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 [Fe1/2Zn1/2(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–.