ATEM SEM STUDY OF THE OXIDES DEVELOPED IN SCC CRACKS AND AT THE SURFACE OF NICKEL BASED ALLOYS EXPOSED IN PRIMARY WATER

摘要

Chromium is known for its protective effect against corrosion and stress corrosion cracking behavior of nickel based alloys. In this paper we study the oxide growth on nickel based alloys in PWR primary water as a function of different parameters: location (outer surface, SCC cracks), mechanical polishing and stress. This study is conducted on nickel based alloys with Cr contents ranging from 5% up to 30% (Ni balanced, other elements equivalent to industrial alloys). A detailed characterization of the oxide scale is performed by SEM and ATEM. The results reveal that: 1) mechanical polishing modifies the beneficial effect of chromium, 2) the applied stress leads to a decreased chromium concentration in the protective layer, 3) in SCC cracks observed on a reversed U-bend (RUB test) made of alloy 600, the outer crystallites have the same morphology as those observed at the outer surface on Ni based alloys with lower Cr concentrations, but the Cr protective layer is not symmetrical on each side of the SCC cracks. The results will be presented and discussed based on the oxide morphology, structure and chemical composition.