Inhibitory Protection of Low-Carbon Steel in a Flow of Acid Solutions with Different Anionic Composition Containing Iron(III) Salts

Abstract—The corrosion of low-carbon steel is studied in static and dynamic solutions of HCl, HCl + H₃PO₄, and H₃PO₄ containing Fe(III) salts, and also with addition of a two-component inhibitor IFKhAN-92 + KNCS. In these media, the following partial reactions proceed on steel: the anodic ionization of iron and the cathodic reduction of H⁺ and Fe(III). The reactions of iron ionization and proton reduction are controlled by kinetics, whereas the reduction of Fe(III) is controlled by diffusion. In these solutions, the addition of 4.5 mM IFKhAN-92 + 0.5 mM KNCS suppresses all partial reactions on the metal surface due to the formation of a protective polymolecular layer on the surface. As a result of inhibition of all the partial reactions on the steel surface in the HCl, HCl + H₃PO₄, and H₃PO₄ solutions containing Fe(III) salts, the steel corrosion slows down. The mixed inhibitor hinders the steel corrosion in 2 M H₃PO₄ and 1 M HCl + 1 M H₃PO₄, but poorly protects the metal in 2 M HCl. This is explained by the different ability of anions contained in the aggressive media to bind Fe(III) cations to complex compounds. In contrast to chloride anions, the phosphate anions bind stronger the Fe(III) cations to complex compounds, which leads to the lower diffusion coefficient D_Fe(III) of these cations in phosphate media. The protective film formed by the inhibitor on the steel surface hinders most efficiently the diffusion-controlled reduction of Fe(III) compounds present in solution and characterized by the relatively low diffusion coefficient.