Joint effect of Fe(II) and Fe(III) cations on corrosion of low-carbon steel in mixture of sulfuric and phosphoric acids containing composite inhibitors based on IFKhAN-92

Abstract

Corrosion of 08PS low-carbon steel was studied in a wide temperature range t = 25—100 °C in 1 M H2SO4 + 1 M H3PO4, 2 M H2SO4 и 2 M H3PO4, inhibited by the binary mixture IFKhAN-92 + KNCS (a molar ratio of components 9:1) and a three-component mixture IFKhAN-92 + KNCS + urotropine (9:1:400), in the presence of high concentrations of Fe(III) and Fe(II) salts, as well as in their joint presence. It is shown that under experimental conditions the corrosion inhibitors under study provide effective protection of steel in 1 M H2SO4 + 1 M H3PO4 even in the presence of Fe(III), Fe(II) salts or their mixtures. The highest protective effects are provided by a three-component inhibitor mixture. Under similar conditions in individual 2 M H3PO4 and, especially, 2 M H2SO4, the protective effects of inhibitors are lower. A stronger deceleration of steel corrosion by composite inhibitors based on IFKhAN-92 in H2SO4 + H3PO4 solutions containing Fe(III) salts in comparison with similar solutions of individual H2SO4, is largely due to the binding of Fe(III) cations by phosphate anions into complexes, that significantly reduces their chemical activity and, as a result the corrosion rate of steel.