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

The effect of Fe(III) and Fe(II) salts on the corrosion of low-carbon steel 08PS in 1 M HCl+1 M H3PO4, 2 M HCl and 2 M H3PO4 was studied in the temperature range of t =25-100°C.Mixtures of chemical compounds containing IFKhAN-92, a 1,2,4-triazole derivative, were studied as steel corrosion inhibitors in these environments.The inhibitory effect of the binary formulation IFKhAN-92+KNCS (molar ratio of components 9:1) and the three-component formulation IFKhAN-92+KNCS+hexamethylenetetramine (HMT) (9:1:400) on steel corrosion in these media was considered.The IFKhAN-92+KNCS+HMT formulation was shown to be promising for the protection of low-carbon steel in 1 M HCl+1 M H3PO4.It was found that under the experimental conditions, the three-component mixture provides protection for steel in 1 M HCl+1 M H3PO4, even if the solution contains Fe(III), Fe(II) salts or their mixtures.The protective effects of the inhibitors studied are weaker under similar conditions in 2 M HCl alone.The stronger inhibition of steel corrosion by the composite inhibitors based on IFKhAN-92 in HCl + H3PO4 solutions containing Fe(III) salts in comparison with similar solutions of HCl 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.