Synergistic Extrema of the Mixtures of Corrosion Inhibitors for Metals in Aqueous Salt Solutions
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SYNERGISTIC EXTREMA OF THE MIXTURES OF CORROSION INHIBITORS FOR METALS IN AQUEOUS SALT SOLUTIONS V. M. Ledovs’kykh,1,2 S. V. Levchenko,1 and S. M. Tulainov1 We investigate the influence of individual components and synergistic mixtures of the surface-active substances with inorganic passivator on the electrochemical and corrosion behaviors of steel in neutral aqueous salt solutions. The character of relationship between the ratio of the concentrations of components of the compositions in the corrosive medium and their influence on the electrochemical and corrosion behaviors of the metals (isomolar series) is established. It is also demonstrated that, for the synergistic maxima the inhibition of the anodic reaction of ionization of the metals is especially pronounced the changes in the parameters of their passive state (corrosion potential, current density of complete passivation, and pitting potential) are maximum. In this case, the corrosion protection of steel becomes practically complete. Keywords: inhibitor, synergistic mixtures, adsorption, synergistic extrema.
For the protection of metals against electrochemical corrosion, it is customary to use combined inhibitors, namely, the mixtures of substances capable of exhibiting the elevated efficiency as compared with the individual components. The indicated synergistic phenomenon is observed for the inhibition of corrosion of metals in acid and neutral media and in the atmosphere. Most often, these are mixtures of cation- and anion-active surfactants in solutions of acids [1–14], organic surfactants with inorganic passivators [15–19], and combinations of passivators of oxidizing and sedimentary action in aqueous salt media [1, 12, 20, 21]. The relationship between the molecular structure of organic adsorption surfactants and the anticorrosion efficiency of their synergistic compositions in the process of corrosion of ferrous metals in acid [13] and neutral media [17–19] obeys the principle of linearity of free energies (LFE) and, thus, the influence of substituents in the molecules of surfactants on the inhibiting characteristics of their mixtures can be estimated by using the Hammett–Taft constants [14, 17] or by the method of model reaction series [18, 19]. It was established [6, 13] that, in the case of inhibition of the acid corrosion of iron by synergistic mixtures of cationic organic surfactants with organic and inorganic anions, the curves of the dependences of protective action on the ratio of concentrations of the components of mixtures have extrema and, hence, it is possible to get the maximum protecting effect. We assume that these extrema are also typical of combined inhibitors in the case of corrosion protection of the metals in neutral aqueous salt media. The aim of our investigation is to study the influence of the ratio of concentrations of the components of combined inhibitors, namely, of the adsorption surfactants with inorganic passivators, on the electrochemical and corrosion behaviors of steel in aqueous salt solutions by an example of amine-ni
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