Soutenance de thèse : Kathleen JAFFRE

Influence of mechanical surface treatments on oxide films properties formed on 304L stainless steels in simulated BWR and PWR primary water

Doctorante : Kathleen JAFFRE

Laboratoire INSA : Mateis
Ecole doctorale : ED34 : Matériaux de Lyon

As some mechanical surface treatments may be applied during service life of nuclear components, the objective of this work is to determine their influence on surface state of the 304L stainless steel (SS) and its oxidation behaviour during exposures in a simulated BWR and PWR environment. In this regard, the oxidation features of SS modified surfaces by dry grinding or mechanical polishing were compared. To describe the microstructural modification induced by the surface treatment, subsurface characterizations were first performed. The characterizations tools used are scanning electron microscopy (SEM), transmission electron microscopy (TEM), 3D optical profilometer, and X-ray diffraction. Each surface treatment affects differently the surface state. The underneath surface of the ground specimens presents an ultrafine grain layer and a strong plastic deformation, while the thickness of the ultrafine grain layer is related to the final polishing stage for the polished specimens. Grinding also induces high residual compressive stresses and high roughness compared to polishing. Then, the reactivity of the modified surface was evaluated under different conditions. The electrochemical protocol, based on electrochemical impedance spectroscopy coupled with Mott-Schottky analysis, was firstly set up to study the properties of passive films formed on SS in a boric acid/borate buffer solution at room temperature as a function of the surface treatment. Then, the reactivity of the surface oxidized in autoclave was determined ex-situ by the same method complemented by morphology and composition characterizations by either SEM, TEM, or X-ray photoelectron spectrometry. Whatever  the simulated environment, the mechanical surface treatment mainly influences the thickness and doping density of the inner oxide and the composition and morphologies of the outer oxide precipitates in terms of shape and size. Finally, the impact of the considered environment on oxidation features was also discussed.