ISSN 2305-6894

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

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

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