Efficacy and Mechanism of the Action of Zinc Complex Nitrilo-tris-Methylenephosphonic Acid As an Inhibitor of Corrosion in Low-Carbon Steel in Neutral Bromide- and Iodide-Containing Aqueous Media

A study is performed of the effect zinc complex nitrilo-tris-methylenephosphonic acid Na₄[Zn{N(CH₂PO₃)₃}]⋅13H₂O has on the corrosion–electrochemical behavior of low-carbon steel in neutral aqueous media with Br^– and I^– ions. X-ray photoelectron spectroscopy with layer-by-layer etching is used to investigate the composition and structure of passive films formed at different potentials and compositions of the medium. The optimum concentration of inhibitor is 0.2–2.0 g/dm³ in a borate buffer solution (pH 7.4) at a 1.4 mmol/dm³ concentration of Br^– and I^– ions. Br^– ions exhibit greater corrosive activity than I^– ions and cause uneven corrosion. I^– ions are predominantly adsorbed on unoxidized iron and do not destroy the oxide–hydroxide part of the film. When using inhibitor Na₄[Zn{N(CH₂PO₃)₃}]⋅13H₂O, protective films form on a surface of steel. Such films contain heterometallic polynuclear complex [Fe_1/2Zn_1/2(H₂O)₃μ-H₄{N(CH₂PO₃)₃}]_n in addition to iron oxides and hydroxides. The degree of conversion of the inhibitor into this complex in environments containing I^– is higher than in ones containing Br^–.