Alloys, iron aluminides

Iron alloys, selenium and metallurgy of, 22 98. See also Ferrosilicon Iron aluminide... 

High Temperature Corrosion Behaviour of Iron Aluminides and Iron-Aluminium Chromium Alloys... 

The high temperature corrosion behaviour of different iron aluminides and iron-aluminium-chromium alloys containing 6-17 wt% aluminium, 2-10 wt% chromium and additions of mischmetal has been investigated in both air and hot process gases. 

A set of four iron aluminides (FA 49, FA 56, FA 57 and FA 58) was supplied by Shell Research, with compositions given in Table 1. These contained about 5 wt% Cr and aluminium concentrations from about 8% to 16 wt%.They all included a small amount of zirconium while three of them also contained yttrium, to assist in maintenance and retention of the protective A1203 scale. For comparison, two other Al203-forming alloys and two Cr203-forming alloys were included in the programme.The former alloys contained much more chromium than the aluminides but much less aluminium MA... 

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. 

Under these conditions, the presence of 0.2 to 0.3 wt% Y in the iron aluminides has little influence on the effectiveness of the thin A1203 scale in giving protection however, extensive intergranular precipitates, rich in yttrium and zirconium, are formed in alloys containing 0.3 wt% Y. 

It is more difficult to establish a protective A1203 scale at lower temperatures than at higher temperatures, particularly on alloys of relatively low aluminium concentrations thus, an iron aluminide containing 8 wt% Al formed more extensive sulphides at 500°C than at 700°C, although a protective oxide scale prevented significant sulphidation of an alloy containing 12 wt% Al at either temperature. 

Selective solution of the aluminum from the alloy using a volatile metal, such as mercury, lead, bismuth, cadmium, magnesium, or zinc, has been investigated. After extracting the aluminum from the original alloy into the volatile metal, the volatile metal is distilled, leaving pure aluminum. Neither electrolysis nor volatile metal extraction can extract aluminum from iron aluminide [12004-62-5], FeAl, titanium aliiminide [12004-78-3], TiAl, or Al C. ... 

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... 

Corrosion of iron aluminides by a highly aggressive, oxidizing molten salt used in chemical air separation processes (NaN03-KN03-Na202) at 650 °C has been shown to proceed by oxidation and slow release of an Al-rich product layer into the salt. Alloys richer in Al were significant-... 

Intermetallic hardening alloys, for example, nickel-base superalloys, austenitic stainless steels, and iron-aluminides... 

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