Contribution to the knowledge of the dissolution mechanism of stainless steel in nitric acid highlight Cr(VI) in the oxide layer
Résumé
The materials aging (mainly stainless steels) of the spent nuclear fuel reprocessing plant is the focus of an important RetD activity at CEA. The control of this aging will be achieved by a better understanding of stainless steels corrosion mechanisms in the dissolution medium. This medium is mainly hot concentrated nitric acid and sometimes in the presence of oxidizing ions. Thus it's really important to describe precisely the mechanism of corrosion during the dissolution and find an indicator of the transition between passive field and transpassive field where the intergranular corrosion lead to an important increase of corrosion rate. First, a literature highlight by electrochemistry experiment (potential imposition) the presence of Cr(VI) in the oxide layer of stainless steel 304L in nitric acid [1]. Secondly a study of chemical corrosion was undertaken through immersion tests on stainless steel 304L in boiling nitric acid and which contain sometimes oxidizing species (Ce(IV), V(V)) [2]. By using several analytical techniques (X-ray photoelectron spectroscopy, Scanning Electron Microscopy, Ultravioletvisible spectroscopy ), those experiments have improved the knowledge of the dissolution mechanism including highlighting the presence of an extreme surface layer of Cr(VI) that could be related to a grain marking and a preliminary step of intergranular corrosion.