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 aluminium 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 Cr2O3 layer at the breakaway, oxide-substrate interface. Conversely, molybdenum-free alloys from an internal-oxide zone in the substrate adjacent to this interface, rather than a Cr2O3 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 (1150 degrees 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, which is effective in inhibiting breakaway oxide from spending, cannot be established at the breakaway oxide-substrate interface for the molybdenum-free alloys.