Evolution of the Oxide Microstructure and Oxide Growth Induced Residual Strains During Waterside Corrosion of Zirconium Alloys

Open Access
Author:
Spengler, David Joseph
Graduate Program:
Nuclear Engineering
Degree:
Master of Science
Document Type:
Master Thesis
Date of Defense:
None
Committee Members:
  • Arthur Thompson Motta, Thesis Advisor
Keywords:
  • zircaloy-4
  • corrosion
  • zirconium oxide
  • synchrotron radiation
  • X-ray diffraction
Abstract:
The oxide layer formed on Zircaloy-4 during high temperature waterside corrosion was characterized in an attempt to understand the mechanisms behind the corrosion process and the oxide transition. The samples examined were Zircaloy-4 and zirconium corroded with pure water at temperatures between 270 °C (520 °F) and 360 °C (680 °F) in an autoclave environment. The oxides were characterized using microbeam synchrotron x-ray diffraction and fluorescence, 3-D Laue x-ray microscopy, and scanning electron microscopy. The crystal structure and chemical composition of the oxide as a function of position within the oxide layer were determined at the sub-micron scale using the x-ray microbeam at the 2-ID-D beamline at the Advanced Photon Source (APS) at Argonne National Laboratory (ANL). This facility is unique in that it combines high spatial resolution with high flux and allowed us to examine the oxide in an extremely detailed manner to produce new information on the oxide structure. The oxide was found to be comprised primarily of monoclinic ZrO2 with a small amount of tetragonal ZrO2 as well. Well-defined periodic variations in the diffracted intensity from both phases as a function of distance from the oxide-metal interface were observed, with an average period of 1.9 µm corresponding to the oxide transition thickness. Strong in-phase relationships were observed between several of the main monoclinic and tetragonal diffraction peaks, and an out of phase relationship was seen in one monoclinic peak. The oxide grains were shown to grow with their monoclinic (200) planes parallel to the oxide-metal interface. A Zr3O “suboxide” phase was observed in a small (1-2 µm) region of metal immediately adjacent to the oxide-metal interface. This observation corresponds well to previous transmission electron microscopy studies which found a region of high oxygen content at this location. The measured tetragonal phase fraction was found to be highest at the oxide-metal interface, reducing over a range of approximately the oxide transition thickness to a constant bulk oxide value. Both the monoclinic and tetragonal grain sizes were observed to be smallest at the oxide-metal interface and grow in size to a maximum value with residence time in the oxide. The maximum tetragonal grain size was seen to be smaller than the monoclinic grain size. These studies helped us investigate the basic corrosion mechanisms through observation of the oxide structure which helped us answer questions on the role of each oxide phase during the corrosion process. Strains in the metal substrate induced by volume expansion during oxide formation were investigated using the 3-D x-ray microscope at beamline 34-ID-E at the APS at ANL. This beamline has the capability to measure very small changes in d-spacing as a function of depth into a material, allowing us to measure changes in d-spacing (and thus calculate strain and stress) in metal grains immediately beneath the oxide layer. Since stresses in the oxide correspond to those in the metal, we were able to indirectly examine the oxide stress state and level as well as investigate the effect of oxide thickness and the oxide transition on accumulated stresses. Experiments were performed at room temperature and with the samples at the corrosion temperature to mitigate the effects of differential cooling between the metal and oxide. The oxide grown on Zircaloy-4 was shown to induce some level of both plastic and elastic deformation in the underlying metal. Plastic deformation was visible in the streaking of the Laue x-ray spots. The degree and mode of plastic deformation were not calculated but were qualitatively observed to vary with position between metal grains or even within different locations in a single metal grain. Bare metal Zircaloy-4 and crystal bar zirconium did not show evidence of plastic deformation in the metal. The level of stress in the oxide may be a result of the oxide thickness and oxide transition. Oxides just before transition were found to have a higher level of stress compared to oxides just after transition, and high stresses were again observed in oxides nearing their second transition thickness. The stress state in the metal was found to vary with position, as both tensile and compressive stresses were measured in different samples and grains in the same sample despite our expectations of an overall tensile stress in the metal. Temperature was shown to play a significant role in the measured strains in the metal. Stresses calculated from high temperature strains were lower than room temperature stresses, indicating that the stresses present during corrosion are lower than those measured at room temperature.