The protective effect of 3-mercaptoalkyl derivatives of 5-amino-1H-1,2,4-triazole against corrosion of α-brass in a chloride media was studied using polarization curves, electrochemical impedance spectroscopy, and full-scale corrosion testing. The brass electrode remains passive up to the activation potential, which is much higher than in solutions without organic additives, and it increases with the concentration of the inhibitor. The protection degree of all the studied inhibitors reaches its maximum of over 99% in solutions with the concentration of the inhibitor C_inh ≥ 0.10 mM. The protective effect increases with the length of the alkyl chain. All the studied derivatives are effective against atmospheric corrosion of α-brass. A protective film is formed on the brass surface, and it most probably includes oxides as well as complex compounds of zinc and copper with the molecules of the inhibitors. The impedance spectroscopy demonstrated that the presence of the inhibitor results in a decrease in the double-layer capacitance and an increase in the polarization resistance, which proves that the protective film actually forms on the brass surface. The quantum chemical analysis of the optimized molecular structures demonstrates that all the studied inhibitors should have a similar protective effect, which agrees with the experimental results.

A set of electrochemical and full-scale corrosion testing methods demonstrates that 3-sulphinylalkyl-5-amino-1H-1,2,4-triazoles can be used as inhibitors of copper corrosion in neutral chloride solutions and under atmospheric corrosion. It was shown that the inhibition effect of 3-sulphinylalkyl derivatives of 5-amino-1H-1,2,4-triazole increases with the growth of the chain of the additive. By means of the impedance spectroscopy method, the authors determined that the polarisation resistance and the Warburg impedance increases whereas the double-layer capacitance decreased with the growth of the inhibitor chain and the concentration of the inhibitor. In addition, the period when the first signs of corrosion attack appear on the copper plates in the salt spray chamber increased with the growth of the inhibitor chain and its concentrations. The lowest value of this period was measured for 3-sulphinylpropyl derivatives. However, it is still 12 times higher that this value measured without any inhibitors. Full-scale testing in 1% HCl solution showed that the corrosion rate slows down by 10–20 times upon the introduction of the derivatives of 3-sulphinylbutyl and 3-sulphinylpentyl at a concentration of ≥5.00 mM. If the concentration of 3-phenethylsulphinyl is equal to or higher than 1.00 mM, then the protection level is about 85%.

New substituted 4-aryl-8-methyl-2-thioxo-1,2,3,4-tetrahydro-6H-pyrimido[1,2-a][1,3,5]triazin-6-one 8a–b and ethyl 4-aryl-6-oxo-2-thioxo-1,3,4,6-tetrahydro-2H-pyrimido[1,2-a][1,3,5]triazine-7-carboxylate 8c–e were synthesized by the reaction of the corresponding 4-oxopyrimidin-2-ylthioureas with arylaldehydes. The formation of only one regioisomer was proven using complex spectral data and its structure was characterized. It was found that the interaction of 6-amino-4-phenyl-3,4-dihydro-1,3,5-triazine-2(1H)-thione with ethyl acetoacetate and diethyl ethoxymethylenemalonate leads to the formation of the same regioisomer. That is, changing the sequence of stages in this cascade process does not affect the structure of the final reaction product. All synthesized compounds exhibit antibacterial activity against E. coli and S. aureus cultures at a concentration (MIC) of 256 µg/mL.