Structural Evolution of a Ni Alloy Surface During High-Temperature Oxidation
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We show that considerable structural transformations occur at a Ni alloy surface during the transient stages of high-temperature oxidation. This was demonstrated by exposing the alloy to high-temperature CO2 for short times at both atmospheric and supercritical pressures. A protective Cr-rich oxide layer formed after only 5 min at 700 °C and persisted for longer exposures up to 500 h. Voids formed and grew over time by the condensation of metal vacancies generated during oxidation, while the alloy surface recrystallized after sufficient oxidation had occurred. The oxygen potential established at the oxide/alloy interface led to oxidation along the newly formed grain boundaries as well as adjacent to and inside of the voids. Al, the most stable oxide-former and present at low concentration in the alloy, was preferentially oxidized in these regions. The results provide an improved understanding of the internal oxidation of Al and its role in enhancing scale adhesion for this class of Ni alloys.
KeywordsNickel alloy Transient oxidation Chromia former Void formation Recrystallization
This work was performed in support of the US Department of Energy’s Fossil Energy Crosscutting Technology Research and Advanced Turbine Programs. The Research was executed through NETL Research and Innovation Center’s Advanced Alloy Development Field Work Proposal. This research was supported in part by an appointment (RPO) to the NETL Research Participation Program sponsored by the US Department of Energy and administered by the Oak Ridge Institute for Science and Education. Research performed by AECOM Staff was conducted under the RES contract DE-FE-0004000. The authors thank Dr. Jinichiro Nakano and Dr. Anna Nakano for performing the high-temperature confocal scanning laser microscope (5 min) exposure in the Corrosion Electrochemistry Laboratory at NETL.
This project was funded by the Department of Energy, National Energy Technology Laboratory, an agency of the United States Government, through a support contract with AECOM. Neither the United States Government nor any agency thereof, nor any of their employees, nor AECOM, nor any of their employees, makes any warranty, express or implied, or assumes any legal liability or responsibility for the accuracy, completeness, or usefulness of any information, apparatus, product, or process disclosed, or represents that its use would not infringe privately owned rights. Reference herein to any specific commercial product, process, or service by trade name, trademark, manufacturer, or otherwise, does not necessarily constitute or imply its endorsement, recommendation, or favoring by the United States Government or any agency thereof. The views and opinions of authors expressed herein do not necessarily state or reflect those of the United States Government or any agency thereof.
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