Reduction of oxidizing properties of mineral acid solutions containing Fe(III) salts by additives of urotropine

A potentiometric study of 2,0 M HCl; 2,0 M H2SO4; 2,0 M H3PO4 and their mixtures (20−95 °C), containing cations Fe(III) and Fe(II), made possible to examine details of the protective action mechanism of a composite inhibitor, containing 3-substituted 1,2,4-triazole (IFKhAN-92), KNCS and urotropin, on the corrosion of low-carbon steels in acid solutions containing Fe(III) salts. It has been found out that a decrease in the oxidizing ability of such systems is observed with the simultaneous presence in their composition of phosphoric acid and urotropine. A decrease in the oxidizing ability of such systems is a result of the binding of Fe(III) in complex compounds with phosphate anions and urotropine. Such Fe(III) complexes have a low oxidizing ability. Inhibitory protection of steels in solutions of mineral acids, in the case of accumulation of an additional oxidizer, namely Fe(III) cations, is possible under the following conditions. First, the oxidizing ability of Fe(III) cations must be reduced by binding them in complex compounds by anions formed during acid dissociation. Individual H3PO4 or mixtures thereof with other acids can be used as such an acid. Second, it is necessary to use an effective inhibitor of acid corrosion, which is capable of simultaneously appreciable slowing down the reduction of protons and Fe(III) complex compounds with anions of acid residuals. Composite inhibitors based on triazole derivatives and sulfur-containing compounds correspond to this property.