A potentiometric study of an H2SO4–H3PO4–H2O system containing Fe(III) and Fe(II) cations

The article summarizes the literature data and patents on the protection of steels and nonferrous metals in phosphoric acid solutions with various corrosion inhibitors. Some aspects of practical use of H 3 PO 4 solutions, their properties and corrosivity toward various metals are reviewed. It is shown that the overwhelming majority of individual organic and inorganic compounds, as well as composite inhibitors offered for the protection of metals in this medium are of low efficiency due to weak adsorption on metal surfaces. A promising method for the protection of steels in phosphoric acid media involves the use of formulations containing triazole derivatives and sulfur-containing additives. The components of these mixtures form polymolecular protective layers on the metal surface that are highly efficient in slowing down corrosion processes even at temperatures up to 100С. An important property of these protective layers is a strong chemical bond of the inhibitor molecules with the metal surface and with each other inside the layer. Inhibited H 3 PO 4 solutions resistant to the accumulation of corrosion stimulators-Fe(III) saltsmay become a new and promising field of use for this acid. The bibliographic list includes 170 sources.

Section: Ligandsmentioning

confidence: 99%

Section: Ligandsmentioning

confidence: 99%

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

2019

“…Previously, the potentiometry method was used to study an H 2 SO 4 -H 3 PO 4 -H 2 O system containing Fe(III) and Fe(II) cations [5,6]. It was shown that a probable reason of the efficient protection of mild 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.…”

Section: Introductionmentioning

confidence: 99%

“…Cyclic voltammograms of platinum electrode in 2.0 M H 2 SO 4 + Fe 2 (SO 4 ) 3 at different concentration of Fe(III). Scan rate: 0.10 Vs -1 at 25C.The values of current maxima on the current-potential curves is determined by the Randles-Sevcik equation: 69105 is the equation constant at 25C; z is the number of electrons participating in the redox process (z = 1 for the redox pair Fe (III)/Fe (III)); S is the surface area of the electrode (m 2 ); C is the concentration of the electrochemically active substance (molm -3 ); D is the diffusion coefficient (m 2 •s -1 ); and v is the potential sweep rate (V•s -1 ). The Randles-Sevcik equation describes redox processes that occur in equilibrium systems.…”

mentioning

confidence: 99%

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

2019

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).

“…It was previously shown [6] that a decrease in the oxidizing ability of acids containing Fe(III) salts makes it possible to effectively use corrosion inhibitors in these environments to protect mild steel. A decrease in the oxidizing ability of these solutions is possible by binding these cations into complex compounds, which have a lower oxidizing potential in comparison with hydrated Fe(III) ions.…”

mentioning

confidence: 99%

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

Avdeev¹,

Andreeva²,

Panova³

et al. 2019

Solutions of HClO 4 , HBr, H 2 SO 4 , HCl, H 3 PO 4 , 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: HClO 4  HBr  H 2 SO 4  HCl  H 3 PO 4  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 H 2 SO 4 ) with a weak ability of anions to bind Fe(III) cations to complex compounds, it is possible to add acids (H 3 PO 4 or HF) that generate anions binding Fe(III) into stable and hardly reducible complexes. In the H 2 SO 4 -H 3 PO 4 -H 2 O, H 2 SO 4 -HF-H 2 O, HCl-H 3 PO 4 -H 2 O, and HCl-HF-H 2 O 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 H 3 PO 4 is the most promising additive that reduces the oxidizing properties of HCl and H 2 SO 4 in solutions. In the HCl-H 3 PO 4 -H 2 O 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 H 3 PO 4 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 + H 3 PO 4 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 H 2 SO 4 solutions containing Fe(III) salts with a composite inhibitor (IFKhAN-92 + KNCS + hexamine) can be effective when H 3 PO 4 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.