A study of the H2SO4–H3PO4–H2O–Fe(III) system by cyclic voltammetry

Abstract

The electrochemical behavior of Fe(III) cations on a platinum electrode in the H 2 SO 4-H 3 PO 4-H 2 O, H 2 SO 4-H 2 O, and H 3 PO 4-H 2 O systems (25-95°C) has been studied by cyclic voltammetry. The oxidizing ability of the 2.0 M H 2 SO 4 + H 3 PO 4 system containing 0.10 М Fe(III) decreases as the molar fraction of H 3 PO 4 increases. The observed effect results from the formation of Fe(III) complexes with phosphate anions that are weaker oxidants than the corresponding hydrate and sulfate complexes. An increase in temperature causes an increase in the oxidizing ability of the H 2 SO 4-H 3 PO 4-H 2 O-Fe(III) system. This effect is most pronounced in the H 3 PO 4-H 2 O-Fe(III) system. Addition of H 3 PO 4 to H 2 SO 4 solutions converts Fe(III) cations from hydrate and sulfate complexes to phosphate complexes, which leads to a decrease in their diffusion rate in aqueous acid solutions. The decrease in the diffusion coefficient of Fe(III) cations is about 1/3 of the initial value. A probable reason for efficient protection of low-carbon steel by triazole-based composite inhibitors in H 3 PO 4 or its mixtures with H 2 SO 4 containing Fe(III), in comparison with similar solutions of H 2 SO 4 alone, is that phosphate complexes of Fe(III) are formed in these media. They have a lower oxidizing ability and diffusion rate in comparison with aqueous and sulfate complexes of Fe(III).

Protection of metals in phosphoric acid solutions by corrosion inhibitors. A review

“…In solutions of acid mixtures (HCl + H 3 PO 4 or H 2 SO 4 + H 3 PO 4 ) containing Fe(III) salts, the oxidation capability decreases with an increase in the relative content of H 3 PO 4 in the system [165,166]. In addition, examination of the system H 2 SO 4 -H 3 PO 4 -H 2 O-Fe(III) by cyclic voltammetry (CVA) shows that binding Fe(III) ions into phosphate complexes significantly decreases the rate of their diffusion in a corrosion environment [167]. This plays an important role in inhibiting Fe(III) reduction on the surface of corroding steel because this process is diffusion-controlled [126].…”