The effects of molybdenum on the high-temperature oxidation resistance of thin foils of Fe-20Cr-5Al at very high temperatures
Thin foils of Fe-20Cr-5Al alloys are susceptible to breakaway oxidation once the aluminum content of the substrate has fallen below some critical value. The combined addition of 0.1 wt.% lanthanum and 0, 1, or 2 wt% molybdenum has a beneficial effect on the high-temperature oxidation of such foils. Lanthanum has the well-known reactive-element effect on adhesion of the protective alumina scale, thereby increasing the time to onset of breakaway oxidation, while, for alloys containing molybdenum, breakaway oxide spreads relatively slowly over the specimen in comparison to alloys that contain no molybdenum. In particular, molybdenum-containing alloys are able to develop a protective Cr{sub 2}O{sub 3} layer at the breakaway oxide-substrate interface. Conversely, molybdenum-free alloys form an internal-oxide zone in the substrate adjacent to this interface, rather than a Cr{sub 2}O{sub 3} layer, so breakaway oxide spreads rapidly. A martensitic phase is observed in the substrate adjacent to the breakaway oxide formed on Fe-20Cr-5Al-La specimens, which means that the {alpha}-phase has transferred to the {gamma}-phase at the temperature of the oxidation test (1,150 C). Conversely, {alpha}-phase is retained in the molybdenum-containing alloy, even after breakaway takes place, since molybdenum, which is a strong ferrite former, is enriched in the alloy adjacent to areas of breakaway oxide. The diffusion rate of chromium is slower in the {gamma} than in the {alpha}-phase so a continuous chromium-rich oxide layer, which is effective in inhibiting breakaway oxide from spreading, cannot be established at the breakaway oxide-substrate interface for the molybdenum-free alloys.
- Research Organization:
- UMIST, Corrosion and Protection Center, Manchester (GB)
- OSTI ID:
- 20050533
- Journal Information:
- Oxidation of Metals, Vol. 53, Issue 5-6; Other Information: PBD: Jun 2000; ISSN 0030-770X
- Country of Publication:
- United States
- Language:
- English
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