Corrosion inhibition of cold rolled steel in 1 M HClO 4 solutions by N-naphtyl N′-phenylthiourea

Abstract

N-naphtyl N′-phenylthiourea (NPTU) synthesized in our laboratory, was tested as inhibitor for the corrosion of cold rolled steel in 1 M HClO 4 by electrochemical measurements. Polarization curves show that NPTU acts as mixed type inhibitor. Changes in impedance parameters (charge transfer resistance, R t, and double layer capacitance, C dl) were indicative of adsorption of NPTU on the metal surface, leading to the formation of protective film. The degree of the surface coverage of the adsorbed inhibitors was determined by ac impedance technique, and it was found that the adsorption of this inhibitor on the cold rolled steel surface obeys the Langmuir adsorption isotherm. The effect of the temperature on the corrosion behavior with addition of 2.5 × 10 − 4 M of NPTU was studied in the temperature range 25–50 °C. Results show that the rate of corrosion of mild steel increased with increasing temperature both in the presence of inhibitor and in its absence. Activation energies and enthalpies of activation in the presence and absence of NPTU were obtained by measuring the temperature dependence of the corrosion current. The use of the transition state equation to calculate the apparent entropy of activation for dissolution of metals and alloys was criticized.