Electrochemical Properties of Passive Film Formed on Supermartensitic Stainless Steel in a Chloride Medium

The electrochemical behavior and the electronic properties of passive films formed on a super martensitic stainless steel (SMSS) used in oil and gas industries were investigated in aqueous 0.6 M and 2.1 M NaCl solutions with additions of sodium acetate and acetic acid (pH 4.5). Open-circuit potential transients, electrochemical impedance spectroscopy, cyclic voltammetry, and x-ray photoelectron spectroscopy were measured to characterize the passive film formed on SMSS. The electrochemical behavior of the steel in an aqueous solution of 0.6 M NaCl presented the highest pitting potential and the highest polarization resistance in relation to the NaCl/NaAc solution. The passive film of SMSS in an aqueous solution of NaCl presented a thickness of 18.40 nm, three times the thickness of the oxide film in NaCl/NaAc, and consisted of FeO, Cr₂O₃, MoO₂, and spinels such as FeCr₂O₄ species that are a p-type semiconductor, but may also contain a small fraction of the Fe₂O₃ and MoO₃ oxides. Additionally, it was shown that the passive layer after immersion in a saline solution also contains hydroxides such as FeOOH and Cr(OH)₃.