Corrosion inhibition of structural steel by organophosphonate complexes

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/dm 3 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/dm 3 to 5 mg/dm 3 .Based on the experimental data obtained, 200 to 1000 mg/dm 3 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.