Effect of anionic composition of solutions of mineral acids containing Fe(III) on their oxidizing properties

• 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, Moscow, 119071 Russian Federation

Abstract: Solutions of HClO4, HBr, H2SO4, HCl, H3PO4, HF and some of their mixtures containing Fe(III) and Fe(II) cations were studied at 20–95°C by potentiometry. The oxidizing ability of such solutions of individual acids decreases in the series: HClO4 > HBr > H2SO4 > HCl > H3PO4 > HF. This effect is the result of the enhancement in this series of the ability of anions of acids to bind Fe(III) cations into complex compounds that are less prone to reduction in comparison with its aqua complexes. To decrease the oxidative ability of acid solutions (HCl and H2SO4) with a weak ability of anions to bind Fe(III) cations to complex compounds, it is possible to add acids (H3PO4 or HF) that generate anions binding Fe(III) into stable and hardly reducible complexes. In the H2SO4–H3PO4–H2O, H2SO4–HF–H2O, HCl–H3PO4–H2O, and HCl–HF–H2O systems containing Fe(III) and Fe(II), the dependence of the redox potential of the Fe(III)/Fe(II) couple on the anionic composition of the medium showed that H3PO4 is the most promising additive that reduces the oxidizing properties of HCl and H2SO4 in solutions. In the HCl–H3PO4–H2O system taken as an example, the main regularities of changes in the oxidizing properties of two-component acid mixtures containing Fe(III) and Fe(II) are revealed. Variation in the total content of an equimolar Fe(III)+Fe(II) mixture (C = 0.01–0.10 M) in 1 M HCl + 1 M H3PO4 nearly does not affect its oxidative ability. Conversely, an increase in the relative content of Fe(III) in the system considerably increases its oxidative potential. At a fixed concentration of the Fe(III) + Fe(II) mixture (0.04 M) in the HCl + H3PO4 system, an increase in the concentration of these acids from 1 to 4 M slightly reduces the oxidative properties of the system. Improving the corrosion protection of steel in HCl or H2SO4 solutions containing Fe(III) salts with a composite inhibitor (IFKhAN-92 + KNCS + hexamine) can be effective when H3PO4 is added. This effect is largely determined by the binding of Fe(III) cations to phosphate complexes, which have a lower oxidizing capacity compared to its aqueous, sulfate and chloride complexes.

Keywords: corrosion inhibitors, potentiometry, perchloric acid, hydrobromic acid, hydrochloric acid, sulfuric acid, phosphoric acid, hydrofluoric acid, Fe(III)/Fe(II) redox couple

Int. J. Corros. Scale Inhib., 2019, 8, no. 1, 139-149 PDF (492 K)
doi: 10.17675/2305-6894-2019-8-1-12

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