosion inhibition characteristics of the quadridentate macrocycle 1, 4, 8, 11 tetra-azacyclotetradecane (cyclam) on steel in sulfuric acid media are investigated at 25, 35, 45, and 55~ and the thermodynamic energy parameters calculated. In addition the inhibition characteristics of the open-chain polydentate amines, ethylenediamine (en), diethylenetriamine (dien), triethylenetetraamine (trien), and tetraethylenepentamine (tetren) were investigated at 25~ Activation-free energies, enthalpies, and entropies for the inhibition process of both cyclam and tetren were determined from the rate constants data measured at four temperatures viz., 25, 35, 45, and 55~ at one or two fixed inhibitor concentrations. All data were compared and fitted to a previously published kinetic-thermoydnamic model in terms of an active site occupancy parameter y and a binding constant K of the inhibitor to the active site. The results obtained are compared with fits obtained from the application of Fumkin and Flory-Huggins adsorption isotherms. The first isotherm attempts to treat the deviations from the ideal Langmuir isotherm by the use of a lateral interaction parameter a while the second uses a relative-size parameter x which is a measure of the relative size of the inhibitor molecule to the surface-adsorbed water molecule. Good agreement was obtained between the kinetic-thermodynamic and Flory-Huggins models. The Frumkin isotherm model, however, agreed only in cases where the active site occupancy, or the relative-size parameters were equal to or close to unity. A discussion of the strengths and weaknesses of each treatment is given. In addition, the results obtained are correlated to the chemical structure of the inhibitors, the number of donor atoms, and the macrocyclic effect.

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Corrosion inhibition characteristics of four selected Salkylisothioronium halides on steel in sulfuric acid (H 2 SO 4 ) 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.