The effect of thermodynamic parameters of the adsorption surfactants by lead on the lead?s corrosion rate

Abstract

Surfactants are widely used as corrosion inhibitors. However, in spite of many studies of the mechanism of surfactant action on the corrosion rate [1][2][3][4][5], the problem is far from being solved, because the adsorption of surfactants at a metal electrode has not yet been sufficiently investigated.The adsorption of surfactants from bulk solution at the metal or oxide surface is always a complex process [6] that involves squeezing an organic molecule from the bulk to the interface, desorbing solvent from the metal or its oxide, and, lastly, forming a bond between the molecule and the surface. The surfactant concentration in the bulk solution is low; therefore, the adsorption isotherm is commonly [1, 2, 6-8] presented as follows(1)where c org and X org are the molar concentration (mol/l) and the mole fraction of surfactant ( c org /55.5), respectively; B is the adsorption constant; ∆ is the standard free energy of adsorption in the case of conventional asymmetrical choice of standard state (assuming that the surfactant activity coefficient reaches a limit equal to unity at infinite dilution); θ is the surface coverage with surfactant species, and f ( θ ) is a function of θ .From (1), it is seen thatIn electrochemistry, it is commonly believed [7, 8] that parameter ∆ characterizes the energy of organicHowever, this is not true. The metal surface coverage with adsorbed molecules is always determined by two effects: the squeezing of surfactant molecule to the interface and the formation of adsorbed molecule-metal bond. The squeezing force is based on hydrogen bonds between water molecules which are broken by the hydrocarbon part of a surfactant molecule. Tending to restore their bonds, water molecules squeeze SA-molecule to the surface. The component of ∆ associated with this process, is determined by the bulk properties of surfactant-water system.In this work, for several surfactants, we will determine those thermodynamic parameters that characterize the adsorbed molecules-metal surface bond, and study the effect of these parameters on the lead corrosion rate. FUNDAMENTAL EQUATIONS FOR CALCULATING THERMODYNAMIC PARAMETERS OF SURFACTANTSIt is known [9, 10] that the physical essence of standard Gibbs energy characterizing surfactant adsorption depends on the chosen standard state. The choice of symmetrical standard state instead of the aforementioned asymmetrical one means the assumption that the activity coefficient of organic substance γ org 1 at X org 1 [9]. This standard state corresponds [10, 11]Abstract -For several surfactants, free energies of adsorption ∆ are calculated relative to a symmetrically chosen standard state. It is shown that they characterize the strength of organic molecule-metal bond. The effect of these surfactants on active corrosion of lead in acetic acid solution, as well as correlation between the inhibition coefficient and the free energy of adsorption ∆ , is studied by measuring the polarization resistance. The corrosion rate of lead in acetic acid solution with and without adding sur...