Papers by Author: Olivier Raquet

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Authors: Mohamed Sennour, Loïc Marchetti, Stéphane Perrin, Régine Molins, Michèle Pijolat, Olivier Raquet
Abstract: The oxide film formed on nickel-based alloys in Pressurized Water Reactors (PWR) primary coolant conditions (325°C, aqueous media) has been investigated by Transmission Electron Microscopy (TEM). TEM observations revealed an oxide layer divided in two parts. The internal layer was mainly composed of a continuous spinel layer, identified as a mixed iron and nickel chromite (Ni(1-x)FexCr2O4). Moreover, nodules of Cr2O3 were present at the interface between this spinel and the alloy. The external layer is composed of large crystallites corresponding to a spinel structure rich in iron (Ni(1-z)Fe(2+z)O4) resulting from precipitation phenomena. The influence of alloy surface defects was also studied underlining two main consequences on the formation of the passive film e.g. the internal layer. On one hand, the growth kinetics of the internal spinel rich in chromium increased with the surface defect density. Besides that, when the defect density increased, the oxide scale became more finely crystallized. This result agrees with a growth mechanism due to a rate limiting process of diffusion through the grain boundaries of the oxide. On the other hand, the quantity of Cr2O3 nodules increased with the number of surface defects, revealing that the nodules nucleated preferentially at defect location.
Authors: Loïc Marchetti, Stéphane Perrin, Olivier Raquet, Michèle Pijolat
Abstract: Oxidation mechanism of Alloy 690 has been investigated in Pressurised Water Reactor (PWR) primary coolant conditions (325°C, aqueous hydrogenated media). Experiments performed with gold marker and RBS technique reveal that the passive film formation is the consequence of an anionic mechanism. This result is confirmed by experiments achieved with two sequences of corrosion in a H2 16O media and in a mixed H2 16O/ H2 18O media. The localisation of 18O by SIMS analysis in the thin passive layer underlines an oxidation mechanism due to oxygen diffusion by short circuits (like grain boundaries) in the oxide scale. Moreover grain boundary diffusion coefficient in chromite like oxide was estimated to be in the range 2 10-18 – 1 10-17 cm2.s-1 and compared to values extrapolated from higher temperature.
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