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 Na 4 [Zn{N(CH 2 PO 3 ) 3 }]⋅13H 2 O 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/dm 3 in a borate buffer solution (pH 7.4) at a 1.4 mmol/dm 3 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 Na 4 [Zn{N(CH 2 PO 3 ) 3 }]⋅13H 2 O, protective films form on a surface of steel. Such films contain heterometallic polynuclear complex [Fe 1/2 Zn 1/2 (H 2 O) 3 μ-H 4 {N(CH 2 PO 3 ) 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 – .