Many studies have been made on the ability of thiourea and its substituted derivatives to inhibit corrosion of iron alloys by an acidic environment.1-4 A recent publication which is especially relevant to this paper interprets the inhibiting efficiency of thiourea and its derivatives as being the resultant of two opposing effects--an inhibition by adsorbed molecular species and an acceleration of corrosion by adsorbed protonated species.5 Pillai and Narayan propose that alterations in the bulkiness, solubility, and symmetry of the substituted thiourea molecules change the adsorption characteristics of the thiourea-type molecules and hence influence the inhibiting efficiency. It is the purpose of this paper to investigate relative effects on corrosion rate of sulfur electron density and molecular topology. A more quantitative estimation of the relative effects of these two parameters is described. The electron density of the sulfur atom is derived from self consistent field molecular orbital calculations as outlined by Pople and Beveridge.6 The structural properties of the thiourea molecules are parameterized using the Hosoya Index7 . The thiourea molecules chosen for study include: thiourea, N-methylthiourea, N-ethylthiourea, N, N’-dimethylthiourea, N, N’-diethylthiourea, N, N, N’, N’-tetramethylthiourea, and N, N’-sec-dibutylthiourea.
