Aluminium iron aluminides

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... 

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. 

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. 

Klower J. High Temperature Corrosion Behaviourof Iron Aluminides and Iron- Aluminium-Chromium Alloys In Grabke HJ, Schiitze M, editors. Oxidation of intermetaUics. Weinheim - Germany Wiley-VCH Verlag GmbH 1997. p. 203-20. 

Lee and Lin (1999) studied the oxidation, mixed oxidation-sulphidation and hot corrosion of ductile iron aluminide Fe3Al with Cr addition at temperatures of 605-800°C. They observed that hot corrosion of iron aluminide was significantly more severe than oxidation and mixed oxidation-sulphidation. According to Lee and Lin, this can be attributed to the formation of aluminium sulphide at the metal-salt interface as a result of high sulphur potential in the molten salt at the oxide-metal interface. 

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%. 

Several interesting features were apparent when the aluminide specimens were observed in plan after exposure at 700°C (Figure 3). In all cases, there was little evidence for loss of scale by spallation. However, second-phase particles were observed on all the surfaces, particularly of the lower aluminium-containing alloys, FA 56 and FA 57, while the number and size of such particles for a given alloy were less for preoxidized specimens than for those exposed directly in the mixed gas. Detailed examination and EDX analysis of the particles (e. g. labelled, C, A and B respectively in Figures 3(c), 3(d) and 3(f)) indicated them to be rich in iron and sulphur although too small for precise analysis, they were undoubtedly sulphide nodules. These nodules were always small and discrete, with no evidence that extensive sulphidation of the substrate had... 

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