Alumina iron aluminides

There should be no significant difference between alumina scales formed on nickel aluminide and those formed on iron aluminide. Compared to Ni3Al, however, Fe3Al has some major advantages ... 

Figure 6>16. In sulfidizing gases iron aluminides are significantly more resistant than alumina-forming FeCrAl alloys. Sulfidizing environment (Pq = 4 X 10" atm Psj = 9 x 10" atm) at 875 °C. For Fe-Al the transition to protective behavior occurred between 18.7 and 22 at.% A1 (data of Natesan, 1997).

Esp] Espinosa-Medina, M.A., Casales, M., Martinez-Villafane, A., Porcayo-Calderon, J., Martinez, L., Gonzalez-Rodriguez, J.G., Hot Corrosion of Atomized Iron Aluminides Doped Wifli Boron and Reinforced With Alumina , Mater. Sci. Eng. A, A300(l-2), 183-189 (2001) (Experimental, Meehan. Prop., Morphology, 18)... 

The diffusion coefficient of aluminium in a ferritic iron-aluminium matrix is some orders of magnitude higher than in austenitic Ni3Al. Because of the low diffusion coefficient, diffusion in nickel aluminides is slow and aluminium depletion beneath the alumina scale and formation of nonprotective nickel oxides has been observed in Ni3Al. [4]. 

As expected, no carburisation attack at all was detected on iron-aluminium-chromium alloys after 1000 hours exposure in CH4/H2 environments at 850°C, 1000°C and 1100°C. Since the formation of chromia and iron requires relatively high oxygen partial pressures, alumina is the only stable phase at the low partial pressure of the used gas. If once formed, alumina is impervious to carbon, provided the scale remains intact [20], Excellent resistance to carburisation was also found for other alumina forming alloys like nickel aluminides [21] and Ni-Al-Cr alloys [22], The results of the present work show that 10 wt% aluminium are sufficient to prevent carburisation. It is expected, that the minimum aluminium concentration is even lower than 10 wt%. 

---