Evolution of non-stoichiometric iron sulfide film formed by electrochemical oxidation of carbon steel in alkaline sour environment

Non-stoichiometric iron sulfide films (Fe_xS_y) were formed electrochemically on a 1018 carbon steel/1 M (NH₄)₂S, 500 ppm CN^- interface, using cyclic chronoamperometry for different time intervals. The films showed great stability in medium typical of the catalytic plants of PEMEX Mexico (0.1 M (NH₄)₂S, 10 ppm CN^- as NaCN, pH 8.8). Characterization of the films by cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) show two different behaviors depending on the growth time. For films grown at times <15 min, oxidation of the film was continuous, while oxidation ability for the film formed at times >15 min was lower. Film characteristics were more clearly defined by EIS experiments, as the Nyquist diagrams show depressive loops with high, real impedance values (Zr>1 kΩ cm²) for films grown at longer times. Structural characterization by X-ray diffraction (XRD) and Scanning Electron Microscopy (SEM) correlated electrochemical behavior with topographical changes and chemical composition of the films formed. The abundance of sulfur and pyrrhotite is evident in the samples grown for increasing times, and is likely due to electrochemical/chemical oxidation of iron sulfides during film growth. The sulfur-rich layer is responsible for the passive character of these films. The equivalent electrical circuit describing the EIS spectra for films formed over longer times has fewer elements than that used to model EIS spectra for films grown at shorter times. In particular, diffusion of atomic hydrogen is not apparent in sulfur-enriched films, and the charge transfer is carried out at the metal/film interface with values that are insensitive to film thickness and chemical nature.