Síntese e avaliação da atividade anticorrosiva de tiossemicarbazidas e sais mesoiônicos da classe 1,3,4-tiadiazólio-2-aminidas

Abstract

Six (6) thiosemicarbazides and five (5) 1,3,4-thiadiazolium-2-phenylamine hydrochloride were synthesized for this dissertation; two (2) of them are being reported for the first time. Thiosemicarbazides were synthesized via three (3) different methodologies: stirring with temperature variation (Method A), microwave irradiation (Method B), and solid-solid reaction (Method C). Mesoionic salts were also synthesized by different methods; firstly, under reflux in the presence of 1,4-dioxane as solvent, from cinnamic acid chlorides and N1,N4-substituted thiosemicarbazides (Method A). And secondly, using microwave irradiation in the presence of thionyl chloride, from cinnamic aldehyde and N1,N4-substituted thiosemicarbazides (Method B). The compounds were obtained in high purity and satisfactory yields: between 65% and 92% for thiosemicarbazides, and between 54% and 99% for mesoionic salts. All compounds were characterized by spectroscopic techniques. In order to obtain information on the structural and electronic properties that could contribute to the prediction of corrosion inhibition efficiency, the structures of the synthesized compounds were investigated using SPARTAN-PRO program in semi-empirical method. Mesoionic salts showed the lowest values forthe difference between energy levels of the frontier orbitals (HOMO and LUMO), which indicates that these compounds could be more effective in inhibiting corrosion than di-ortho-tolylthiourea. All synthesized compounds were experimentally evaluated against the corrosion of AISI 1020 carbon steel in HCl 1 M. The results generally showed significant percentages of corrosion inhibition; specially, for the mesoionic salt that contains the methoxy group in the cinnamic portion of the molecule, which reached levels of 77% and 94% in the technical Electrochemical Impedance Spectroscopy and Potentiodynamic Polarization, respectively. In general, the methodologies used for experimental evaluation of corrosion activity and the technique of molecular modeling gave correlated results.