Synergistic influence of iodide ions on inhibition of corrosion of mild steel in H 2 SO 4 by N -phenyl thiourea

Abstract

The synergistic action of iodide ions on the inhibition of corrosion of mild steel in 0·5M H₂SO₄ by N-phenyl thiourea (N-PTU) has been investigated using weight loss measurements, potentiodynamic polarisation studies, linear polarisation studies, and impedance measurements. Inhibition efficiency is found to increase with increase in the concentration of N-PTU. The inhibiting action of N-PTU is considerably enhanced by the addition of potassium iodide. The adsorption of this compound either alone or in combination with iodide ions on the metal surface is found to obey Langmuir's adsorption isotherm. The increase in surface coverage 0 in the presence of iodide ions indicates joint adsorption of N-PTU and iodide ions. The synergism parameter is defined and evaluated from the values of inhibition efficiency and surface coverage. This parameter is found to be greater than unity, which indicates that the enhanced inhibition efficiency caused by the addition of iodide ions is only due to synergism and that there is a definite contribution from the inhibitor molecule. Thus N-PTU is adsorbed by coulombic attraction on the metal surface where iodide ions are already chemisorbed, which lowers the corrosion rate.