Corrosion inhibition of structural steel by organophosphonate complexes

• B.N. Driker¹, N.V. Tsirulnikova², N.N. Styagov¹ and E.K. Musikhin¹

¹ Federal State Budgetary Educational Institution of Higher Education “Ural State Forestry Engineering University”, 35 Sibirsky Trakt, 620100 Yekaterinburg, Russia
² National Research Center “Kurchatov Institute–IREA”, 1 Akademika Kurchatova St., 123182 Moscow, Russia

Abstract: Unbalanced complexonates of nitrilotri(methylenephosphonic) acid (NTMP) with zinc or magnesium ions (at a molar ratio of NTMP:Me=2:1) are used for simultaneous inhibition of corrosion and mineral scaling. The assumed mechanism of corrosion inhibition involves the formation of a protective film on the corroding metal surface via nucleophilic–electrophilic substitution reactions. To investigate the conditions for protective film formation, the deposition process was studied on St3 steel tablets with a diameter of 5 mm from NTMP-Me solutions with a concentration of 200 mg/dm3 over an exposure period ranging from 2 to 24 h. The composition of the resulting coating was determined by atomic absorption spectroscopy (ICAP 6300 DUO). It was found that the main component of the coating is an iron-NTMP complex (with a Fe:NTMP molar ratio of 1:1.5), with the amount of zinc or magnesium hydroxides being less than 1%. The coating thickness was measured using an SPEl-7LED ellipsometer. Under the given conditions, the rate of protective film formation was 12–20 nm/h. The thickness of the protective coating affects both the corrosion rate and the duration of the protective effect. A sufficient protective effect was observed at a coating thickness of 50 nm, while coatings thicker than 400 nm provided protection for more than 10 days. The resulting protective film makes it possible to reduce reagent consumption from 20–50 mg/dm³ to 5 mg/dm³. Based on the experimental data obtained, 200 to 1000 mg/dm³ NTMP was additionally introduced for sulfide removal from water using the iron complex of ethylenediaminetetraacetic acid (Fe-EDTA). During the treatment process, the resulting Fe-NTMP complex significantly reduced the steel corrosion rate from over 10 mm/year to regulated levels below 100 nm/year.

Keywords: corrosion, corrosion inhibitor, complexonate, organophosphonate

Int. J. Corros. Scale Inhib., 2025, 14, no. 2, 560-565
doi: 10.17675/2305-6894-2025-14-2-7

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