INHIBITOR PROTECTION OF MILD STEEL IN THE FLOW OF ACID SOLUTIONS OF DIFFERENT ANIONIC COMPOSITION CONTAINING IRON(III) SALTS

Corrosion of mild steel in static and dynamic solutions of HCl, HCl + H₃PO₄ and H₃PO₄ containing Fe(III) salts, including media containing two-component inhibitor IFKhAN-92 + KNCS has been studied. In the discussed media, partial reactions of anodic ionization of iron, cathodic reduction of H⁺ and Fe(III) compounds are realized on steel. Iron ionization and proton reduction proceeds with kinetic control, while Fe(III) reduction is characterized by diffusion kinetics. In the solutions under consideration the addition of 4.5 mM IFKhAN-92 + 0.5 mM KNCS inhibits all partial reactions occurring on the metal surface, which is a consequence of its formation of a polynolecular protective layer on the steel. The result of inhibition by the investigated inhibitor in HCl, HCl + H₃PO₄ and H₃PO₄ solutions containing Fe(III) salts of all partial reactions realized on the steel surface is its slowing down of its corrosion. The mixture inhibitor strongly retards the corrosion of steel in 2 M H₃PO₄ and 1 M HCl + 1 M H₃PO₄, but provides poor protection of the metal in 2 M HCl. This is due to the different ability of the anions of the aggressive medium to bind Fe(III) cations into complex compounds. Phosphate anions, in comparison with chloride anions, more strongly bind Fe(III) cations into complex compounds, determining for them in such media lower values of D_Fe(III). The protective film of the inhibitor, formed on steel, most effectively slows down the reduction of Fe(III) compounds, present in solution, realized in diffusion mode, with relatively low values of the diffusion coefficient.

convection, diffusion kinetics, acid corrosion, corrosion inhibitors, mild steel, hydrochloric acid, iron(III) chloride, phosphoric acid, iron(III) phosphate