The Oxidation off Iron, Nickel and Cobalt-Base Alloys Containing Aluminum

Abstract

By conducting continuous weight-gain measurements from 500 to 1300 C on a variety of Fe-Al, Fe-Cr-Al, Ni-Al and Co-Al alloys in oxygen at 100 torr and examining the reaction products which form, reasons for the enexpected ineffectiveness of preferential aluminum oxidation have been explored. With Fe-4.4 percent Al and Fe-12 percent Al, there are two different regions of parabolic growth- these are associated with the appearance of γ-A12O3 below 900 C and α-Al2O3 above. The latter is normally more protective but possesses a greater tendency for cracking and spalling; adding 24.6 percent chromium promotes its formation at lower temperatures. With Ni-5.0 percent Al, two displaced regions are also found; here, nickel penetrates alumina to provide intermediate NiO · Al2O3 and an outer layer of NiO. With Co-4 percent Al, somewhat less cobalt penetrates γ-Al2O3 to provide CoQ · Al2O3 and an outer layer of Co3O4. When both CoO and α-Al2O3 are stable phases, the latter grows mainly by internal oxidation and offers little protection.

Al2O3scale development on iron aluminides

“…11), where there is no Cr in the alloy. These results are in agreement with the earlier proposals [12,11] …”

Investigation of Iron Aluminide Weld Overlays

“…The results obtained in this study were similar to those found in the literature~oggs; DeVan , 1993z 1993bHagel;Prescott 1992% 1992bSaegusa;Sakiyama;Smith 1987c;Tomaszewski 1978Tomaszewski , 1983Tortorelli , 1993Tortorelli , 1994a. In general, the relative amount of scale development was found to decrease with increasing aluminum content and temperature.…”

“…In terms of the iron oxide that was found to grow on the scale, whether in nodule or continuous scale form, the results are again similar to that observed in the literature~oggs; Hagel;Prescott 1992b;Saegusa;Sakiyama;Smith 1987c;Tomaszewski 1981Tomaszewski , 1983. Growth of the oxides was predominantly by the outward difhsion of Fe cations to form an external layer of F~03 and Fe301.…”

“…While the thin surface scale was not positively identified, enough data exists to assume it as gamma alumina. These facts range from the low weight gain observed to the color of the scales (Hagel (1965) also reports tan, blue, and purple colored scales for gamma formation). Many other researchers working in this temperature regime on similar alloys have also found protection due to the gamma scales~agel;…”

Microstructure and Oxidation Behavior of Low Pressure Plasma Sprayed Iron Aluminides.

“…Hagel reported a-alumina formation below 1 173 K and a-alumina above 1 173 K for Fe-22at%Al alloy. 15) Tortorelli also assigned the oxide as a-alumina from the kinetics curve of mass gain. 3) Additional experiment like isothermal oxidation tests should be conducted to assign the oxide.…”