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Comportement à la corrosion, passivité et mécanismes de transport des ions sur les alliages contenant du Cr

Abstract : The corrosion behaviour and ion transport mechanisms were investigated by surface analytical techniques (Time-of-Flight Secondary Ion Mass Spectrometry and X-ray Photoelectron Spectroscopy) combined with electrochemical measurements on Cr-containing alloys, including CoCrFeMnNi high entropy alloy (HEA), 304L and 316L stainless steels (SS). For HEA, both air-formed native oxide and passive films (formed in sulfuric acid) have a bilayer structure, comprising Cr and Mn inner layer and Cr/Fe/Co mixture outer layer. No nickel is observed in the oxide layer. The effects of exposure of the native film to sulfuric acid and passivation under anodic polarization have been investigated.For SS, both the native and passive films formed on 304L and 316L SS exhibit a bilayer structure, with Fe-rich and Mo-rich (for 316L SS) outer layer and Cr-rich inner layer. The thermal stability of the passive film formed on 316L stainless steel surface was tested during the gradual exposure, under Ultra High Vacuum, of the oxide from room temperature to 300°C. Below 250 °C, the dehydroxylation and dehydration of the surface oxide films were observed by ToF-SIMS in-depth profiling . Above 250 °C, the main modification in the film is linked to the diffusion of Cr3+ cations in the oxide, resulting in the formation of chromium oxide at the expense of iron oxide (thermodynamically less stable) at the internal oxide / external oxide interface. The ion transport mechanism on SS was then elucidated thanks to the development of a 2-step process, where the first step (native or passive oxide growth) is followed by oxidation in oxygen gas at high temperature (300°C) under low pressure of isotopic 18O2. The results reveal that the outward diffusion of cations governs the growth of the oxides. On the 316L stainless steel, the outer layer of Mo oxide prevents the continuous transport of Cr ions to the surface. Passive films, due to their composition and structure, have an oxidation rate significantly lower than that of native oxide films.
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Luntao Wang. Comportement à la corrosion, passivité et mécanismes de transport des ions sur les alliages contenant du Cr. Chimie analytique. Université Paris sciences et lettres, 2020. Français. ⟨NNT : 2020UPSLC014⟩. ⟨tel-03549703⟩

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