Stability Study of Iron Sulfide Films, Electrochemically Grown on Carbon Steel, in Different Electrolytic Media

In this work, nonstoichiometric iron sulfide (Fe_xS_y) films were prepared and grown by applying cyclic pulses of potential to the carbon steel/sour medium interface (1 M ammonium sulfide [(NH₄)₂S], 500 ppm cyanide [CN⁻]) for 15 min. Once the film was formed, it was introduced into solutions of different chemical compositions in order to perform the studies of stability. These studies were initially performed by immersing the film for 18 h in different solutions: 0.1 M (NH₄)₂S, 10 ppm CN⁻ (pH = 8.8), a neutral solution of 0.1 M sodium nitrate (NaNO₃), a neutral solution of 0.1 M sodium sulfate (Na₂SO₄), and a borate buffer solution prepared by mixing 0.614 M boric acid (H₃BO₃) and 0.145 M sodium hydroxide (NaOH) (pH = 8.8). Afterward, the behavior of this film was characterized by electrochemical impedance spectroscopy and scanning electronic microscopy. The results showed that the Fe_xS_y films are stable only in the medium of 0.1 M (NH₄)₂S, 10 ppm CN⁻. In contrast to the studies reported in the literature for iron oxide characterization in a borate medium, in the case of Fe_xS_y films, said medium originates a great modification of the initially formed film.