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The effects of irradiation on the oxidation first stages of 316L austenitic stainless steel

Boisson, Marylou. The effects of irradiation on the oxidation first stages of 316L austenitic stainless steel. PhD, Science et Génie des Matériaux, Institut National Polytechnique de Toulouse, 2018

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Abstract

Core internals of Pressurized Water Reactors (PWRs) are made of austenitic stainless steel (304Lor 316L). These materials are exposed to the primary medium under irradiation and undergomechanical stresses. Under these conditions, degradation of the components by IrradiationAssisted Stress Corrosion Cracking (IASCC) is likely to occur. This complex phenomenon canhave a significant influence on the maintenance time and cost of the internal components ofPWRs. The understanding of the oxidation mechanisms at IASCC crack tip is therefore essentialto better apprehend IASCC. From a practical point of view, this goes through that of the holding ofthe passive layer during the corrosion first stages. The aim of this work is to better understand therole of irradiation on the oxide films formation. To limit the experimental complexity associated withthe activation generated by neutrons, the effect of neutron irradiation was simulated by syntheticproton pre-irradiations. The TEM observations carried out have shown that synthetic irradiationsreproduce defects as well as intra and intergranular segregation. Short time oxidations insimulated primary medium (between 5 min and 96 h) were performed to study the oxidation firststages. Thus, this study focused on the understanding of the phenomena governing the initialstages of formation and growth of the oxide layers formed on 316L alloy. The oxidations weredesigned to obtain, on a same sample, an irradiated and non-irradiated zone in order todecorrelate the effect of irradiation under identical oxidation conditions. The crystallographicorientation of the underlying metal grains was taken into account and its effect was evaluated. Theinfluence of some surface inhomogeneities (cold-work and inclusions) has also been studied. Inaddition, a sample was oxidized under proton flux for 24 hours to study the influence of IrradiationAssisted Corrosion. Finally, the oxides formed at the IASCC crack tip taken from a bolt extractedfrom a French nuclear power plant were studied and compared to those formed on the samplesoxidized in simulated PWR medium. The oxides were studied from the macroscopic scale down tothe nanoscopic scale (optical microscopy, Grazing Incidence X-ray Diffraction, Ramanspectroscopy, Scanning and Transmission Electron Microscopy). The morphology, thickness,structure and composition of the oxides have been analyzed. The oxides are, in all cases, duplexand of spinel structure. Outer crystallites are iron-rich while the inner oxide is rich in chromium,iron and nickel. The comparison of irradiated and unirradiated samples revealed that the natureand density of defects near the metal/oxide interface play a role in the morphology, thickness andcomposition of the oxides. The oxides are similar to those formed at the IASCC crack tip takenfrom the extracted component. This work demonstrates that the crystallographic orientation of thesubstrate influences the oxidation kinetics and that an epitaxial relationship is established betweenthe oxides and the underlying metal grains. Moreover, it has been found that cold-work and thepresence of inclusion affect the oxides formed. All these results have brought new elements ofunderstanding relating to the formation and growth of oxides formed on austenitic stainless steelsexposed to primary water. This work also highlights the effect of irradiation defects on the oxidesformed and the alternative represented by synthetic pre-irradiations to emulate neutronmicrostructures. A corrosion mechanism taking into account the crystallographic orientation, thesurface state of the material and the irradiation defects has been proposed.

Item Type:PhD Thesis
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Institution:Université de Toulouse > Institut National Polytechnique de Toulouse - Toulouse INP (FRANCE)
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Research Director:
Andrieu, Eric and Laffont, Lydia
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Deposited On:08 Nov 2021 10:19

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