Corrosion and Cracking Susceptibility of Austenitic Alloys in Supercritical Water

• Author / Creator

  Behnamian, Yashar

• The present research has been done with the intent of evaluating the corrosion resistance and cracking susceptibility of some commercial candidate alloys that can be potentially used in Gen-IV supercritical water reactor (SCWR). The mechanism of the crack initiation in austenitic alloys exposed to SCW is not entirely understood and experimental data for performance in long-term exposure and upside down situation are not available yet. Fundamentally, austenitic and nickel-base alloys have always attracted a significant attention in power generation industry. However, using these materials for such purposes is limited due to their susceptibility to stress corrosion cracking (SCC). With this in mind, this study was focused on investigation of oxidation of several grades of austenitic stainless steels, and nickel-based alloy exposed to the supercritical water (SCW) at 800 °C for 12 h. Morphology, microstructure, and chemical composition of oxide films formed on stainless steels (SS) 347H, 316L, and 310S, and alloys 625, 214, C2000, and 800H upon SCW exposure were studied using the weight measurement, X-ray diffraction (XRD) analysis, scanning electron microscopy (SEM), and energy dispersive spectroscopy (EDS). It was found that alloying elements can substantially influence the oxidation of austenitic alloys exposed to SCW at high temperature (800 °C) for around 12 h. Results confirmed the presence of a duplex oxide layer consisting of Fe-Cr and Cr-Ni spinel/Cr2O3 on the substrate surface. In addition, the corrosion behavior of SS 316L capsules was investigated after 20,000 h SCW exposure at the temperature of 500 °C. Specimens were analyzed by electron microscopy, energy dispersive spectroscopy, electron energy loss spectroscopy, and X-ray diffraction technique. The film of the corrosion products was composed of an outer layer of magnetite and an inner layer of spinel structure. By increasing the exposure time, a rougher surface and thicker oxide film was approached, which were identified as Fe3O4 + spinel/Cr2O3/Ni-enrichment + SS 316L from the outer to the inner layer. Microcrack initiation was evident around the oxidized grain boundaries. The possible oxidation mechanism was studied and discussed as well. Furthermore, the oxidation behavior of capsules which were made of SS 310S was investigated after 20,000 h exposure to SCW at 500 °C. Later on, samples were analyzed using scanning electron microscopy, energy-dispersive X-rays, Auger electron spectroscopy (AES), and X-ray diffraction analysis. Results revealed that two distinct oxide layers including an outer layer of magnetite and an inner layer of spinel had been formed on the inner surface of the SS 310S capsules. Long exposure to SCW resulted in formation a rougher surface and thicker oxide scale, identified as Fe3O4 + spinel/Cr2O3/Ni-enrichment + SS 310S alloy from the outer to the inner layer. The most probable oxidation and microcrack susceptibility mechanism were proposed and discussed. By employing the complementary characterization techniques, such as transmission electron microscopy, conventional selected area electron diffraction analysis, energy-dispersive X-ray spectroscopy and advanced electron energy loss spectroscopy, it was recognized that the cracks tip were oxidized and had a three-layer morphology in which all the layers tapered toward the crack tip. In the complementary step, the impact of SCW exposure time on the corrosion behavior of 304-ODS (oxide dispersion strengthened) steel in SCW (at the temperature of 650 °C) was investigated. The exposed coupons were analyzed by scanning/transmission electron microscopy equipped with energy dispersive spectroscopy, electron energy loss spectroscopy, X-ray diffraction, and time-of-flight secondary ion mass spectrometry (TOF-SIMS). The results demonstrated that the weight gain increased by escalating the SCW exposure time, following a parabolic behavior. An oxide scale was formed on 304-ODS alloy which was made of two distinct layers, including an outer layer of magnetite (Fe3O4) and an inner layer of FeCr2O4/(Fe,Cr)2O3. Additionally, the oxidation mechanism was discussed.

• Subjects / Keywords

  • Stress Corrosion Cracking
  • Supercritical water
  • Austenitic Alloys
  • Corrosion
  • Crack Susceptibility

• Graduation date

  Fall 2015

• Type of Item

  Thesis

• Degree

  Doctor of Philosophy

• DOI

  https://doi.org/10.7939/R3G737C0K

• License

  This thesis is made available by the University of Alberta Libraries with permission of the copyright owner solely for non-commercial purposes. This thesis, or any portion thereof, may not otherwise be copied or reproduced without the written consent of the copyright owner, except to the extent permitted by Canadian copyright law.