Microstructural Characterization of Oxide Scales Grown on Austenitic Fe-Cr-Ni Alloys Exposed to Supercritical Water (SCW)

Abstract

Microstructural Characterization of Oxide Scales Grown on Austenitic Fe- Ni- Cr Alloys Exposed to Supercritical Water (SCW) S. Mahboubi, G. Botton and J. Kish1 Department of Materials Science and Engineering, McMaster University, 1280 Main Street West, Hamilton, ON, CA mahbos@mcmaster.ca, gbotton@mcmaster.ca, kishjr@mcmaster.ca The aim of this research was to compare the oxide scales formed on Alloy 800 HT and Alloy 3033 after exposure in supercritical water (SCW). Test coupons were exposed to 25 MPa SCW with 8 ppm dissolved oxygen at 550 °C for 500 h in a static autoclave. Auger Electron Spectroscopy (AES) was used for plan view imaging and depth profile analysis of the oxide scales. Transmission Electron Microscopy (TEM) along with Energy Dispersive Spectroscopy (EDS) was used on thin foil TEM specimens prepared by Focused Ion Beam (FIB) milling for imaging, qualitative and semi quantitative analysis of the scale/substrate interface. TEM-EDS line scans and phase maps showed the presence of double oxide layer on Alloy 800 HT consisting of a Cr-rich inner layer and a Fe-rich outer layer. In contrast, only a single Cr-rich layer was found on the surface of Alloy 3033. Selected Area Diffraction (SAD) and Electron Energy Loss Spectroscopy (EELS) were conducted for analysis of the oxide scales. Results revealed that the oxide particles randomly distributed on Alloy 800 HT surface were Fe2O3. The Cr-rich layer on both alloys was identified as Cr2O3 and the Fe-rich outer layer on Alloy 800 HT was identified as a mixture of FeO and Fe3O4. Moreover, descaling procedure showed that Alloy 3033 exhibited a lower weight loss than Alloy 800 HT. The manner in which the Cr-rich layer formed on Alloy 3033 is more protective than that formed on Alloy 800HT is being examined within the context of diffusion process. Keywords: Characterization, Oxide Scales, Austenitic, SCW.