Relationships demonstrated between (sets of) parameters of molecules and the efficiency of corrosion inhibition may be used to design new derivatives, even if the mechanism by which the corrosion inhibitor interacts with the metal/metal-oxide surface is not fully understood. The aim of this work was to find correlation between parameters related to the electronic structure of molecules and their ability to block corrosion processes. Quantum chemical calculations were done by using the Hückel molecular orbital method, and several molecules were also considered by using the AM1 method. Earlier results obtained for corrosion inhibitors of mild-steel (in acid solution) indicated that correlation exists between inhibition efficiency and the difference (D ) between the energy of the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO). It was found that efficiency (related to 0.001 M inhibitor concentration) is optimal at D = 1.3 beta if pyrimidines, benzothiazoles and amino acids are considered. This result was also confirmed by considering the inhibition efficiencies of 1,3,4-oxadiazole derivatives (FIGURE). - Inhibition efficiencies of aromatic corrosion inhibitors of copper (thiophenol derivatives) and zinc (imidazole derivatives) were also considered. The efficiencies were correlated with values of D of the inhibitor molecules. It was found that in the case of copper an optimal value for inhibition exists and it is D = 1.7 beta, while for zinc the optimal value of D (if it exists) should be less than 1.1 beta. Optimal values of D of the investigated systems decrease in the following order: copper-iron-zinc. Differences in the optimal values of D explain the fact that a molecule, being a good corrosion inhibitor of iron does not inhibit corrosion of copper.

. The inhibitors - pyrimidines (D), amino acids ( o ), benzothiazoles (  ), 1,3,4-oxadiazole derivatives (  ) – were used in 0.001 M concentration.