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2.0 M HCl, 2.0 M H2SO4 and 2.0 M H3PO4 solutions and their mixtures containing Fe(III) and Fe(II) cations were studied by the potentiometry method at 20–95C. It was shown that addition of urotropine to the HCl+H3PO4+Fe(III)+Fe(II) system containing at least 20% H3PO4 of the total acid amount decreased the oxidizing ability of the system. The decrease in the oxidizing ability of the system upon addition of urotropine is the stronger, the higher the content of H3PO4 in the acid mixture. In the H2SO4+H3PO4+Fe(III)+Fe(II) system, the effect of urotropine on the decrease in the oxidizing properties is weaker. The decrease in the oxidizing ability of the Fe(III)-containing acid mixtures being studied upon addition of urotropine can result from binding of Fe(III) into complex compounds with phosphate anions and urotropine. Such Fe(III) complexes have low oxidizing ability. The two-component inhibitor IFKhAN-92+KNCS is recommended for the inhibitory protection of steels in mixtures of mineral acids based on H3PO4 if there is a potential for the accumulation of Fe(III) salts in these solutions which significantly increases the system corrosivity. Addition of urotropine is used to increase the efficiency of this inhibitory formulation. It was found that urotropine causes a more considerable increase in the protection of low-carbon steel by this formulation in the HCl+H3PO4+Fe(III) system than in the H2SO4+H3PO4+Fe(III) system due to a stronger decrease in the oxidizing ability of the former system by this compound.

Effect of urotropine on the potential of the Fe(III)/Fe(II) redox couple in mineral acid solutions