Cyclic voltammetric study of the HCl–H3PO4–H2O–Fe(III) system

• Ya.G. Avdeev, T.E. Andreeva, A.V. Panova and Yu.I. Kuznetsov

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

Abstract: The electrochemical behavior of Fe(III) cations on a platinum electrode in the HCl–H₃PO₄–H₂O, HCl–H₂O, H₃PO₄–H₂O systems (25–95°C) has been studied by cyclic voltammetry. The oxidizing ability of the 2.0 M HCl+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 chloride complexes. An increase in temperature causes an increase in the oxidizing ability of the HCl–H₃PO₄–H₂O–Fe(III) system. Addition of H₃PO₄ to HCl solutions converts Fe(III) cations from hydrate and chloride 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 in 1.0 M HCl+1.0 M H₃PO₄ is about 70% of the initial value. A probable reason for more efficient protection of low-carbon steel by triazole-based composite inhibitors in H₃PO₄ or its mixtures with HCl containing Fe(III), in comparison with similar solutions of HCl 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 chloride complexes of Fe(III).

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

Int. J. Corros. Scale Inhib., 2020, 9, no. 2, 538-549 PDF (367 K)
doi: 10.17675/2305-6894-2020-9-2-9

Download PDF (Total downloads: 1445)

Back to this issue content: 2020, Vol. 9, Issue 2 (pp. 372-779)