EFTEM and EELS analysis of the oxide layer formed on HCM12A exposed to SCW Jeremy Bischoff ⇑ , Arthur T. Motta Department of Mechanical and Nuclear Engineering, Pennsylvania State University, 227 Reber Building, University Park, PA 16802, USA article info Article history: Received 23 December 2011 Accepted 12 June 2012 Available online 20 June 2012 abstract The inner-diffusion layer interface of an HCM12A sample oxidized in 600 °C supercritical water (SCW) was analyzed using EFTEM and EELS. The EFTEM analysis showed the presence of chromium-rich zones linked with the porosity within the inner layer, as well as a nanometric iron–chromium separation, which may be linked with the presence of both Fe 3 O 4 and FeCr 2 O 4 in this layer. The diffusion layer was charac- terized by large chromium-rich oxides located at the tempered martensite lath boundaries, which sug- gested the preferential grain boundary diffusion of oxygen and the preferential oxidation of the chromium carbides present at these boundaries. The metal grains of the diffusion layer contained nano- metric chromium-rich spinel oxides. The presence of large chromium-rich oxide precipitates in the dif- fusion layer appears to help improve the corrosion resistance of these alloys. Ó 2012 Elsevier B.V. All rights reserved. 1. Introduction 1.1. Background As part of the Generation IV forum, the Supercritical Water Reactor is envisioned for its high thermal efficiency and simplified core [1] and is designed to operate at high outlet temperature (be- tween 500 °C and 600 °C). Consequently, the goal is to find clad- ding and structural materials that can perform at these elevated temperatures for extended exposures. Because of their resistance to irradiation and stress corrosion cracking, ferritic–martensitic steels, such as HCM12A, are candidate materials for the supercrit- ical water reactor [2]. Nevertheless, the process of uniform corro- sion of HCM12A has to be better understood in order to better predict and control it. The oxide layers formed on HCM12A during exposure to super- critical water have been previously studied using scanning electron microscopy (SEM), X-ray diffraction (XRD), and electron backscat- ter diffraction (EBSD) [2–4]. These studies have shown that HCM12A forms a dual layer structure at 500 °C with Fe 3 O 4 in the outer layer and spinel (Fe,Cr) 3 O 4 in the inner layer with some evi- dence of Cr 2 O 3 [4]. At 600 °C a diffusion layer is also observed, con- taining a mixture of oxide precipitates and metal grains [2]. The present article follows a previous experiment, which ana- lyzed the oxide layer microstructure of HCM12A using microbeam synchrotron radiation diffraction and fluorescence, and studied the influence of the base alloy microstructure on the advancement of the oxide front using energy filtered transmission electron micros- copy (EFTEM) [5]. That study showed that the carbides present at the tempered martensite lath boundaries were oxidized preferen- tially, leading to the formation of chromium-rich oxide precipitates along these boundaries in the diffusion layer [5]. The present arti- cle shows a more detailed analysis of the inner and diffusion layers of an HCM12A sample exposed to 600 °C for 2 weeks in supercrit- ical water (SCW), using EFTEM and electron energy-loss spectros- copy (EELS) in order to better understand the advancement of the inner layer into the diffusion layer. The focus is to obtain a micrometric and nanometric distribution of elements in these two layers as well as their oxide microstructure. 1.2. Previous EELS characterization of iron oxides Several EELS studies have been performed on iron oxides, show- ing differences in the EELS spectra from FeO, Fe 3 O 4 and Fe 2 O 3 [6– 10]. The study of both the oxygen and the iron edges can generate information on the oxide phase being analyzed. Since it is difficult to distinguish between the various iron oxide phases using only the iron edge, we focused our analysis on the oxygen edge, which also gives information on the oxidation state of iron and the iron oxide phase the oxygen atoms belong to. Colliex et al. have studied the iron oxides using EELS [6]. Fig. 1 shows the variations of the oxygen edge spectrum corresponding to the different iron oxide phases [6]. The oxygen edge contains four main peaks referred to as (a–d) (marked in the a-Fe 2 O 3 quadrant) in Fig. 1. The relative height and position of these peaks give information on the electronic structure and coordination chemistry of the absorbing O atoms and can also help measure the valence state of iron, the average interatomic dis- tances between the absorbing O atom and its nearest neighbors, 0022-3115/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jnucmat.2012.06.017 ⇑ Corresponding author. Tel.: +1 814 865 0036. E-mail address: bischoff.jeremy@gmail.com (J. Bischoff). Journal of Nuclear Materials 430 (2012) 171–180 Contents lists available at SciVerse ScienceDirect Journal of Nuclear Materials journal homepage: www.elsevier.com/locate/jnucmat