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

• Ya.G. Avdeev, T.E. Andreeva, A.V. Panova and E.N. Yurasova

A.N. Frumkin Institute of Physical Chemistry and Electrochemistry, Russian Academy of Sciences, Leninskii pr. 31, Moscow, 119071 Russian Federation

Abstract: The electrochemical behavior of Fe(III) cations on a platinum electrode in the H₂SO₄–H₃PO₄–H₂O, H₂SO₄–H₂O, and H₃PO₄–H₂O systems (25–95°C) has been studied by cyclic voltammetry. The oxidizing ability of the 2.0 M H₂SO₄ + H₃PO₄ system containing 0.10 М Fe(III) decreases as the molar fraction of H₃PO₄ 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 H₂SO₄–H₃PO₄–H₂O–Fe(III) system. This effect is most pronounced in the H₃PO₄–H₂O–Fe(III) system. Addition of H₃PO₄ to H₂SO₄ 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 H₃PO₄ or its mixtures with H₂SO₄ containing Fe(III), in comparison with similar solutions of H₂SO₄ 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).

Keywords: corrosion inhibitors, cyclic voltammetry, sulfuric acid, phosphoric acid, redox couple Fe(III)/Fe(II), diffusion coefficient

Int. J. Corros. Scale Inhib., 2019, 8, no. 2, 411-421 PDF (582 K)
doi: 10.17675/2305-6894-2019-8-2-18

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