Thermodynamics of Steel Corrosion in Acid Media Containing Organophosphorus Derivatives

Abstract

A kinetic–thermodynamic model describing the process of adsorption of organic inhibitors on the surface of metals has been applied to ascertain its validity and to determine the number of active sites and the binding constant. Structure–protection correlations for seven organophosphorus compounds have been studied over the temperature range 25–50°C using potentiodynamic polarization and gasometry techniques. Ultraviolet absorption spectra show that the inhibitors are stable in 1 M and 5 M H₂SO₄ solutions at 50°C. The activation parameters have been determined using the Arrhenius and Eyring equations and it has been shown that E_a and ΔS are increased by a factor of 1.5 in the presence of the inhibitors over the situation in the acid solution, indicating a decrease in the corrosion rate. The adsorption isotherms of vinyl triphenylphosphonium bromide demonstrate the presence of two progressive steps for all temperatures examined due to the formation of multilayers of the adsorbed inhibitor on the steel surface. Additionally, the use of a 0.001 M concentration terminates the effect of temperature on the protection efficiency of vinyl triphenylphosphonium bromide as a result of the strong inhibitor film chemisorbed on the surface. The process of adsorption leads to an increase in the enthalpy of the corrosion process, i.e., it diminishes the number of hydrogen atoms adsorbed on the metal. The values of the thermodynamic parameters, ΔG°_ads., ΔH°_ads. and ΔS°_ads., have also been computed and are discussed.