The corrosion of low-carbon steel 08PS in 2 М HCl, 2 М H 2 SO 4 , and 2 М H 3 PO 4 solutions as well as in their mixtures at t = 20-80°C has been studied. Addition of a mixture of the IFKhAN-92 inhibitor, which is a triazole derivative, with KCNS slows down the corrosion of the low-carbon steel in these media. The presence of Fe(III) in the solutions weakens steel protection by the inhibitor formulation studied. The presence of phosphates in an acid solution decreases the adverse effect of Fe(III) as it binds Fe(III) cations into strong phosphate complexes. This effect is most distinct in H 3 PO 4 and H 2 SO 4 + H 3 PO 4 solutions with high H 3 PO 4 content. A mixture of IFKhAN-92 and KCNS forms a polymolecular inhibitor layer on the steel surface. This layer can efficiently slow down the reduction of Fe(III) phosphate complexes. In steel etching or in cleaning of steel equipment in acidic solutions, where Fe(III) salts are accumulated and accelerate steel corrosion, it is recommended to use solutions of H 3 PO 4 or its mixtures with H 2 SO 4 (at least 50%) inhibited by a formulation of IFKhAN-92 with KCNS.