Chemical characterization of corrosion films electrochemically grown on carbon steel in alkaline sour environment

Surface reactivity and the chemical nature of corrosion films electrochemically formed on carbon steel in a 1 M (NH₄)₂S and 500 ppm CN^-1 medium for different growth times were studied by scanning photoelectrochemical microscopy (SPECM) and X-ray photoelectron spectroscopy (XPS). SPECM diagrams corresponding to films grown for times <15 min revealed their heterogeneous and protective nature, while for times >15 min these films showed surface activity and homogeneity in the characterization medium. The XPS spectra of Fe 2p, S 2p, and O 1s on the film surface indicated that FeS₂ and FeO are present in the protective (heterogeneous) films; meanwhile, Fe(OH) and FeS are the chemical species present in nonprotective (homogeneous) films. The O 1s peak analysis using a mix of Gaussian and Lorentzian functions indicated that during film growth both H₂O and/or hydroxyl groups are incorporated into the film structure. These results prove that the corrosion film composition was modified as a function of electrochemical oxidation time and this composition is directly related to the passive properties of these films.