The effect of temperature on the corrosion of mild steel in acidic media in the presence of some sulphur-containing organic compounds

The effects of methylthiourea (MTU), phenylthiourea (PTU) and thiobenzamide (TBA) on the corrosion of mild steel in 0.1 M H2SO4 solution at temperatures of 30 degrees C, 40 degrees C and 50 degrees C have been studied. AC impedance, polarization resistance measurement and polarization curves measurement techniques have been used to obtain experimental data. The percent inhibition efficiency (IE%) and activation energy (E-a) have been determined in the presence and in the absence of these compounds. The results showed that all compounds have good inhibtion properties at all temperatures studied. The value of activation energy (E-a) found in the presence of PTU and TBA is smaller than that in the uninhibited solution while it is greater in the presence of MTU. The reasons that cause the deviation of a Nyquist plot from a semicircle have been investigated. Differential capacitance (Q(dl)) has been used instead of double layer capacitance (C-dl) to represent the experimental deviations of a Nyquist plot from a semicircle. Whether the difference in real impedance at lower and higher frequencies correspond with the charge transfer resistance (R-t) has been discussed by taking the resistance contribution of diffuse layer into account. The resistance measured with AC impedance technique has been divided into charge transfer resistance (R-t) and diffuse layer resistance (R-d). The corrosion current density (i(corr)) values have been determined by making use of the variation of corrosion potential with pH. (c) 2005 Elsevier B.V. All rights reserved.