Electrochemically grown passive films on carbon steel (SAE 1018) in alkaline sour medium

• Published in: Electrochimica acta 2003-05, Vol.48 (12), p.1665-1674
• Main Authors: Sosa, E, Cabrera-Sierra, R, Oropeza, M.T, Hernández, F, Casillas, N, Tremont, R, Cabrera, C, González, I
• Format: Article
• Language: English
• Subjects: Alkaline sour media; Applied sciences; Carbon steel; Corrosion; Corrosion mechanisms; EIS; Exact sciences and technology; Iron sulfide film; Metals. Metallurgy; SEM; SPECM; XPS
• ISSN: 0013-4686; 1873-3859
• DOI: 10.1016/S0013-4686(03)00145-2

Corrosion films were prepared by applying cyclic potential pulses to the 1018 carbon steel–sour medium interface (1 M (NH 4) 2S, 500 ppm CN −) for 1 min. Electrochemical behavior and surface morphology of these films were determined using electrochemical impedance spectroscopy (EIS), scanning electron microscopy, and scanning photoelectrochemical microscopy (SPECM). EIS diagrams and SPECM images show the passive properties and homogeneity of the films. Furthermore, X-ray photoelectron spectroscopy (XPS) was used to characterize their chemical nature and structure. XPS results show that different oxide and sulfur structures were developed during the electrochemical oxidation of carbon steel in concentrated sour media. The analysis of O 1s data indicated that, during film growth, H 2O and/or hydroxyl groups are incorporated into the film structure. The XPS spectra of Fe 2p show iron bonds with S as iron sulfide (FeS 2 and FeS) and the corresponding peak of O 1s shows those bonds with oxygen as Fe 2O 3 and/or FeO. XPS depth profile analyses for the film showed that the ratio of FeS and FeO increases from film surface to film–carbon steel interface. This corroborates the diffusion of iron ions through the film during its electrochemical growth. The chemical shift through the film for the peak associated with Fe 2p signal proves that transport mechanism of iron ions through the film is carried out by chemical diffusion.