Inhibitory Protection of Low-Carbon Steel in a Flow of Phosphoric Acid Solutions Containing Iron(III) Phosphate

The corrosion of low-carbon steel is studied in a flow of H3PO4 solutions containing FePO4 and also with addition of a mixture of corrosion inhibitors, particularly, a 3-substituted derivative of 1,2,4-triazole (IFKhAN-92) and KNCS. In such solutions, the partial reactions of the iron anodic ionization and the cathodic reduction of H+ and Fe(III) cations proceed on steel. The former two reactions are controlled by kinetics, whereas the latter reaction is diffusion-controlled. The accelerating effect of FePO4 on the steel corrosion in H3PO4 solutions is mainly due to the reduction of Fe(III). In acid solutions containing inhibitors, Fe(III) cations continue to accelerate all partial reactions on steel. Despite this accelerating effect, the mixtures of IFKhAN-92 and KNCS still exert a strong inhibitory effect on the electrode reactions on steel, which is noteworthy. The data on the corrosion of low-carbon steel in such flows, obtained based on the mass loss of metal samples, adequately agree with the results acquired when studying the partial electrode reactions. The accelerating effect of FePO4 on steel corrosion in a flow of H3PO4 solutions is observed, including the media containing inhibitors. In these media, the steel corrosion is determined by convection, which is typical of diffusion-controlled reactions. The mixed inhibitors IFKhAN-92 + KNCS are shown to considerably inhibit the steel corrosion in a flow of H3PO4 solutions containing FePO4, which is a result of the efficient suppression of all partial electrode reactions on this metal.