THERMODYNAMICS OF MILD-STEEL CORROSION INHIBITION IN PHOSPHORIC-ACID BY ETHYLENE TRITHIOCARBONATE

Ethylene trithiocarbonate has been tested as corrosion inhibitor for m ild steel in 35% H3PO4 - 10% butanol by weight loss method and potenti odynamic technique. The kinetics of the corrosion process has been inv estigated over the temperature range 25 to 50-degrees-C at different i nhibitor concentrations. The anodic dissolution process at concentrati ons less than 5 x 10(-4)M was found to be under activation control. Li miting current at higher concentrations is observed for the anodic dis solution of steel. At a critical concentration of the inhibitor (2 x 1 0(-4)M)a high protection efficiency is maintained over the temperature range examined. This is thought to relate to a high surface coverage or relatively compact adsorbed film, which resists desorption at eleva ted temperatures. Hill de Boer adsorption isotherm and El-Awady et al kinetic model describe well the experimental findings. The number of a ctive sites, the binding constants and the change in free energy of ad sorption as well as the thermodynamic functions such as DELTAH, DELTA S, E(a) have been discussed in detail.