STUDIES ON THE INFLUENCE OF IODIDE-IONS ON THE SYNERGISTIC INHIBITIONOF THE CORROSION OF MILD-STEEL IN AN ACIDIC SOLUTION

The synergistic action caused by iodide ions on the corrosion inhibiti on of mild steel in 0.5 M H2SO4 in the presence of dicyclohexylamine ( DCHA) has been investigated using potentiodynamic polarization, linear polarization and a.c. impedance techniques. DCHA inhibits the corrosi on of mild steel in 0.5 M H2SO4 even at lower concentrations, The inhi bition efficiency decreases with increase in the concentration of the amine. The addition of iodide ions enhances the inhibition efficiency to a considerable extent. The adsorption of this compound is also foun d to obey Langmuir's adsorption isotherm, thereby indicating that the main process of inhibition is by adsorption. The increase in surface c overage values in the presence of iodide ions indicates that DCHA form s an insoluble complex at lower amine concentrations by undergoing a j oint adsorption. The synergism parameter (S) is defined and calculated both from inhibition efficiency and surface coverage values. This par ameter in the case of DCHA is found to be more than unity, indicating that the enhanced inhibition efficiency caused by the addition of iodi de ions is only due to synergism and there is a definite contribution from the inhibitor molecule. Thus, DCHA is then adsorbed by coulombic attraction on the metal surface where the I- is already chemisorbed an d thus reduces the corrosion rate.