Inhibitory protection of low-carbon steel in a mixed solution of sulfuric and phosphoric acids containing Fe(II) and Fe(III) salts

• Ya.G. Avdeev¹, A.V. Panova¹ and K.L. Anfilov²

¹ A.N. Frumkin Institute of Physical Chemistry and Electrochemistry, Russian Academy of Sciences, Leninsky pr. 31, 119071 Moscow, Russian Federation
² Bauman Moscow State Technical University (Kaluga Branch), Bazhenov str. 2, 248000 Kaluga, Russian Federation

Abstract: The effect of Fe(III) and Fe(II) salts on the corrosion of 08PS low-carbon steel in 1 M H₂SO₄ + 1 M H₃PO₄, 2 M H₂SO₄, and 2 M H₃PO₄ solutions was examined in a wide temperature range, t=25–100°C. Formulations of compounds containing a 1,2,4-triazole derivative, IFKhAN-92, used as steel corrosion inhibitors in these environments were studied. The binary mixture of IFKhAN-92+KNCS (molar ratio of components 9:1) and a three-component mixture IFKhAN-92+KNCS+urotropine (9:1:400) were explored. Under the experimental conditions, two- and three-component corrosion inhibitors provide efficient protection of steel in 1 M H₂SO₄+1 M H₃PO₄ solution even if it contains a high concentration of Fe(III) and Fe(II) salts or mixtures thereof. The most significant protective effect is provided by the three-component inhibitor mixture. Under similar experimental conditions, the protective effects of the inhibitors in individual 2 M H₃PO₄ and especially in 2 M H₂SO₄ solutions are lower. The stronger inhibition of steel corrosion by the composite inhibitors based on IFKhAN-92 in H₂SO₄+H₃PO₄ solutions containing Fe(III) salts, in comparison with similar solutions of H₂SO₄ alone, is largely due to the binding of Fe(III) cations by phosphate anions into complexes, which significantly reduces their chemical activity, and hence, the corrosion rate of steel.

Keywords: acid corrosion, sulfuric acid, phosphoric acid, corrosion inhibitors, triazoles, low carbon steel, iron(III) sulfate, iron(III) phosphate, iron(II) sulfate, iron(II) phosphate

Int. J. Corros. Scale Inhib., 2023, 12, no. 4, 2375-2390
doi: 10.17675/2305-6894-2023-12-4-49

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