Application of the Kinetic–Thermodynamic Model to the Adsorption of Organophosphorus Compounds on to a Steel Surface

Abstract

The inhibitive action of organophosphorus derivatives on the dissolution rate of steel in H 2 SO 4 has been determined by gasometry and weight loss methods and then compared to rates measured using the potentiodynamic polarization technique. Additionally, UV spectroscopy was used to study the thermal stability of these compounds in different acid concentrations. An empirical kinetic relationship has been presumed correlating the molecular structure of the inhibitors to their inhibition efficiency. The shape of the adsorption isotherms of the compounds shows that some exhibit one S-shaped curve whilst others are characterized by two-consecutive steps. This behaviour was correlated to the proposed skeletal representation of the adsorption mode of the inhibitors on the metal surface. Electrochemical results indicated that all the compounds investigated acted as cathodic-type inhibitors. The data obtained provide a good fit to both the Temkin adsorption isotherm and the kinetic–thermodynamic model recently proposed. Estimated values of the binding constant, lateral interaction, number of active sites and the change in free energy of adsorption are presented.