Inhibition effect of hydantoin compounds on the corrosion of iron in nitric and sulfuric acid solutions

The inhibition of corrosion of iron in 2M nitric acid and 2M sulfuric acid solutions by substituted phenylhydantoin, thiohydantoin, and dithiohydantoi n compounds was measured using thermometric, weight loss, and polarization methods. The three methods gave consistent results. The polarization curves indicated that the hydantoin compounds act as mixed-type inhibitors. The a dsorption of the inhibitors were found to obey the Temkin adsorption isothe rm. The higher inhibition efficiency of the additives in nitric with respec t to sulfuric acid solution may be attributed to the reduced formation of s oluble quaternary nitrogen salts in nitric acid medium, favouring adsorptio n of the parent additive on the metal surface. The obtained results indicat e that the corrosion rate of iron in both acids increases with increasing t emperature, both in absence and presence of the tested inhibitors. Kinetic- thermodynamic model functions and Temkin isotherm data an compared and disc ussed. The synergistic effect of halide anions on the inhibition efficiency of the hydantoin compounds was also investigated.