A Mössbauer study of the oxidation of Fe-Ni alloys at 535 and 635°C

Abstract

Fe⁵⁷ transmission Mössbauer spectroscopy, supported by metallography, SEM and X-ray diffraction analysis, has been employed to study the oxidation of Fe-Ni alloys at 535 and 635° C in 1 atm. of air. With increasing Ni content of the alloy, the composition of the scale changed and the oxidation rate decreased. For an alloy containing 0.9% Ni, the oxide scale produced at 535° C was Fe₃O₄ covered by a thin outer layer ofα-Fe₂O₃, while at 635° C FeO was additionally present as a major phase. The scale formed on a 10% Ni alloy at both 535 and 635° C was similar to that observed for the 0.9% Ni alloy oxidized at 535° C (i.e. of Fe₃O₄ andα-Fe₂O₃), although theα-Fe₂O₃ layer tended to be relatively thicker. For a 49% Ni alloy, the scale at both 535 and 635° C comprised an inner layer of Ni _x Fe_3−x O₄ (withx≈0.5, on average) and an outer layer ofα-Fe₂O₃, of similar thickness. Finally, on an 83% Ni alloy oxidized at 635° C, the scale consisted of roughly equally thick layers of NiO (next to the metal) and NiFe₂O₄, and a thin outer covering ofα-Fe₂O₃. The decrease in oxidation rate with increasing Ni content of the alloy is discussed briefly in relation to the changing composition of the scale and diffusion in the alloy.