HRTEM
,
HREELS
analyses and modelling of nanometric oxide layers formed on
316L
in simulated Pressurized Water Reactor (
PWR
) conditions
Authors: Legras Laurent , Mansot Jean‐Louis , Baranek Philippe , Soulas Romain 
Published: December 2016

The development of a passivation layer on stainless steels in Pressurized Water Reactor (PWR) environment is a key phenomenon in Stress Corrosion Cracking (SCC) behavior. Although several works pointed out the presence of a duplex oxide layers, the contribution of the oxide layers themselves is still poorly explored and appears as a crucial factor in the understanding of SCC in PWR medium. The present work studies the oxides growth during short time corrosion tests (0h up to 24h) in a specific corrosion loop running in PWR conditions. Post mortem investigations are carried out using various characterization techniques ranging from macroscopic to atomic scale
1
. HRTEM images, acquired on an objective lens aberrations corrected TITAN at Marseille IM2NP
2
, are analyzed using ASTAR software
3
in order to investigate the crystallographic structure of oxides and the evolutions as a function of oxidation time (Fig 1&2). The outer oxide crystallites present a spinel crystal structure. Their size increases with oxidation time. The inner oxide layer is amorphous during the first stages of growth and becomes fully crystalline after 10 mn leading to mono‐crystalline domains on every metal grains. These mono crystals have a FCC spinel structure in epitaxy with the underlying metal grains. EELS analyses showed that the Cr/Fe ratio in the outer layer decreases and becomes close to pure magnetite after 10mn whereas the Cr/Fe ratio in the inner layer is increasing up to 1 showing a Cr enrichment. Depending on the ratio of Cr/Fe, the structure of the monocrystals in the inner oxide layer might be either normal or inversed spinel
4
. In order to investigate this question, the evolution of the Fe‐L
2,3
Near Edge Structures (NES) were followed using condensor lens aberrations corrected TITAN with a mono‐chromated gun (ΔE<200mV) equipped with Gatan Quantum HR (EDF R&D). The experimental Fe edges were reconstructed using linear combination method
5
of spectra obtained on pure FeO, Fe
2
O
3
and Fe
3
O
4
(Fig3) as standards. Whereas a good match is obtained for the outer oxide layer, the reconstruction method fails for the inner layer. Thus,
ab initio
simulations were performed in order to understand the influence of the Fe substitution by Cr on Fe
3
O
4
crystal structure and electronic properties. A hybrid functional, combining PBE exchange‐correlation functional with Hartree‐Fock exchange, was parameterized in order to reproduce the various magnetic phases of this spinel, first of all the spinel inverse phase of Fe
3
O
4
which is the ground state of this oxide. Then, the crystal structure and electronic properties of Fe
(3‐x)
Cr
x
O4 were determined (Fig 4). For each ratio, the spin repartition was determined in order to minimize the total energy of the system. A semi‐metal to insulating phase transition was found for 0 < × < 0.05. All the obtained data are discussed in terms of lattice parameters, band structures, DOS and mulliken charges evolutions.