The effects of surface-applied oxide films containing varying amounts of yttria, chromia, or alumina on the high-temperature oxidation behavior of chromia-forming and alumina-forming alloys

In order to study the effects on oxidation behavior of changing the composition of oxide scales without changing the composition of the underlying alloys, thin films, which contained various amounts of Y2O3, Cr2O3, or Al2O3 have been deposited on Fe-25%Cr and Fe-25%Cr-5%Al prior to oxidation. There were major differences between the (Fe-25%Cr)-(Cr2O3-Y2O3) and (Fe-25%Cr5%Al)-(Al2O3-Y2O3) systems. A film of Cr2O3 on Fe-25%Cr5%Al reduced the oxidation rate and Cr2O3-Y2O3 films reduced it still further. A film of Al2O3 on Fe-25%Cr-5%Al reduced the rate considerably, but Al2O3-Y2O3 films exhibited rates that were greater than those for Al2O3 alone. A film, which consisted entirely of Y2O3, greatly decreased the oxidation rate of Fe-25%Cr, but increased that of Fe-25%Cr-5%Al. The changes in oxidation rate produced by Y2O3 in the (Fe-25%Cr)-(Cr2O3-Y2O3) system were much greater than those produced in the (Fe-25%Cr-5%Al)-(Al2O3-Y2O3) system. As far as oxidation rate is concerned, the optimum amount of Y2O3 in the (Fe25%Cr)-(Cr2O3-Y2O3) system was found to be greater than or equal to 40% and less than 100%: in the Fe-25%Cr-5%Al)-(Al2O3-Y2O3) system it was approximately 10%. In thermal-cycling tests, the results for the (Fe-25%Cr)-(Cr2O3-Y2O3) system followed the same pattern as that for the isothermal tests; a film of Y2O3 reduced the oxidation rate and a film of Cr2O3-33%Y2O3 reduced it still further.