Realization of synergism in 1-, 2-, 3-benzotriazole trimolybdate, tungstate, and chromate corrosion inhibitors
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REALIZATION OF SYNERGISM IN 1-, 2-, 3-BENZOTRIAZOLE TRIMOLYBDATE, TUNGSTATE, AND CHROMATE CORROSION INHIBITORS Z. V. Slobodyan, L. A. Mahlatyuk, and H. M. Nykyforchyn
UDC 620.197.3
The inhibiting action of new 1-, 2-, 3-benzotriazole trimolybdate, tungstate, and chromate compounds is explained by the presence of intramolecular synergism as a result of which the coefficient of inhibition of the corrosion rate of steel in water becomes much higher than the total inhibition coefficient of the rate of corrosion by compositions based on benzotriazole and the corresponding oxoanions. The 1-, 2-, 3-benzotriazole trimolybdate, tungstate, and chromate compounds are inhibitors of mixed action inhibiting both electrode reactions and shifting the compromise potential to less negative values. According to the data of impedance tests and the results of evaluation of the degree of packing of these compounds on the steel surface, we conclude that their inhibiting effect is caused by chemisorption satisfactorily described by the Frumkin– Temkin isotherm.
The inhibitor protection with inorganic compounds in neutral media is most often based on the principle of formation of passive films of oxide or salt types. These films decrease the rate of transfer of oxygen to the metal surface [1]. However, they are not continuous, which promotes the process of local corrosion [2]. On the other hand, the organic adsorption-type inhibitors do not guarantee the required degree of corrosion protection of steel. This is caused by insufficiently high degrees of packing of inhibitors on the metal surface [3] explained by the fact that the repulsive forces between adsorbed particles exceed the corresponding attracting forces. The efficiency of inhibitors can be improved by using organic compounds with several functional groups of different polarities or compositions some components of which have functional groups of different polarities. The first and second cases correspond to intramolecular and intermolecular synergism, respectively [4]. In some compounds formed by an organic cation and an inorganic anion [5] and characterized by low degrees of dissociation [6], one may also observe the combination of the indicated two types of synergism. In this case, the inorganic anion passivates the surface of the metal and the organic anion is adsorbed (chemisorbed) in the nonpassivated regions. Moreover, the nondissociated molecules can also interact with the metal. As passivating inhibitors of iron and its alloys, we can mention the oxoanions of transition elements containing d-electrons and vacant d-orbitals [7, 8]. First of all, this is true for elements from Group VIb (chromium, molybdenum, and tungsten) (Table 1). In the series Cr – Mo – W, the ionization energy increases and the electron shells of atoms become more compact. However, due to close atomic and ionic radii, the affinity between molybdenum and tungsten is higher than between the indicated elements and chromium [9]. Thus, in particular, Mo (VI) and W (VI) frequently form polymeric oxoanions
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