The structure–activity relationship of three oxyphosphonate inhibitors differing in heteroatom type (C, N, S) was examined to clarify their influence on the pitting corrosion resistance of ferritic–martensitic steel EP-450 in chloride media. Gravimetric tests in 10% FeCl₃, supported by surface microscopy and adsorption analysis, showed that EP-450 is highly susceptible to localized attack, with pits nucleating preferentially at carbide-enriched, chromium-depleted regions. Addition of dimethyl(1-hydroxycyclohexyl)phosphonate reduced the corrosion rate from 49 to 33 mm/year at 2.0 g/L, corresponding to ≈33% protection, while the nitrogen-containing dimethyl[1-(2-ethoxyethyl)-4-hydroxypiperidin-4-yl]phosphonate produced the largest decrease in mass loss, exceeding a 55% reduction under identical conditions. The sulfur-bearing dimethyl(4-hydroxytetrahydro-2H-thiopyran-4-yl)phosphonate afforded an intermediate effect. Adsorption analysis for the cyclohexyl derivative suggested mixed physisorption–chemisorption with limited surface coverage, while heteroatom substitution (N or S) is consistent with a change in adsorption configuration and interfacial packing that can yield a more compact protective layer. The observed inhibition efficiency increased in the sequence C < S < N, which is interpreted empirically in terms of heteroatom-dependent adsorption geometry and film integrity rather than conjugation-driven activation of the P=O group.

The article is devoted to the study of the resistance of structural materials of the BN-350 reactor to pitting corrosion in an aqueous medium containing chlorine ions in the presence of organic corrosion inhibitors. The results of accelerated testing of samples of austenitic steels 12Cr18Ni10Ti, 08Cr16Ni11Mo3Ti and Cr13Mo2NbVB (EP-450) ferrite-martensitic steel for pitting corrosion in a 10% aqueous solution of iron three-chloride hexahydrate without an inhibitor and in the presence of various concentrations of corrosion inhibitor are presented. The effect of heat treatment on the pitting resistance of structural steels is studied and the role of carbide precipitates of MC and M₂₃C₆ types in pitting defects formation is discussed.