Study of Corrosion Inhibitor/Stimulator Characteristics of Guanidine Derivatives

Abstract

The corrosion of pure iron in neutral aqueous solutions in the presence of some guanidine derivatives such as dicyandiamide, guanylurea and biguanide has been studied at different temperatures. The low corrosion rates were specially chosen in order to study the behaviour of the adsorbed additive layer – whether catalytic or inhibitive. The inhibition of the corrosion of pure iron by biguanide has been shown to follow the Langmuir adsorption isotherm over a wide concentration range and the activation energies to decrease with increasing biguanide concentration. The adsorbed molecules of dicyandiamide and guanylurea, however, behave as catalysts towards the hydrogen discharge reaction, although dicyandiamide, like biguanide, is found to be an endothermic adsorbing species and the effective inhibition increases with increasing temperature. It has been concluded that, for the adsorption type of inhibitors, the observed corrosion rate is the sum of the corrosion rates of the uncovered suiface and of the surface covered by the adsorbing species. Some adsorbing inhibitors like biguanide, effectively protect the metal suiface from corrosion showing that the contribution of the latter rate becomes negligibly small or zero.