Environment iron aluminides

Examination of the specimens after exposure confirmed that thin protective scales had developed and were retained on the iron aluminides and on the other alloys, except where large weight gains were recorded, in all three environments. The scales on the iron aluminides at 700°C were essentially A1203, as confirmed by X-ray diffraction... 

Iron aluminides containing 5 wt% Cr, 0.2 wt% Zr and 8 to 16 wt% Al show better resistance to a mixed-gas, H2/1.5 %H2S/4.2%H20 environment at 700°C than conventional A1203- and Cr203-forming alloys, with the extent of sulphide formation for the aluminides increasing with decreasing aluminium concentration in the substrate. 

Given that Fe-Al intermetallics were among the earliest intermetallics investigated, a historical perspective of Fe-Al alloy development will be presented. This is followed by a discussion of the oxidation behavior of Fe3Al and FeAl in air. Because of interest in their use in complex, corrosive environments, a detailed discussion of the oxidation behavior of iron aluminides in intermediate temperature (<800°C) environments containing species such as sulfur, chlorine, and carbon will also be presented. 

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).

In carburizing environments, if the relative partial pressures of oxygen and carbon favor AI2O3 stability and the temperature is such that the kinetics of AI2O3 formation enable rapid coverage of the alloy surface, iron aluminides would be expected to have very... 

P. (1997), Mater. Sci. Eng. A 234-236, 22-29. Bakker, W. T. (1998), Corrosion of Iron Aluminides in HCl-Containing Coal Gasification Environments, Paper No. 185, Corrosion/98. Houston, TX NACE International. 

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%. 

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