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The corrosion resistance of stainless steels is determined by the thickness, structure, composition and electronic properties of the oxide layers grown on their surface and isolating the metallic substrate from the environment. In the present work, ex situ XPS (X-Ray Photoelectron Spectroscopy) and ToF-SIMS (Time of Flight Secondary Ions Mass Spectrometry) have been combined to in situ PCS (PhotoCurrent Spectroscopy) in order to perform an integrated physicochemical characterization of surface oxide films grown on bright annealed ferritic stainless steel (AISI 434) as-received and after thermal post-treatment in air. The surface oxide film on as-received samples has a bilayer structure with an inner layer close to pure Cr(III) oxide and an outer layer of mixed Fe(III) and Cr(III) oxide enriched in Cr. After thermal post-treatment in air at 350°C and 550°C, the surface oxide films thicken and the outer layers are enriched in Fe(III). Linear voltammetry and electrochemical impedance spectroscopy analysis evidence a decrease of the polarization resistance in the passive range, a cathodic shift of the corrosion potential and an increased cathodic current after thermal post-treatment. The bilayer structure, composition and electronic properties of these surface oxide films and their modifications are discussed to rationalize their corrosion behavior.
Franco et al. (Sun,) studied this question.