Evolution of the Oxide Structure of Ferritic-Martensitic Steels Exposed to Supercritical Water

Open Access
Author:
Bischoff, Jeremy
Graduate Program:
Nuclear Engineering
Degree:
Master of Science
Document Type:
Master Thesis
Date of Defense:
None
Committee Members:
  • Arthur Thompson Motta, Thesis Advisor
Keywords:
  • martensitic
  • ferritic
  • supercritical water
  • oxide
  • steel
  • synchrotron
  • X-ray diffraction
Abstract:
The microstructure of the oxide layers formed on ferritic-martensitic steels exposed to supercritical water was characterized as a means to understand the oxidation mechanism of such alloys in supercritical water. This is of interest because a supercritical water reactor design is envisioned as one of the Generation IV reactors. Three alloys were studied: 9CrODS, HCM12A and HT9. 9CrODS is an oxide dispersion strengthened steel that contains nano-particles of Y2O3. These alloys were corroded in the supercritical water corrosion loop at the University of Wisconsin at two temperatures (500ºC and 600ºC) during different exposure times. Both 9CrODS and HCM12A were corroded at both temperatures during 2, 4 and 6 weeks. HT9 was only corroded at 500ºC during 1 and 3 weeks. The oxide microstructure was characterized by using synchrotron microbeam X-ray diffraction and fluorescence using the APS synchrotron facility at the Argonne National Laboratory. This technique has a high spatial resolution enabling a detailed analysis of the oxide structure. Both elemental information with the fluorescence data and microstructural information with the diffraction data are acquired simultaneously. The three alloys had similar oxide microstructures with Fe3O4 in the outer oxide layer, with a mixture of FeCr2O4 and Fe3O4 in the inner oxide layer, and with a diffusion layer often containing chromium rich oxide precipitates. The oxide precipitates in the diffusion layer were mainly FeCr2O4 in the 600ºC samples, but in the 500ºC samples the diffusion layer contained few oxide precipitates and was a solid solution of oxygen ahead of the oxide. The corrosion resistant interface was characterized by chromium enrichment in the fluorescence data and the presence of Cr2O3. In most cases this interface was the inner oxide-diffusion layer interface. Nevertheless, in the 9CrODS 600ºC 4 and 6 week samples, the corrosion resistant interface shifted to the diffusion layer-metal interface, where a uniform Cr2O3 ribbon had formed. The presence of such a Cr2O3 ribbon is thought to have an important role in the oxidation resistance of the material. HCM12A and HT9 had similar corrosion behaviors but 9CrODS formed the most protective oxide even though it contains the least amount of chromium. The enhanced corrosion resistance of the 9CrODS is likely due to the fine dispersion of the Y2O3 nano-particles. Finally, an oxidation mechanism was proposed, where the outer oxide layer is formed by outward diffusion of iron cations, and the inner oxide layer is formed by inward diffusion of oxygen anions.