Internal Oxidation–Nitridation of Ferritic Fe(Al) Alloys in Air
Exposure of undoped Fe(Al) and Fe(Al)+Cr ferritic alloys in laboratory air at 900–1,000 °C resulted in significant internal attack after 5,000 h, including oxides and underlying nitrides. In the most severely attacked alloys, kinetics based on mass gain and maximum penetration depth were linear; also, the deepest penetrations were a significant fraction of the specimen thickness, and were thickness-dependent. Little internal attack was observed at 700–800 °C where these compositions may be used as coatings. The extent of internal attack did not decrease with increasing Al or Cr content which may indicate that rather than classical internal oxidation this attack is related to the permeation of N through a defective external scale. No internal attack was observed in alloys doped with Y, Zr, Hf or Ti where the substrate-alumina scale interface was flatter.
KeywordsFerritic Fe(Al) Internal oxidation Nitridation Kinetics
The author would like to thank G. Garner, J. Moser, J. Vought, H. Longmire and L. Walker at ORNL for assistance with the experimental work; J. Regina and A. Marder from Lehigh Univ. for the initial alloy specimens; D. J. Young from the Univ. of New South Wales, Australia, for several discussions on these results; and I. G. Wright, M. P. Brady and P. F. Tortorelli for comments on the manuscript. MJD worked at ORNL as part of the SULI program and RMD as part of the HERE program, both administered by Oak Ridge Associated Universities. The research was sponsored by the U.S. Department of Energy, Fossil Energy Advanced Research Materials Program and work at the SHaRE User Facility by the Division of Scientific User Facilities, under contract DE-AC05-00OR22725 with UT-Battelle, LLC.
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